MOSFET linear regulator circuit

[ZeTeX]

sure I fully understood the ground as positive output, but how will this affect other stuff? like other circuits like LCD, buttons, other ics, ...etc. which are usually connected to a 5v regulator and to the ground which is the battery - terminal, just like the switching supply itself. I am not sure how will all this mix?

It wont affect anything, think about the ground as positive and that's it, what you connected is going to see a normal positive voltage, its not going to see negative (ground).

[VEGETA]

I understand that, the output voltage and the user is gonna get positive and negative. However, what about the other parts of the circuit? also can this supply be paralleled with the same or different supply?

Say if I want to add another circuit like LCD stuff or other analog control doing something else, what is the power supply rails that I connect it to? remember the ground of the battery is now the output voltage.

[ZeTeX]

I understand that, the output voltage and the user is gonna get positive and negative. However, what about the other parts of the circuit? also can this supply be paralleled with the same or different supply?

Say if I want to add another circuit like LCD stuff or other analog control doing something else, what is the power supply rails that I connect it to? remember the ground of the battery is now the output voltage.

Yes, the ground of the battery is the positive output voltage in respect to the positive side of the battery, usually the positive voltage is in respect to ground, here its the opposite, so you just assume that the ground of the battery is the positive and connect whatever you want there like usual.
always think of voltage as respect to something, usually one assume its in respect to ground but here its the opposite, but at the end its just a matter of how do you connect the load so it will see positive and not negative voltage.
any power supply basically can be paralleled as long as it is floating, so the ground of #1 power supply is not the same ground as #2 power supply.

[Kleinstein]

Other circuitry that has to work with the measured voltage, ADCs, MCU and similar would be power through the DCDC converter as well. One thing that does not work well is to use the same MCU to do things like control of charging or under voltage lockout of the battery. This would need an extra µC or extra circuitry.

The circuit from the other thread can also work, but it is not that much simpler. To get a low drop, if would need the extra positive (e.g. 30 V) and negative supply, and it takes an extra OP to bring the ref signals to the right place. Usually one also wants a fast current limit and thus a second "shunt". It is a little easier to understand and calculate, if you have to do it by hand (or experiment). The difference in the number of parts is not really big - so it is more like on par.

[VEGETA]

Ground which is battery negative is positive rail for the output... But it is not this way for the switching supply input. So you think I should connect the negative of the battery to the input of the sepic while the positive of the battery to the ground pin of it??

I understand that voltage is with respect to something but if this linear supply is the final stage...  Shouldn't this have any effect on the previous stages?

Anyway... This means should connect stuff backwards like voltage regulators having gnd as positive rail. Right?  If so, then I should put proper naming in the schematic such as switching ground and vcc to make it more readable.

Is there any chance of making this design ground referenced or not? If not then no choice but accept it xd

I wonder if this type of circuits is used in modern professional supplies since it is a new knowledge to me. To my understanding, the best benefit of it is the fast cc mode compared to the normal one.

Maybe one question I didn't ask is... Are there any professional supplies having this low dropout and same specs or not,  and why? I bet it is the only one with batteries too.

[Kleinstein]

Quite a lot of lab supplies use this type of floating regulator circuit. 
I don't know if this is still true with those that use a SMPS instead of a transformer.
The adjustment used in this simulation is already faster than on usually finds in commercial supplies, so there output caps are usually larger. Also the extra transistor used here to do some kind of anti-windup to speed up the CC-CV transition is usually not used. In real life one might have to keep the speed a little lower, as parasitic inductance can make things a little tricky. This is a general problem if you want a fast low impedance circuit - just like the more common case with parasitic caps in fast high impedance circuits.

Normally main powered supplies don't need an extremely low drop out and a little more dropout helps to react better to fast load changes without the need for a really fast SMPS part. A little more voltage at the MOSFET also reduces capacitance and thus could reduce output ripple.

The negative side of the battery will still be the negative side of the output. It is just that the reference point in the control circuit and for the ADC/DACs will be the positive output. This is why we need the insulated DC/DC to power this part. This type of control has no effect on how the SMPS part is connected.

The interface to the tracking SMPS part gets a little different with a full GND referenced version, that has the MOSFET and shunt on the low side. Aus this is a kind of low drop negative regulator. This type might be the easiest one, as it could get away without an extra supply voltage, but one has to expect slight poorer performance: e.g. less ripple suppression, maybe slower and more residual DC output resistance.

