EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: bloguetronica on September 06, 2018, 04:28:58 am
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Hi,
I'm currently thinking of doing a variable power supply. In order to be more efficient, I wanted to use a variable DC-DC converter before my pass Darlington element, basically to keep the input voltage a couple of volts above the output voltage, and to reduce the dissipation on that pass Darlington. I don't have a circuit yet, but it would be something in the likes of the one attached, but adding lots more power besides that DC-DC converter before the pass element (in this case will be a Darlington and not a simple NPN).
I wonder, is there any DC-DC converter controlled directly by voltage? Say, a tracking DC-DC converter? I think I can make a control circuit that just adds 2 or 3 volts and outputs the voltage to be tracked, no problems there. I want this to be very accurate, and that's why I'm not planning to use an off the self DC-DC converter + LDO in the same package or something.
Kind regards, Samuel Lourenço
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DC-to-DC converters which use an external feedback divider can be used to do this. One complication however is that the DC-to-DC converter will need to operate over a wide voltage range.
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Thanks David. But using a DC-DC converter like this wouldn't have the opposite effect? I mean, if the DC-DC converter sees a too low voltage on its feedback loop (from the DAC, or the output - to be decided), won't it attempt to ramp up its output voltage to compensate?
Kind regards, Samuel Lourenço
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Thanks David. But using a DC-DC converter like this wouldn't have the opposite effect? I mean, if the DC-DC converter sees a too low voltage on its feedback loop (from the DAC, or the output - to be decided), won't it attempt to ramp up its output voltage to compensate?
Isn't that what it is suppose to do? The idea is to use the DC-to-DC converter to regulate the voltage across the linear regulator while the linear regulator regulates the output voltage. If the output voltage rises, then the output voltage from the DC-to-DC converter rises to compensate and maintain the same voltage across the linear regulator.
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Hi David. That was the idea. The DC-DC converter has to keep a certain voltage difference from the linear output, or from the DAC. I see that my logic was flawed. I was thinking about the feedback return seen on many DC-DC converters. So, what I have to use is a variable DC-DC converter that has its voltage set by a voltage divider, right?
Kind regards, Samuel Lourenço
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See Dave's video #260 & #329:-
https://www.youtube.com/watch?v=iTxKCQYMHbY (https://www.youtube.com/watch?v=iTxKCQYMHbY)
https://www.youtube.com/watch?v=myqytW9ecww (https://www.youtube.com/watch?v=myqytW9ecww)
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Thanks, intabits. The circuit with the PNP transistor looks like a solution that I can use. It seems I can use any boost DC-DC converter. I wonder if I can use a buck converter. Anyway, if I can find a variable isolated DC-DC boost converter, I can ditch the buck converter after the fixed isolated converter and use it as pre-regulator.
Kind regards, Samuel Lourenço
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Yes you can use a buck converter. Such solution I've used in EEZ H24005 based on LTC3864 (see Power board (http://www.envox.hr/eez/bench-power-supply/power-board.html)). Regarding "PNP tracker" don't forget to add 470 pF or so as suggested in Dave's video. Pre-regulator use 48 Vdc on its input and gives about 3-43 V on output.
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Thanks prasimix,
It seems that I'll have to use a buck converter. Can't find any isolated DC-DC converter with a suitable output voltage and/or with external feedback.
I've noticed that the boost or buck converter have a fixed output voltage of 5V, on their standard application. Is there any reason? Does that have any influence? This is hard to understand, BTW.
Kind regards, Samuel Lourenço
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Or -- stick with me here -- how about making a DC-DC converter quiet enough that it doesn't need a postreg? ;)
I guess this is the beginner's dual-control project, in which case, by all means proceed. I don't see that it's ever beneficial or capable of delivering on its stated or implied goals as well as a simpler solution though.
Tim
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Hi,
I can't use a off-the-self DC-DC converter without any kind of "postreg", since I want both low noise and accuracy. Hence the topology with the Darlington being served with a precision DAC and a precision op-amp. The voltage after the DC-DC doesn't need to be accurate at all. It only needs to have a certain difference to mitigate the dissipation on the Darlington., yet still providing enough overhead.
Kind regards, Samuel Lourenço
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Ah, so you're not going to attempt an SMPS design yourself, only looking for off-the-shelf?
There's still a lot you can do (e.g., servo the output of an inaccurate module with a more accurate reference and opamp), though the noise levels of COTS modules are probably going to suck no matter how you cut it (passive or active filter).
Tim
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Thanks prasimix,
It seems that I'll have to use a buck converter. Can't find any isolated DC-DC converter with a suitable output voltage and/or with external feedback.
I've noticed that the boost or buck converter have a fixed output voltage of 5V, on their standard application. Is there any reason? Does that have any influence? This is hard to understand, BTW.
Kind regards, Samuel Lourenço
Yes, in almost 100% of all app. notes and eval. boards output voltage is fixed since it's easier in that case to ensure stability of the circuit despite the load connected and input voltage variations. But, many of them can be make stable regardless of broad output voltage range. So far, I didn't find any example of converter presented as pre-regulator. Usually people mentioned a couple of Jim Williams LTC app. notes that include some basic switching pre-regulator but nothing based on "modern" SMPS controller. If you're going to design your own SMPS pre-regulator take care about PCB layout and try to use as much as possible SMT parts since they are smaller and can be better organized for lower EMI and circuit stability. If possible (don't know your budget and final goal) try to use 4-layer PCB instead of min. two layers.
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Well, I will have to weight all the options carefully. Probably a self made DC-DC converter will do, although I saw an application based on the LM2576 from Texas Instruments. In my application, I don't require a sophisticated DC-DC converter. However, I was not expecting that the DC-DC converter would be one having a fixed voltage, which is counter intuitive. With that feedback loop, the output voltage is now variable?
Difficult to grasp for me, IMO, for now. I want to get there and fully understand this concept from head to toe. Only then, when I understand the whole concept, and when I can customize it, I can call myself a designer of this project. Probably, a discretely designed DC-DC converter, customized to the task at hand, having minimal parts, would be ideal. A design simple, elegant, and one that I can understand. I have to research, research, research... and reinvent the wheel if necessary.
As for the PCB, you gave a good advice. A 4-layer board is definitely in the requirements for this project.
Kind regards, Samuel Lourenço
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You could relay switch between a few different converters for different ratios. Then each of them can be optimized for that voltage and a specific load current. For very low currents you can bypass all of them.
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Well, I will have to weight all the options carefully. Probably a self made DC-DC converter will do, although I saw an application based on the LM2576 from Texas Instruments. In my application, I don't require a sophisticated DC-DC converter. However, I was not expecting that the DC-DC converter would be one having a fixed voltage, which is counter intuitive. With that feedback loop, the output voltage is now variable?
