I use this MOSFET:
in a basic boost converter topology in DCM mode (L=100µH). The gate is driven by an ATtiny402 through a 100 Ohm resistor. My oscilloscope shows some odd behaviour: the MOSFET turns off more than 500ns after the gate has reached 0V. Turn-on is practically instantaneous.
I also measured the current into the gate. It shows some constant-ish current coming out of the gate in the delay phase.
As a test I used a different MOSFET, where the waveforms look exactly as expected:
Unfortunately this well-behaved MOSFET is not suitable for the final application.
Now to my question(s):
What is happening here? At Vgs=0.7V Rds>30kOhm so it isn't just a low threshold voltage. I tried soldering in a new one of the same type, but the problem is the same. If possible, how can I improve turn-off performance? And no, shorting the gate directly to the pin of the ATtiny did not make a difference.
Update with some clarifications and some additional experiments:
I reduced the gate resistor to 10 Ohm, it did not change the turn-off delay. It did, however, slightly change the gate current waveform:
For those that requested a schematic, it really is just your standard textbook boost converter:
other tests I did:
- added a 1.5nF capacitor across the 100 Ohm gate resistor, no effect on turn off delay.
- reduced VCC of the circuit from 3.8V to 2.5V, the delay was shortened by 25%.
- reduced Rg to 10 Ohm, no effect. (I/O pin resistance ~ 30 Ohm)
- yes, I did measure directly at the gate, not at the MCU pin.
- yes, the MOSFET is supposed to be able to be driven from a MCU pin (700pC)
- where I bought the weird behaving MOSFET: reichelt.com
- the second MOSFET I tested (that did not behave weird) was a BSS138 https://www.onsemi.com/pdf/datasheet/bss138-d.pdf
clarification edits in the original section above:
- changed the Ig waveform picture to be more clear and show the remaining current better.