DIY Scalable Bench Power Supply Design

[dannyf]

Why do you need such a big fan I wonder?

When the output voltage is low and output current is high, the linear adjuster portion of the circuit will dissipate tons of heat. Thus the fan.

The existing design essentially replicated a low-power laptop power supply via the switching regulator - I think you are far better off just use a spare laptop power supply in lieu of it.

If you are going to use a LM2596 or the likes, two ways to reduce the power dissipation on the linear regulator:

1) configure the LM2596 as a pre-regulator so the linear regulator has a constant voltage drop over it -> the output of the LM2596 tracks that of the linear regulator;

2) apply feedback on the LM2596 so its output is no longer constant.

[void_error]

I don't like the way you put the heatsink and fan... it would make sense to have the fan above the heatsink, pulling air from the bottom of the case or the sides though the heatsink fins and pushing the hot air out.

I would screw the power devices onto the heatsink and then screw the heatsink onto the back of the case (if it's metal), this way the case itself would also act as heatsink, potentially making fan not needed at low power dissipation. I don't quite like how the power devices are supposed to be attached to the pcb and the heatsink and have the spacers there as well, seems like difficult to service if there's a problem, and there may be a problem with solder joints from fan vibrations or just accidental knocks of the power supply.

The fan will pull air in from the bottom and push it towards the back of the case through the heatsink's fins and past the transformer. BTW, that's a rough sketch of how everything will be slapped together. The case will be metal and the heatsink will be attached to the bottom of the case. Actually I intend to bolt the whole assembly down using 4 screws (maybe 6, 2 for the front panel). I'll make sure everything is vibration-proof.

Why make three boards when you could maybe compact everything into one or a couple of boards?

It'll make testing/troubleshooting a lot easier and currently the largest board size I can currently make at home is 60x120mm. I also like to keep things fairly modular so if I want to build another similar but slightly different power supply I don't have to do too much PCB redesigning.

Do you really need an 8 bit port expander? Maybe you could use a simpler shift register for some stuff (like sending data to lcd display for example). I would just go for something more powerful and with more IO pins, like... maybe pic16f1947

The control boards (with the knobs and buttons) will be a separate board. With a serial I/O expander I get additional interrupt capabliity (I'll be polling for buttons changes 100 times a second) and only 6 wires are required for SPI including +5V and ground. After all the pin assignments I'll be left with 2 pins to be used as chip select for future expansion modules.

Also, since I'm not a microcontroller expert yet, I'll keep things simple and use a dumber MCU. At some point I was thinking of using a PIC18F18xx for its USB connection.

Yes, it's surface mount, but TQFP-64 is easy to solder and there's cheap tqfp to dip on ebay if needed, for example here.

I'm already using a TQFP-44 package for the MCU and SO for all the other ICs to avoid the tedious task of drilling dozens of holes.

[mij59]

The fan will pull air in from the bottom and push it towards the back of the case through the heatsink's fins and past the transformer. BTW, that's a rough sketch of how everything will be slapped together. The case will be metal and the heatsink will be attached to the bottom of the case. Actually I intend to bolt the whole assembly down using 4 screws (maybe 6, 2 for the front panel). I'll make sure everything is vibration-proof.

If there's not enough space between the fan and the bottom the fan will be noisier.
You can't stack an other item on top of the power supply.
May be you can use it as coffee warmer, but be careful, don't spill the coffee!

[void_error]

Looks nobody got the idea so I made another sketch.
Arrows indicate airflow. All the connectors have been omitted for the sake of simplicity. The boards are green, the white rectangles are cutouts, the black rectangle is the fan.
Everything can be removed by unscrewing a bunch of screws which will make servicing easy.

[miguelvp]

Looks nobody got the idea so I made another sketch.

Of course they got it.

But you didn't answer the clearance question from the bench to the case.
And you didn't answer the concern about the stackability of your design. If on top of a hot device air will be preheated before entering your case and even if you decide to reverse the flow then it will heat the device under it.

[akis]

I must say there comes the time to stop designing and start building!

