Bidirectional mosfet switch with AC source inductor load

[Salvador12]

I have attached an image/drawing of how I plan to switch a couple of parallel coils that are put on an axial type stator, the idea is to pass AC of about 40V through the coils but selectively therefore I need to switch them ON/OFF which means that any one coil can get switched OFF at either the zero crossing of sine or peak, I'm bit tired here after long work, are the mosfet body diodes enough to counter inductive flyback here in this scenario or is something more sophisticated needed?

thanks.

 

[FvM]

Substrate diodes provide no overvoltage protection. If you switch off non-zero current in this circuit, you get inductive overvoltage and possibly transistor avalanche breakdown. You should use bipolar TVS diodes across the coil to absorb stored energy and sufficient switch voltage rating.

 

[Salvador12]

I wonder how do they do it BLDC motors.

 

[mtwieg]

I wonder how do they do it BLDC motors.

BLDCs are driven by inverters (half bridges) powered from DC, not a series switch powered from AC. Completely different. With a half bridge, the body diodes can carry the load freewheeling current without overvolting the FETs.

 

[Salvador12]

BLDCs are driven by inverters (half bridges) powered from DC, not a series switch powered from AC. Completely different. With a half bridge, the body diodes can carry the load freewheeling current without overvolting the FETs.

If you can check out my other thread on this very subforums (power electronics) about the axial flux stator and the coils, now if i have my pole pair coils in parallel isn't it the case then that if they get switched ON/OFF one by one adjacent to the next one, since they share the same core, the flux that was going through the coil that gets switched off is also shared through the adjacent coils that are still ON, in theory I think the freewheeling current should then dissipate through the adjacent coils that are still ON?

I think it is somewhat similar to having multiple coils on the same core, say a toroid, if one coil is switched OFF with current through it while others are still ON, then the collapsing field dissipates through the adjacent coils that are ON.

 

[FvM]

Despite of distracting setup details, the coil of interest has less than perfect coupling to other coils. The leakage inductance will still induce high voltage during current switch off which must be clamped by suitable means to protect the transistors.

 

[Salvador12]

I guess one way one could manage this is by having two separate coils on the same inductor , then each coil could have a freewheeling diode but still each coil could only be switched with current in one direction aka DC current for this to work, so in order to get AC like flux, I would need to mount each coil wound in opposite direction, but this then would mean, IIRC, that each coil would need active freewheeling rectifier because with passive diodes as one coil would get current through it it would induce current in opposite direction in the other coil and that current would then be short circuited by the diode.

 

[BradtheRad]

The diode-clamped converter might suit your purpose. It adds two freewheeling diodes, each turned On or Off by additional transistors. These form loops (together with the load) which continue to conduct briefly after the chief transistor shuts off. These absorb spikes generated by the inductive load.

I used analog-switches in my simulation. The screenshot is just after the positive polarity shut off. A loop still conducts through the led (or diode), absorbing the spike.