Easy -- start with an adjustable regulator. The feedback pin ties to a voltage divider sensing the output.
So, suppose we tie another resistor to the feedback pin. If we tie the other end to ground, it's in parallel with the lower resistor of the divider, and the ratio drops, so the output voltage rises. Simple enough? :)
Suppose we tie the other end to a voltage reference. If that voltage is 0V (GND), we have the above situation. If VREF = V(FB), nothing happens, because there's no voltage across the resistor, no current flow and no one's the wiser. If we tie it to a higher voltage, then some of the current into the bottom divider resistor comes from this one, and some comes from the original (top divider) resistor. The bottom resistor always draws the same current (because V(FB) is kept constant by the internal control), so that current gets divided between the two top resistor.
This is a linear circuit, so there's no distinction between any of these cases, they're all equivalent. In short, biasing the FB pin down causes the output to rise, and vice versa.
Or, overall, the FB pin is the -in of an inverting opamp, so putting current into that pin (with the feedback divider connected as usual) causes the output to fall proportionally. A simple geometric leverage problem. :)
So it's very easy to set the output with a pot or opamp or DAC. :)
Tim
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I thought about doing this a few times but I never saw the benefit of not just using a large transformer and heat sink. Only got interested when I thought about running power MMIC circuits. But they are so expensive I am scared of not just using a commercial PSU because I might not anticipate a failure mode. But with a 800 $ IC you can throw together a giant heat sink and the most inefficient regulators imaginable and it will still be worth it........
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Well, I will have to weight all the options carefully. Probably a self made DC-DC converter will do, although I saw an application based on the LM2576 from Texas Instruments. In my application, I don't require a sophisticated DC-DC converter. However, I was not expecting that the DC-DC converter would be one having a fixed voltage, which is counter intuitive. With that feedback loop, the output voltage is now variable?
Easy -- start with an adjustable regulator. The feedback pin ties to a voltage divider sensing the output.
So, suppose we tie another resistor to the feedback pin. If we tie the other end to ground, it's in parallel with the lower resistor of the divider, and the ratio drops, so the output voltage rises. Simple enough? :)
Suppose we tie the other end to a voltage reference. If that voltage is 0V (GND), we have the above situation. If VREF = V(FB), nothing happens, because there's no voltage across the resistor, no current flow and no one's the wiser. If we tie it to a higher voltage, then some of the current into the bottom divider resistor comes from this one, and some comes from the original (top divider) resistor. The bottom resistor always draws the same current (because V(FB) is kept constant by the internal control), so that current gets divided between the two top resistor.
This is a linear circuit, so there's no distinction between any of these cases, they're all equivalent. In short, biasing the FB pin down causes the output to rise, and vice versa.
Or, overall, the FB pin is the -in of an inverting opamp, so putting current into that pin (with the feedback divider connected as usual) causes the output to fall proportionally. A simple geometric leverage problem. :)
So it's very easy to set the output with a pot or opamp or DAC. :)
Tim
Oh, now I see! Hence the PNP transistor controlling the FB pin. So, essentially, if the output voltage (after the linear pass element) drops, the base of the PNP is pulled up, then the collector current rises and pulls up the FB pin (as if the output voltage of the DC-DC was too high), and then the voltage of the output of the DC-DC converter drops accordingly. That's actually simple and brilliant. Essentially, the PNP inverts the compensation effect of the FB pin, right? In this case, it shouldn't matter if the DC-DC converter is fixed or not.
Thanks Tim!
I thought about doing this a few times but I never saw the benefit of not just using a large transformer and heat sink. Only got interested when I thought about running power MMIC circuits. But they are so expensive I am scared of not just using a commercial PSU because I might not anticipate a failure mode. But with a 800 $ IC you can throw together a giant heat sink and the most inefficient regulators imaginable and it will still be worth it........
It is worth it, IMHO. No relays or transformer taps, no relays, you can use a smaller pass element and much smaller heatsink, and less heat drifting the precision of your voltage reference (but that last reason is debatable, and I'm nitpicking here).
Kind regards, Samuel Lourenço
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I thought about doing this a few times but I never saw the benefit of not just using a large transformer and heat sink. Only got interested when I thought about running power MMIC circuits. But they are so expensive I am scared of not just using a commercial PSU because I might not anticipate a failure mode. But with a 800 $ IC you can throw together a giant heat sink and the most inefficient regulators imaginable and it will still be worth it........
Using commercial PSU is no warranty that something does not go wrong. @dmg learned (https://www.eevblog.com/forum/projects/disabled-power-supply-blows-up-expensive-board-how/msg1380006/#msg1380006) that in hard way. Neither large transformer and heat sink (i.e. classical linear regulator) alone is enough that sensible load can be safely powered (just imagine for example a large power up overshoot). You need a properly designed voltage and current control loops combined with various protection mechanisms to decrease chance of failure to the minimum.
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Oh, now I see! Hence the PNP transistor controlling the FB pin. So, essentially, if the output voltage (after the linear pass element) drops, the base of the PNP is pulled up, then the collector current rises and pulls up the FB pin (as if the output voltage of the DC-DC was too high), and then the voltage of the output of the DC-DC converter drops accordingly. That's actually simple and brilliant. Essentially, the PNP inverts the compensation effect of the FB pin, right? In this case, it shouldn't matter if the DC-DC converter is fixed or not.
Not quite. I think their intent was to have a current source into the pin, but the current is proportional to output voltage minus control voltage. It's a high-side current source, and therefore referenced to the output rail.
So the current mirror really doesn't accomplish anything, and it looks like another resistor in parallel with the top divider resistor.
I suppose that's not inherently a bad thing, it just reduces the gain. The current mirror does invert the control signal, though, so it acts as a follower rather than an inverter.
If the control input is in turn set by the output (postreg) voltage, it should track, give or take gain and offset, which I'd have to write down and figure out to be sure about.
I think I would prefer to use the two-way resistor divider, and drive the control node with an op-amp that computes the required gain and offset. At least for starters.
I might then decide to optimize it, potentially to as simple as a single transistor -- in which case, it would be roughly equivalent to what's shown, give or take resistor values and offsets and all that. Again, I'd have to write it down.
The rest of the circuit looks a bit sausage to my eyes, it could be simplified, and linearized better for more stable control and probably lower noise at the same time, or something like that. That's just more to the point that almost all lab supply circuits on the internet are crap, just to varying degrees. I've only seen one that's mostly good (unfortunately, the author got very defensive when given constructive criticism).
