4 ms·

2oz copper plus fat track, multiple via stitches, huge ground planes as well as isolation between the sampling and power delivery parts of the boards. There’s a

by cjk2 2y ago

2oz copper plus fat track, multiple via stitches, huge ground planes as well as isolation between the sampling and power delivery parts of the boards. There’s a lot of design that goes into a simple power supply PCB. Check out an Agilent supply for detail :)

Yossarrian22 2y ago
I think you mean Keysight these days
- cjk2 2y ago
  No I mean Agilent. The stuff since keysight rebrand is mostly garbage.
  - Yossarrian22 2y ago
    Too true
  - ramenbytes 2y ago
    Care to expand? I have some gripes with them myself, but I haven't demanded enough of their stuff to find technical issues yet. My problems are of the "oscilloscope is pestering me about windows updates" and "service manuals suck" variety.
    - cjk2 2y ago
      Lack of service information, poor treatment of customers compared to HP/Agilent (try getting anything fixed without a service contract now), design shortcuts (U8002A is a buggy piece of shit), absolutely nightmare trying to order parts which at least in the UK means someone random calls you from Spain and asks for your credit card details and you may or may not get the parts. Oh and the whole fact that half the gear seems to turn into a brick fairly quickly compared to older models. I will always look elsewhere now. If you have to throw something away every 2 years, might as well buy some Chinese junk instead (Siglent / Rigol etc). Aim-TTi are still good though - the last bastion of stuff that isn't shite.
formerly_proven 2y ago
Lab power supplies have always been a staple project but most just aren't very good (which is fine). This one certainly has many of the staple ingredients: TL072 op amps, slow current limit, large output capacitance (look at C3||R22 x hFe(Q4) x hFe(Q5), there's probably additional capacitance directly on the outputs that I'm overlooking in the schematic), CC/CV cross-over saturates the other regulator and thermal design. Besides poor response, these are usually unstable with various loads, there's often a loss of regulation when switching on or off, they don't survive a dead short or a sustained load (especially the "not quite dead short" case is problematic for designs like this without current fold-back [1]). Most blow up when you do the file test. Those are all good things though. You're going to find these issues when using the PSU and try to fix them. A really good lab PSU is, like you say, a surprisingly tricky thing to engineer. Some major compromises as well. There's a reason why a great many of them have used the circuit invented at HP some time in the late 60s (which in itself makes a number of compromises). [1] f.e. the suggested TIP35C: Ptot = 125 W, which is already less than the >150 W you need to dissipate when the supply is shorted. But also heed the conditions: Ptot is at Tcase = 25°C. Does a small heatsink keep Tcase at 25 °C while dissipating 125 W?
- cjk2 2y ago
  Yup. The killer on these things is usually actually feedback phase shift. Sometimes the output load can have a complex reactance which turns the supply into a convenient power amplified oscillator. Fat output capacitor solves most of these problems but sometimes that has its own problems (old HP supplies had nice barrier strips so you can deal with them yourself :). Obvious trade off is step response there etc but your load should be properly decoupled anyway. Lots of problems on power dissipation there as well as you state. The HP/Agilent designs tend to use an SCR pre-regulator which reduces Pd on the pass transistor considerably. But of course the principal cost in these things now is shipping and profit margin so it works out cheaper to cost cut even more and shift a small heatsink with a loud ass fan on it that does your ears in. Grr. (this is one of my many reasons for disliking Keysight) The old HP designs are very robust. I've owned a few. Almost impossible to blow up, even the big ones. I actually had a Harrison one built in 1967 that was still working unrepaired and unmodified until I sold it recently. Bob Pease did an interesting "zero output capacitance" supply article a couple of decades back. That was surprisingly stable.

by cjk2 2y ago

Yossarrian22 2y ago

I think you mean Keysight these days

cjk2 2y ago
No I mean Agilent. The stuff since keysight rebrand is mostly garbage.
- Yossarrian22 2y ago
  Too true
- ramenbytes 2y ago
  Care to expand? I have some gripes with them myself, but I haven't demanded enough of their stuff to find technical issues yet. My problems are of the "oscilloscope is pestering me about windows updates" and "service manuals suck" variety.
  - cjk2 2y ago
    Lack of service information, poor treatment of customers compared to HP/Agilent (try getting anything fixed without a service contract now), design shortcuts (U8002A is a buggy piece of shit), absolutely nightmare trying to order parts which at least in the UK means someone random calls you from Spain and asks for your credit card details and you may or may not get the parts. Oh and the whole fact that half the gear seems to turn into a brick fairly quickly compared to older models. I will always look elsewhere now. If you have to throw something away every 2 years, might as well buy some Chinese junk instead (Siglent / Rigol etc). Aim-TTi are still good though - the last bastion of stuff that isn't shite.

formerly_proven 2y ago

Lab power supplies have always been a staple project but most just aren't very good (which is fine). This one certainly has many of the staple ingredients: TL072 op amps, slow current limit, large output capacitance (look at C3||R22 x hFe(Q4) x hFe(Q5), there's probably additional capacitance directly on the outputs that I'm overlooking in the schematic), CC/CV cross-over saturates the other regulator and thermal design. Besides poor response, these are usually unstable with various loads, there's often a loss of regulation when switching on or off, they don't survive a dead short or a sustained load (especially the "not quite dead short" case is problematic for designs like this without current fold-back [1]). Most blow up when you do the file test. Those are all good things though. You're going to find these issues when using the PSU and try to fix them. A really good lab PSU is, like you say, a surprisingly tricky thing to engineer. Some major compromises as well. There's a reason why a great many of them have used the circuit invented at HP some time in the late 60s (which in itself makes a number of compromises). [1] f.e. the suggested TIP35C: Ptot = 125 W, which is already less than the >150 W you need to dissipate when the supply is shorted. But also heed the conditions: Ptot is at Tcase = 25°C. Does a small heatsink keep Tcase at 25 °C while dissipating 125 W?

cjk2 2y ago
Yup. The killer on these things is usually actually feedback phase shift. Sometimes the output load can have a complex reactance which turns the supply into a convenient power amplified oscillator. Fat output capacitor solves most of these problems but sometimes that has its own problems (old HP supplies had nice barrier strips so you can deal with them yourself :). Obvious trade off is step response there etc but your load should be properly decoupled anyway. Lots of problems on power dissipation there as well as you state. The HP/Agilent designs tend to use an SCR pre-regulator which reduces Pd on the pass transistor considerably. But of course the principal cost in these things now is shipping and profit margin so it works out cheaper to cost cut even more and shift a small heatsink with a loud ass fan on it that does your ears in. Grr. (this is one of my many reasons for disliking Keysight) The old HP designs are very robust. I've owned a few. Almost impossible to blow up, even the big ones. I actually had a Harrison one built in 1967 that was still working unrepaired and unmodified until I sold it recently. Bob Pease did an interesting "zero output capacitance" supply article a couple of decades back. That was surprisingly stable.