MOSFETs donot turn off even after VGS falls to 0!

Thread Starter

haneesh_as

Joined Jan 12, 2020
6
I did a full bridge DCDC high frequency rectifier with the MOSFET IRF840. The idea is to control the duty ratio for regulating the output. The diagonally opposite switches are turned off and on simultaneously for a period less than 40% of duty ratio. Hence there is a situation at which all switches are turned off between each diagonal switch pair is turned on alternately. When I observed the voltage across primary of HF transformer, it looked like a waveform which swings between Vdc and -Vdc with exactly 50% switching period. Initially I started with 50kHZ and then reduced it to 10kHz. When the individual switch voltage waveforms were observed along with its gate wave form, it was found that the device turns on even in the absence of gate pulse prior to the rise of VGS and there is no situation in practice at which all gets turned off. The on time is exactly 50% of switching time. Instead of HF primary when I connected a 100Ohms resistance the expected waveform was obtained with the device status following gate command. Somebody please help to resolve the issue. My driver IC is FAN 7392.Rg is 10 Ohms.
 

Alec_t

Joined Sep 17, 2013
10,716
Welcome to AAC!
Please post your schematic plus screen shots of any relevant waveforms, so that we have a better understanding of your setup.
 

SLK001

Joined Nov 29, 2011
1,543
Sometimes the gate capacitance will hold the gate "ON". You might need a large value resistor from gate to ground to keep the gate honest.
 

SLK001

Joined Nov 29, 2011
1,543
but when i see the gate voltage waveform, it has a perfect shape
Wow... How do you "see" the voltage waveform? The mere act of "seeing" this waveform briefly provides a high resistance to ground, FIXING (albeit temporarily) your problem.

And, I'll add to the chorus, SHOW YOUR SCHEMATIC!
 

Thread Starter

haneesh_as

Joined Jan 12, 2020
6
Welcome to AAC!
Please post your schematic plus screen shots of any relevant waveforms, so that we have a better understanding of your setup.
Schematics . The primary winding of the HF transformer is connected between A and B. The waveforms were observed in DSO. unfortunately it was not recorded. but it was like that plotted in attachement.
 

Attachments

Last edited:
On the FAN7392, the "Vs" needs to connect to your nodes "A" and "B" for proper bootstrapping .... You will need two of the FAN7392's ... one for the "A" connection and another for the "B" connection.

Also since it is a boot strap topology, the PWM can never be at 100% or else the boot strapping does not charge
 

Thread Starter

haneesh_as

Joined Jan 12, 2020
6
On the FAN7392, the "Vs" needs to connect to your nodes "A" and "B" for proper bootstrapping .... You will need two of the FAN7392's ... one for the "A" connection and another for the "B" connection.

Also since it is a boot strap topology, the PWM can never be at 100% or else the boot strapping does not charge
Vs is connected to node A and there is one more FAN for the other limb.
 
Top