I mean, I'd design the perfect supply to end all supply design threads -- but I don't need one, and no one's giving me any money to. :P
Tim
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Thanks Tim! I'll have to consider all the options. Gain is not important. Probably, I could sort of track the DAC voltage to have a faster response (so that the voltage after the pass element doesn't have to depend on the voltage of the DC-DC converter, which in turd depends on the former).
However, I'll add current limiting, and I have to create an unusual circuitry to compute the "should be" output voltage (which is limited to a value when the current is "limited" to a set value, according to the load resistance - or is equal to the DAC voltage when the current is not limited). Probably, on second thought, I'll track that computed voltage indirectly without the use of the funny transistor, although tracking the output voltage directly via that transistor would be much simpler. So, if I add a voltage to that computed value, via an op-amp or even a Zener pulled up, I'll have the tracking reference for the DC-DC converter. Thus, the computed value would be the reference for the op-amp that controls the pass element, and the added value (via the Zener) would be the reference for the DC-DC converter.
Lets see where this goes.
Kind regards, Samuel Lourenço
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You don't want the SMPS output to track DAC/setpoint output, because it won't follow under a current limit condition. That's why it's made to follow the actual output instead. :)
(But you can get just as stable DC with a servo amp setting the SMPS correctly; and, a reasonably fast, stable and well-filtered SMPS can do as well on AC noise, as the postreg can, without the extra loss).
Tim
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I'm planning to do something as represented in the attached sketch.
There are some things that were not represented there:
- The amp circuitry that controls the pass transistor, in this case a Darlington, will include lead impedance/resistance compensation (and the possibility of having an external voltage sensing at the load). This is fairly easy to implement, and it only requires 6 precision resistors and a few protection diodes.
- The DC-DC converter control is not that straightforward, and probably is grossly misrepresented here. The voltage that controls this stage is the same voltage that controls the final stage, but with a Zener drop added. Probably instead of the Zener, I'll use the PNP transistor and drive its base with the voltage from the "computation block".
- The "computation block" is fairly simple. You have two DACs, one that sets the voltage, and one that sets the current. It the voltage resulting from the current sensed at the output equals or exceeds the voltage given by the "Iset" DAC, the compute block will output the voltage so that these previous voltages are equal. If not, the "compute block" will output the exact same voltage as the "Vset" DAC.
This will require some extra precision op-amps and comparators, but it allows for a very precise voltage and current control, which is the main goal of this project. As for taking out the post regulator altogether, and replacing it by a filter, well, you will need a huge filter. I want to keep the noise as low as possible, and on any load conditions.
Kind regards, Samuel Lourenço
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- The amp circuitry that controls the pass transistor, in this case a Darlington, will include lead impedance/resistance compensation (and the possibility of having an external voltage sensing at the load). This is fairly easy to implement, and it only requires 6 precision resistors and a few protection diodes.
Hm, if you are talking from experience than you'll be well aware that is not "fairly easy to implement" since there is a lots of places where things can goes wrong. Many of them are even easily visible in simulation and you can spent hours and hours in front of simulation just to address various "use cases" and still not be 100% sure that everything can be fine with real circuit.
- The DC-DC converter control is not that straightforward, and probably is grossly misrepresented here. The voltage that controls this stage is the same voltage that controls the final stage, but with a Zener drop added.
- The "computation block" is fairly simple. You have two DACs, one that sets the voltage, and one that sets the current. It the voltage resulting from the current sensed at the output equals or exceeds the voltage given by the "Iset" DAC, the compute block will output the voltage so that these previous voltages are equal. If not, the "compute block" will output the exact same voltage as the "Vset" DAC.
This will require some extra precision op-amps and comparators, but it allows for a very precise voltage and current control, which is the main goal of this project. As for taking out the post regulator altogether, and replacing it by a filter, well, you will need a huge filter. I want to keep the noise as low as possible, and on any load conditions.
It seems that you're really interesting in making a pre-regulator (mind the prefix in that name :)) control overcomplicated. All what you need is a simple PNP tracker connected to output as you are already instructed in some of previous posts. That tracker will faithfully follow output voltage regardless of mode of operation, i.e. CV-constant voltage or CC-constant current when due to current limitation voltage starts to drop and your pre-regulator output voltage will follow that change!
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- The amp circuitry that controls the pass transistor, in this case a Darlington, will include lead impedance/resistance compensation (and the possibility of having an external voltage sensing at the load). This is fairly easy to implement, and it only requires 6 precision resistors and a few protection diodes.
Hm, if you are talking from experience than you'll be well aware that is not "fairly easy to implement" since there is a lots of places where things can goes wrong. Many of them are even easily visible in simulation and you can spent hours and hours in front of simulation just to address various "use cases" and still not be 100% sure that everything can be fine with real circuit.
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Well, I've used the concept already in another project, but I'm yet to test it, truth to be told. Not complicated at all. If you open the schematic attached to the OP, you'll see four precision resistors near the op-amp. That is for internal lead compensation.
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- The DC-DC converter control is not that straightforward, and probably is grossly misrepresented here. The voltage that controls this stage is the same voltage that controls the final stage, but with a Zener drop added.
- The "computation block" is fairly simple. You have two DACs, one that sets the voltage, and one that sets the current. It the voltage resulting from the current sensed at the output equals or exceeds the voltage given by the "Iset" DAC, the compute block will output the voltage so that these previous voltages are equal. If not, the "compute block" will output the exact same voltage as the "Vset" DAC.
This will require some extra precision op-amps and comparators, but it allows for a very precise voltage and current control, which is the main goal of this project. As for taking out the post regulator altogether, and replacing it by a filter, well, you will need a huge filter. I want to keep the noise as low as possible, and on any load conditions.
It seems that you're really interesting in making a pre-regulator (mind the prefix in that name :)) control overcomplicated. All what you need is a simple PNP tracker connected to output as you are already instructed in some of previous posts. That tracker will faithfully follow output voltage regardless of mode of operation, i.e. CV-constant voltage or CC-constant current when due to current limitation voltage starts to drop and your pre-regulator output voltage will follow that change!
Probably I'm over-complicating. But having a voltage (from the computation block) that is equal to the output voltage, and deriving the DC-DC control from there provides faster response, right? Then I could drive the PNP tracker with the voltage from that "computation block" instead of using the output voltage for that. Yes, I dropped the Zener idea, meanwhile.
Without using any "computation block", I could track the output voltage directly, and implement CC-constant by starving the base of pass transistor. I'm not sure about the precision if I do that. Probably I can get away with just another DAC, an instrumentation amplifier for the current sensing (or a dedicated amplifier), a comparator and another NPN transistor. The NPN transistor would be wired in an analogous fashion as depicted in the OP schematic, but its base would be connected to the comparator. I think this last approach is more simple and could be tested first.