[VEGETA]

so you mean I should connect the battery + to the + input of the SMPS and the - to - input which is the GND pin? and then the ground of the whole design is the positive output of the supply while the negative side is the GND pin of the SMPS according to the schematic.

I have other circuits like battery protection, battery charging, lcd stuff, buttons and knobs, and linear regulators like 5v... < how should I wire these? there will be no external adc/dac stuff since i wanna use those in the MCU itself.

So you think I should make 2 grounds, one for the power circuit and one for other stuff? You said the control circuit is being done like always which means a voltage regulator of 5v connected to battery + and its negative side is the battery negative also, which we can call ground. However, the output stage only has the reference point to the positive input which confuses me. If the control circuit has the ground at battery negative terminal which is also the output + how can this work? if I can understand this, the design is nearly finished I guess.

So this leaves us with the isolated supply which supplies little current such as 100mA or so, is it enough? what are the stuff to be connected to it from those which I listed above?

[VEGETA]

I think I made something working after a bit of thoughts and trials... the circuit is in the attachments, please check it out. It is a combined SEPIC pre-regulator with this linear post-regulator design that we did. Never mind the way to set pre-regulator voltage for now, I will make it tracking later on.

I've set the sepic to 18.4v and the linear at 15 with current limit at 1A. It works nicely!! As I said, never mind the pre-regulator performance for now as it is somehow slow and needs a bit of time to regulate the current. One thing that bothers me is the extra 1.5mA in the output.

Anyway I've attached the negative terminal of SEPIC converter and everything else to the OUT- terminal so now it is the active ground so to say while the true GND is on the OUT+ as you suggested. So now everything I need to put must have its negative terminal connected to OUT- right? like a 5v linear regulator connected to positive side of the battery from its positive "In" terminal while its "GND" terminal is connected to the OUT-. After that I can connect whatever I like to this 5v regulator and they work perfectly. <<< is this correct? I hope!

Now as for the MCU with other stuff like op-amps... why not connecting it to that 5v regulator? since it is gonna be pure +5v voltage across its terminals as well as the op-amps which will get say +12v voltage.

One thing that is odd for me is that the mosfet gate voltage reaches 20v while the OPs supply is +11v differential (+6 to -5 from "GND" symbol)... this is the voltage from the mosfet gate to the OUT- terminal so I guess it is not the proper way of reading it right?

__________

can you name modern professional power supplies using this circuit? BTW, I would not care if it is not used or famous if it works, I am asking for information only xD.

I cannot say "thanks" enough for you.

[Kleinstein]

In principle the circuit is OK. The OPs supply should come from an DCDC converter.

The relevant gate voltage is relative to source, so more towards the GND point or out+.

The filtering inductor could likely be smaller (e.g. 5 µH or even less should be enough, maybe even less). One might want a resistor in parallel to damp a possible resonance.
The feedback compensation at the switched mode regulator does not look perfect. AFAIK the ringing could be less. The drop in the intermediate voltage is still a little large. The compensation part is anyway changing when going to tracking feedback - so no need to adjust it for constant voltage.

There is a way to get rid of the extra shunt and use the resistor between source and GND for current measurement. This also make the set voltage directly relative to GND.

[VEGETA]

If the OPs power is from the isolated supply, then how will it react to the mosfet and the rest of the circuit? its own current loop is not like them. How to simulate that in ltspice? having the ground symbol between the 2 sources makes it relative to the output. We agreed to have an isolated DC-DC converter of low power (<100mA) to power op-amps and MCU (what else?) to be the source, here it will be like 12v output only, so what and where should we connect its negative output terminal? to OUT-? Notice that the negative voltage of this isolated supply is not yet discussed, we only said it is gonna be a shunt regulator or diodes.

You made me notice one thing I didn't, which is the MCU/ADC set voltage, it is relative to OUT+ not OUT-! so the chip that outputs this voltage must be like that, but how? if the "OUT+" is the negative terminal of the MCU or w/e, then what is the positive rail? By having a DC-DC from +Battery to -Battery gives us an isolated 12v, call it +D and -D... then we connect a 5v 7805 regulator to it which gives us +5v relative to -D. Now MCU will output 2v relative to -D to the op-amp to make output go to 20v. How can this be functional?

