DIY Scalable Bench Power Supply Design

[void_error]

Then I had another idea - to add an option to disable the pre-regulator (the LM2596 can go up to 100% duty cycle - internal switch on all the time) while limiting the output current to a value that does not cause excessive heating of the series pass transistor. It cand be done by shorting the feedback pin to ground. This will basically result in a cleaner output which might be handy sometimes.

It especially make sense when you are using MCU so you can limit a total output power that you're not limited only by voltage or by current.

About that other idea... it doesn't need a MCU, just a schmitt trigger comparator to switch between modes: for example, if the current limit is set lower than 500mA it disables the pre-regulator by shorting the BE junction of Q1, if it's higher than 550mA it enables the pre-regulator. Actual values will be decided according to the max allowable dissipation with the 2.2 K/W heatsink I'll be using. Dirt-cheap LMV321 + some resistors + 1NPN + 1PNP will do the job and maybe an LED to indicate the mode. As it is a one-off project this is basically for free as I'll have more parts than I actually need here.

[macboy]

Oh, I completely forgot that shunt is BEFORE serial transistor Sorry.

This is a poor design decision as the base current of the transistors also goes into the load. Sure, with a darlington drive, this is a relatively small current, but it is not close enough to zero to be neglected.

[void_error]

Oh, I completely forgot that shunt is BEFORE serial transistor Sorry.

This is a poor design decision as the base current of the transistors also goes into the load. Sure, with a darlington drive, this is a relatively small current, but it is not close enough to zero to be neglected.

Instead of criticizing everything you might want to be useful and suggest something better than the INA168, preferably one available here or here. I already have a second option.

Also, FYI, the BDX33 has a minimum gain of 750 which means 4mA maximum base current at 3A. Slightly above 0.1% error.

[prasimix]

What's about hall-effect based sensor such as ACS-712? Another possibility is INA193/196 that goes down to -14V with nice bandwidth (up to 500KHz). Huh, this sentence I found in device description:

The 500kHz bandwidth simplifies 20V/V, 50V/V, and 100V/V use in current control loops.

[void_error]

The ACS712 is quite inaccurate. The other IC I found is the AD8211. It has a lower output error (according to the graphs in the datasheet) than the INA193 at low input voltages and it's also available on TME so I don't have to pay shipping from two different suppliers.

[prasimix]

Looks very promising, and price is right! Nice suggestion.

[void_error]

By using two of them like in LT's AN105, page 117 - Figure 215, allows for higher sensitivity at lower currents. Looks like I'll have to redesign the current sensing circuitry. Time to look for MOSFETs. An N-channel type with low V_GS is required since the P-channel will require a negative supply rail.

The ranges should be 0-300mA and 0-3A.

Or I could just stick with INA168 and also sense the base current then add it to the collector current. It would make switching between ranges easier, using a P-channel MOSFET.

[prasimix]

Are you still thinking about 10 resistor in parallel for shunt? Now you'll need 20

[void_error]

Now you'll need 20

Looks like you got that one a bit wrong. Only 1 extra resistor will be required for the low range. The 10 resistors were used to handle the power dissipation at maximum load current. For a 1 ohm resistor at 300mA the dissipation is below 100mW.

[prasimix]

Yes, but you still need to find cheap 100mohm 1% (or even 0.1%) resistor, or you are going with something like 1ohm for low range?

[void_error]

Yes, but you still need to find cheap 100mohm 1% (or even 0.1%) resistor, or you are going with something like 1ohm for low range?

I'm switching to low-side current measurement since the supply will be floating anyway. This will simplify things a lot. Or I could use an INA213.

[prasimix]

Ok, what you are going to do with short circuit detection/protection in that case?

[void_error]

Ok, what you are going to do with short circuit detection/protection in that case?

Here's a bunch of application notes, this might clear things up for you as far as current limiting goes:
http://cds.linear.com/docs/en/application-note/an105fa.pdf
http://www.intersil.com/content/dam/Intersil/documents/an18/an1827.pdf
http://www.diodes.com/_files/products_appnote_pdfs/zetex/an39.pdf
http://www.analog.com/library/analogdialogue/archives/44-12/high_side.pdf

[void_error]

Well if you are going to publish out your design you have to expect that. Everybody has an opinion on how to do things, whats the best part and what specifications are important etc..

You clearly haven't read this whole forum topic.

This is far from the first power supply design attempted on this forum and likely won't be the last.

Guess why I ended up designing my own... because I haven't found any design that I liked. And I did google a lot.