Kind regards, Samuel Lourenço
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You may find this helpful for the "computation block":
(https://www.seventransistorlabs.com/Images/Limiter2.png)
The op-amp could be the internal error amp of the converter, so you can set variable current limiting, and then the postreg only needs to be a C-mult.
Tim
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You may find this helpful for the "computation block":
(https://www.seventransistorlabs.com/Images/Limiter2.png)
The op-amp could be the internal error amp of the converter, so you can set variable current limiting, and then the postreg only needs to be a C-mult.
Tim
I'm not sure if that was what I had in mind. How does that circuit work? I see some kind of current mirror to limit the current (transistor at the bottom), then the input stage of an op-amp, and then a transistor pulling up the negative input of the amp-op, which I presume is to implement CC limiting.
Kind regards, Samuel Lourenço
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Right, the transistor stuff wraps around the existing error amp, say of a CV power supply to add CC operation. The transistors have limited gain, a necessity to deal with the already high loop gain of the op-amp without introducing too much slop themselves (it will transition from CC to CV mode in the span of maybe 100mV at the output pin).
This is better than two opamps wired-OR with diodes, because there is no integrator windup. It acts ~instantly, going from CC to CV mode.
Tim
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Hi, a design I saw was from Jim Williams with an LT1074 preregulating an LT1083. ChangPuak also used this idea in a wide range lab PSU:
https://www.changpuak.ch/electronics/PETH-581.php (https://www.changpuak.ch/electronics/PETH-581.php)
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Hi Wolfgang,
I just don't want to copy a design. I want to build a design that I can understand. That is one of the points of the project. Anytwy, I appreciate your suggestion.
Right, the transistor stuff wraps around the existing error amp, say of a CV power supply to add CC operation. The transistors have limited gain, a necessity to deal with the already high loop gain of the op-amp without introducing too much slop themselves (it will transition from CC to CV mode in the span of maybe 100mV at the output pin).
This is better than two opamps wired-OR with diodes, because there is no integrator windup. It acts ~instantly, going from CC to CV mode.
Tim
Thanks Tim!
Anyway, I've decided to ditch the whole computational block idea, since it wont calculate the voltage well. So, I will base the DC-DC pre-regulator feedback voltage on the output voltage via the PNP transistor. I decided to implement constant current on another way, using a transistor to starve the base of the pass transistor (perhaps using a MOSFET instead of a BJT to starve the base of the main transistor is better?).
That transistor (BJT or MOSFET?), in turn, will have its base connected to the output of a op-amp wired as a comparator. The reason I'm using an op-amp here instead of a comparator is because I want the op-amp to operate in its linear region. At the inputs of the op-amp, is connected the output of an INA180 current sensing amplifier and the output of a DAC that sets the current.
I've also decided to ditch the voltage sensing. After all, I don't need the extra precision. I'll post the drawn schematic later.
Kind regards, Samuel Lourenço
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Hi,
I never suggested to build something you dont understand. I never do that, either.
Nobody stops you from learning and understanding designs from others (including simulation) and improving or adapting them on your own.
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Sorry for misunderstanding. I never taken you as being that "assertive", and it was not my intention to do so, hence the word "suggestion".
Anyway, I have an idea for a schematic, but I'm having a busy day. Will post it as soon as possible.
Kind regards, Samuel Lourenço
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So you want something you created and understand, but that is worse than the standard approach?
Tim
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Hi Tim,
How it is worse? I didn't presented the schematic yet! What is the standard approach? Am I taking the wrong approach by basically not copying a design (at least blindly)? Although Wolfgang presented a pretty much valid and working design, I decided not to use it, but thanked anyway.
What can I say? I like to reinvent the wheel, despite the fact of often being criticized for that. I like to experiment, and even if I do mistakes it is not important, because I can learn from them. Nothing beats our own experience.
I also don't know the concept of "standard" in electronics. There is just different ways of doing things. And since I was over-complicating I just decided to be back to the drawing board (the "computation block" would never work as intended, BTW, because I was thinking wrong in the first place). Simpler, more elegant design, with less problems and caveats.
Regarding the pre-regulator DC-DC, I'll use the PNP transistor to control it, and it will be based on the output voltage. As you suggested earlier:
You don't want the SMPS output to track DAC/setpoint output, because it won't follow under a current limit condition. That's why it's made to follow the actual output instead. :)
...
Kind regards, Samuel Lourenço
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It's a behavior I've seen a lot in audiophile circles, and that's probably why I snapped at it. They can get away with it, because audio amplification is a very easy problem (tolerant of poor solutions), and few of their designs ever have to face the rigors of reality.
It works, for some range of "works", but that doesn't mean it's a good idea.
It's very much a good thing to be different, to explore the available design space -- there are many valid ways to wire up components (although very, very many more invalid ways in total!), all with different tradeoffs. Having the experience to know which combinations of tradeoffs are best for a given application, is the payoff.
The problem is if you get far enough into the project that it works, then you stop without going the extra step to finish it up, to make it work consistently and reliably -- that's the part that bugs me the most.
Another way to put it: is this a learning experience, or a challenge towards a stated goal?
If it's the former, definitely do play around and test -- emphasizing in particular, the many ways a circuit can go wrong. Test the extremes of inputs and outputs, of environmental conditions (temperature and such), of component ratings. See how things fail, and understand why manufacturers choose ratings as they do.
While, if it's the latter, you have a clear goal in mind -- a power supply with some minimum efficiency, maximum noise, operating range, functionality and so on (probably with size and cost being secondary considerations) -- there is a far smaller space to choose from, that will meet those goals.
I'm probably wrong for emphasizing a goal here, and that's probably because my experience points me, very quickly, to a set of possible solutions. I should reflect more often on what it was like without that experience...
Tim
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I think you are mixing the fact that I want a linear power supply with what happens inside the audiophoolery circles. These are two separate goals. One involves (me) designing a power supply with acceptable noise levels, even if it is not 100% efficient, the other revolves around expensive capacitors, unidirectional cables and snake oil.
There are far lesser efficient PSUs than the one I'm designing now, believe me (and that includes two that I've designed myself, that are very inefficient, emphasis on very). The DC-DC pre-regulator is not the center of the design, but a secondary aspect that is there just to avoid excessive dissipation on the pass element. Making it the primary regulation element, and then adding a pretty heavy filter, may not solve the noise completely. In fact, I consider using a DC-DC converter a necessary evil, just to keep the design more compact. I'm just using it, because the evil is already done since the whole contraption will be fed by an isolated and unregulated DC-DC converter.