I tried connecting the ground symbol to the battery positive but didn't work too.

[Kleinstein]

Simulating the DC7DC converter is simple:  it just provides an 12 V source you can put somewhere.

I did a little cleanup in the circuit, used a single 12 V supply for the floating part and changed current regulation to use a single low ohms resistor. I also added tracking for the SMPS  - it might still need a little tweaking.

The 2 K resistor at the 12 V supply is to add some current used by the µC and similar and just in case the simulation does not include supply current of the OPs.

[VEGETA]

I've understood the circuit of tracking pre-regulator, the 10k resistor sets the dropout. It doesn't always get it to 1v accurately, sometimes is more. I didn't play with it yet so I don't know if this is fixable.

I noticed 2 small problems:

1- huge ripple of the switching supply: I fixed it by putting more capacitance, multiple 220uF. Dunno if it is good without side effects.

2- the output voltage doesn't reach the maximum I want of 20v: this is because the switching regulator itself can not reach that far because you kept R2 and R1 the same as my last experiment which is about 18v max... I changed it for like 21.8v max which is nice for this design.

You can try it in the circuit below.

[Kleinstein]

To get a more or less constant 1 V dropout, one should replace R13 with something like 22 K and a capacitor (e.g. 10 nF range ?) in series. Also remove C16, as the new cap. does the job. This way the fast changes come from before the extra filter and the DC drop is only set by the 10 K resistor at the transistor.

Due to the large capacitance after the filter one does not see it that much, but there see, so be a kind of heavy ringing and oscillation in the 16 kHz range at the SMPS part. So I think the compensation is not yet good. Also R24 looks like very small - at least the simulation showed way to high current peaks on startup.

So I think the SMPS part still needs some work.

A large capacitance at the filter can effect the reaction to fast load changes. There might be some glitches if the drop out gets too small, and MOSFET might have to dissipate more power on a very dynamic load.  It limits the speed how fast the voltage can recover after overload like a short. The 3 x 220 µf might be still acceptable.

[VEGETA]

R24 is like that because it is for the output current to be suitable. I read in the datasheet that it must be smaller to allow for bigger maximum current spike, and I think I tried smaller values but didn't allow the regulator to give more output power. It's formula is R24 = 80mV/I_switching_max.

As for other compensation and values, I didn't touch them. Just took them from ltspice file from linear.com page of the lt3757. I just altered the output cap, coupled inductor, R24.

well, if you put a cap in series with R13 this will not allow dc voltage, and why 22k exactly? you seem confident that R10 will set the value although in the eevblog video of the circuit it seemed without effect.

[Kleinstein]

The voltage difference (nominal dropout) is set to U_BE + R9/R12*U_ref. To avoid trouble with too much gain R9 should not be larger than R12. So to get a smaller drop out one might have to subtract a little (e.g. at the base side).

The 22 K value is just a first guess, so to have some feedback from before the filter. For better stability a smaller value might be better. Feedback from behind the filter is coming too late (which is bad for loop stability). Feedback through that extra capacitor tends to be earlier than normal, which can be an advantage, but one has to find a good balance of the two.
It all depends on the compensation settings at the SMPS. Finding good values here can be a little difficult, as with a SMPS the compensation settings can depend of the used voltages. Also the speed of the output stage is rather low compared to a linear regulator - so there is usually not that much of reserve find a good compromise in speed and performance. So would expect that one really has to find suitable values for this application worst case one might need something like voltage dependent part / switches in the compensation circuit.
Usually the data-sheets provide more information on how to set the compensation part. Having the feedback from the difference is already a first complication, though not much. Without a good hint from the DS one might have to do it the hard way: find (measure, calculate or use simulation) the transfer function of the power stage and than calculate the desired parts. The full simulation in transient mode is rather slow - so a little more than just pure try and error is likely a good idea. One might do that without the linear output stage to keep the simulation fast and simple.

[VEGETA]

I didn't quite understand most of that, but my point is that I got the circuit from the part's page on linear.com and didn't touch it. The big question is: Is the circuit good enough as it is now? I mean, will it oscillate or do errors..?

Also, what about replacing our MOSFET with another one... namely: FDZB33N25. It is modern and has SMD version.

I made R9=5k and the result is: for 5v output voltage, 6.27v pre-regulator voltage. Same result when I make a resistor divider at the gate of Q2. Also can we replace C16 with an already used value like 1n or 10n?

