i am making a circuit with power mosfets
IRFZ48N
but getting a problem that when i start the motor..
the mosfet connected to Vdd gets hot even at .4A..
but the mosfet whose Vss is connected to ground is very cold..
so what is problem..even when i uses the same basising for both
power mosfets..

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Rafhan Aslam

 

post your circuit

 

The problem is that the N-MOS that is connected to VDD requires a voltage higher than VDD to turn it on.

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I have tried ur solution ..great. :shock:
the circuit is performing on very cold state even at 2A
so can any body suggest how to on mosfet in a h-bridge of a robot with a higher voltage than the supply voltage..

 

simple
MAKE A PROPPER GATE DRIVE!!!!

I have said this time and time again abt the Power-electronic cct that are flying around this forum. They all have common problems and that is complete lack of gate-drive to the power-switch!

This isn't low-power infomation that is getting delt with here THIS is POWER, be it only a few watts its a damb sight more then the uW contained within a LOGIC on/off or analogue signal


To switch a FET in anger (ie with real current through it) take energy/power to do that job and THAT requires a decent GATE-drive. IT can be as simple as a BJT push-pull. IT NEEDS TO BE PUSH-PULL!!! not like what is in this cct



The gate-drive ( or the incompetance of) in this cct is its main problem! a 10k is used to turn-OFF the N-types as well as a 10k to turn-ON the P-types. That is HUGH!!!! you need a push-pull (with gate resistor of a few 100R) to enable the charge to be pushed into and pulled out of the FET correctly



Anyway enough of the RANT

What is needed: a floating gate-drive
required:

1x OPTO (6n137 are good)
1x isolated DCC converter, NVU/C&D do fantastic very small DCC converters at around 1W, they have 5Vin to +/-12V out, 12Vin 5Vout lots-o combinations

1x PNP BJT
1x NPN BJT
and the odd few resistors and capacitors.


So for the top two FETS you now have a floating gate-drive which you connect the isolated 0V to the FET-source and the output of the PUSH-PULL to the GATE, isolates/floating power via the DCC, isolated/floating signal via the opto


All in all kids, think of the gate-drive.

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Once a problem is realised, the rest is just details

 

i am a novice of electronics so could not understand what u wanted to say..
please give ur answer in more detail...
thanx

 

His response has enough detail. The main idea is to make a better gate drive for your power transistors in the bridge. Styx suggests a floating drive which is fine, but it might not be necessary. Some H-Bridge configurations use P-Channel high side parts so floating or bootstrapped gate drives might not be necessary.

However, the gate drive should be driven by a low impedance push-pull stage. There are very large capacitances that need charged & discharges on the power devices if you are going to get them to switch fast. The only way to do that is with a low impedance gate drive. Transient gate currents of AMPS is not unusual for fast switching.

 

will using the p-channel power mosfets will solve the problem..
or it will also require some extra circuitry like push pulls or ..
Actually i am working this for a robot and wanted to simplify the circuit ..
just guide me in this respect..

 

I have modified the circuit a bit hoping for some improvement. The both "1" state is still "not allowed" but if any control signal is already "1", the going high of another would have no effect until the corresponding channel control signal returns to LOW. This would help a little bit.

Styx?

Attached Images

nch-brdgx.gif (11.7 KB, 444 views)

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L.Chung

 

Still got SHITE gate-drive (will cause a soft shoot-through)
and that LED still exist which will fail the moment the potential switches

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It's a tri-colour, so is two LED's back to back.

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Proper gate drive is only needed if the OP wants to use PWM for speed control. For just forward/reverse action, the design is sufficient in a hobby sense.

There will alway be a big difference between driving MOSFETs in industrial situation and in hobby circuit.

__________________
L.Chung

 

Thats answers that, correct symbol would be handy tho

__________________
Nothing is impossible.
Once a problem is realised, the rest is just details

 

For just fwd/rev fixed speed (based on link potential) yes it is fine BUT if it is to be PWM'ed (and that doesn't have to be industrial, there are plenty of places where hobby PWM is used) this cct will FAIL

__________________
Nothing is impossible.
Once a problem is realised, the rest is just details

 

The IRFZ48n is not a logic level mosfet, so if you are driving from logic, it won't turn on reliably. The transistors I chose are logic level. The NPN driver is designed not to saturate, to avoid that delay. Even so, efficiency will be improved if you provide some small delay between turn on and turn off of A and B drives.

Attached Images

h-bridge_163.png (12.6 KB, 427 views)

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