DIY Scalable Bench Power Supply Design

[void_error]

Hmm... I might as well go full discrete then, but I'll still need to 'OR' the voltage control loop with the current control loop which uses floating high-side current sensing (there's a schematic in one of my previous posts). Fortunately the current sensing circuit output is a current source so it's pretty easy to reference to ground. What I'm trying to do here is to design a version easily scalable for different output voltages and currents - just change some component values but not the PCB as I'll get 3 anyway from OSHPark.

Driving Q10 directly with the MCP6021 seemed really unstable.

[T3sl4co1l]

Nice thing about discrete is, you can plug whatever you want into it -- you can tag on another diff+VAS in parallel with the first to do exactly that, without having to worry about making decisions between op-amp outputs (and the slew rate limited decisions allowing overshoot!).

Tim

[void_error]

Made some progress
No. of opamps used: 0 (integrated ones)
Only simulated it for 5V so far, common mode input range should include ground.
Here's the result.


V_OUT
Q4-base - actually Q5 base but I didn't bother renaming it
Q2-G
It only overshoots 0.23V - load from 1% to 100% in 1us which probably won't happen in real life.

[T3sl4co1l]

Nice!

It should poop out maybe 0.2V above ground, because of diff pair Vbe minus load Vbe (the current mirror or voltage gain transistor, respectively), plus Vce(sat) headroom.  Do mind that "pooping out" means full output voltage (i.e., phase reversal), so this will be necessary to avoid.

You could simply bias the inputs up with resistor dividers, or biased diodes, or PNP emitter followers, to push the common mode range back into place, or use yet another variant on the diff amp (like a folded cascode) to extend operating range.

Also shows you don't need the 5V anymore, since it's just supplying a current.  Pretty handy.

Tack on a current limiter and you've got a pretty passable bench supply!

Tim

[void_error]

On to the current sensing part now... -5V rail needed, also more current mirrors for level shifting. All done on the output. All that's left is to figure out how to add this to the circuit... any ideas?



I_OUT
I_SENSE

[T3sl4co1l]

Eww... you've got three things wrong with that, which is 90% of the circuit... the darlington is going to be sloppy (mainly slow to turn off, but probably still stable), and not just one but two current mirrors, that are discrete; they might be good enough (a few %), but geez, why play ping-pong when you can use a diffamp part directly?

AD8210 comes to mind.  Probably don't need one that fancy, but that type of (5-ish terminal) "diffamp with voltage output" current sensor is the way to go.  There's also the three terminal high-side sensors, which are what you're doing here, but they're less accurate -- and as you know, don't work with zero dropout!

I'd rate the "diffamp with resistors you need to trim" method as better than this.  But even better still, yeah, this is one of those cases where you really just want to give in and use something pre-trimmed.

Tim

[void_error]

A BSS84 would be a better replacement for the darlington, or I could use a LTC6101 for current sensing. However it will still need to be referenced to -5V so I'll still have to play ping-pong with its output voltage in order to reference it to ground. I have two options here: 1. current mirrors 2. opamps, with the first one being a buffer and the other one a level shifter which is the first idea I tried. Decisions, decisions...

Transistors are dirt cheap though and therefore more tempting

[T3sl4co1l]

Transistors are cheap, but you end up paying for it in space, design time and so on.

I mainly play with transistors because it's fun and "good enough".  I wouldn't recommend it for precision (sub-1%) work -- it'll be too damn drifty, even after calibrating it.

Sadly, it's rarely economical as a design staple, these days -- development costs are that much greater than a few cents here or there for the parts, and the cost of transistors versus ICs varies with where you're buying parts from, and assembling them at.  Automated SMT is definitely the leader around here, but through hole may still be cheapest from some Chinese factories!

Tim

[void_error]

I'm only going to make 3 of these power supplies, not 9k+, so they're going to be hand-built. Development costs don't matter that much since I'm using this project to learn new stuff.

Now here's the catch: I'm trying to design a power supply that's scalable for different output voltages/currents while keeping the same circuit board. Oh, and there's another thing: it must use only one DC input voltage.

EDIT: Changed the thread title as the whole thing went south...

[void_error]

Had a little play with current mirrors today and everything that I was expecting to happen just happened. As it was pointed out to me they are very sensitive to temperature - any temperature difference between the transistors will cause a relatively large imbalance. The hotter transistor to conduct more current which is obvious given the -2mV/K of the BE junction. I did equalize the currents first using a multi-turn trimmer in the same way it's used to null opamp offset.

