I'm controlling a P-Ch MOSFET on a circuit,

  • VIN is 8-30V
  • TTL_IN is a 0-5V signal generated by a MCU, which can source/sink currents up to 15mA
  • Zener Izt 5mA (BZT52C15)
  • MOSFET Vgs limit 25V

I'm currently using this circuit:


I find many different implementations for this situation; I see voltage dividers at gate, emitter resistors for current limiting, replace NPN with a small N-ch FET, gate driver IC's etc.

My questions are:

  1. Is my circuit is valid and safely usable? (I actually use it with no obvious effects (heating etc.) but I want to be sure)
  2. For relatively fast (5KHz) switching speeds, what should I need to change?
    • I previously used 10K for R12 and it cannot charge the gate fast enough, leading slow turn off times. 1k is much better for regarding speed but I'm not sure R9 limits the current enough.


  • \$\begingroup\$ 5 MHz is relatively fast gate drive ;-) But you are correct, with 1 k pull up, even 5 kHz becomes an issue. \$\endgroup\$ – winny Apr 7 '18 at 20:49
  • \$\begingroup\$ You can make both R9 and R12 bigger and put a small capacitor across R9 to speed up the circuit. I think it will work. Simulate first. The basic circuit seems OK. I think you can get it to work. It might be easier if you use NMOS on the bottom (if your application allows that) and use a gate driver IC to drive the NMOS gate. \$\endgroup\$ – mkeith Apr 8 '18 at 1:37
  • \$\begingroup\$ @mkeith Thanks for your suggestion, I'll try and report back. Unfortunately my application requires high side drive, so I'm stuck to P channel FET's. \$\endgroup\$ – sabbath Apr 8 '18 at 21:37
  • \$\begingroup\$ It is possible to do high side control with NMOS, and it can work well provided that there is an upper limit on the duty cycle less than 100%. If you are interested you can look into high side gate drivers. But PMOS may just be easier. \$\endgroup\$ – mkeith Apr 8 '18 at 23:23
  • \$\begingroup\$ Even at DC there is a major problem. At 30V VIN, you'll have 15V across R9/T1 leading to 100mA. 100mA is the ABS MAX rating for a BC847. R9 will need to be at least 2 watts. D2 will be dissipating 1.5W which is 3 times the max rating on the data sheet. I'm going to guess that you didn't try this at 30V, correct? \$\endgroup\$ – Randy Nuss Apr 7 at 14:13

Your 1N4007 freewheel diode is far to slow ,In fact it could cause the mosfet to blow if your load has inductance .Replace this with something faster .If V in was 30 V and the duty cycle was high R9 and Z2 will get very Hot .What I do is use a current source to limit the worse case power disipation .Your 1K R 12 is the only thing that turns the mosfet off .Reducing it will speed things up but waste more power in the gate drive .You could Augment R 12 with some active turnoff scheme .


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