h-bridgeis not working using MOSFET irf840

hi everyone
i am trying to drive a H-bridge using high rating mosfet irf840 and gate driver ic ir2112.but this circuit is not working as i am using a dc motor as load. while this circuit works efficiently for MOSFET irf3205S. i am untable troubleshoot. can anyone ple tell me the problem due to which this circuit is not working properly?
my circuit is shown in attachment

You can improve your question explaining what "not properly" means to you.

Then report all pertinent voltages and waveforms and layout photos from debugging your test results.

If there is some valid reason why you made this substitution, state it.

for Rdson
IRF3205 8.0mΩ
IRF840 0.85 Ω

this should be <<5% of Rs coil resistance of motor for efficient operation

Show your results.

What is your Rs of the motor?

SunnySkyguy said:

You can improve your question explaining what "not properly" means to you.

Then report all pertinent voltages and waveforms and layout photos from debugging your test results.

If there is some valid reason why you made this substitution, state it.

for Rdson
IRF3205 8.0mΩ
IRF840 0.85 Ω

this should be <<5% of Rs coil resistance of motor for efficient operation

Show your results.

What is your Rs of the motor?

Click to expand...

thanks alot for ur reply
actually my aim is to drive an ac motor while giving a pwm signal at gate driver IC . but before that i was testing it using a simple dc motor as load
u highlighted a very important point regarding load resistance
can u plz elaborate it a bit more
regards

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Rs of motor used in the circuit is 0.6ohm. one more point that i want to mention is. the rating of irf3205S is 55v but it drives the motor above 55v..why so? is every power mosfet require more voltage than its rated value?

How fast are you trying to switch the mosfets? The 4007 diode is not good choice for the bootstrap diodes...too slow.
as FVM says you need mosfets with <<0.05 ohm to drive a 0.6 ohm motor load, also a strong source of DC power...

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According to your circuit, U3 is back to front for generating you gate drive, also you have no dead time generated in your circuit, this will lead to problems...!

SunnySkyguy said:

You can improve your question explaining what "not properly" means to you.

Then report all pertinent voltages and waveforms and layout photos from debugging your test results.

If there is some valid reason why you made this substitution, state it.

for Rdson
IRF3205 8.0mΩ
IRF840 0.85 Ω

this should be <<5% of Rs coil resistance of motor for efficient operation

Show your results.

What is your Rs of the motor?

Click to expand...

Istian said:

Rs of motor used in the circuit is 0.6ohm. one more point that i want to mention is. the rating of irf3205S is 55v but it drives the motor above 55v..why so? is every power mosfet require more voltage than its rated value?

Click to expand...

1) When you apply V+ thru IRF840 0.85 Ω to 0.6 Ω motor at start,

Let me assume V+ = 14.5V , then 14.5V /(0.85+0.6)Ω = 10A to motor on startup surge.

Most of the power is dissipated in the MOSFET,
With 10A^2* 0.85Ω = 85 Watts it may overheat depending on ('C/W)

Therefore do not substitute with a higher RdsOn unless you know the heat rise.

2) Driving a DC motor is simple with DC but driving a synchronous AC motor requires starting at DC and ramping up frequency so that current sense does exceed max.

Orson Cart said:

How fast are you trying to switch the mosfets? The 4007 diode is not good choice for the bootstrap diodes...too slow.
as FVM says you need mosfets with <<0.05 ohm to drive a 0.6 ohm motor load, also a strong source of DC power...

- - - Updated - - -

According to your circuit, U3 is back to front for generating you gate drive, also you have no dead time generated in your circuit, this will lead to problems...!

Click to expand...

thanks for your reply actually i am trying to generate 50Hz sinewave by applying a pwm signal at gate driver ic.having maximum pwm freq 6.4KHz .can u plz suggest me tha appropriate diode for proper switching ?

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SunnySkyguy said:

1) When you apply V+ thru IRF840 0.85 Ω to 0.6 Ω motor at start,

Let me assume V+ = 14.5V , then 14.5V /(0.85+0.6)Ω = 10A to motor on startup surge.

Most of the power is dissipated in the MOSFET,
With 10A^2* 0.85Ω = 85 Watts it may overheat depending on ('C/W)

Therefore do not substitute with a higher RdsOn unless you know the heat rise.

2) Driving a DC motor is simple with DC but driving a synchronous AC motor requires starting at DC and ramping up frequency so that current sense does exceed max.

Click to expand...

i appreciate your concern for my question.actually i am working on MOSFETs for the very first time
and many of my question would seem to be quite stupid but your answers are reallly a source of learning for me
i am unable to get your 2) point u mentioned .
as my question was, why i need to give more than 55v to irf3205s(rated at 55v) for driving the motor.while Rds(on) of irf3205s is far less than the coil resistance of dc motor.

You need some dead time in your gate drive, so that all fets are off for 500nS before you turn the next ones on. Any ultra-fast diode (Trr<75nS) with high enough V and 1A or more current capability will do for the bootstrap diodes, when the low side duty cycle is very small there may not be proper charging of the upper bootstrap supply - so be careful of this. At star-tup until the lower fets have been switched on a few times there will be no volts on the upper boot-strap caps...