problem in h-bridge using Power MOSFET irf840

Thread Starter

syed hassan

Joined Mar 11, 2015
13
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 to troubleshoot. can anyone plz tell me the problem due to which this circuit is not working properly?
i can show u my circuit.

regards
 

RamaD

Joined Dec 4, 2009
328
Can you please explain "not working properly"?
The Rds(on) of IRF840 is 850 mOhm as against IRF3205's 6.5 mOhm. So, voltage drop across IRF840 would be quite high comparatively, leaving lower voltage to the motor!
 

Thread Starter

syed hassan

Joined Mar 11, 2015
13
Can you please explain "not working properly"?
The Rds(on) of IRF840 is 850 mOhm as against IRF3205's 6.5 mOhm. So, voltage drop across IRF840 would be quite high comparatively, leaving lower voltage to the motor!
thanks for ur reply
actually i am trying to run a ac motor with pwm signal applied at gate driver IC.but before that i intended to to check it for simple dc motor as load.and logic toggle at gate driver ic. u highlighted a very important point..can u plz guide me a bit more that do i need to chose mosfet on basis of my load resistance or something else?
and plz elaborate it further about ur explanation regarding Rds
regards
 

GopherT

Joined Nov 23, 2012
8,012
thanks for ur reply
actually i am trying to run a ac motor with pwm signal applied at gate driver IC.but before that i intended to to check it for simple dc motor as load.and logic toggle at gate driver ic. u highlighted a very important point..can u plz guide me a bit more that do i need to chose mosfet on basis of my load resistance or something else?
and plz elaborate it further about ur explanation regarding Rds
regards
R(ds) is the equivalent resistance of the MOSFET when it is ON. MOSFETs range from 1mOhm to nearly 10 ohms. The lower the on-resistance, the lower the voltage across the MOSFET when it is on (ohms law, voltage drop = current x resistance). So, for a 3 amp motor and an on-ressitance of 0.8 ohms, the voltage drop will be 2.4 volts. If you have 2.4 volt drop on the P- and the N-mosfet, you are dopping the total of the two mosfets of the H-Bridge. P-type usually have higher on-resitance than the N-type if made by the same manufacturer and are the same 'matched' pair. E.g. IRF530 and IRF9530.

Newer mosfets with logic-level gates will be as low as a few milliOhms, and, therefore, allow currents of 10 amps and more with essentially no heat sinks. 5 mOhms x 10 amps = 0.05 volt, 0.05V x 10 amps = 0.5 watt.
 

RamaD

Joined Dec 4, 2009
328
The MOSFETs selected should have acceptable voltage drop (as low as possible), when it is switching the maximum current in the circuit.
In general, higher Voltage MOSFETs tend to have higher Rds(On).
 

PeterCoxSmith

Joined Feb 23, 2015
148
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 to troubleshoot. can anyone plz tell me the problem due to which this circuit is not working properly?
i can show u my circuit.

regards
When you say circuit not working, can you be more specific? What volts are you applying and what are you seeing across the motor?
 

Thread Starter

syed hassan

Joined Mar 11, 2015
13
R(ds) is the equivalent resistance of the MOSFET when it is ON. MOSFETs range from 1mOhm to nearly 10 ohms. The lower the on-resistance, the lower the voltage across the MOSFET when it is on (ohms law, voltage drop = current x resistance). So, for a 3 amp motor and an on-ressitance of 0.8 ohms, the voltage drop will be 2.4 volts. If you have 2.4 volt drop on the P- and the N-mosfet, you are dopping the total of the two mosfets of the H-Bridge. P-type usually have higher on-resitance than the N-type if made by the same manufacturer and are the same 'matched' pair. E.g. IRF530 and IRF9530.

Newer mosfets with logic-level gates will be as low as a few milliOhms, and, therefore, allow currents of 10 amps and more with essentially no heat sinks. 5 mOhms x 10 amps = 0.05 volt, 0.05V x 10 amps = 0.5 watt.
thanks a lot you gave me very useful pieces of information.
i have one more query that if i use irf3205s mosfet having voltage rating of 55v then for driving the motor i need to give more than 55v below 55v the simulation gives an error
what could be the possible reason for it?
 

PeterCoxSmith

Joined Feb 23, 2015
148
thanks a lot you gave me very useful pieces of information.
i have one more query that if i use irf3205s mosfet having voltage rating of 55v then for driving the motor i need to give more than 55v below 55v the simulation gives an error
what could be the possible reason for it?
simulation errors can occur in this type of circuit where diodes are connected to capacitance, even the gate of a mosfet. I put a low value resistor, say 1R in series with all the diodes in the circuit.
 
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