Switch selection for high-power H-bridge

[stube40]

I'm trying to design an H-bridge that can control the output polarity of an electromagenetic coil with inductance of around 50mH and very low resistance. The source voltage is around 150V at 40A and hence the design and the switch selection aren't trivial.

The timing constraints make things even tougher - the circuit needs to be able to switch within a few ms of the control signal trigger going high. Once switched, the circuit will stay in the same polarity for around 100ms or so. Hence, it's far more about response time and lag than it is about actual switch frequency.

I've considered both solid state relays (IGBTs and MOSFETs) as well as electro-mechanical relays. The solid state stuff seems to have considerable power losses due to the voltage drop between emitter and collector. The electro-mechanical relays suffer from being too slow and/or occasionaly bouncing when driven hard (which is likely to blow my PSU)

Does anyone have any suggestions for a good solid state solution that has low power losses, or an electromechanical solution that is fast enough and wont bounce?

[Hero999]

I would probably suggest MOSFETs.

Is there any DC?

If it's AC only, then you could use a pulse transformers to control the high side, rather than N-channel MOSFETs, which will loose less voltage.

I can't think of any other solutions than MOSFETs or IBGTs but at 150V, the former is probably the least lossy.

[stube40]

Thanks for that.

It's 150V DC, but a quick calculation I did showed 72W dissapation at 40A and I have to multiply that by 2 for normal H-bridge operation.

I wondered if I couldn't use electromechanical relays to achieve the same thing. My time constraints are pretty tight (eg make the circuit at +/- 1ms of the target time), but my switching frequency is low at around 10Hz max and I will know at least 100ms in advance when the switch has to be made. So maybe I can pre-trigger the relays to fire close to the target provided that the relay has reliable response characteristics and that it doesn't bounce on the contacts (which will blow mp PSU).

Or maybe I should take the power loss hit on the solid state solutions?

[Hero999]

72W power loss isn't too bad, it's 98.8% efficient which is quite good.

You can buy 150V MOSFETS with a resistance of 16mΩ which will gut the loss to just 51.2W but you'll need a way of driving the high side gates above 150V.
http://www.fairchildsemi.com/ds/FD/FDB2532.pdf

A simpler solution is just to connect the MOSFETs in parallel which will halve the resistance and therefore the losses. It will obviously double the cost but it'll probably still be cheaper than contactors. It will also double the gate capacitance and slow down the switching time but 1ms should still be easily attainable.

[stube40]

72W power loss isn't too bad, it's 98.8% efficient which is quite good.

You can buy 150V MOSFETS with a resistance of 16mΩ which will gut the loss to just 51.2W but you'll need a way of driving the high side gates above 150V.
http://www.fairchildsemi.com/ds/FD/FDB2532.pdf

A simpler solution is just to connect the MOSFETs in parallel which will halve the resistance and therefore the losses. It will obviously double the cost but it'll probably still be cheaper than contactors. It will also double the gate capacitance and slow down the switching time but 1ms should still be easily attainable.

Thanks for your comments. I hadn't considered connecting the MOSFETs in parallel - cost is not a problem and I could put 10 in parallel if needbe (and it was physically possible to do so on a PCB).

Are there any disadvantages to connecting MOSFETs in parallel and/or any caveats that you are aware of? You have already mentioned the increased gate capacitance but I'm not 100% sure if that will have a detrimental effect in my application (I'm actually a digital electronics guy who's working on his first power electronics project!).

[Hero999]

The capacitance is normally only a problem for op-amps and circuits with high output impedance which can limit the switching time.

If all the MOSFETs parrallel have a combined capacitance of 50nF and the output resistance of the logic circuit and gate resistor is 500R the time constant will be 25

[stube40]

The capacitance is normally only a problem for op-amps and circuits with high output impedance which can limit the switching time.

If all the MOSFETs parrallel have a combined capacitance of 50nF and the output resistance of the logic circuit and gate resistor is 500R the time constant will be 25