Timeline for Multiple Buck Converters kills dead Pi Pico?
Current License: CC BY-SA 4.0
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| Jul 11 at 21:31 | comment | added | Math Keeps Me Busy | Actually, there is a slight error in my explanation in the last comment. The max current will flow before the Zener reaches its knee, so the resistor has some extra volts across it when it is passing the most current to the gate. So, we don't need the Zener to pass quite the same current as the maximum for the gate. But even so, the series resistor needs to be small and consequently the power lost will be large. | |
| Jul 11 at 21:21 | comment | added | Ben Voigt | Ahh that's definitely a dealbreaker | |
| Jul 11 at 21:17 | comment | added | Math Keeps Me Busy | @BenVoigt That was my original thought. It would work (with say 12 Volt Zeners or more), but I have a somewhat stingy attitude toward power. A Zener shunt regulator for the gate (Zener between ground and gate, resistor between gate and power rail) has a constant current draw, that needs to be a least as great as the maximum draw of the MOSFET gate. I would want peak current into the gate to be at least 200 mA, to ensure quick turn on. (The whole point is to protect other components). But 200 mA dropped across 19 V is 3.8 W! That seems excessive to me. | |
| Jul 11 at 19:09 | comment | added | Ben Voigt | Would it make sense to swap the Zener and resistor of those voltage dividers, so that the gate drive voltage is constant across a range of charger voltages? | |
| Jul 11 at 18:35 | history | edited | Math Keeps Me Busy | CC BY-SA 4.0 | deleted 12 characters in body |
| Jul 11 at 4:27 | comment | added | ashbygeek | Tested the circuit. I implemented your second circuit, the voltage difference between ground coming to my circuit and the ground at the pico dropped to 0.15V with the pico up and running and 0.1V with no load. So I went ahead and tested the USB and didn't seem to have any trouble. I'll do some more testing, let it run longer to make sure, but I went ahead and accepted your answer. Many thanks for all the help! | |
| Jul 11 at 3:48 | vote | accept | ashbygeek | ||
| Jul 3 at 18:09 | comment | added | Math Keeps Me Busy | @ashbygeek I added an explanation of the purposes of R4, R5, and R6 to the answer. As you surmised, R1 just limits the current into the gate so that the diodes don't burn out from excessive current. PS, don't use the Zeners if you are going to use a low voltage supply, for example 12 V. If you shave 5 volts of the 12 volts, you only get 7 V, and while that will turn on the MOSFETs, there will be more resistance than necessary, and this resistance will have some voltage drop. Remember getting rid of the voltage drop was the goal of this redesign. | |
| Jul 3 at 18:07 | history | edited | Math Keeps Me Busy | CC BY-SA 4.0 | added 1078 characters in body |
| Jul 3 at 17:41 | comment | added | ashbygeek | Awesome, I didn't have any suitable mosfets onhand, so I've ordered some and breadboarded your schematic, should be able to test it next week. To make sure I understand things. R4 is just a pulldown resistor to make sure M1 won't get triggered by transients, right? And R1 just limits how fast M1's gate can be pulled to zero so , correct? How did you choose 100 Ohms for R1? | |
| Jul 3 at 1:48 | history | answered | Math Keeps Me Busy | CC BY-SA 4.0 |