This is just a concept that I'm developing, that may turn or turn not into a fully fledged project. And, if that fails, a learning experience. Essentially, most of my projects started that way.
Kind regards, Samuel Lourenço
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I think you are mixing the fact that I want a linear power supply with what happens inside the audiophoolery circles. These are two separate goals. One involves (me) designing a power supply with acceptable noise levels, even if it is not 100% efficient, the other revolves around expensive capacitors, unidirectional cables and snake oil.
There actually are true directional cables - they have multiple layers of shielding with the outer layers grounded at only one end. The intent being that induced noise gets conducted to the device that is less sensitive to noise (usually the source) or has a more direct path to ground.
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I think you are mixing the fact that I want a linear power supply with what happens inside the audiophoolery circles. These are two separate goals. One involves (me) designing a power supply with acceptable noise levels, even if it is not 100% efficient, the other revolves around expensive capacitors, unidirectional cables and snake oil.
There actually are true directional cables - they have multiple layers of shielding with the outer layers grounded at only one end. The intent being that induced noise gets conducted to the device that is less sensitive to noise (usually the source) or has a more direct path to ground.
I think you should elaborate on that. If shielding is well done, the behavior of the cable should be the same lengthwise. But we are drifting a tad off-topic here.
Kind regards, Samuel Lourenço
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I think you are mixing the fact that I want a linear power supply with what happens inside the audiophoolery circles. These are two separate goals. One involves (me) designing a power supply with acceptable noise levels, even if it is not 100% efficient, the other revolves around expensive capacitors, unidirectional cables and snake oil.
FYI, there is a more technical, design-centered facet of audiophoolery, that likes to play with novel circuits -- diyAudio and Audiokarma are probably good examples. Audiophoolery is a huge sliding scale, from good engineers with a bit of a kink (an analogous and somewhat topical example might be Dave Berning's DC-transformer (chopper based) designs, worth a look), to the casual rich, looking to improve the chakra of their music with crystal pyramids supporting hand-woven silver cables.
Don't take it as an insult -- it's a sign of creativity. Just don't get disgusting with it, that's all. ;D
Making it the primary regulation element, and then adding a pretty heavy filter, may not solve the noise completely.
Let's talk about that --
How much attenuation is "complete"? 40dB (100x)? 80dB? 200dB? Below the noise floor (whatever that might happen to be)?
Because it of course can never be complete-as-in-actually-zero, not in the real world, not at finite temperature and over finite time!
How much attenuation do you expect to obtain from the linear regulator stage?
It's not as easy a problem as you may envision, and you're open to common mode noise regardless of what circuitry is on either side of the converter. The converter needs to be laid out properly to avoid this, regardless, and some additional filtering will inevitably be required (for some values of "complete" ;) ).
So anywhere you're using a nasty rattling converter, you have to be careful to filter it, on all sides. The only true solution is going full linear, which as you already understand is quite inefficient, and is still not the greatest because you still have to deal with mains noise, diode recovery noise, and supply ripple. The noise floor is set by the output transistor, and/or overall output impedance*, which defines how much filtering you need, to reach that floor from some given noise source, whatever that might be.
*Note that capacitors have noise, just as everything else does: take the impedance at a given frequency, and convert it into noise with the Johnson noise relation:
https://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise#Thermal_noise_on_capacitors
Since the impedance is usually low (~ohms), this is a small contribution (below 1nV/rtHz), but still nonzero.
The easiest low-noise source is a battery: the effective capacitance is extremely large, and as long as the current draw is small, the chemical overpotential is small as well. Noise is usually due to thermal and ionic motion, having a 1/f spectrum; keeping wind currents away, and current draw low, helps greatly.
Anything else is more complicated, whether a linear, switching or hybrid supply.
I recommend a well-filtered switching supply, because you may need filtering regardless of what topology you choose, and filters are easier to make than low-noise linear regulators: just throw more inductors and capacitors at it!
Tim
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Hi,
Didn't take any offense, not at all. However, I was thinking that you took me for an audio-follower or something.
As for the filter, I'm not concerned about its attenuation, but what can be achieved with varying loads. Using a filter right at the output is kind of novel, although a DC-DC already uses an inductor and a capacitor, basically a filter. Going full linear unfortunately is not an option, for the reasons mentioned above. So I think it is better to go midway and compromise.
Anyways, using a low to medium frequency DC-DC converter is within my plans. Although lower frequencies are harder to filter, typically the PSRR of the linear stage is much better for those frequencies.
Kind regards, Samuel Lourenço
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Hi,
I've been very busy lately. Only today I've came up with a schematic. The DC-DC pre-regulator will be designed according to the "PNP tracker" configuration shown.
There are some things that I want to explain about the schematic:
- The first half of the OPA2703 is required for buffering, while the second half is responsible for the feedback control. The added resistors will provide a gain of four over the voltage from the AD5551 DAC, while also providing voltage sensing and lead compensation. The OPA2703 was chosen because of its low offset. This block will require 0.05% resistors to maintain the overall precision.
- The current control is done via the LTC2640 DAC. The INA180 is an instrumentation amplifier specialized for current sensing, and the OPA705 works as a comparator of sorts. I didn't used an actual comparator here because I want the OPA705 to operate in its linear region. The 2N3904 will starve the base of the pass transistor.
- The pass transistor is a MJD122 Darlington. Unfortunately, it is not properly drawn.
- The 0.1 \$\Omega\$ resistor will make sure that the inverting input of the INA180A will not be pulled down to 0, maintaining the operation in the linear region.
- I'll have to replace the OPA705 and OPA2703 with higher voltage parts, or decrease the supply voltage and also the voltage gain (overlooked that part). Edit: I will use the OPA2180 as a replacement for the OPA2703. The OPA705 is fine to use, since I can supply 5V to it.
Kind regards, Samuel Lourenço
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Hi,
Been simulating the voltage regulator circuit with current limiting, and it worked as expected. VG1 is the reference that sets the output voltage, which is 4x bigger than the value of this reference voltage. On the other hand, VG2 sets the current limit to a value that corresponds to this reverence voltage divided by four. In other words:
Vout = 4 x VG1
Iout = VG2 / 4
I've used the OPA703 in the simulation, instead of the OPA705 I'm going to use in the end product. The transistor at its output can be either a BJT or a MOSFET (the later should have a low Vgs). In the near future, I'll implement the DC-DC pre-regulator circuit in the simulation. So far, it worked like a charm, even when defining small output voltages and currents.
