MOSFET Fall time Increased when switching at high frequency.

Thread Starter

Tankesh singh

Joined Jan 21, 2017
13
Hello Everyone,
I am facing a problem to switching the MOSFET. I attach the circuit detail. i am switching the resistive load(12v bulb) at 27Khz at 47% duty cycle. when i do it fall time is increase which change the duty cycle. tell me how can i perfect switch my circuit.

Note : 1) fig. 1 show the square wave which peak volatge is arround 12v (47% duty cycle) is applied at gate of mosfet.
2) fig. 2 yellow color show Gate to source waveform.
3) fig. 2 blue color show drain to source waveform.
 

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crutschow

Joined Mar 14, 2008
23,487
You need a lower impedance driver for the MOSFET gate, to rapidly charge/discharge the gate capacitance.
What is driving the gate now?
 

dl324

Joined Mar 30, 2015
9,111
The problem is likely in the circuit driving the gate. Post a schematic that includes the gate driver.
 

Thread Starter

Tankesh singh

Joined Jan 21, 2017
13
circuit is based on sg3524. which may capable to drive mosfet. i change the trasformer to load and also change R and C value to get desired frequency.
 

Attachments

crutschow

Joined Mar 14, 2008
23,487
The outputs of the sg3524 are single NPN transistors (below) and you are attempting to use them as emitter-follower outputs to drive the MOSFETs, but there are no emitter resistors.
You thus need to connect the collectors (pins 12 and 13) directly to 12V and move the 2kΩ resistors to between the emitters and ground (pins 11 and 14), to rapidly pull down the MOSFET gate voltage when the transistors turn off.

If that isn't sufficient, then you will need to add a buffer driver to each output.
upload_2018-10-4_8-27-8.png
 

Thread Starter

Tankesh singh

Joined Jan 21, 2017
13
thanks crutschow. I add a 10uF capacitor between emitter and gate of Mosfet. falling time is decrease when i add a resister(2K ohm) between emitter and ground but not so much. when i decrease resistor value falling time is also decrease. so i add 68ohm resistor thats make a perfect square wave with falling edge but thats create a new problem resistor is staring heating and voltage across gate is decreased 12v to 9.2v.
 
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