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| how does blackdog's PSU work? |
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| Cerebus:
--- Quote from: exe on March 19, 2018, 07:36:36 pm ---Is a separate isolated power supply needed? Can I just use a charge pump to generate some voltage above the output? Let's say I can sacrifice some noise performance (my PSU enclosure is not shielded (plastic), so I'm not going to get microvolts of noise even if I try hard). --- End quote --- Strictly it needs to be floating with respect to the output 'ground' as opposed to fully isolated from all the other supplies, i.e. it needs to move with the final PSU output voltage. Obviously you need galvanic isolation from the primary side, but you'll still get that if you steal a few 10s of mA from the bulk supply. The current demand is modest, but not trivial, so all the usual charge pumping tricks (voltage doublers, IC charge pumps and the like) are going to be a bit marginal. You need current consumption figures for the actual control circuity under worst case conditions to know if you'd be able to get away with it. A dirty little trick is, if you are using a toroidal transformer for your 'bulk' supply, you can add a couple of minor windings yourself to get your secondary supply. If you've got a 200VA transformer, it's not going to care about a couple of 2VA windings added on the outside. Obviously you've got to use the right materials, right insulation etcetera, etc. Bone up on transformer construction techniques before even thinking about attempting this, so you know how to do it safely. I say toroidal transformer because it's easy to add windings. Obviously you could do the same with an EI transformer or similar, but it would be so much physical work that you'd be better off ordering a custom transformer from the start. |
| alm:
--- Quote from: Cerebus on March 19, 2018, 07:44:40 pm ---So, the question is: What is a reasonable slew rate for an applied step load when testing PSU regulation recovery times? Are there industry standards? (I couldn't find evidence of any.) Or do people just test with whatever slew rate the electronic loads available to them support? --- End quote --- This Keysight application note specifies that the load should have a rise time at least five times faster than that of the power supply. Obviously this is not particularly helpful if you are trying to establish the latter. I guess that means increasing the slew rate until you observe no change in power supply behavior? |
| T3sl4co1l:
A charge pump would be workable, but probably too noisy (even if you don't need as low noise as the original design was aimed at!). If the floating negative supply can be eliminated (say by using a RRIO opamp, and omitting the LEDs, using schottky diodes instead?), then only the positive would be left, and a voltage doubler could be used to make an extra-high supply, which powers a CCS which powers the floating rail. RRIO amps don't tend to be very low noise, but that's alright. Tim |
| Cerebus:
--- Quote from: alm on March 19, 2018, 08:18:07 pm ---This Keysight application note specifies that the load should have a rise time at least five times faster than that of the power supply. Obviously this is not particularly helpful if you are trying to establish the latter. I guess that means increasing the slew rate until you observe no change in power supply behavior? --- End quote --- That's helpful, thanks. The minimum possible rise time of the PSU is pretty much limited by the ft of the pass transistors and is relatively easy to calculate/measure as is the actual rise time once the whole control loop is taken into account. My problem was just understanding what was an acceptable load profile for measuring transient response. Obviously the response overshoot/undershoot (and any ringing) of a PSU gets worse with faster edges on the test load, and gets better with slower edges. I suppose that my question really ought to have been "What's the slowest slew rate that is regarded as an acceptable test?". If one sets the criteria as 5x faster than the supply rise time then that seems reasonable to me as you're basically saying "faster than the supply can respond". Kind of obvious really, but experience tells me that I'm more likely to miss the obvious than the subtle. |
| T3sl4co1l:
Directly applicable example: I was testing a new DC-DC module against my bench charger/discharger supply. The former has loop bandwidth in the mid 10s of kHz, while the latter has a rise time of a few milliseconds. I was unable to detect any perturbation on the module output, while switching the discharger on and off. In contrast, a MOSFET + resistor + function generator easily shows the step response of the system, which in this case was limited by the module's output filtering network (amazingly, no changes needed on the compensation network!). Tim |
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