Bidirectional current switching with single type transistor

[artis]

Some bidirectional switch topologies shown at the beginning of this paper:

https://www.researchgate.net/public...g_SiC-MOSFET_for_Power_Converter_Applications

I had seen this paper before, some interesting options, not sure how well BJT's take prolonged operation near their SOA current wise, doesn't it cause heating of the die and secondary breakdown or what was the thing happening to them when the die temp increased beyond a limit?

Since this is high-current low-voltage, if you connect two N-channel MOSFETs source to source, and gate to gate, do you not have a bidirectional switch between the drains, controlled by ±15 volts on the common gate terminal?

In theory yes, by the way isn't it the case that in common source mode for two series N fets the voltage drop for either current direction forms only across one of the two switches given the other is bypassed through the body diode therefore before applying voltage to gate to turn ON the PD is across one switch in either current direction?

 

[Baluncore]

In theory yes, by the way isn't it the case that in common source mode for two series N fets the voltage drop for either current direction forms only across one of the two switches given the other is bypassed through the body diode therefore before applying voltage to gate to turn ON the PD is across one switch in either current direction?

When both MOSFETs are turned on, neither body diode conducts.
When both MOSFETs are turned off, one body diode conducts, the other blocks current.

The source floats when off, but quickly resolves the situation if it is negative, since a MOSFET then turns on sufficiently, to turn both off. The source cannot float high, because then a body diode will conduct.

 

[Rive]

need the capability to block current in both directions which with a single mosfet isn't possible due to the body diode.

I'm still struggling with understanding the circuit you intend to build, so please just take it as noise if does not fit: once we had a high current switch circuit with two serial MOSFETs where OFF-current was terminated by keeping the middle point voltage level close to the output point (by an extreme input resistance opamp circuit).
Keeping the body diode closed just did the trick nicely.

 

[artis]

When both MOSFETs are turned on, neither body diode conducts.
When both MOSFETs are turned off, one body diode conducts, the other blocks current.

The source floats when off, but quickly resolves the situation if it is negative, since a MOSFET then turns on sufficiently, to turn both off. The source cannot float high, because then a body diode will conduct.

But I can also ground the source with respect to the gates at all times within the gate drive circuit, can I not?
It shouldn't affect the voltage/current of the main path through the FET's nor the gate drive circuit itself because the two circuits - the power and gate drive one don't interact.

 

[Baluncore]

 

[Baluncore]

But I can also ground the source with respect to the gates at all times within the gate drive circuit, can I not?
It shouldn't affect the voltage/current of the main path through the FET's nor the gate drive circuit itself because the two circuits - the power and gate drive one don't interact.

I don't think it is necessary to ground the sources if the current path is low-voltage and the gate voltage is high enough to turn the switches on, while being small enough to not turn on the Zener diodes used for gate voltage protection.

You can ground the sources if you do not ground any other part of the switched current circuit. There may be problems with grounding the sources if the inductors share a common node.

You need to consider protection of the MOSFETs from the flyback of the inductor voltage.

 

[artis]

Ok, so I've thought about it and here is a partial solution I've come up with.
The coils are all parallel shown by the numbers "1 and 2" , so each parallel coil has the switching part in the middle of the of the coil (because there are two coils in series) So as @Baluncore already suggested , I'm using a plain "old" N mosfet bidirectional switch made from two back to back common source N fets.
Then I am using a zener in series with a diode arrangement. Making two of these with opposing polarities since inductor current is bidirectional.

So whenever the mosfets open up into OFF position, the flyback voltage is clamped by the zener + diode series clamp, but the zener addition I think clamps the inductor current at higher voltage which might be beneficial for faster inductor energy discharge as compared to clamping at regular diode ~0.7 volts .

The zener also makes it possible to use the bidirectional mosfet switch at all since while switching the regular current through the inductors the zener will allow not to short circuit and circumvent the actual N fet switch, otherwise simple diodes would conduct all the time , everytime the voltage is above 0.7

Additionally I have two diodes parallel to each mosfet body diode as they are fast recovery and can handle larger currents.