Attached is a DC-DC sweep of the reference voltage VG1. You can see how the output voltage increases until the current limit is reached, because a 16 \$\Omega\$ load resistor was used, and the current limit is set to 500mA (VG2 = 2V).
Kind regards, Samuel Lourenço
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Hi,
Meanwhile I did a few tweaks to the circuit. I've added a CC LED indicator that light up when the threshold current is exceeded. Basically, the output of the OPA703 goes from approx. 0V to around 600mV, which is the voltage required to bias the CC control transistor. The comparator compares that to a reference voltage of around 300mV, given by the BAT54, and lights up the LED when the CC condition is detected.
I've noticed that the base current of the transistor is very stable, and stays around 60 to 70uA when the circuit is in CC mode. So, there is no need of a base transistor to limit the current, since, as expected, the negative feedback loop takes care of that. Thus, the transistor is always correctly polarized and works in the active region when passing current.
Kind regards, Samuel Lourenço
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An op-amp driving a base directly? Oh dear.
Tim
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An op-amp driving a base directly? Oh dear.
Tim
Hi Tim,
What will happen? I'm trusting the simulation too much? A base resistor doesn't seem to make a difference there, since the current is too small. Do you think it might over-saturate the transistor in case of a transient, case I don't use a resistor? Or is is just a convention?
Kind regards, Samuel Lourenço
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Yeah, you completely made an oscillator, U3 has no feedback or compensation and the transistor is wide open. It might simulate stable under some circumstances but it will definitely oscillate in practice.
Tim
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Hi Tim,
Did some transients and steady state simulations, and indeed that oscillation happens. So, I've decided to stop using an op-amp as a comparator and decided to use the LMV331 comparator instead. The datasheet doesn't mention an hysteresis figure, and also suggests that can the comparator can be used where no hysteresis is desirable (with the due PCB layout cautions).
So, I've decided to redraw the circuit. But the transient and steady state simulations seem to show convergence errors. The DC sweep works as expected. The new circuit shows a much more precise current limiting (indicative of problems with the old circuit). It seems I'll have to build the circuit using the comparator to see how well it works. The pre-regulator will be figured out later, but I'll use the bipolar transistor solution presented before here, somewhere. The CC LED indicator will not be used, because it is not possible to use in this solution without messing up the circuit.
See "Sim2.png" for the new circuit. A comparator should be stable. I didn't used before because I was worried about the hysteresis that these guys normally show. However, since I have some degree of negative feedback, the voltage at the inputs will have a tendency to be equal in the CC condition (as also simulated with the OPA703 op-amp). That means the comparator will work in its "active" region (perhaps "unstable" by other means).
Thanks, and lets see how this goes.
Kind regards, Samuel Lourenço
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Nooooo no no no!
A comparator just oscillates more!
Are you trying to make a fault latch instead? Then you need a latch. You can't have a comparator turn off the signal that turns it on, that's the definition of a (relaxation) oscillator.
But a stable, linear current limit, just control the loop gain and phase so it's stable. Which means, limiting the gain of that transistor (typically, a base voltage divider and an emitter degeneration resistor are added), and compensating the amp (an R+C from OUT to -IN, and a series resistance to -IN from whatever source is feeding it, if necessary).
Edit: also, have you checked the LMV331 datasheet? It's a 5V part, and propagation delay is probably as high as the rest of the circuit. Sanity check! I don't know what model is inside that part there, but I suspect if you test it alone you will find it bears no resemblance at all to a real LMV331!
Tim
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Hi Tim,
No, a fault latch is very undesirable. I just want to implement current limiting, and that is why I opted for an op-amp in the first place, because I want the circuit to operate in the linear region (no hysteresis whatsoever, in any shape or form). I was expecting that this comparator could be used like so, since it is stable in open loop and supposedly has no hysteresis. If it has, if even by a tiny ammount, then the whole circuit would act indeed as a relaxation oscillator.
So, how do I limit the gain of the transistor? How do I stabilize the unstable op-amp? R+C in parallel from the output to the input? Or in series? A rough schematic of this sub-circuit will do. Then I would tweak the values. I can't find examples on how to use an op-amp as a comparator of sorts.
As for the 5V supply, it is implemented in the simulation and will be implemented in real life, from a completely independent regulator that is fed by the 24V rail. No worries!
Kind regards, Samuel Lourenço
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Your problem is thinking it's a comparator at all -- a comparator is a one-bit analog-to-digital converter. An ideal comparator has no linear range whatsoever. We could maybe use one in a PWM circuit, but there's nothing linear and stable about one!
A fault latch isn't uncommon in power control functions -- if not in bench supplies -- and that would be an expected use for a comparator. A latch is a digital logic element, so a comparator would trigger it -- hence why I asked to make sure. :)
Rather, what you're looking for is more of a "fuzzy logic" condition: you want the current error amp to be inactive (saturated output-low) when the output current is in the range 0...IREF, and active above there. What's more, you require active operation to be limited in speed, so it isn't chasing its tail (oscillating) in the active range.
Yes, R+C in parallel with the amp, you may remember this:
https://www.seventransistorlabs.com/Images/Limiter2.png (https://www.seventransistorlabs.com/Images/Limiter2.png)
The op-amp is the current error amp, and the R+C across it is the compensation. R5 is the series input resistor. In this case, you can ignore everything else in the circuit (but using this circuit instead of the two error amps is far superior!).
Tim
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Hi Tim,
In my case, I need to control the current more or less precisely to the milliamp. The current limit is set by a DAC, and it has to be set proportionally and precisely. That's whay I'm using a full op-amp as a comparator. I can't dispense the voltage control op-amp either for a similar reason (because I want a very precise voltage controll, even far more precise than the current control).
So, I'll use the resistors around the op-amp as you suggested. Also, the voltage divider to control the base of the transistor is also a good idea to diminish the sensitivity (I could use a MOSFET, but unfortunatily it has a too high of a Vth gs).
Kind regards, Samuel Lourenço
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Hi Sam,
are you sure you have understood how control loops work ? What you do here is running them uncompensated, which is a major design flaw.
You should think about your gain and phase margins under several load and current/voltage mode conditions.
A good idea is to simulate load step responses and tune compensation so that overshoot and settling time is minimized and no "ringing" occurs.
The voltage/current steering solution is also not optimal. Normally you try to use switching diodes to priorize the active op amp.
In order to speed up the switchover, some minimum amp loading is also beneficial.
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Hi Wolfgang,
For the voltage control op-amp, I make sure to use a unity stable op-amp. The op-amp used for the current control is the loose end here. The voltage control part works very well and also compensates the output lead inductance (the reason behing the odd resistor configuration around the op-amp).
