Hans, The proposed circuit has problems but the concept shows promise.? I didn't look at it very carefully. I saw the issue of "?the transistor will be shorting the processor's GPIO pin directly to ground. In other words the transistor will be fighting whatever transistor is inside the microcontroller." That could be fixed by adding a resistor in series with the GPIO output pin to limit the GPIO pin's current when the transistor turns on.? I haven't looked at the other more difficult questions you bring up or whether this fixes the input voltage jump issue at all or is it not a robust fix.? I do though like the idea of a "dead-man switch" so no matter what the software is doing, the circuit should have some hardware protection if it can be implemented at very low hardware cost with relatively few components.? OMG the density, the density of that wonderful 3D art packaging.? One can't appreciate it until you get the plugging all the boards together in the final assembly steps!
-Steve K1RF
------ Original Message ------
Date 9/3/2023 7:05:52 AM
Subject Re: [QRPLabs] #qmx Don’t Use USB-C PD to power your QMX?!
Hi Gunnar, all
A few comments/questions...
1) this circuit has been proposed, but has anyone actually tried it for real, and then verified that on the step change from 6V to 12V it does protect the circuit without killing the zener?
2) As far as I can see the transistor will be shorting the processor's GPIO pin directly to ground. In other words the transistor will be fighting whatever transistor is inside the microcontroller. Does this worry anyone??
3) More fundamentally... Does it work at all, even theoretically? So the voltage input suddently jumps from 6V to 12V. The SMPS is then running at a too high duty cycle and the voltage at the SMPS therefore jumps. Ordinarily the 5.6V zener is supposed to eat this spike by passing the current to ground. It has to do that for up to 2ms until the processor control loop detects the situation and acts to fix it. Apparently that couple of milliseconds is a bit too much for a 0.5W zeners and there is a possibility of killing it. Which brings me to my question. Now that same current is going through the zener and though the base-emitter junction of the transistor. It could be for a much shorter time because it will (hopefully) win the fight with the GPIO pin and shut down the PWM; still, by that time substantial energy has been stored in the 330uH inductor and now that has to dissipate as a current pulse onto the base-emitter junction of the transistor. Why is the transistor going to survive this?
