Switch selection for a high power H-bridge

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 at least 100ms, depending on the speed of the actuator at the time. Hence, it's far more about response time and lag than it is about actual switch frequency. Saying that, the target switch time is predictable with reasonable accuracy and hence it's possible to negate lag by advance switching (assuming the switch has consistent lag times).

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?

Hi,

You can safely use MOSFETs and IGBTs with heatsink. The heat won't be that great.
Say, if you choose IRFP450 which has RDS(on) of 0.33ohm, your heat at max 4A load will be (4*4*0.33) = ~6W.

You could go for lower heat, if you choose STW55NM60ND if heat is really that big a consideration, and you'll have heat loss of (4*4*0.06) = ~1W only. [Although this would probably be overkill, using a 51A MOSFET.]

MOSFETs would be the best choice as they switch fast, heat isn't THAT big an issue.
You could use IGBTs as well.

Hope this helped.
Tahmid.

Your description seems to demand a constant current control loop, so you would have to provide current measurement and a PWM
controller. With a contactor, the current is only limited by the "very low resistance" of the coil. Or is it just O.K. to connect
the coil to a 150V supply and operate the H-bridge as solid state switch?

The 50 mH inductance implies a current rise time of at least 13 ms to 40 A with 150 V(assuming constant current control and a
neglectable DC resistance). That's more than "a few ms".

For transistor selection, the actual switched coil current should be known. It's not clear if you mean it to be 40 A.

Tahmid said:

Hi,

You can safely use MOSFETs and IGBTs with heatsink. The heat won't be that great.
Say, if you choose IRFP450 which has RDS(on) of 0.33ohm, your heat at max 4A load will be (4*4*0.33) = ~6W.

You could go for lower heat, if you choose STW55NM60ND if heat is really that big a consideration, and you'll have heat loss of (4*4*0.06) = ~1W only. [Although this would probably be overkill, using a 51A MOSFET.]

MOSFETs would be the best choice as they switch fast, heat isn't THAT big an issue.
You could use IGBTs as well.

Hope this helped.
Tahmid.

Click to expand...

Thanks Tahmid,

What do you think of the IXYS IXFK230N20T? It has an RDSon of only 7.5mOhm and can handle 200V. At 40A power is 12W.

Added after 12 minutes:

FvM said:

Your description seems to demand a constant current control loop, so you would have to provide current measurement and a PWM
controller. With a contactor, the current is only limited by the "very low resistance" of the coil. Or is it just O.K. to connect
the coil to a 150V supply and operate the H-bridge as solid state switch?

The 50 mH inductance implies a current rise time of at least 13 ms to 40 A with 150 V(assuming constant current control and a
neglectable DC resistance). That's more than "a few ms".

For transistor selection, the actual switched coil current should be known. It's not clear if you mean it to be 40 A.

Click to expand...

I am using a 3rd party PSU that has on-board current control, although it's behaviour is far from ideal (that's a different story). Hence, the H-bridge is purely a switching mechanism in this case.

I agree that 13ms amounts to more than "a few ms", but the real important issue here is to accurately control the timing/start of the 13ms charge cycle. In other words, I hav to be able to control the switching event to within a few ms. Does that make more sense?