One solution would be to use dual transistors, pretty hard to get compared to single ones but at least they'll be at almost exactly the same temperature. Not liking this. Another option might be to keep the transistors physically close which probably wouldn't be difficult with SOT-23 devices.

Either way I'm stuck here.

[blackdog]

Hi void_error,

Please, dont make the mistake because Spice tels you its stable (power on, Power off and Load variation)

I did extensive testing on power supply's, and i can tell you, its is difficult to get it all wright.
Just test the real thing, you will be amazed how much Spice is lying to you.

Take a look @ this Dutch website (a lot of pictures!) how i measured the AC Ri. of a powersupply i'am designing.

http://www.circuitsonline.net/forum/view/110029/8/noise+amp+power+supply

Be aware that i use 2x 0,5M kabel to my Dummy Load, in the Dummy Load there is a 6,8uF capacitor (High Q)
This make's the ringing on the output of the power supply.
The better the power Supply the smaller the ringing
(its complicated, bandwith of the opamp, bandwith compound stage, ESR and L of the output capacitor, wiring etc.)

With de best compound output stage, and a fast opamp, there was only a view mV over and undershoot @ almost 10A change in load.
Thise is only possible with good wiring technics.

Keep up the good work!

Kind regarts,
Blackdog

[void_error]

Hi void_error,

Please, dont make the mistake because Spice tels you its stable (power on, Power off and Load variation)

I did extensive testing on power supply's, and i can tell you, its is difficult to get it all wright.
Just test the real thing, you will be amazed how much Spice is lying to you.

I'm not entirely relying on simulations, just using them to get a general idea of how the circuit will perform and which components affect which parameters.
Unfortunately I don't have the test equipment you have (no DSO, just a crappy analog scope and a bunch of crappy multimeters) so it's going to be a bit of a pain in the arse to test the whole thing.

Take a look @ this Dutch website (a lot of pictures!) how i measured the AC Ri. of a powersupply i'am designing.

http://www.circuitsonline.net/forum/view/110029/8/noise+amp+power+supply

Be aware that i use 2x 0,5M kabel to my Dummy Load, in the Dummy Load there is a 6,8uF capacitor (High Q)
This make's the ringing on the output of the power supply.
The better the power Supply the smaller the ringing
(its complicated, bandwith of the opamp, bandwith compound stage, ESR and L of the output capacitor, wiring etc.)

With de best compound output stage, and a fast opamp, there was only a view mV over and undershoot @ almost 10A change in load.
Thise is only possible with good wiring technics.

Keep up the good work!

Kind regarts,
Blackdog

I couldn't get away with using an integrated opamp because of the relatively high supply voltage involved in case of a higher voltage version. I know I could use another winding like a few HP/Agilent/Keysight/(wonder what's next) use to bias some stuff but that translates into another separate transformer and I'd like to keep this bench supply as simple as possible - only one transformer winding required.

I'll post an updated list of project goals soon. The one on the first page has become obsolete.

[T3sl4co1l]

One solution would be to use dual transistors, pretty hard to get compared to single ones but at least they'll be at almost exactly the same temperature. Not liking this. Another option might be to keep the transistors physically close which probably wouldn't be difficult with SOT-23 devices.

Duals don't actually save you much, because the lateral thermal resistance (from one die to the other -- no, they don't use monolithic transistors in the duals, unless they very specifically say so) is not much less than the thermal resistance to ambient for a single device.  So if the powers are unmatched, the temperatures will be, and you'll have a predictable current mismatch.

One thing you can do to address this is try to enforce equal conditions on both transistors.  The Wilson mirror cascodes the output device with another, so both transistors at the bottom (which are doing the mirroring) see low collector voltages.  They're still not exact (indeed, the error goes the opposite direction now), but it's a big help if you're dropping more than a few volts.

Either way I'm stuck here.

No need to be stuck.  Just do it with op-amps: they have much more predictable Vbe.

Tim

[void_error]

Here are the updated design goals:

Analog part

• Scalable for different output voltages/currents - up to 40V & 5A output using the same PCB
• Step-down switchmode pre-regulator which can go up to 100% duty cycle - for low output currents it will automatically be disabled to provide a cleaner output
• Not use highly specialized/hard-to-get parts
• Only one transformer winding fed into a bridge rectifier + filter caps or any fixed DC supply with the required voltage/current output used to power it
• Voltage/current can be set either using DACs or regular multi-turn pots

Digital part (optional, incomplete)

• MCU control
• Voltage/current and maybe other things displayed on an LCD
• USB interface
• Auxiliary 5V and/or 3.3V earth-referenced output to power logic circuitry (the USB connection to a PC will be mains earth referenced anyway)

As always suggestions are welcome.