A few years ago I built a PSU based on a 300VA mains transformer, 2 channels, 0-30V/5A. Used a bypassed 317/337, insufficient heatsinks on the 317/337s, insufficient rectifier diodes (3A...), ended up melting components near the diodes and breaking down from time to time. The next design uses a complimentary pair for the pass transistors, better diodes, better cooling (4 small fans inside and 1 large fan outside), heatsink temperature controlled fans and thermal shutdown at 100C. Extra sense lead to monitor voltage on load. Relays that connect/disconnect the load with momentary push buttons. Still 30/5A and two channels.

There could be a next design, but this works damn well. When it melts down, if it does, then it will be time.

[void_error]

But you didn't answer the clearance question from the bench to the case.
And you didn't answer the concern about the stackability of your design. If on top of a hot device air will be preheated before entering your case and even if you decide to reverse the flow then it will heat the device under it.

A clearance of 2cm should probably do.

As for the stackability, the PSU doesn't entirely rely on the fan for cooling, it's only going to kick in when the heatsink temperature reaches 70C. I estimated the heatsink's thermal resistance at around 3K/W (this one http://www.tme.eu/en/details/rad-a5723_60/radiators/#), although it's probably lower, which translates into a 75C rise above ambient at 25W without the fan running. Assuming that with the fan on the thermal resistance of the heatsink drops another 1K/W I think the PSU won't overheat.

I must say there comes the time to stop designing and start building!

There's going to be a lot of PCB designing to be done. Plus the software part isn't finished yet as the digital hardware has suffered a few changes. I've built quite a few things before and what I've learned is that I have to be more careful with the way the whole thing comes together otherwise final assembly will be a huge pain in the ass.

I still have to test another version of the linear regulator because I had an idea. This way I could lose the negative rail and use 5V rail-to-rail opamps. The current design requires opamps than can handle a fairly high supply voltage and I personally don't like that.

[dannyf]

Negative rails are not necessary unless you want to get to 0v output.

A cheap R2R opamp will get you within 100mv of the negative rail / GND. How often do you need to output something that low?

[akis]

In my system the op-amps go near to ground but not quite (using MC33072 single supply op-amps). The op-amp that senses current on a sense resistor has an offset base with a biased diode to ground so the V+input is always 650mV above ground and that is its baseline. That means it can sense very small currents which would otherwise go undetected.

For high side sensing there is also a problem so I feed the op-amps a higher voltage. On the transformer based PSU this is done easily with a voltage doubler (2 diodes and 2 capacitors off the mains transformer) followed by a linear regulator down to 40V. The MC33072 accepts up to 44V if I remember correctly.#

Another solution would be to use jFET op-amps eg the TL082, or the much more modern TLE2142, for high side sensing if your sense resistor has one side permanently connected to the raw supply input (sense resistor before anything else).

[dannyf]

it can sense very small currents which would otherwise go undetected.

Sensing small (differential) voltage / current isn't a problem - you just need lots of gain and that's easy to do.

The difficulty with high-side sense amplifier is common mode range, ie the amplifier continues to function when the input voltages (on the + and - pins) goes very high or very low, sometimes even beyond the supply rails of the amplifier. Many, if not most, opamps will fail here because of that.

Another solution would be to use jFET op-amps eg the TL082, or the much more modern TLE2142, for high side sensing if your sense resistor has one side permanently connected to the raw supply input (sense resistor before anything else).

Both suggestions would make the designs more challenging: TL082  (and most jfet opamps) cannot swing to the rails (not R2R), and the 2nd approach unnecessarily expands the required common mode signal range - it is best to put it after the regulator which helps you reduce the opamp's common mode range requirement.

[mij59]

A clearance of 2cm should probably do.

As for the stackability, the PSU doesn't entirely rely on the fan for cooling, it's only going to kick in when the heatsink temperature reaches 70C. I estimated the heatsink's thermal resistance at around 3K/W (this one http://www.tme.eu/en/details/rad-a5723_60/radiators/#), although it's probably lower, which translates into a 75C rise above ambient at 25W without the fan running. Assuming that with the fan on the thermal resistance of the heatsink drops another 1K/W I think the PSU won't overheat.

