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PWM and MOSFET problem ?

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hameeds01

hameeds01

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irf460 amplifier

Hello guys,
I am driving DC motor 12volts 6Amp via MOSFETs with Heat sink & cooling fan, the problem is that when I decrease the duty cycle below 20% the MOSFETs become very very hot (some times burns out),
It works fine above 50% duty cycle runs motor under normal temperature.

How I can protect MOSFETs from burning??
 

mosfet problem

Do you really need 4 100 PLUS AMP MOSFETS.

Use a mosfet driver and an opto.TD351 etc

Code: 
http://www.st.com/stonline/products/literature/ds/10977/td351.pdf

Or just use a good opto

PS9552, FOD3180 etc.

Code: 
http://www.cel.com/pdf/datasheets/ps9552.pdf

Code: 
http://www.fairchildsemi.com/ds/FO/FOD3180.pdf


I'm surprised it only heats up at low duty.
 

pwm and mosfet

At low speed, motors sink more currents

+++
 

fod3180 circuits

The circuit implies rather slow on and off switching of the MOSFETs. Without a special driver IC or at least a complementary emitter follower between OC and MOSFET, a suitable switching can hardly be achieved.

As another point, paralleling MOSFETs without individual gate resistors has a certain risk of producing high frequency oscillations.
 

The gate's parasistic capacitance charges through your opto + 100 ohms resitror : it's okay, but it discharges through your 10k resistor, it's a waaaaay too much!! Replace it with an 1k one maximum!!

It may resolve your overheating problem undoubtly.
 

Hi,
Use a single MOSFET, like IRF460, with small heatsink.
Introduce an emitter follower circuit (may be with BC547) between Opto and MOSFET.
 

A push-pull driver as discussed in this thread would be better.
 

The voltage across Gate and Source is 11.63v , still I require the driver circuit (Push-Pull) ??

Added after 8 minutes:

here is the specifications
 

Hello Hameed...its not about Vgs...though u get 11 volts at G and S ...but the 22 resistor make the mosfet slowly turn on...and the 10K resistor make very slow turn off...more the mosfet spends time in transition region the more it gets hot...that is why we all are prefering push-pull driver...it will turn on and off with high speed...and thus will produce lesser heat...
 

Hameed....Used the driver as used in this pdf file, it will make the gate turn off with high speed. I think u will understand it. its simple
 

For proper risetime control, you need something like:

The best choices for component values vary with FET characteristics,but are, for the most part, not critical.
<als>
 

An arbitrary mix of useful, many dubious and some definitive faulty circuits, to my opinion. Better to consider the previous suggestions made in this discussion.
 

you must use a series coil with motor terminals to smooth current ripples.
Actually in DC motor current =(Vin-Va)/(equivalent motor winding resistance), so when motor speed decreases, Va decreases too, and motor instantinuse current increased. this led to over heating of power switchs (MOSFETs).

regards
 

you must use a series coil with motor terminals to smooth current ripples.
Click to expand...
Never say must. Many motor PWM drives are actually using armature inductance as only energy storage. As one drawback, this causes additional skin and proximity effect losses at the winding, but still bearable in most cases.

But it's advisable anyway to determine the current ripple and to use either an additional inductance or a higher PWM frequency, if it's beyond acceptable limits. The unloaded motor current can be expected discontinuous, but I wouldn't regard this as an issue.
 

now its working fine ..........

do i need a diode in reverse biase across motor to kill inductive kicks?? as MOSFET has builtin Diode ..........
 

U have to use a fast diode across the inductive load to suppress the transients....MUR860(diode) handles about 8 amperes....
 

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