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Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Hi guys, The motivation sent me back to the Coilgun I have some verry nice IRGB30B60K (TO220 IGBTs for 600V, 75A) and tested them - they surived 200A for several milliseconds. I am planning to parallel them 64x for some higher current. They have a positive temp. coeff so current sharing is not the problem.
I want to focus on switching times here. Such a large array of IGBTs needs several gatedrivers. What timing do i need to guaranty?
1) Turn on timing: a mismatch between the gatedrivers is neglectable, because our load is an inductor, so there is no immeadiate turn-on-current. 2) Turn off timing: every IGBT conducts some current after switched off. But this is just a residual charge carried away to clear the junction, right?* Can this current can rise above the current that the IGBT conducted in its on-state? Can this current destroy the IGBT? In my understanding there is a specific energy stored in the junction - if that energy can not destroy the IGBT, everything is all right, isnt it?*
Worst case: every IGBT switches of at the same time but one does not - that one conducts 300ns longer. What happens? Of course the tail current (first 100ns) is consumed by the coil, then the poor slow IGBT needs to source the "5kA" (haha) What now? Does this IGBT blow up? Maybe.
If my timing varies within the turn-off-time of the IGBTs - is there any problem?*
What if i set the gate voltage to a value so that the IGBT can only conduct 120A (which is ok, respecting the datasheet) Then the "5kA" wil simply rise the collector-emitter-voltage until the halfbridge-diodes kick in. Under this condition 120A is drawn from the IGBT while there is the 350V capacitor-voltage applied. This condition is the short-circuit-condition for the IGBT, which it can surive for about 10us. (a bit longer since it can handle 120A at 600VUce for 10us - i have lower voltage)
Hmmh. Does limmiting the gate voltage really makes switching off that easy?* I now have some microseconds margin?? Of course there are new problems then.. the gate-threshold voltage has a negative temp coeff, so the hotter the IGBT gets, the more current it can conduct with the same gate voltage. Will the lower gate voltage increase problems with current sharing?* This is relevant due to the following stages fire while the IGBTs could still be heated.
The gate voltage needs to be selected that unter normal condition the IGBT is saturated, while under stress the current is limited within the SOA.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Pinky's Brain: of course there is a half bridge with diodes^^ that are the ones that limit the voltage, after turn off. Sry i should have called it an asymetric halfbridge.
Yandersen: there is no point in using the IGBTs seperately in single stages since the stages can be seperated with big SCRs. If one does not want to wait until the current falls back to zero, one can use two halfbridges in an alternating arrangement. (maybe i will try this)
But i really want to discuss the gate drive timing. Maybe i split my halfbridge into two.. but it doesnt matter: i need the paralleling and therefor i need the timing to be handled.
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Switching off parallel IGBTs in half bridge config
I have some verry nice IRGB30B60K (TO220 IGBTs for 600V, 75A) and tested them - they surived 200A for several milliseconds. I am planning to parallel them 64x for some higher current. They have a positive temp. coeff so current sharing is not the problem.