A clearance of 3 cm would be better, or place the fan on the rear of the unit.
You could run the fan at low speed and ramp up as the temperature increases.
A heat sink with no data on the thermal resistance

[void_error]

A clearance of 3 cm would be better, or place the fan on the rear of the unit.
You could run the fan at low speed and ramp up as the temperature increases.
A heat sink with no data on the thermal resistance

My PC case uses lower clearance for the 12cm bottom fan and for the PSU fan so I guess anything above 2cm should do. The surface area of the fan intake shouldn't be higher than the surface area through which the air comes in below the case if you know what I mean... not to cause an airflow bottleneck (?).

Was thinking of using a PWM fan and freeing up a MCU PWM pin. This will keep the noise low at least.

Yup. Dirt cheap though. I've done some thermal resistance measurements on my junkbox heatsinks (I do have a lot of them) a few years back and got some data. This one is quite similar surface area and fin spacing wise to what I have in my junkbox. Cooling on this PSU will still be overkill for reliability reasons.

[mij59]

A clearance of 3 cm would be better, or place the fan on the rear of the unit.
You could run the fan at low speed and ramp up as the temperature increases.
A heat sink with no data on the thermal resistance

My PC case uses lower clearance for the 12cm bottom fan and for the PSU fan so I guess anything above 2cm should do. The surface area of the fan intake shouldn't be higher than the surface area through which the air comes in below the case if you know what I mean... not to cause an airflow bottleneck (?).

Yes, high air speeds means also high noise level.
When the intake of the fan is close to the surface, the rotation speed of the fan increases, and the air flow decreases.

[void_error]

Apparently I've had no luck with my second idea for the linear regulator part of the PSU so I'll stick with the first one with a few minor changes.

Swapped the MC14584 charge pump with a 7660 and will probably go back to the INA138/168 for the current sensing part after I test everything again.

I might reduce the negative rail to a lower value because I'm planning to use roughly the same circuit for a 0-30V PSU (got a smaller transformer out of an old printer) and I don't really want to change the opamp. As soon as this is dealt with I'll start designing the PCBs and finishing the software part.

[mij59]

Apparently I've had no luck with my second idea for the linear regulator part of the PSU so I'll stick with the first one with a few minor changes.

Swapped the MC14584 charge pump with a 7660 and will probably go back to the INA138/168 for the current sensing part after I test everything again.

I might reduce the negative rail to a lower value because I'm planning to use roughly the same circuit for a 0-30V PSU (got a smaller transformer out of an old printer) and I don't really want to change the opamp. As soon as this is dealt with I'll start designing the PCBs and finishing the software part.

The INA138 has minimal common mode voltage of 2.7V, so you can't use it on the output.
You need a current sensor with a range from zero to the max. output voltage.
If you use the current sensor you can also reduce the VCC of the opamp.

[void_error]

The INA138 has minimal common mode voltage of 2.7V, so you can't use it on the output.
You need a current sensor with a range from zero to the max. output voltage.
If you use the current sensor you can also reduce the VCC of the opamp.

There's one way around that but it has at least one drawback. If I measure the current before the series pass element the common mode voltage will be 4V + V_OUT but then I will have to do more math in the MCU to get the actual output current which is an unnecessary complication.

As for reducing the VCC of the opamp, in the current design the output has to swing quite close to VCC to get 20V output. I'll have to do some more measurements to determine the least difference between VEE (in the last schematic) and ground to get 0V output.

So far I could as well build the current version that works but I still think there's a bit of room for improvement.

I'd appreciate it if someone would point me to a circuit that uses opamps with lower voltage than the PSU output voltage.

[akis]

Just general ideas:

The pass elements, they can be P-channel FET, or NPN/PNP complimentary. For example the P-channel opens at 0V.

Use a jFET op-amp to sense current on the high side voltage and produce as an example 1V-5V output for say 0..5A current draw maybe with a raised ground (one diode) to get over the first 1V, eg TLE2072. Use another op-amp to drive your pass element depending on the voltage requirements.

I have found the easiest way to do it, is to have 5-10V over the "Vcc" to power the op-amps then I know that I can deal with anything in the circuit (plus a raised ground of course).

[void_error]

Just general ideas:

The pass elements, they can be P-channel FET, or NPN/PNP complimentary. For example the P-channel opens at 0V.