[prasimix]

Hi void_error,

Please, dont make the mistake because Spice tels you its stable (power on, Power off and Load variation)

I did extensive testing on power supply's, and i can tell you, its is difficult to get it all wright.
Just test the real thing, you will be amazed how much Spice is lying to you.

Take a look @ this Dutch website (a lot of pictures!) how i measured the AC Ri. of a powersupply i'am designing.

http://www.circuitsonline.net/forum/view/110029/8/noise+amp+power+supply

Be aware that i use 2x 0,5M kabel to my Dummy Load, in the Dummy Load there is a 6,8uF capacitor (High Q)
This make's the ringing on the output of the power supply.
The better the power Supply the smaller the ringing
(its complicated, bandwith of the opamp, bandwith compound stage, ESR and L of the output capacitor, wiring etc.)

With de best compound output stage, and a fast opamp, there was only a view mV over and undershoot @ almost 10A change in load.
Thise is only possible with good wiring technics.

Keep up the good work!

Kind regarts,
Blackdog

Hi blackdog, the link that you providing is full of interesting pictures and diagrams. Do you possibly have any related english text?

[void_error]

Do you possibly have any related english text?

You could use google translate to translate from dutch to chinglish and then try to decipher the chinglish, it's doable.

[prasimix]

Here are the updated design goals:

Analog part

• Scalable for different output voltages/currents - up to 40V & 5A output using the same PCB
• Step-down switchmode pre-regulator which can go up to 100% duty cycle - for low output currents it will automatically be disabled to provide a cleaner output
• Not use highly specialized/hard-to-get parts
• Only one transformer winding fed into a bridge rectifier + filter caps or any fixed DC supply with the required voltage/current output used to power it
• Voltage/current can be set either using DACs or regular multi-turn pots

Digital part (optional, incomplete)

• MCU control
• Voltage/current and maybe other things displayed on an LCD
• USB interface
• Auxiliary 5V and/or 3.3V earth-referenced output to power logic circuitry (the USB connection to a PC will be mains earth referenced anyway)

As always suggestions are welcome.

I'd like to propose a modular design with some sort of "motherboard" and plug-ins with various functionality. In that case we can share the best part of many designs like:

• input filtering, protection and rush-in limitation (in case of huge toroidal transformer)
  
• PFC or transformer
• pre-regulator stage which can be DC-DC (step-down or even step-up battery or solar panel powered!) or AC-DC
• bias power supply
• post-regulator (SMPS or linear, serial or shunt)
• voltage reference
• OVP, OCP
• control section: manual or digital (ADC/DAC, PIO)
• MCU with with various types of connectivity to PC, other equipments, etc.
• control panel section
• output filtering and protection, etc.

That could be a group effort with different level of commitment when we have to define physical boundaries, type of connection, levels and signals (both analog and digital). That possibly slow down a progress on the beginning of the process but can be beneficial on the long run since we can reuse and much easier multiply certain functionality with such buildings blocks. I'm aware that due to the nature of analog circuits it's not so simple to do that like in digital world but we can examine existing solutions and see what is applicable for DIY level or construction and usage. At the end I believe that will be possible to organize various group buys where participants don't necessarily need to select the same configuration depends of their taste and requirements.

[void_error]

That's quite close to what I'm aiming for and as you said the analog part is the tricky one because of the initial requirements - having no separate floating bias supply like the E361xA does. However this has some advantages with one being the possibility to use an open-frame AC-DC switchmode power supply.

I'm planning to make the whole thing modular with the minimum configuration being the following:

• Rectifier + filter caps - in case a (single winding) mains transformer is used
• Switchmode pre-regulator - optional but recommended for higher output power
• Analog board - adjustable linear regulator (the post-regulator) with adjustable output voltage and current limit and maybe electronic fuse mode current limiting
• Control board - powered by the analog board, contains a voltage reference, connections to off-board pots to adjust output voltage/current limit and off-board panel meters as well as an output-enable switch/relay

[prasimix]

Great, do you think that we can open i.e. a new topic where we can start to define a general specification which will be open for suggestion and discussion with all interested members?

[void_error]

I'll start a new topic as soon as I have a schematic, do some tests and settle on the specs.

I've also figured out relatively simple way to do all the level shifting on the current sensing part, looks good on the simulation. The TLC277 probably isn't the best choice when it comes to accuracy but it's cheap and meets the input voltage requirements with a reasonably low input offset voltage.

Yes, I'm playing ping-pong with voltages and currents to get the output referenced to ground, but at least supply voltage variations won't affect the circuit.