Author Topic: Current sharing in QCW inverters  (Read 229 times)

Offline TDAF

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Current sharing in QCW inverters
« on: March 29, 2018, 08:57:18 AM »
Exactly how does transformers or Split MMCs force the bridges to share current??

Offline Steve Ward

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Re: Current sharing in QCW inverters
« Reply #1 on: April 07, 2018, 11:03:05 PM »
Transformers:

Each transformer primary gets its own inverter.  The secondaries are then wired IN SERIES (this only works in series).  Since the secondaries are in series, their currents must be equal.  If the secondary currents are equal, the primary currents must therefore be equal.  Even if the inverters output slightly different voltage (maybe your bus wiring has unequal resistance or whatever) the currents will be the same, but their power contributions will be proportional to the voltage.  Essentially you are putting the inverters in series, rather than parallel.

Split capacitors:

Considering only currents at the resonant frequency, the capacitors represent the impedance of the tank circuit which is significant, so it's pretty safe to assume that the current through each section of the capacitor will be close to equal.  Variations in inductance making these connections can skew the balance some as the impedances add (complexly, however).  When using split capacitors and full-bridge drives, you should use what looks like "equi-drive" configuration, where MMC caps are placed on both sides of the primary, or else you are forced into directly connecting h-bridge outputs together which is no good.  The number of caps on each side of the primary need not be equal, but ideally they would be so that the capacitors offer maximum impedance.

If this idea still makes no sense, consider the alternative which is directly connecting bridge outputs together with some sort of wires or whatever.  The impedance of the H-bridge is tiny (if you short circuit it, you get huge currents till it fails).  The impedance of the capacitors is usually several ohms, so its very much like how paralleling transistors often include some sort of series resistor with each parallel part to help "even out" the resistance of each parallel branch and force a more uniform current distribution.

This scheme is not as fool-proof as the transformer scheme.  If there were, for some reason, a mismatch in switch timing (or if a bridge leg fails to switch) then the capacitor impedance to the very fast switching voltage is pretty much zero and you will see "shoot through" currents between bridge legs.  What i mean is, imagine 1 bridge output switched hi, but the other "parallel" one was still low, you are now applying 2 different voltages to the same node in your circuit, and if the impedance between those voltages is small (remember, capacitor impedance approaches 0 at very high freq) then currents are big.

Offline Hydron

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Re: Current sharing in QCW inverters
« Reply #2 on: April 15, 2018, 03:35:07 PM »
This scheme is not as fool-proof as the transformer scheme.  If there were, for some reason, a mismatch in switch timing (or if a bridge leg fails to switch) then the capacitor impedance to the very fast switching voltage is pretty much zero and you will see "shoot through" currents between bridge legs.  What i mean is, imagine 1 bridge output switched hi, but the other "parallel" one was still low, you are now applying 2 different voltages to the same node in your circuit, and if the impedance between those voltages is small (remember, capacitor impedance approaches 0 at very high freq) then currents are big.

This does concern me for the coil I am building - I have done some simulations and the results aren't pretty if one half-bridge fails for some reason.
I plan to try and do some sort of sensing to shut everything off if there is a fault like this (potentially by looking at current or voltage difference between half-bridges - I have a separate CTs for each so have a few options for how I do it).
I also am looking at how to limit fault current from any failure so that I'm not dumping several kJ from the main storage caps into a failed IGBT - am currently looking at a simple DC rated fuse, or potentially using a low-VCE(sat)/Rds(on) IGBT/MOSFET as a fast switch for reacting to a fault situation (in this case with more flexibility than a simple fuse).
Steve (and others) - any suggestions/ideas on either of these issues would be welcome!

The other question I have for Steve is regarding the impedance of his transformer coupled coil - I'm interested in what the primary tank values are and what the transformer ratio is to see what the tank circuit impedance looks like to each half-bridge (and to compare it to what I have, which has no transformer but a relatively low tank impedance for a QCW coil).

