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Drive a 24V 180W 7.5 A(rated value) dc motor.....

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Full Member level 4
Dec 7, 2005
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Hi all, i need to drive drive this dc motor. i build my H-bridge using high rated current and voltage Mosfets(50V 50A max) and control it using a PIC uC. I even include the free-wheeling diode accross the Mosfets and interface optoisolator between the uC and the H-bridge. But when i increase the load of motor suddenly, it just damage my Mosfets......any suggestions??

Hi wakaka!

Could you tell us what the gate drive voltage is?
Disconnect the motors, and just give the gate drive signal from PIC or manually. Poor gate drives (<10V Vgs) can damage mosfets at high currents because power dissipation will be higher.

Also tell us if the freewheeling diodes are damaged or not.


Make sure that under no circumstance you get one leg of the H bridge to turn on causing a high shoot-thru current in the MOSFETS. Use of a lock-out circuit can prevent this from happening.

Hi wakaka,
If you choose correct mosfet and correct excitation voltage, mosfet going to
linear (ohmic) region, when turned on.
In this case power dissipation will be RDS * I².
I is maximum motor current.
RDS-on can be determind from transistor datasheet.
If power dissipation in each transistor are greater than 600-800mW it needed heatsink.


Added after 38 minutes:

sorry i dident saw current rating before.
I think you must use good power MOSFET like IRFP150 or better with use of a good heatsink.

i din short one of the leg of the mosfets, they just burn when i increase the load of the motor suddenly and tremendously. Actually i just stop the motor from turning by using a clamp or something, just to test whether it can withstadn the high current spikes. I'm using 2SJ334(PMOS) and 2SK2173(NMOS). The gate drive voltage is 4V. Id(max) around 30A, Vds(max) around 50V.

So how to protect the H-bridge circuit from burning if there's spikes occurs?
I can implement the protection circuit with my microcontroller is necessary.

Although these MOSFETS are said to be 4V min gate drive, I would make the gate drive much higher (at least 10V). Looking at the datasheet, you can get much more out of the device with a gate drive of 10V. Also when stalling the motor current is only limited by armature resistance as there's no back-emf and you may draw more than 30A. If your circuit does not have some type of current limit or I×t trip you will blow the devices.

so how can i limit the current??

You can use one series resistor through bridge ground. (eg. 0.01-0.02 ohm 3W ).
then amplify resistor voltage, which proporatinal to output current.
and control PWM so current not exceeded maximum motor rating.

er sorry for that, how to connect the series resistors???? wat u mean bridge ground? This is my circuit.

Hi wakaka!

Your high side gate drivers won't work. use high side/low side driver IR2110. The purpose of optoisolator itself is defeated in your circuit.


i also intend to add one resistor to limit the current, but do there exist any cement resistors that can withstand 180W???and i need minimum 7.5A to drive my dc motor. by applying P=I(sqr)R, the resistance is about 3ohms. ...

Resistor is just for current measurment!
it's voltage drop must as low as possible.
i suggest 0.01 ohm is good and its rating:
0.01 * 10^2=1W (for 10Amp motor)
but usually for this value we choose 2-3Watt

voltage across resistor are 0.01 * 10A = 0.1 Volt
you can amplify this voltage or simply use comparator with reference of 0.1 volt.
when voltage going to higher than reference (0.1 volt), comparator output changed.
connect comparator output to one of microcontroller inputs.

ic....thanks a lot....but, is it possible to set the ref voltage to 0.1V?or 0.2V?wat resistors should i use?A variable or a fix resistor?

I advice you to measure your motor current under load before proceeding to correctly select mosfets. Then you can think about rest.

Not knowing what the max motor current will make you one of the biggest investors in mosfet manufactoring industry.)

ic....thanks a lot....but, is it possible to set the ref voltage to 0.1V?or 0.2V?wat resistors should i use?A variable or a fix resistor?
yes of course, you can use 2 resistor as voltage divider.
i think it is better to use fix resistor.

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