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.

 

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

 

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.

 

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.

but when i see the gate voltage waveform, it has a perfect shape

 

Post the schematic.

Trying to imagine what is going on through your "wall of text" is not efficient or productive.

 

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!

 

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.

 

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!

 

I viewed it in DSO and it showed Vgs dropping down to zero after the ON period.

 

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

 

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.