Hi, sorry for the late reply,
Hi,
You cant set up a switch to switch between the two modes? Or you want to do it digitally?
An analog switch comes to mind without thinking about this too much yet.
Sometimes you can set it up to do both, as long as the current mode can be adjusted high as that will keep the current mode from kicking in too soon when in voltage mode. In current mode you dont care if it tries to regulate voltage because the voltage will never get high enough to start to regulate anyway. That's the way most simultaneous voltage and current regulators work anyway. During the current mode, the voltage should be pretty darn low due to the series resistance of the inductor, but if it's not then just set the voltage regulation higher...assuming you can do that already somehow.
A switch (be it digital, or analogue, controlled by a microcontroller, ) would be a convenient way, however, the Vref and Iref (max voltage and max current) are set by a 12-bit DAC, so of course these can be set to maximum, or minimum in order to turn on/off each part of regulation. So yes, in this way a CC/CV supply would be fine. You rightly pointed out the issue of setting the voltage part 'low' during current regulation, because as I'll be testing power inductors (perhaps up to Irms > 8A) their resistance will be super low. Although I realize that a really fine resolution for resistance is not easy, say if an inductor has DCR of 12mOhms, a 12-bit DAC (Vref 4.096) would give an ideal resolution of 1mv. So a current of 1Amp through that inductor, would only give 12mV which is so tiny to set, even a rail-to-rail DAC would struggle to keep that close to the ground rail (although the DAC would be connected directly to an opamp's input, high impedance). Perhaps I could use a small signal mosfet to pull the input to the opamp pretty much down to ground, even when the DAC outputs '0'. A series 10k resistance from the DAC and a 2n7000 would nail that.
I considered using analogue switches to change both the input to the opamp feedback (a voltage representing output voltage, or output current) ... AND... switching the compensation. So I can have a very fast transient response for voltage regulation, and a slow one for current - to reduce ringing when switching in a relatively high value inductor. However, I'm cautious about inserting switches into feedback loops. If the output voltage goes up to max (say, 4V), I will not have much of a load on it without the inductor switched in.. so it will take a while to discharge the output cap.
As I was googling for ways to have 'separate control loops' controlling the same pass element, I came across this wonderful project:
**broken link removed**
He chose the second method, with one control loop, it seems he was after perfecting the
transition between CV and CC - something which I do not need, as they will be completely separate tests. So the top schematic would be ideal. There are however differences between that and what I'm doing.
Firstly, He is using a MOSFET pass element, I'll be using a darlington (a high current, high gain main NPN, driven by a smaller NPN, Perhaps FZT1051A - 40v, 10A peak I, with a 2n222A).
**broken link removed**
So thats current driven, as opposed to voltage driven.
Secondly, he is using high-side current sensing. I looked into this, as it is generally preferential in all situations, (making load voltage measurements much easier) but had trouble deciding on a high-side monitor that was fast enough, as well as kept errors low even with small sense voltages (accurate at say 5A, but 20% error at 50mA). I have some MAX4172's somewhere, which dont' seem to be used as much as the MAX4173, but gain can be adjusted and appears to be fairly accurate. Only 800kHz bandwidth, at full scale range, which could be difficult for fast current ramps with low value (<4.7uH) inductors.
https://datasheets.maximintegrated.com/en/ds/MAX4172.pdf
I only have the LTspice model for the MAX4173T which seems both very accurate, and fast enough - that however, has double the bandwidth of the 4172.
I chose a low side measurement, because, despite the real hassle of using a differential amp to measure voltage across a load, it is straightforward, can be very accurate, and I can choose a high speed (>6MHz), very low offset opamp for it. I am unsure how to determine the maximum bandwidth I would need, for a given gain, to accurately keep track of the current ramp. worst case, say a 1uH inductor, at 1V, current sense resistor at 0.1Ohm. That will give an input ramp of 0- 0.1V, in 1us. Perhaps the maxim chips 800Khz (200Khz when input is low) would be fast enough?
But back to that first schem on the page:
**broken link removed**
The current amp uses a diode on the output to pull the voltage to the pass element low. Would this work with a NPN pass element? I perhaps I'm over thinking, but I can see the current regulation, overrides voltage. So if I set voltage to say 1V. And current to 0.2A, and put on a load with a low resistance to CC kicks in... the voltage regulator opamp would output as much current as it can, trying to bring the output voltage up - but the current opamp would pull that low via the diode. Seems like we're just shunting a few miliamps.
The second method he uses, despite his very cool use of compensation networks (and a brilliant explanation of them I might add) I fear would just be too slow to maintain output within 2% voltage with a sudden load ramp (not step) of say 5A within 5us.
Apologies for a very long, and somewhat boring post. But I think I'm missing something here, all the CV/CC supplies I have seen, use quite slow loops - and rightly so for bench supplies to keep things stable. Do you think the first circuit on that page is close to what I'm after?
Cheers!
Blueteeth