I'm going to give you probably the best advice yet, first design and build your PSU the way you like then publish it here. If people don't like it or prefer different components they are free to do their own. Second piece of advice, have realistic and obtainable goals.

I started this thread because I had a bunch of questions and an idea of how the power supply should be. If I would have designed it before that I wouldn't have even bothered posting anything.

You are not going to be able to build a PSU that will have ideal qualities for every conciveable loading, just ain't going to happen.

Thanks captain obvious.

[prasimix]

Ok, what you are going to do with short circuit detection/protection in that case?

Here's a bunch of application notes, this might clear things up for you as far as current limiting goes:
http://cds.linear.com/docs/en/application-note/an105fa.pdf
http://www.intersil.com/content/dam/Intersil/documents/an18/an1827.pdf
http://www.diodes.com/_files/products_appnote_pdfs/zetex/an39.pdf
http://www.analog.com/library/analogdialogue/archives/44-12/high_side.pdf

Thanks for that. I didn't saw before that Zetex app note. They have some nice examples there.
Almost all documents talks about high side sensing and in Linear (an105, pg.2) there is a clearly stated disadvantages of the low side sensing which you'd like to deploy:

• Load activated by accidental short at ground end load switch
• High load current caused by short is not detected

From my existing experience I'd like to say that accidental short circuit condition is the reality not an exotic case which don't deserve serious attention. It can hurt serial transistor/mosfet badly because it easily push condition outside the SOA (Safe Operating Area). Linear in their app note on pg.16 and pg.18 show low side current protection but it is for -48V and to me it looks like "high side". Anyway I think that one should be ready to test PS with current limit set to rated max. value and try "controlled" short circuit condition after the PS is switched on (another case is trying to power it up with shorted output terminals which should be easier to manage).

[void_error]

About the SOA... I might replace the darlington with two paralleled transistors just to be sure I don't exceed the SOA. Emitter resistors will be required to balance the currents.

After I build it one test will be shorting the output at 10kHz or something and look at the response on the scope. Starting up with a shorted output won't be an issue with the MCU control in place as the output will be zero by default at startup. May as well set the LM2596 up for delayed startup to ensure the control circuitry gets powered before the high power stuff.

[prasimix]

I found INA206 interesting. Its bandwidth is up to 500KHz and comes with two comparators and internal reference. Good thing is that comparators inputs are reachable from outside. Thanks to that you can use the same DAC signal to set max. ("trip") current which could be i.e. 20% higher than allowed current for CC (constant current) mode. Comparator has latch and reset. Another comparator could be used for monitor "under current" condition.

[void_error]

I found INA206 interesting. Its bandwidth is up to 500KHz and comes with two comparators and internal reference. Good thing is that comparators inputs are reachable from outside. Thanks to that you can use the same DAC signal to set max. ("trip") current which could be i.e. 20% higher than allowed current for CC (constant current) mode. Comparator has latch and reset. Another comparator could be used for monitor "under current" condition.

Unfortunately, like most high side current sense chips, it has roughly a 1:10 accurate measurement range which translates into 300mA - 3A for my design and that's not enough. Below 1/10 of the full scale current the output error goes through the roof.

With that thought through I've moved to low-side current sensing and made a few changes to the pass transistor + driver circuit. Managed to get rid of a few parts without any drawbacks... or at least that's what I'm going to think until I breadboard it and test it for stability.

If anyone is interested let me know and I'll post the revised schematic. I bet you're all sick of this thread already.

[liquibyte]

If anyone is interested let me know and I'll post the revised schematic. I bet you're all sick of this thread already.

I haven't commented but I've been watching the progress you're making and I may build one of these if I ever solve my own problems.  So, nope, not sick of it yet unfortunately.  I don't like watching videos to try and grep concepts, doesn't work for me.  Watching threads progress with ideas, concepts, and differences of opinion does though.  Hell, I'm still trying to wrap my head around all the different ways you can use op amps.

[prasimix]

If anyone is interested let me know and I'll post the revised schematic. I bet you're all sick of this thread already.

Yes, please. Nothing sick so far, just lots of exchanging of good ideas. I'd like to know if you had a chance to test short circuit condition and if any overshooting exists during power up and power down phase. How is going with switching pre-regulator? Did you test tracking?

[void_error]

I'd like to know if you had a chance to test short circuit condition and if any overshooting exists during power up and power down phase. How is going with switching pre-regulator? Did you test tracking?

Next week.

