MrNobody

Hey..
I need help in designing a H bridge motor driver circuit to drive a DC permanent motor.
The H bride is similar to the attched circuit.
But instead of using IGBT, I need to use MOSFET.
The circuit also need to have optocoupler (each connected to the MOSFET).

The optocoupler will be controlled by a function generator (around 5V).

The motor specs are:
Type: dc permanent magnet motor.
Voltage rating: 180 Volts dc
Power: 500W

The supply specs are:
High voltage, high current (60V, 3A) fixed dc is to power the motor.
An external low power transformer with primary rated for 230V, and two secondary coils of 12V ratings is available to provide the power required for the gate drive circuit.



The difficulties I have is I don't how to choose the MOSFET i need to use, don't know how to choose the optocoupler I need to use.
I read somewhere that I need to use both N-MOSFET (enhanced) and P-MOSFET (enhanced). Why need to use enhanced MOSFET and not depletion MOSFET..? The only thing i know about MOSFET is for N-MOSFET, the source is connected to GND and for P-MOSFET, the drain is connected to GND.

I really need help.. Can somebody please help me.. Thanks..

Attached Images

Motor Drive.jpg (21.7 KB, 111 views)

Sceadwian

Look up photo voltaic generators, which are good for the high side mosfets. Any logic level FET and generic opto coupler will do. The photo voltaic generators eliminate the need for P channel fets.

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MrNobody

Thanks.. I will look into photo voltaic generators.
But, jst to try to understand, why do some of the H-Bridge circuit use 2 N-MOSFET at the top near the V source and 2 P-MOSFET near the ground..?
Why don't they just use 4 N-MOSFET..?
What is the advantages of using combination of N and P MOSFET..?

When choosing for a MOSFET, several thing I need to consider are:
V(DSmax) - the higher it is, the more voltage is allowed to pass to the motor without burning the MOSFET
I(D) - The amount of current that pass through the MOSFET
Vth (voltage to turn the MOSFET on - the lower the better)
R(DS) during on time (the lower the better)
P(tot) - power disspation - the lower the better.

Is my understanding of the MOSFET correct..? Did I leave anything out..? It is the first time I design a circuit using MOSFET so I am quite nervous..
I am new to reading datasheet and also new to using MOSFET as well...

Thanks for your time..

MrNobody

Hmm.. I found an N-MOSFET.. Here is the datasheet..
http://www.ortodoxism.ro/datasheets/irf/irfr120n.pdf
The V(DSmax) is 100V, I(D) is 9.4A..
Was searching for V(DSmax) of 70V and I(D) of 4A but that is the cheapest and closest I can get..

Can I use that MOSFET..?

Hero999

Hmm I smell college work. . .

I'm glad to see you've started asking in the correct manner.

There are many ways of doing this but it depends on what you need to do. Do you need to implement so form of spees control or do you just want forward/reverse?

There are plenty of MOSFETs around that will do this, try looking at a component distributer like Farnell or RS.

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Styx

you wouldn't be able to use P-types in this case anyway. The link potential is 60V

Now depending on the quality of the control (and more specifically switching freq) thuse use of photo-voltaic generators to drive teh gate of a FET is out of the question, they do not source/sink that much current and to get switching speeds up you need current (at least 1A if using FET's)

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MrNobody

Hero999:
Yeah.. its college work..
It need to have forward/reverse control and speed control.. but the speed is entirely control by the function gen. I only need to design the H-Bridge and the optocoupler side..

Styx:
If that is the case, I use 4 N-MOSFET rite..?


I am still curious about MOSFET... I still don't know why people use 2 N-type at the top and 2 P-type at the bottom instead of all N type... I have tried to look on the net and the only thing i can find is for P-MOSFET, the drain is connected to ground but for N-MOSFET, the source is connected to the ground... Another thing is that for P-MOSFET, the V(th) is negative..
In the college, all they taught is N-MOSFET so P-MOSFET is a new thing to me.. Dats why I ask so many questions about it.. Hope to be forgiven..

Sceadwian

MrNoby. A FET is controled by it's gate to source voltage. The high side N-mosfet of an H-bridge would require a voltage higher than it's supply voltage in order to turn on. You either need to use a boost converter to obtain higher than VCC, or use a floating supply.

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Roff

A quick look at solid state relays driven by photovoltaic generators shows turn-on times of several milliseconds, and turn-off times of around 0.5ms. This is a neat concept, and works well if speed is not an issue, but for PWM H-bridges, it seems to me like they would be too slow.

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Ron

Sceadwian

You don't necessarily have to PWM both sides of the H-bridge to control speed to a DC motor. You could just PWM the low side. Some motor control methods require both ends though it wouldn't be appropriate for them, there'd be a lot of ohmic losses in the FET during switching.

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Analog

True. The PIC18F4520 does this automatically.

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MrNobody

Here is the driver circuit..
I haven't had much time so I haven't enter the values in yet..
Will do so soon...
Is the general design ok..?

Attached Images

Schematic 01.jpg (52.1 KB, 103 views)

Hero999

For a start those MOSFETs will blow up, the gate voltage is limited to 15V to 20V depending on the type.

Secondly how are you planning to discharge the gate capacitances to turn the MOSFETs off?

Thirdly I don't think those optocouplers will be fast enough for PWM.

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MrNobody

Sorry, there is some mistake in the circuit.. The voltage that triggers the MOSFET gate is 12V and not 5V.

Will it help if I add 1M Ohm resistor..? Or that resistance will be too high..?

Will it help if I connect a 1K Ohm resistor between the Gate terminal and the Source terminal...? This will cause the gate to be discharged to the drain when the circuit is off..

Which optocoupler do you recommend..?

MrNobody

Below is the updated circuit with the changes.. I added a Free Wheeling Diode for each MOSFET.. Just a note, the VCC for the BJT is 5V, when I tried to change that VCC to 5V, all the other VCC also changes so I cannot change the VCC..

Attached Images

Schematic 02.jpg (60.6 KB, 83 views)