Problem in an H-Bridge

[audioguru2]

When Mosfets are used for pulse-width-modulation to control a motor's speed, they switch on and off at up to 100kHz. Then a gate current of up to 1A from a Mosfet driver IC is used to charge and discharge the high gate capacitance quickly. The Mosfets are heating for the time that they are not completely on or off, so the quicker they switch then the cooler they will be.

A 1/4W resistor will be extremely hot at 1/4W and will overheat unless it is in free cool air. On a pcb it is not free because the pcb and parts near it block air flow. So on a pcb its max dissipation is about only 200mW when it will be extremely hot. At half its rated power it will be hot but not too hot on a pcb but it will last a long time and parts beside it won't be damaged.

The Mosfets you selected are old and have a fairly high on-resistance which reduces the max speed and max power of your motor.

 

[yscheok]

hai......

The Mosfets you selected are old and have a fairly high on-resistance which reduces the max speed and max power of your motor.

itz high on resistor have to refer datasheet at RDS(on), if more higher then the speed will go slow??

did u have any recomend using what type of MOSFET? if using IRF 1405, itz can maximum the speed motor??

thanks for replying me.........

 

[audioguru2]

itz high on-resistance have to refer datasheet at RDS(on), if more higher then the speed will go slow??

No, on-resistance is the resistance of the Mosfet in series with the motor. The motor's current through the resistance cause a voltage drop that is reduced from the voltage for the motor. The lower is the on-resistance then the lower is the voltage loss.
The new Mosfet is more than 30 times better than the old Mosfet. The P-channel Mosfet has twice as much resistance and voltage loss as this one.
The on-resistance has nothing to do with switching speed.

did u have any recomend using what type of MOSFET? if using IRF 1405, itz can maximum the speed motor??

There are thousands of Mosfets. This one is very good but its voltage rating and its price might be too high. A P-channel Mosfet with the same spec's will be more expensive. Both Mosfets must be good for your motor to be at its fastest speed.

 

[yscheok]

hai audioguru......
thanks a lot and speed time with me....

 

[yscheok]

hai audioguru, pls help me.....
still remember that H-bridge circuit that u have been modified for me........, i have test it and every thing is ok. But got 1 problem is when i doing the switching of the h-bridge circuit fast, the mosfet will over heat and burn the mosfet. i have put a heatsink for every mosfet but still very heat. If i just switch on the h-bridge circuit, the mosfet wont be over heat..... is there any way to reduce the heat so that the mosfet wont burn???

 

[audioguru2]

The Mosfets are old and therefore have a high on-resistance. They will get hot if the motor's current is high.

Mosfets have a very high input capacitance that needs a fairly high drive current to charge and discharge quickly. The 10k resistor values are much too high for pulse-width-modulation control of motor speed because then the input capacitance of the Mosfets will slow down the switching speed so that the Mosfets get hot while switching slowly in a ramp
The 10k resistors were for the Mosfets to turn the motor on, then tuirn it off occasionally.

If you have high frequency PWM motor speed control then the h-bridge needs to have a Mosfet driver IC that has a fairly high output current.

 

[ante1]

I believe some zeners to protect the MOSFETS wouldn

 

[yscheok]

hai audioguru,
can u recommand what type of mosfet driver ic?
i have try to put the diod across the the drain and source mosfet, but the result is same. if i using the snubber at every mosfet to cut out the over shoot current when turn on and off the mosfet, itz can reduce the over heat?? 

thanks for ur reply.....

 

[audioguru2]

I don't know which Mosfet driver ICs you can buy, there are many of them.
Get one that has a bootstrap diode for a boosted voltage to the gates of its high side Mosfets.

Your Mosfets overheat because the 10k resistors charge and discharge their high input capacitance slowly and therefore they are ramping instead of quickly switching.

 

[yscheok]

Hai audioguru,

if i using the snubber at every mosfet to cut out the over shoot current when turn on and off the mosfet, itz can reduce the over heat?? 

Your Mosfets overheat because the 10k resistors charge and discharge their high input capacitance slowly and therefore they are ramping instead of quickly switching.

u mean the charging and dischagrging slow will cos the mosfet heat?? Can i reduce the resistor 10K? so that the charging and discharging time will more faster and the heat will reduce.

is there any way to modified the circuit so that the mosfet won't over heat?? cos i didn't have much time to change type of mosfet again.

thanks for ur replying..........

 

[audioguru2]

A 10k resistor charges or discharges the 3nF capacitance of a Mosfet in about 30us. A Mosfet would be heating for that 30us.
If your PWM frequency is 50kHz then each half-cycle is only 10us long so the Mosfets are never fully turned on and are not fully turned off. They are heating all the time.

A Mosfet driver iC provides about 1A of current to charge and discharge the capacitance of a Mosfet very quickly so it is linear for a short time so it stays cool.

Reduce the PWM frequency to 500Hz to see the Mosfets become much cooler when they are turned on and off with 10k resistors.

 

[yscheok]

hai audioguru,
thanks for ur info, i will try it. thanks for reply me.