Besides I think that by having two such diodes parallel to the mosfet ones, I can actually just commutate one FET at a time by leaving the other open or closed depending on the need.

Anyway what do you think?
Remember the current through the inductors/coils is bidirectional so I can switch both while it passes one way as well as it passing the other way so flyback is both directions

 

[Baluncore]

Anyway what do you think?

Zeners are a reverse breakdown diode with normal conduction forwards. Therefore, two Zeners, reversed in series, will clamp bi-directional transients. The breakdown voltage will not be perfectly symmetrical since two different Zeners are being used.

Zener diodes are more expensive than power diodes, so a single Zener in a rectifier bridge, is the lowest cost solution, and has voltage symmetry.

Additionally I have two diodes parallel to each mosfet body diode as they are fast recovery and can handle larger currents.

I don't think the FRDs are necessary.

The decision whether to use common source or common drain MOSFET pairs will have implications to how the gates of the bidirectional switches are biased and driven.

 

[DaveE]

Therefore, two Zeners, reversed in series, will clamp bi-directional transients.

Indeed. You can buy them that way as one part.

https://www.littelfuse.com/~/media/.../littelfuse_tvs_diode_1_5ke_datasheet.pdf.pdf

 

[DaveE]

I would also consider leaving out the FRD diodes and turning both fets on together so that you get lower on voltage drop as the current flows through both channels and not the body diodes (synchronous rectifier).

Anyway, I don't think reverse recovery is an issue since the turnoff time is only dependent on turning off the forward biased fet. It doesn't matter how long the other forward biased diode takes to switch, does it?

 

[artis]

Zeners are a reverse breakdown diode with normal conduction forwards. Therefore, two Zeners, reversed in series, will clamp bi-directional transients. The breakdown voltage will not be perfectly symmetrical since two different Zeners are being used.

Right I forgot about that simple fact that I can use only zeners since they too function also as a regular diode below the reverse breakdown voltage by which it starts to conduct in the opposite direction.

Zener diodes are more expensive than power diodes, so a single Zener in a rectifier bridge, is the lowest cost solution, and has voltage symmetry.

I'm not sure I imagined correctly how this can be made possibly in a N fet bidirectional switch?

The decision whether to use common source or common drain MOSFET pairs will have implications to how the gates of the bidirectional switches are biased and driven.

I have the impression that using them with sources tied together is more forgiving in terms of driving them , especially if the overall switched voltage is low.
Given those switches would come parallel to one another I could tie the common sources all together but in order to not create a ground loop I could tie them together using a resistors, what do you think? Or is that a bad idea? It would probably decrease the available upper switching frequency

Anyway, I don't think reverse recovery is an issue since the turnoff time is only dependent on turning off the forward biased fet. It doesn't matter how long the other forward biased diode takes to switch, does it?

That is true.

Anyway so here's my updated schematic based on the suggestions/corrections

 

[Baluncore]

I'm not sure I imagined correctly how this can be made possibly in a N fet bidirectional switch?

You can use B2B Zeners, or a Zener in a bridge. But Zeners and mains frequency rectifiers are slow, so you may need snubbers to reduce the slew rate.

An alternative would be to use two FRDs on each inductor, connected to the supply rails. That recovers the magnetic energy and stores it where it can be recycled, just make sure it cannot pump the supply voltage up beyond what is safe.
Here is the bridge circuit, and my preferred FRDs to the supply rails.

Given those switches would come parallel to one another I could tie the common sources all together but in order to not create a ground loop I could tie them together using a resistor, what do you think?

Without a greater circuit diagram, I cannot comment.

I would try to avoid placing the bidirectional switch between two inductors, because that requires twice as many flyback protection diodes, and moves the source further from the ground or common.

 

[DaveE]

Be mindful of the power/energy dissipation in those zener diodes. IDK your situation, but it's easy to blow up a normal tiny zener in PS clamping circuits. This is an advantage in clamping to the PSs with normal diodes as @Baluncore suggested. That's a much more common approach (except when you can't, LOL).