Use a jFET op-amp to sense current on the high side voltage and produce as an example 1V-5V output for say 0..5A current draw maybe with a raised ground (one diode) to get over the first 1V, eg TLE2072. Use another op-amp to drive your pass element depending on the voltage requirements.

I tried a PNP (actually a PNP driving a NPN) as the pass transistor but I couldn't get the thing stable, it would oscillate at some point. That was the whole idea in using lower voltage opamps. That way I could drive the PNP/P-channel with a NPN (grounded emitter) driven by an opamp. The opamp's supply voltage would be +5V/GND as it would only have to supply the NPN's base voltage/current.

[akis]

Surely the idea would be to have the NPN driving the PNP so the load is on the emitter of the NPN so it acts as common collector? Then it is (more) stable by definition. I only use two transistors, MJE15028G driving a MJL1302A - driven by a simple op-amp no probs to 30V/5A. The op-amp is fed by 40V.

Oscillations are caused by the op-amp driving - you need 10nF feedback on the op-amp and no other feedback.

The NPN doing the driving is not required. OK, I actually use it TO SHORT the base of my NPN/PNP pair, as the current limiter control device. The op-amp does the voltage adjustmest straight into the base, and the other op-amp opens the NPN to act as a short.

It looks like this: U3.2 is the voltage controller. Another op-amp, not shown, opens the Q3 to "fight" the U3.2 and provide current limit. Maybe it can be done better - get two op-amps to agree between them on a voltage level

[void_error]

I've seen similar circuits in books from the 80's and several magazines from that era.

My idea was to supply the opamps with a lower voltage than the input voltage so I had to use a PNP driving a NPN but it's not a stable configuration.

I guess I'm sticking with the last schematic. I'll have to replace the opamps with higher voltage ones for a 0-30V version or use a circuit to boost the output voltage swing.

[dannyf]

I couldn't get the thing stable, it would oscillate at some point.

You have just made a LDO. Yes, it is quite difficult to stablize them.

[void_error]

You have just made a LDO. Yes, it is quite difficult to stablize them.

I was aware of that and I was expecting it to be unstable. Not even a 100uF cap across the output would make it stable.

Now I have a question:

Does anyone know about a circuit to boost the opamp output voltage swing past the opamp supply rails?

For example, let's say the opamp supply is +/-5V ground referenced and I want the output to swing from -5V (less would also do) to around 40V. I suppose such a circuit exists but I haven't found one suitable for my application yet unless I can remember where I found a circuit that might be suitable...

Manged to trick google into displaying this among the search results:



This is going to be the opamp voltage booster, with the following modifications:
If I get rid of R and C and use an opamp to drive the base of the PNP it should still work.
I'll also replace the P-channel MOSFET with a PNP.
R4 & R3 set the gain.
I'll also add a current sink across the output.
I'll be really happy if this works.

Current sensing will be handled by an AD8211 http://www.analog.com/static/imported-files/data_sheets/AD8211.pdf

[dannyf]

let's say the opamp supply is +/-5V ground referenced and I want the output to swing from -5V (less would also do) to around 40V.

Yes, if there is a 40v rail somewhere.

[void_error]

let's say the opamp supply is +/-5V ground referenced and I want the output to swing from -5V (less would also do) to around 40V.

Yes, if there is a 40v rail somewhere.

Thanks Captain Obvious 
With this circuit the positive supply rail is going to be the rectifier+filter output, so with a 24V AC transformer it'll be around 33V DC at no load.

Anyway, If it's possible to get a PSU output swing down to 0V without a negative rail it's even better.

I'll breadboard the thing today and do some measurements. If everything's stable with no load I'll start abusing it by connecting some nasty non-linear loads. If it's still stable I'll post a schematic.

[IanJ]

Anyway, If it's possible to get a PSU output swing down to 0V without a negative rail it's even better.

Out of interest I am using OP295 op-amps (single supply rail to rail) in my design without any negative supply and I get down to about 50mV minimum with a 25v span.

PS. How are you getting on with the LM2956-ADJ..........are you noticing any noise (on load) on the output. Albeit, less than 10mV at the switching freq?

Ian.