One more thing, we have a separate supply for the op-amps and other stuff like MCU\ADC\DAC... right? so these stuff are all must be powered from the isolated supply that has the true GND (which is OUT+ terminal) as its negative terminal and thus supplies positive voltage to the MCU and other stuff. My concern is about everything else like LCD, other ICs,... etc. Should all of them be powered from the Isolated one or just the ones that has relationship of the power controlling loops? in this case, some ICs like external ADC\DAC will be I2C or SPI... which might cause an issue right?

We will end up with 2 linear supply ICs (LM317 for example): one is powered from battery positive terminal (after protection) which is labeled as "IN" in the spice file, and has the battery negative as its negative (called OUT-)... While the other one is power from the isolated supply's positive terminal and the GND as its negative terminal.

^ Which is for which?

I forgot how measuring and sensing of the output voltage and current work in this topology!! I tried to remember but failed with it, the only thing I do remember is the transistor Q3 is used for anti-windup function... probing voltages didn't give me any clue, the V_sense one has the proper 0.2v value while current sensing pin of the opamp gives odd values like uV! how does it sense output voltage and current? and exactly how can I read them into the MCU?

thanks!

[VEGETA]

UPDATE: I understood the current part, it takes a voltage directly from after the mosfet (before the shunt resistor) relative to ground which will be similar to current setting voltage. So 200mV = 2A. Can we make it 1v for 1A without headache? xD

I am still remembering the voltage sensing part though...

[Kleinstein]

The voltage drop at the shunt also determines the heat dissipation and to a large part the initial voltage drop on a load change. So it is a good idea to not make the shunt so large. So something like a 100-300 mV drop for full scale is about acceptable. A higher drop is usually causing more trouble than good.

For changing the linear mode MOSFET, one has to keep the SOA in mind. Modern MOSFETs often have a rather poor SOA and thus not attractive for the linear stage. This is different for the switched mode stage itself, if discrete MOSFETs a re used. Here modern type are usually better as they need less driving power or have lower R_on. Here one has to find a compromise for low driving power plus transition loss and the R_on. Beginners tend to choose a FET that is larger than optimum.

The compensation is not easy for a large voltage range of the SMPS. One really has to check the design equations. The feedback from the difference in voltage is also a little different from normal regulation it is more like a strong feedback, as one gets it at a low voltage. So it is more than just using the equations / diagrams - one has to understand them to know how they change with the different feedback. I have not done that myself, but worst case one might have to include nonlinear parts to make is work really well.
Normally there is some freedom in choosing the RC values, like using smaller caps and larger resistors to a certain extend. However there are limits.

[VEGETA]

The shunt is 0.1R as you can see which makes 200mV for maximum current allowed. 0.2v*2A = 0.4W of power which is good enough.

You didn't answer my last 2 posts, about how to know or measure output voltage and current. I guess I knew for current but not voltage. Plus what I said about power sources.

I told you I haven't played with compensation, I just took it from the SPICE file on the part's website page. So I assumed they are working nicely. Or do you mean the compensation of the linear stage? I don't think you do.

Can you name the comp. pins you mean? are they SS, RT, and Vc? also C15 shows a "x2" mark on it, what is that? does it mean 2 parallel ones?

[Kleinstein]

In the last LTspice ciruit (new_combined-2.asc) the capacitor C16 is just a left over - 1 pF is essentially 0, so no need for this cap anymore. This sometimes happens in such simulations, that one initially has parts but optimization shows you don't need them. To have an easy way back I often give them extreme values, like 1 pF, 1 fF or 1mOhms.

Compensation of the SMPS part is mainly C5,R16 at the VC pin, but the circuit at the feedback pin FBX also is part of this.

With much of the circuit powered from the isolated supply, there will be not much need to have a SPI or similar connection to the battery referenced part. Essentially the only part that needs to be powered from the battery referenced part is the DCDC converter and the part for monitoring the battery voltage. An SPI or I2C connection to the µC would need a special level shifter or maybe opto-coupler. One might prefer an analog signal transfer and only a simple under-voltage lockout at the battery side.

Monitoring the output voltage would need an inverting amplifier, as the output is negative relative to out+.

[VEGETA]

C16 is 500pF not 1pF. As for SMPS compensation, what is the problem that you think lies in this compensation?

