PWM (MOSFET) strange problem

[Korawy]

Hi all,,

im using MOSFET to control speed of two motors, the drive is from pic18f452 ccp1 and ccp2 and through a driver circuit which is as fig:



The motors work perfectly when i type this in the software (using mikroc) : "PWM_CHANGE_DUTY(255);"
But when i change the duty cycle the motor doesnt work.

I think that the problem is in the transistors used in the driver circuit which are (2n3904,2n3906), But not sure.
The resistors that i use are:

Rgate= 330 ohm.
Rb= 120 ohm.
R=10 kohm.

These values are not determined in the datasheet where the figure is.

Hope to find help..

 

[Tahmid]

The problem is in the MOSFET driving. Remove R (short it) or reduce it drastically. Reduce Rb and Rgate. What is the frequency are you using for PWM?

 

[Korawy]

Hi Tahmid...

how can i short R while Vbias not equal Vdrv and they have the same ground ?!

The frequency used is 20KHz...

Thank u

 

[Tahmid]

Hi,

If you're using different power supplies for VBIAS and VDRV, then remove R. I thought you were running them off the same supply VBIAS.

Reduce Rb to about 10-22Ω and Rgate to about 4.7-10Ω. Use transistors with a little higher current rating, eg PN2222 and PN2907.

Hope this helps.
Tahmid.

 

[Korawy]

Vbias and Vdrv are from the same batteries but Vbias is 5v and vdrv is 12v.

I will try to reduce the resistors and give u the feedback.

Thanks

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[Korawy]

i remove R and make Rb = Rgate = 10 ohm, but the same problem occurs