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Over heating mosfet

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Hi,

I made a motor controller (double full H.bridge). When I power it over an amp meter (DMM) the mosfets runs cool up to 6.5 A, but when I connect it directly to the power supply it functions well but the mosfets heats much (less than 10 sec) even at 0.5-1 A to the point that it burns the finger tips.

Then what's wrong about this ? What is happening inside the amp meter that doesn't happen when connected directly ?

The power supply is a 12V AC transformer, Diode bridge, 0.1uF and 2 x 2200 uF capacitors. part of the h-bridge is this:

Thanks.
myHbridge_.PNG
 
Why have you given us only part of the schematic?
Where is the DMM connected?
 
I sent only part of the schematic because the same part repeats and I touogh the problem lays in the mosfet switching logic or mosfet protection. The DMM isn't part of the schematic i connected it for debug purposes then I noticed that once being monitored by the DMM it doesn't heat anymore.
The DMM was connected trough V+ (positive lead of the power supply to the high side voltage supply of the mosfets / source)

PS. the circuit is running a BLDC motor (pulse driven, alternating phases)
 
The ammeter should have little effect, so I don't understand what is happening. Are you sure all the connections are correct?
Measure the voltage drop across the drain-source of the transistors when they are on. It should be only a few tenths of a volt.
 
The DMM was connected trough V+ (positive lead of the power supply to the high side voltage supply of the mosfets / source)
I'm guessing that the impedance presented by the DMM current shunt and probe leads could have provided a small amount of negative feedback for the P-FET, sufficient to reduce its gain below a critical point at which oscillation occurs. If the FET were oscillating it would get hot.
 
I think you have the meter connected up incorrectly.
I was going to say the switch on/off times for the fets was too slow as the circuit doesnt have any drivers, but a meter in the circuit wouldnt make much difference to this.
 
A piece of aluminum heatsink (4 x 5 x 0.5-1 cm) is bolted to 2 mosfets almost no air flow. When i said that it burns the finger tips, I was testing without the heat sinks. The heat sinks gets warm after a while (still not satisfiying).

I'm having a doubt about the switching commands coming from the uC, I'll try to change somethings there. (I was allowing the fets to turn off before setting the fets to on but I'll check again)

Thanks.
 
The resistors on the gates of the Mosfets have high values so they charge and discharge the high gate capacitance of the Mosfets slowly. Then the Mosfets ramp slowly on and off. They heat a lot while ramping.

Use complementary emitter-follower transistors to drive the gates through 22 ohm resistors.
 

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A piece of aluminum heatsink (4 x 5 x 0.5-1 cm) is bolted to 2 mosfets almost no air flow. When i said that it burns the finger tips, I was testing without the heat sinks. The heat sinks gets warm after a while (still not satisfiying).

I'm having a doubt about the switching commands coming from the uC, I'll try to change somethings there. (I was allowing the fets to turn off before setting the fets to on but I'll check again)

Thanks.

Those are NOT Suitable Heatsinks.
I recommend you get Proper Fin type, Aluminum Heatsinks.
At Least 5 x 5 cm Square with 1 cm High Fins.
Mosfets are Only at Rated Specifications, When PROPER HEATSINKS are Mounted on them.

Also consider Audioguru's, GOOD ADVICE.
 
Or when properly switched in and out of saturation with fast rise and fall times to minimize the time they spend in the linear region, as well as providing adequate deadtime to minimize shoot through currents which is the prime reason for heating of mosfet halfbridges in switching applications.
 
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