Kind regards, Samuel Lourenço
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Hi Sam
this is a basic misunderstanding. Even if your op amp is unity gain stable, the control loop as a whole can still oscillate. I am afraid you need to dig yourself a little bit more into the theory of loop stability. Why not try some Jim Williams Appnotes from LT ? Or even Wikipaedia is a good place to start.
Why do you think that even a methusalem like the LM723 has a frequency compensation pin ?
I am absolutely sure that lead inductance is of no importance here. The delays created inside the op amps and in the power stages including filtering are much more important for stability. Not all people like math, but here its needed to understand what is going on, sorry.
Another very good reading is Horowitz and Hill Art of Electronic 3rd Edition Chapter 9 about power supplies and regulators.
regards
Wolfgang
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Hi,
I've been simulating ways to solve the oscillation issue. First, I've implemented the regeneration resistor plus the voltage divider. It improved the performance a bit, but not solved it. Then I proceeded to implement the op-amp compensation circuit, but saw no significant difference. The circuit is still oscillating. It is better to consider taking the whole current control circuitry to the garbage and implement a solution that I can understand - one that only implements the voltage control plus the well needed lead impedance compensation circuitry (aka, remote sensing).
Tested this scheme without the current control and worked very well. I'm ready to trow the towel on this CC crap thing and just use a fixed SC protection as I did before.
What is the point of implementing someone else's circuitry if it was done already? If I'm to reinvent the well, I'll not copy it, especially if I don't understand it. The simple solution, on the other hand, I understand it very well. I can call it a solution of my own.
The only thing "new" in this circuit will be the DC-DC pre-regulator, but if that fails as well, I'll ditch it too. No problems with that.
...
I am absolutely sure that lead inductance is of no importance here.
...
Lead impedance (not inductance) is important and has to be compensated. Remote sensing is the key word. Anyway, it is not the lead compensation circuitry that causes the oscillation. It is the damn current control circuit (unless the simulator is wrong).
Kind regards, Samuel Lourenço
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R16 is probably too small. Sanity check: making C1 ever larger, won't do any good when C1 * R16 is already much longer than the duration of the oscillation. :)
Tim
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Why not just have a diode between the CC op-amp and the Base of T1? Or a PNP Emitter follower. Then there will not be extra gain between the op-amp and the Darlington.
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Why not just have a diode between the CC op-amp and the Base of T1? Or a PNP Emitter follower. Then there will not be extra gain between the op-amp and the Darlington.
I guess that's not original enough. :-\
Tim
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Why not just have a diode between the CC op-amp and the Base of T1? Or a PNP Emitter follower. Then there will not be extra gain between the op-amp and the Darlington.
I guess that's not original enough. :-\
Tim
This is. In my Agilent U8002A, the CC op-amp, via a diode, is able to pull down the Comp2 pin on the CV op-amp which is actually a connection between the transconductance stage and the input to the op-amp's output stage. But HP thought of it first. :)
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Why not just have a diode between the CC op-amp and the Base of T1? Or a PNP Emitter follower. Then there will not be extra gain between the op-amp and the Darlington.
I guess that's not original enough. :-\
Tim
Never though of that.
Kind regards, Samuel Lourenço
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There could be complications. The Base of T1 needs to be pulled down to 1v. If you try the PNP follower idea, the op-amp might need a load resistor on the output pin to help get the voltage down.
The transistor's B-E junction will also need to be protected from more than 6v reverse bias somehow.
There is no negative control rail?
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Why not just have a diode between the CC op-amp and the Base of T1? Or a PNP Emitter follower. Then there will not be extra gain between the op-amp and the Darlington.
I guess that's not original enough. :-\
Tim
Never though of that.
Kind regards, Samuel Lourenço
On second though, bad idea to use the diode. The op-amp will not not drive the pass transistor properly. Had to replace the op-amp with a 24V part, and still didn't work. Anyway, if HP, aka Agilent, aka Keysight (they are the same) uses this, it is probably patented.
Kind regards, Samuel Lourenço
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Hi,
Did a new simulation using different values. Seems to be stable if I do a transient analysis for DC point operation. It shows oscillation if I do transient analysis based on the initial values or on zero values. At least, there is promise.
Anyway, if I do a longer analysis, say, 100ms, I see that the voltage drifts upwards from the CC setpoint of 5V to the CV setpoint of 8V.
Kind regards, Samuel Lourenço
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I doubt that HP would mind someone using their idea for a one off project. It may not have originally been their idea anyway.
I'm mainly curious as to how the simulator deals with it, the re-purposing of the Comp2 pin in that way.
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Well, the simulator is not making any sense. There is no reason why the voltage should drift from 5V to 8V in the first place. I'm not blocking any DC with that added capacitor, except for the feedback loop. Thus, DC wise, the op-amp still works as a comparator of sorts. Thus, that behavior is impossible in real life.
It seems I'll have to build this circuit myself and then tweak some component values. In the simulation I forgot to add a capacitor to the output, which is, in any case, essential. The power supply will oscillate without one.
Attached is the circuit I'm going to test. The voltage generators will be replaced by DACs. There is no better simulator than real life.
Kind regards, Samuel Lourenço
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An unstable system, initialized to a stable point (this is what the DC analysis does), will not move from that point unless perturbed by noise. Transient analysis does not include noise unless you add it yourself. So, simply, it's stable because nothing has pushed it off.
It's like stacked bowling balls, except those can remain upright because they have rough surfaces.
The zero-initial-conditions result is the correct one here.
Tim
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Hi,
More testing with TINA-TI made me to conclude that the simulator is doing AC analysis to evaluate the transient response. Anyway, didn't make any changes to the circuit, and found out that it doesn't oscillate, no matter how I run the simulation. The drifting was caused by the AC analysis.
Thanks to T3sl4co1l for suggesting a series of measures, such as the degeneration resistor, the voltage divider and the feedback network. The feedback network makes sense, because the op-amp has a gain very close to 1 in AC while acting as a comparator regarding to DC voltages. The transistor has its gain reduced, but the current never falls out of regulation since the op-amp doesn't hit the rails, while still showing a wider variation on its output.
Kind regards, Samuel Lourenço
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Hi Tim,
Can I exclude R15? I mean, the value is so small in comparison to R16. What is the cost of doing so?
P.S.: It seems to be stable now, even when simulating with zero initial conditions.
Kind regards, Samuel Lourenço
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Yes, that is a correct insight.
However, did you try larger values and smaller C values?
The step response is going to be ass with a cap that big. (Try putting a pulse current in parallel with the load resistance.)
