Problems with H bridge

Hi,
I am trying to drive 24v 120w dc motor using H-bridge of IRF840 driven by IR2110.at Appr.=11khz pwm frequency.I am using PIC184431 to control the motor.
I am using 22.1184 mhz crystal & PR2=124 with timer2 prescaler =4.
The circuit works fine.
But I encounter following problems.

1. At resistive loads it works fine i.e I can get linear dc o/p from 0 to 23 corresponding to 0% to 95% of duty cycle. But when motor is connected it doesn't start till duty cycle = appr.40%.then I get the linearity.
2. The circuit works fine at lighter loads.At higher load the mosfets get heated up and even at 95% of duty cycle unable to carry the load.

Please help.

 

I think you have a very bad choice of MOSFET. The data sheet for the IRF840 says it has a channel resistance of .85 ohms, and if you want to run several amps through it, you will certainly see some heating. Also your motor won't get the full voltage because so much is being lost driving current through the FETs.

 

Sir,
Will you please suggest one?

 

All right, the Infineon IPB230N06L3 G. 56 cents at Mouser Electronics, 23 milliohms resistance. I haven't checked all the data for you, but this is the kind of thing to look for.

http://tinyurl.com/ctpmp4z

 

Thank you for your suggestion.
Sir what is the reason for my 1st problem? Is it related to high channel resistance? or anything else? i.e. I can measure only 1.5v at motor terminals at 50% duty cycle and there onwards it goes up to 16v at 99% of duty cycle.

 

How much current is flowing with either the resistive load or the motor? And is the motor running free or is it loaded down with something?

 

current flowing is 2a. & MOTOR has to overcome spring load.

 

You haven't said whether energy is stored in the spring, and whether the motor has an increasing load. And then if it's an H-bridge you must want to reverse the motor. What happens when it goes the other way?

 

yes energy is stored . Basically it dooor open -close application.