0
\$\begingroup\$

So I’m trying to run a 3.7V motor off of a mosfet, and it works perfectly fine when I provide a 3.7V PWM signal to the gate of a FQP30N06L.

However, when I swap out that mosfet for the RFP30N06LE, it does not work. It did work when I provided it 9V however. I don’t understand why this is the case, I have compared the two data sheets and they are nearly identical, yet the LE does not seem to be even reaching its threshold voltage at 3.7V.

I have tried connecting the gate directly to a 3.7V rail instead of PWM pin but to no effect. I have 220 ohm resistors on the gate and I have tried with and without a 10k pulldown resistor, still nothing. Any ideas?

\$\endgroup\$
  • \$\begingroup\$ Can you add a schematic to show exactly how you're connecting this stuff? \$\endgroup\$ – BeB00 Sep 27 '17 at 2:03
  • 1
    \$\begingroup\$ You sure you have the correct MOSFET? The RFP30N06 has a Vth of 2V, did you maybe receive a high Vth part instead? \$\endgroup\$ – John D Sep 27 '17 at 2:33
  • \$\begingroup\$ THese specs are far from identical at 3.7V \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 27 '17 at 3:07
  • \$\begingroup\$ Did you just use the flange as the Drain? That would explain it. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 27 '17 at 3:15
  • \$\begingroup\$ @TonyStewart.EEsince'75 But even so its far above the threshold voltage, so shouldn’t it still be allowing current through? \$\endgroup\$ – Matthew de Soto Sep 27 '17 at 3:15
2
\$\begingroup\$

i agree with @Leroy105 on the fact that the fets may be different, however i wonder if you are connecting the source to the load rather than to 0v, that would rise the source voltage hence the gate source voltage would be lower which in turn may reduce the output current, and according to the datasheets of both fets that should happen in a more apparent in the RFP30N06LE since it has a slightly higher vTh which means that the voltage at the load should be lower for the fet to activate and hence lower would be the current at the load

\$\endgroup\$
  • \$\begingroup\$ I have the drain connected to the load and source directly to ground \$\endgroup\$ – Matthew de Soto Sep 27 '17 at 18:11
  • \$\begingroup\$ in that case id recommend you to do a sweep in the gate to determine the threshold voltage, maybe you got a bad fet as leroy105 said \$\endgroup\$ – diegogmx Sep 27 '17 at 18:28
1
\$\begingroup\$

I've seen this before with a circuit using a name brand FET and a cheap cheap Asian FET. This is in a consumer product. One cent FET vs 12 cent FET.

The RdsOn value, even if stated as the same in datasheets, may be different...

I didn't believe it at first, I deep dived this circuit and checked everything and that was the final conclusion. Datasheet specs were claiming same RdsOn but clearly not. Don't always believe what you read...

We are still working on sourcing a suitable FET for a penny. Haha

\$\endgroup\$
  • 1
    \$\begingroup\$ What is the value of unreliable Sources to Drain your reliability. This is the Gate to failure. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 27 '17 at 13:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.