So you mean powering nearly everything from the isolated supply? the power supply is not just this circuit, there will be LCD,ADC,DAC,buttons,encoders, other ICs... the best isolated DC-DC module I found with reasonable price is 12v-to-12v with only 80mA of maximum power! will it be enough to get all that?!

as for voltage, connect a op-amp with /10 gain then feed it to the adc?

[Kleinstein]

Yes for the voltage it could be something an OP as a gain of -0.1 or so.

For the C16 value, it depends on what you want - the last version I have got away without it. Instead it has a RC feedback from before the filter. This is likely the better version. The Version with C16 can have trouble with to much filtering.

Except for an possible LCD backlight something like 80 mA should be well enough - I would even guess 20 mA should be OK, if one really cares one might even get down to the about 8 mA minimum load for many of the small DCDC converters.

For the SMPS compensation, there is no easy fast simulation that directly shows you loop gain. So you essentially have to do it the classical way, getting a transfer function for the PWM output stage. The problem here is that the PWM stage gain can depend on the output voltage - so it is about finding a good compensation that work with one part that is variable. Also the modified feedback part means the ready calculated circuit do not match up, but we have to use a different feedback. The modified feedback might turn out to be even easier, but it is different so we can't expect it works without changing the rest.

A usual way is to replace the PWM generation and PWM + inductor/diode stage with a calculated theoretical transfer function - I don't know if this is given in the DS. There are than still quite some parts to adjust: the RC at the compensation pin of the SMPS controller, the feedback circuit (1 or 2 RC elements) and the filter / output caps of the switched stage. With something like 8 free parameters this is way beyond pure guesswork.

[VEGETA]

I made a new version with an op-amp in the attachments, it is good but will it need precision resistors of 0.1% or just the normal 1% ones? is there a need for RC compensation for it? I guess that the power supply for it is going to be the isolated one, exactly like the ADC that will take its value and the DAC that will supply control voltage for CV and CC.

Making a transfer function and especially for these 8 parameters is way more than my abilities, even at college I wasn't good with control stuff like transfer function and matlab. So I am going to search for another method. right now, the circuit functions properly so maybe datasheet can give good way to pick the Vc parts. Assume worst case of leaving it like this, what will happen? will it oscillate or something?

I removed C16 and it worked good => goodbye C16!! my concern is about other caps and resistors, I must consolidate as much as possible.

As for the isolated supply, I found this one: http://www.digikey.com/product-detail/en/murata-power-solutions-inc/NXE2S1212MC-R7/811-3184-1-ND/6009745

it is 12v to 12v by 167mA which is enough for back light too I guess, as well as any future additions (TFT screen?), all with 4$. Now it only requires El Cheapo boost converter as it's input to get the battery voltage from 8.4v to 12v which is by far more economical than other isolated modules. What do you think?

One thing that bothers me, it is how the CV op-amp works?! I totally forgot the principle of it! We worked on the principle of having the drop voltage supplied to it in the previous topology which didn't work... but what about this one? I don't seem to get it really. All I see is that the - input is connected to ground which is out+ all the time while the DAC voltage is supplied into a resistor divider to make it near ground too. can you explain?

[VEGETA]

I wish to use PIC microcontroller for the PSU (only one MCU) and make a PCB with CircuitMaker. My laptop is bad so I will get a new one in the last of February which will be suitable xD. That is why I need to finalize (or nearly) this main circuit so I can work on the other ones like battery pack and protection... etc.

what price do you think such a PSU can be sold at? just curious ~

[Kleinstein]

With simple PIC µC, the suitable display is more like a text based one with maybe 2 lines of 8 characters, or if the µC has a direct LCD interface even 2 simple 4 digit ones.

The resistors for voltage reading don't need to be very accurate (so 1% should be OK) - it depends on the ADC used, how much makes sense. One might not need R29/R32 - as they both essentially go to GND - only if true 4 wire sensing (e.g. for really high resolution) is needed these two could be a good idea.

The feedback and compensation for the switched mode still needs work ! The current plan will likely not work. May guess would be a capacitor in series with R13 and no C16.  If this part will be stable is still not clear - so at least run quite some extra tests, if you can't do the whole program. The compensation for the switched mode stage is about half the design effort - so this project is far from ready- more like 1/3 at best.