Tim
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Hi Tim,
Indeed I find the capacitor too large. The inverting input series resistor (R16) is also a tad large and noise inviting, but still OK.
I think it is better to simulate this on a real simulator, also known as a PCB. TINA-TI is just unrealistic, and is showing abnormal behavior on the CC circuit part, that at least no longer oscillates (not that its feedback loop is closed and there is no chance of having positive feedback locally there). The CC control transistor is working better than ever.
But first, I'll have to implement the pre-regulator. I don't want to have excess dissipation on that pass transistor.
Kind regards, Samuel Lourenço
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Hi,
I finally had time to simulate the pre-regulator circuit using a buck DC-DC converter with the PNP. I suspect that the model is incomplete, and that is why I probably can't simulate this.
Could anyone check if the circuit is done properly? I will probably use the LM2592HV instead of the TPS62140. It has a lower switching frequency that is easier to filter out and it withstands 24V input voltage. The topology should be similar, save a few pins. The circuitry attached to the FB pin is the one I want to analyze and check for correctness.
Kind regards, Samuel Lourenço
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Hi,
Doing late hours results in mistakes. The circuit was all wrong. Attached is the correct one, which makes far more sense. Still, I can't run a simulation.
Anyway, how can I set the overhead voltage, that is, the voltage difference between the DC-DC output and the one after the pass element? I want to set this voltage difference to 3 or 4V above.
Kind regards, Samuel Lourenço
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If your can't run simulation due to circuit complexity you can try to remove everything except SMPS controller/regulator and PNP tracker and apply voltage source on the PNP tracker as for example in this (http://www.envox.hr/eez/component/jdownloads/download/3-spice/10-ltc3864-pre-regulator.html) simulation where Vout_sim is 22 V that give 25 V on the pre-regulator output.
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Hi prasimix,
I can't simulate your circuit, since it doesn't have a schematic. However, my simulation shows Vout skyrocketing.
On another note, it seems that the datasheet of the DC-DC converter I'll end up using (which is the TPS54233, instead of the LM2592HV or the TPS62140) is very pick about maintaining the top feedback resistor a constant 10K. Is there any way we can do this, and put the transistor below? I think the alternative is to reduce all resistors tenfold, guessing that the problem is a too high impedance on the feedback circuit.
Anyway, how is the voltage difference set? Can I set it to 4V? How can I calculate the resistors?
Kind regards, Samuel Lourenço
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I can't simulate your circuit, since it doesn't have a schematic. However, my simulation shows Vout skyrocketing.
The schematic (in .png format) is included into zip file if you followed mentioned link.
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Hi prasimix,
Why so many resistors in that feedback circuitry? Any particular reason?
Kind regards, Samuel Lourenço
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There is just one voltage divider (R3 + R9) that define max. output voltage (here about 60 V) that is well over max. working range (43 V). Max. voltage can be set much lower but it's intentionally in my case much higher then Vin (i.e. 48 Vdc) to push controller into 100% duty cycle by disconnecting PNP tracker to bypass pre-regulator. That is not applicable in your case. You can eventually remove R9 or decrease its value.
(https://i.imgur.com/08nR0iEh.png)
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Thanks! And what about R8 in series with R7, is it to set the voltage difference higher?
Kind regards, Samuel Lourenço
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Hm, it's not really in series with R7: R8 is connected on Q2's base and set it's working point. Try to change its value in simulation and measure current thru it and check how it affect pre-regulator's output voltage (i.e. offset will increase for lower R8 or decrease for higher value).
You can also use this simulation to see how such kind of tracker cannot provide the same offset over the whole range, e.g. for Vout_sim=0 V offset is above 4.5 V while for Vout_sim=40 V offset drops below 2.5 V. Therefore you can expect higher dissipation on post-regulator's pass element for lower voltages.
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Hi prasimix,
Finally had some time to run some simulations. Without R9, the circuit behaves linearly with the offset pretty much constant. I've then adapted the circuit to my needs (also changed the PNP transistor to the one I'm using). I'll not use the same DC-DC converter, but I hope the behavior is the same. The offset is set to around 3.6V, which I think is the ideal compromise, as it give some headroom so the pass element can regulate.
Anyway, thanks for your excellent help. It was very valuable.
Kind regards, Samuel Lourenço
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Hi,
Finally I've assembled a board based on the TPS54233 from Texas Instruments. The offset measures 3.711V when the control is set to 16V, and measures 3.744 when the control is set to 0V. Unfortunately, I only have a board layout and not the schematic. But the control circuit is the same as used in the simulations, only with a different converter.
Unfortunately, I've damaged the board when assembling it, because I've used a 2N3906 in place of the MMBT3906. Lets say that the "water tower" fell and took two pads with it. Now I'm waiting for a bunch of MMBT3906s to arrive. I'll post pics when I have another board soldered.
Kind regards, Samuel Lourenço
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Hi,
Managed to recreate the schematic from the board layout. See the attached files.
Once I've a board assembled as it should, I'll post some pictures. Anyways, thanks for all the help.
Kind regards, Samuel Lourenço
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Hm, your PCB layout doesn't look optimal at all. Did you study all that app. notes and recommendations and check other people design? I'd try to reduce distance between parts as much as possible, reorganize Cin and Cout to have gnd connections much much closer, move R4 very close to IC1, increase power lines width, connect R3 to output terminal instead of C6, etc.
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Hi prasimix,
Indeed the layout is not optimal at all. This was done quickly (I remember that it took half a day) and there are many things to improve (the diode could be a little closer to the IC, the inductor closer too, and many other things). Anyway, this is just to prove the concept, and actually the board works really well. The ESR imposed by the PCB seems not to be very relevant.
Anyway, the capacitors should have priority over R3 and the remaining control circuitry, which in this case wasn't done. Ideally, C6 and C7 should be right next to the inductor, and R3 should be after the second capacitor, and not in between them. R4 should not be close to IC1, as it doesn't have priority over C2, C34, C4 and R1.
Note that, in the TPS54233 datasheet, the feedback is taken from Vout after the filter capacitor, and loses priority over the remainder of the components. They even use a long feedback trace. The feedback network is closer to GND and near the IC, but de-prioritized over the compensation filter components. In a sense, these components should be near the IC, but as close as possible without moving any other components.
Kind regards, Samuel Lourenço
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Hi,
This is the schematic for the regulator module itself. The "CSENSE_UP" signal is fed back to the pre-regulator, so that its output is set to 3,7V above the voltage of that signal.
As you can see, the regulator module has voltage and current regulation, and over-temperature protection as well.
Anyway, I think it is better to leave this subject for another topic, should problems arise.
Kind regards, Samuel Lourenço