[David Hess]

Almost all documents talks about high side sensing and in Linear (an105, pg.2) there is a clearly stated disadvantages of the low side sensing which you'd like to deploy:

• Load activated by accidental short at ground end load switch
• High load current caused by short is not detected

From my existing experience I'd like to say that accidental short circuit condition is the reality not an exotic case which don't deserve serious attention.

This neatly explains why bipolar supplies do not use simple ground side current sensing; a short to the opposite supply would go undetected.

Tektronix (PS503) used singled ended high side current sensing with current based level shifters to avoid instrumentation amplifiers and matched resistors.  HP/Harrison (HP6236B) made the ground the high side achieving the same thing without level shifters, instrumentation amplifiers, or matched resistors.  The Tektronix design is more flexible and allows remote programming but the HP design is simpler.

I hate using instrumentation amplifiers and/or matched resistors for high side current sensing.

[prasimix]

I hate using instrumentation amplifiers and/or matched resistors for high side current sensing.

What do you think about PGAs (programmable gain amplifiers) that comes with matched resistors?
I spent some time now examining current monitors (theoretically and in practice) and it's really challenging to find a single chip to cover CM (common mode) range of 0-50V with good frequency bandwidth and precision when CM goes below 3-5V or Vsense (voltage drop on shunt) below 20mV. I start to think about dual monitor solution (with two shunts, "auto range") or even to try some PGA with required CM, gain, precision and bandwidth. I'm trying to stay on "high side" monitoring.

[David Hess]

I hate using instrumentation amplifiers and/or matched resistors for high side current sensing.

What do you think about PGAs (programmable gain amplifiers) that comes with matched resistors?

Did you have any specific parts in mind?

Most still have common mode input range limitations and are usually slow and/or noisy compared to the alternatives which may not matter.

Some of the integrated high side current monitoring solutions work like I mentioned using current for level shifting.  It is pretty easy to do the same thing with a discrete solution over wide voltage ranges and with high bandwidth.  This also avoid specialized parts.

One reason I like the single ended designs like in the power supplies I mentioned earlier is that it simplifies feedback loop compensation.  If you have to do a lot of active signal conditioning within the current control loop, the frequency compensation can get out of hand.

I spent some time now examining current monitors (theoretically and in practice) and it's really challenging to find a single chip to cover CM (common mode) range of 0-50V with good frequency bandwidth and precision when CM goes below 3-5V or Vsense (voltage drop on shunt) below 20mV. I start to think about dual monitor solution (with two shunts, "auto range") or even to try some PGA with required CM, gain, precision and bandwidth. I'm trying to stay on "high side" monitoring.

Some designs are amendable to bootstrapping the integrated circuit supply voltages and a cascode can be used to increase the output voltage range.  Nobody likes to use a separate floating bias supply but that also works like in the HP 6271B which is 60 volts and 3 amps.  Where it gets tricky is if you want programmable current and/or voltage when your reference is no longer ground referred.

I have less interest now in power supplies above 30 volts because a floating dual tracking 0 to 20 volt supply can be used as a single 0 to 40 supply.  Doing this doubles the number of circuits though and is probably only desirable if you want the dual tracking supply function.

[prasimix]

Did you have any specific parts in mind?

Nothing yet in particular, still looking what offer i.e. TI, Linear and Analog devices. I found that there is in general two types of PGA: one with digital interface (like SPI) and another one without it.

Most still have common mode input range limitations and are usually slow and/or noisy compared to the alternatives which may not matter.

Some of the integrated high side current monitoring solutions work like I mentioned using current for level shifting.  It is pretty easy to do the same thing with a discrete solution over wide voltage ranges and with high bandwidth.  This also avoid specialized parts.

One reason I like the single ended designs like in the power supplies I mentioned earlier is that it simplifies feedback loop compensation.  If you have to do a lot of active signal conditioning within the current control loop, the frequency compensation can get out of hand.

I'm agree. More parts make frequency compensation difficult. Do you have any reference to discrete solution when we comes to current monitoring?

Some designs are amendable to bootstrapping the integrated circuit supply voltages and a cascode can be used to increase the output voltage range.  Nobody likes to use a separate floating bias supply but that also works like in the HP 6271B which is 60 volts and 3 amps.  Where it gets tricky is if you want programmable current and/or voltage when your reference is no longer ground referred.

Yes, separate floating bias supply add some parts but I'm still fine with it if overall result is good. Talking about HP/Agilent/Keysight solution I don't want to polute this thread with it so I opened a new one where I asked a couple of question about E3634 model. I'll appreciate your feedback.