Hi,
I wanted to present you my latest project. This started just being a proof of concept and later evolved into a full blown project. I was not expecting to get too much precision of this power supply, especially considering that it uses such a basic topology: just an average NPN transistor and a semi precise op-amp polarizing its base and having the output voltage fed back to the inverting input. The schematic is attached.
The output section is controlled via SPI, and is isolated from the input section, which is controlled and fed via USB. To pull this off, I use a CP2130 as my "micro-controller" in the input section (because, for the sake of simplicity, this supply does not have brains). I have used this interface before with relative success. It has its problems, though, because due to a race conditions, the control program has to wait about 100us before disabling or changing CS, or asserting or de-asserting any pin.
The output section is pretty much self-explanatory. It has some simple short-circuit protection that basically starves the base of the pass transistor (Q2) if the current exceeds the vicinity of 259mA (give or take a few tens of mA). That short circuit is comprised of Q3, uses R8 as the current sense resistor, and has R7 so that the output of the op-amp does not get "shorted" in the process. It is true that the OPA705 can be shorted, but R7 makes the short-circuit current of the power supply much more predictable, because you can simply consider that Q3 starts to turn on at 0.65V of base-emitter voltage.
The OPA705 op-amp was chosen because it offers some precision due to its low input offset voltage, while being cheap. It also has adequate GBP for this application, thus providing a reasonable fast response. I could, in the end, have used a better op-amp (like the OPA703), but I was not sure if that would be worth the trouble. I was surprises that, indeed, the input offset voltage was the significant factor at play, and there was no other deviation factors due to the pass transistor or what not.
As for the topology used here, I chose it because it seemed straightforward at the time. There was no special consideration, and I admit that I'm new to this. All I knew is that I didn't needed a Darlington as my pass transistor, although I did considered that option. At 200-250mA, the current gain of the MJD31C is acceptable so that the base current is still negligible and also there is plenty of headroom. One of the caveats that made me not to use it is that the output voltage of the RI3-0509S can go as low as 7V! (also, the USB ports on my computer can go as low as 4V at 500mA due to the PPTC protection on the motherboard - but anyway, that is way out of the USB specs)
As a side note, I must confess I was a bit surprised to see that Q3 works in the active region and not in the saturation region as I wrongly expected. That makes sense, because its Vcb has to be greater than 0V. Essentially Q3 Vcb is equal to Q2 Vbe, and thus the collector-base junction of Q3 is always reverse-biased.
Some specs:
– Maximum output voltage (nominal): 4.09V (minimum is very close to zero)
– Voltage increment/step: 1mV
– Noise (V out = 2V, I L = 0mA): 1.41mVrms
– Noise (V out = 2V, I L = 200mA): 1.56mVrms
– Accuracy: ±(0.782% + 7.5mV)
– Load regulation: 9.72mV/A (measured with the intended internal wiring)
– Short-circuit current (I SC): 259mA
Kind regards, Samuel Lourenço