Offline TDAF

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Re: Current sharing in QCW inverters
« Reply #3 on: April 18, 2018, 02:58:17 PM »
I'd like to ask,
Why don't the transformer cores saturate??

Offline Steve Ward

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Re: Current sharing in QCW inverters
« Reply #4 on: April 23, 2018, 03:42:50 AM »
Quote
Why don't the transformer cores saturate??

Saturation of a transformer happens because there is too much magnetic flux in the core.  Flux is proportional volts*time/number_of_turns.  For a simple inductor, the current is proportional to volts*time/number_of_turns... which means flux is proportional to current.  Now for the "confusing" part... when looking at a transformer, there is another coil coupled in there (the secondary) and it's current interacts with the primary coil current to give a resulting flux in the core.  Any secondary current will generally cancel out the additional primary current, and so despite operating at a high level of current, the magnetic flux in the core is the same as if there were 0 secondary current and only the tiny "magnetizing current" that would be in the primary coil as its energized by the source voltage. 

Quote
Steve (and others) - any suggestions/ideas on either of these issues would be welcome!

Fuses might be fun to try.  I thought about protection circuits of all kinds... you just listed out most of them.  At the end of the day I didn't bother with any of it and focused on making sure i was not stressing components to the point that a failure would be likely.  Of course, if you look at my flickr you can see the gnarly results from when this fat coil DID have a failure.  My take on that was that it incurred more damage due to plasma/sparks emitting from failed IGBTs to non-failed ones, spreading the failures out further.  I would look at ways of mitigating the propagation of failures.  Either add more distance between transistors, or consider some sort of materials that might be able to withstand the blast (think about how IGBT modules use the silicone goo to contain the plasma/sparks). 

I use a big IGBT as a "circuit breaker", like you describe, for some of my motor drive research to reduce failure propagation on a drive that i built which crams 384 GaN mosfets into a 4"x2" cylinder.  The IGBT can interrupt the bus in ~1uS which limits the destruction to just the device that originally failed, rather than letting it spew plasma at all its neighbors which triggers more failures and destruction of PCBs and stuff.  Depending on the source inductance, this IGBT may need snubber capacitors across it so that it can safely open under fault current conditions without over-voltage.  It can help to use a high gate resistance so the turn off is slow... particularly with IGBT modules that have significant intrinsic inductance.

My buck inverter has current limiting as part of its digital control laws.  Unfortunately, feeding failed bridges with a 500A current limited source still makes for a lot of destruction, but the buck transistors were OK :P.  If i were so inclined, i'd program it to notice that the buck output voltage is low for too long despite high buck currents and that this is a fault condition which should turn everything off.

Quote
I'm interested in what the primary tank values are and what the transformer ratio is to see what the tank circuit impedance looks like to each half-bridge (and to compare it to what I have, which has no transformer but a relatively low tank impedance for a QCW coil).

I think the primary C is 13nF on the FAT coil.  The reflected impedance is such that i maximize each bridge's output current to ~75A, or 600A total for all 8 bridges.

Offline TDAF

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Re: Current sharing in QCW inverters
« Reply #5 on: April 23, 2018, 10:58:00 AM »
Any secondary current will generally cancel out the additional primary current, and so despite operating at a high level of current, the magnetic flux in the core is the same as if there were 0 secondary current and only the tiny "magnetizing current" that would be in the primary coil as its energized by the source voltage. 
WHOA!!
That's some wonky-ass science right there!!

Offline Paultesla

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Re: Current sharing in QCW inverters
« Reply #6 on: April 23, 2018, 11:20:53 PM »
Steve is correct. The flux in the core is about the same off load as on load. Transformer cores saturate from too many volts per turn of winding, or a DC voltage applied for too long.

Paul

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Re: Current sharing in QCW inverters
« Reply #6 on: April 23, 2018, 11:20:53 PM »

 


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