I'm currently using an N-channel MOSFET (PSMN0R9-25YLC to drive a load as shown below:


The Arduino is providing a PWM signal with a voltage from 4.2V-2.5V depending on the charge of the battery and driving the gate at 490Hz. The voltage between the drain and the source also reflects the voltages above as well. I'm also pulling about 5 Amps at any given time.

After about 30 seconds of use the MOSFET is extremely hot to the touch and I can actually hear it switching the load. Looking at the specs of the MOSFET I believe it should be able to handle this no problem, so what could be wrong?

  • \$\begingroup\$ What's the value of the resistor between the I/O pin and MOSFET gate? What's the value of the resistor from gate to ground? \$\endgroup\$ – jms Oct 3 '17 at 22:42
  • \$\begingroup\$ 10k between gate and gnd. I actually currently have no resistor between the arduino and the gate, I know that's a no-no; but would that cause any issues? \$\endgroup\$ – Sam W Oct 3 '17 at 22:44
  • \$\begingroup\$ No Rg is ok since Arduino CMOS GPIO port is about 50R RdsOn give or take a wide margin. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 3 '17 at 22:49
  • \$\begingroup\$ OK, given the low PWM frequency and absence of a gate resistor, I think switching losses aren't the cause of the problem (even though you are driving the gate with a microcontroller pin). Instead, I suspect the circuit is suffering from conduction losses caused by low gate drive voltage, as Tony already answered. \$\endgroup\$ – jms Oct 3 '17 at 22:49
  • \$\begingroup\$ Rds(on) not specified below Vgs=4.5V, possibly quite high by 2.5V. \$\endgroup\$ – Brian Drummond Oct 3 '17 at 23:33

Don't feel bad, some student every day makes this mistake of not having enough Vgs.

This 1mΩ FET turns into 5mΩ at 2.5V from fig 12 in datasheet and then increases 50% when it gets hot. from fig 13 and thermal resistance case to ambient depends on a heatsink of the copper substrate ( of course using recommended layout for Power-SO-8) which otherwise will get hot if your load current exceeds 200mW or W/R=I^2 or 0.2W/5mΩ ... or > 6A

There may be other causes as well.

So make sure you have enough Vgs. Charge pumps are often used or PWM with a boost diode cap.

Rule of Thumb

Ensure Vgs is >=3x Vgs(th) threshold.

  • \$\begingroup\$ Is a charge pump the simplest solution to up the gate voltage? Is this the purpose of a gate driver IC or is that to supply more current? \$\endgroup\$ – Sam W Oct 3 '17 at 22:51
  • \$\begingroup\$ just V boost but low Rds also has high Ciss so current is also needed which you can compute for rep rate or use energy from Q to charge. Gate drivers are also useful when Rg/RdsOn >200 when high speed full bridge is needed. here ~50/1mΩ =50k is OK only for low PWM rate. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 3 '17 at 22:52
  • \$\begingroup\$ the FoM or figure of merit in FET choices are RdsOnCoss at high speed output drive response and RgCiss for input drive limits. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 3 '17 at 22:57

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