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Grid tied inverter simulation has shoot through current

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treez

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Hello,
We have simulated a 7kw Grid Tied Inverter with LTspice, as attached.

Unfortunately we have shoot though current coming through the bridge FETs, due to the dead time given by the LTC4449 syncronous FET controller not being enough.

Do you know of a different way of getting the PWM signals (they are the inverse of each other, but need the dead time)
 

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  • GTI _Open Loop.txt
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The LTC4449 uses adaptive shoot through protection, which senses the Vgs on each FET to ensure that both are not on at the same time. For this to work, the driver has to be connected directly to Vgs, no additional buffers and minimal impedance. It's definitely not suited for driving such high power FETs directly, so pick another driver.

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Also I'm pretty sure the shoot through you're seeing is reverse recovery of the FETs' body diodes. For example when the load current reaches its max of 44A, 29A of that if passing through the external schottky diodes, maybe 7A is going through the actual FET channel, and the rest is in the FET's body diode. So pick a much larger FET as well.
 
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I have seen on this site some time ago where diodes shunted by resistors was used. The technique is to delay the rise of the gate drive pulse by driving it via a diode(reverse connected) but shunted by a resistor, so it has got a slow +v edge but a fast -going edge via the turned on diode.
Frank
 

well changing the fet has reduced the spike a good bit, thanks to Mtwieg. (simulation attached shows improvement)
Its still very significant though. I cant use any bigger fet as the bigger ones in the ltspice library have too much rdson.
Ill try the 'diode thing', but the dead time of the LTC4449 is so narrow that any delay at all causes problems. Behavioural voltage sources, which mimmick the ltc4449 outputs are actually driving the fets, so the ltc4449's are not "directly" driving the fets

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Many thanks, to Chuckey & Mtwieg...current spikes almost gone now. as attached (GTI open loop _2)
 

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  • GTI _Open Loop_1.txt
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  • GTI _Open Loop_2.txt
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The RD gate network trick will help, but it means the LTC4449's adaptive shoothrough prevention is going to be useless, maybe even dangerous. If the turn off delay of the RD circuit is greater than the minimum dead time of the LTC4449, then it may actually result in the opposite switch being turned on sooner, leading to less dead time.

In any case you will definitely need more powerful FETs (right now they are burning >50W each), and likely a stronger gate driver. I prefer using ones with separate logic inputs for the high and low side outputs, so I can apply dead time on the input side.
 
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treez said:
Hello,
We have simulated a 7kw Grid Tied Inverter with LTspice, as attached.
Click to expand...
Hi, I can not find the coupling and synchronisation with the grid in the LTSpice schematic.
Can you elaborate how that was done please? Thanks in advance.
 

It is just open loop, it doesnt couple and synchronize with the grid voltage, it was just to see the general working, and to check our calculated fet rms current values, and to check our inductor current value.

As you know, you need a ZCD for the coupling and sync
 
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