NMOS not turning ON, circuit isn't right?

Thread Starter

lolLogan

Joined May 10, 2023
7
Hello, I have a circuit where I have a SCT50N120 SiC MOSFET a MIC4451 to drive it and a 48V DC motor following the schematich attached.

The MIC4451 is powered by external power source with 20V, the power source ground is conected with the MIC4451 ground, the PWM signal from external source is 5V to trigger the MIC4451, the ground of this external PWM source is also linked to the MIC4451 ground.

The MIC4451 outputs a 20V signal that looks exactly that the input PWM as expected.

The problem is the following: When I put to work the circuit it doesn't work, I find 48V between the Drain and the Source of the mosfet which means the MOSFET is not closing, I checked the voltage from the driver and it gives 20V but when I check the Gate-Source voltage it gives a few milivolts, what is the problem? Can someone give me an advice?

Thanks you.
 

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crutschow

Joined Mar 14, 2008
33,971
Sounds like the MOSFET may have a short from gate to source.
To PWM the motor, the diode you show should be in parallel with the motor (cathode to +), not in series with it.
That's to allow the motor current to keep flowing when the PWM signal is low.
The diode in series does nothing.
 

Thread Starter

lolLogan

Joined May 10, 2023
7
Sounds like the MOSFET may have a short from gate to source.
To PWM the motor, the diode you show should be in parallel with the motor (cathode to +), not in series with it.
That's to allow the motor current to keep flowing when the PWM signal is low.
The diode in series does nothing.
Hello, thanks for the reply, I have 8 diferent mosfet and all of them does the same.
 

ElectricSpidey

Joined Dec 2, 2017
2,756
But the circuit itself is properly designed, isnt it? A part from what crutschow said about the diode
You can have a circuit wired correctly as far as the schematic and still have a loose or faulty connection.

What PWM frequency are you using?

If you are testing with the PWM active, make sure that 300ohm resistor is the correct value and not 300M or something.
 

Thread Starter

lolLogan

Joined May 10, 2023
7
You can have a circuit wired correctly as far as the schematic and still have a loose or faulty connection.

What PWM frequency are you using?

If you are testing with the PWM active, make sure that 300ohm resistor is the correct value and not 300M or something.
Hello again, in order to test the MOSFET switch on and off Im not using a PWM signal yet, I just put high state (20V) or low state (0V) to the gate to test the mosfet ON/OFF
 

ElectricSpidey

Joined Dec 2, 2017
2,756
If you connected the drain to negative and the source to the controller thru the 300 ohm resistor and the gate to the motor, that is just the reading I would expect across the body diode.

And I wouldn't expect any MOSFET connected that way with the motor at 48 volts to survive.
 

Thread Starter

lolLogan

Joined May 10, 2023
7
Hello Again, I solved the problem, I was separating the grounds, one for the MIC4451 and the PWM generator and another for the MOSFET and motor, that was making a VGS = 0 so the MOSFET didnt turn ON. Now the problem is different, when the MOSFET is OFF the voltage across the motor is 10V and 38V on the MOSFET, I would expect to have the voltage drop into the MOSFET, doesnt it?
 

Audioguru again

Joined Oct 21, 2019
6,604
With the PWM generator set to zero then its output voltage should be zero volts. measure it.
Also measure the gate voltage of the mosfet that also should be zero volts.
 
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