# What is the fault with my hbdridge design in the below circuit using pchanel and n chanel mosfets?

In the image...

Q1,Q2 - p channel mosfet (IXTH11P50) Q3, Q4,Q5 - n channel mosfet (IRFP460N)

The problem I faced here was...when I test the circuit with 12 volt dc supply, and my load is two leds, it works very fine... When i connect my supply voltage to 300 volts dc with lamp load and when I just turn ON Q1 and Q4(by connecting the pin 1 to 5volt supply and pin 2 to ground )just for checking..the lamp lights up without any fault. Same as when I turn on Q2 and Q3 it also works fine.

when I connect the pins 1 and 2 of optocouplers to microcontroller where the microcontroller alternatively makes the pins 1 and 2 high with a frequency of 2.5HZ (ON time of 400 ms). When i connect my supply voltage to 12 volts with leds as load it is working fine.

But with the frequency of 2.5HZ i connect my supply voltage to 300volts DC and lamp load..its not working at all..the mosfet gets bursted each time. A short ciruit is prevailed either left or right side mosfets. So please suggest me some ideas or the fault am doing with this circuit. I have added two images ..one is for 12 volt dc with leds as load just for testing the circuit.. And the other image is for 300 volts dc with lamp load.

In the lamp load circuit..there will be a 50k resistor with a n chanel mosfet attached, to its drain. The idea here is to make the voltage divider network when the Q5 mosfet turns on.. so the voltage divided with this network will be 286 volts ..then 300-286 = 14 volts for Q1 gate to source. This happens when we turn on Q5 mosfet. If Q5 is turned off then no current flows through 50K resistor and Q5 and no voltage divider exists and 300 volts will headup to gate which means there is no voltage difference between gate and source for Q1.

Please see to my circuitry part and suggest me with some ideas to clear my fault.

Thanks Regards.

• You don't tell what parts you use and anything about your timing. FETs have turn on and off times you know... Oct 25 '17 at 13:01
• Sir i have mentioned it as frequency in the description..my ON time is 400ms for Q1Q4 and other cycle of ON time of 400ms for Q2Q3. Oct 25 '17 at 13:03

MOSFETS have a significant gate capacitance.

That means their turn on time is heavily dependent on the resistors in the gate drive line.

With low voltages these resistors can be small, but with large voltages, with this kind of voltage divider circuit, the resistors need to be large so as not to waste too much power. That means the MOSFET turn on time will quickly become large and make this circuit unusable.

Driving high voltage bridges requires a much different technique. Fortunately, integrated drivers are available for this.

An alternative, since you are using opto-couplers anyway, is to add a -12V supply up at the 300V rail and use that to power the top-side drivers.

simulate this circuit – Schematic created using CircuitLab

BTW: If you are smart you will up those 12V supplies to like 10% less than whatever Vgs_Max is for your selected MOSFETS for maximum efficiency.

• That was quite helpful sir. Please say me a way of adding -12 volts to 300volts rail..i couldnt get an idea about it. Oct 25 '17 at 13:21
• @Prakashveereswar see update Oct 25 '17 at 13:50
• Trevor- sir your idea worked well..finally the probelm is solved. The hbridge with lamp load worked very well. And my main load is an electromagnet which draws around 7 amps of current. Hope the circuit works well for this heavy load also. Yet to test this heavy load. Thanks Regards. Oct 27 '17 at 17:47