Timeline for Can a ESP32 and PCF8575 IO port Expander drive a IRLR7843 MOSFET directly with a 6A Load?
Current License: CC BY-SA 4.0
20 events
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| Feb 13, 2023 at 18:03 | comment | added | DKNguyen | @mike The pulldown is not terribly important. It's mainly to turn the MOSFET off if you cut power before the port expander is able to pull the line low. It's not really for use while operating so I would go 10K. | |
| Feb 13, 2023 at 16:37 | comment | added | mike | Thank you to everyone that answered my post, the information is very useful. I will add a 100ohm on the gate, I also will add a pull down resistor on the gate, I think a 10k will be fine or perhaps a 4k7 would be better? What do you guys think? | |
| Feb 13, 2023 at 7:48 | comment | added | Polynomial | @DKNguyen Yeah, and if we're talking a tens of microseconds or more between switching edges then 100Ω of gate impedance isn't going to make the faintest bit of difference to performance or thermals, even with 4nF of gate capacitance. 1K is definitely way too much. | |
| Feb 13, 2023 at 7:44 | comment | added | DKNguyen | @Polynomial I guess latch up is potential issue. 100 Ohm is much less objectionable than 1K. | |
| Feb 13, 2023 at 7:31 | comment | added | Polynomial | I wrote a fairly in-depth answer about driving MOSFETs for PWM recently, which might be of interest if you are planning on doing any fast switching or PWM driving. | |
| Feb 13, 2023 at 7:30 | comment | added | Polynomial | Worth noting that the absolute maximum in/out current limit per IO on the PCF8575 is 20mA, so a small gate resistor should really be added since you're driving around 4nF of input capacitance on the gate, and without a resistor you will transiently exceed the max IO current rating during switching. Technically you need 250Ω to never exceed the maximum current rating, but since it's an extremely short transient and the latchup limit is 100mA you could go down as low as 50Ω. I'd go with 75Ω or 100Ω personally, unless you need to switch quickly, at which point I'd use a gate driver IC. | |
| Feb 13, 2023 at 5:19 | comment | added | DKNguyen | No, MOSFETS are a capacitor between gate-source. BJTs are a diode between base-emitter. That's the difference. When shutting off the port expander will go LO and drain the capacitor. Unless you are referring the current pulse through the drain-gate parasitic capacitance but that current pulse is not a problem until it is a problem. | |
| Feb 13, 2023 at 5:12 | comment | added | mike | One other question, I thought that while a Mosfet is charging a current could flow back to the ESP32? So wouldn’t I need a resistor to protect it? Thanks. | |
| Feb 13, 2023 at 5:05 | comment | added | DKNguyen | @mike I answered because the post had most of the relevant data already there. | |
| Feb 13, 2023 at 5:00 | vote | accept | mike | ||
| Feb 13, 2023 at 4:54 | history | edited | DKNguyen | CC BY-SA 4.0 |
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| Feb 13, 2023 at 4:54 | comment | added | mike | Thanks I found it. Just seen your update, thank you very much for taking the time to answer. | |
| Feb 13, 2023 at 4:51 | comment | added | DKNguyen | @mike There is an edit button. But I added explanations in my answer. | |
| Feb 13, 2023 at 4:50 | history | edited | DKNguyen | CC BY-SA 4.0 |
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| Feb 13, 2023 at 4:48 | comment | added | mike | The graph shows 3.3v with a output over 10A, as far as i can tell. How do I edit my post to add the graph? | |
| Feb 13, 2023 at 4:46 | comment | added | DKNguyen | If infrequenct switching you typically do not need a driver unless the heating during transition is so large that even a single switching would overheat the transistor (either because the currents and voltages being switched are high or the gate drive current is far too low causing slow switching). | |
| Feb 13, 2023 at 4:46 | comment | added | DKNguyen | If you are using the port expander at 3.3V it will not achieve a good RDson. Whether the RDson is workable in your specific application can be determined by examining the Id vs Vgs curve of the graph in order to calculate the RDson when you apply 3.3V to the gate. | |
| Feb 13, 2023 at 4:43 | history | edited | DKNguyen | CC BY-SA 4.0 |
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| Feb 13, 2023 at 4:38 | comment | added | mike | I will not be switching via PWM. Will the above circuit work? or do i need a driver? thanks for your answer. | |
| Feb 13, 2023 at 4:35 | history | answered | DKNguyen | CC BY-SA 4.0 |