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dude_500

Tue Apr 17 2012, 10:55PM

Registered Member #2288 Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179

I've been working on my large coil for a while which uses big 1700V 1000A IGBT chopper modules. The datasheet says the reverse recovery diode has a minimum on-time of 10uS (as far as I know, this means if you make the current in the diode turn off in less than 10uS of it turning on, it's bad for the diode). It makes sense that this wouldn't matter in a low frequency boost/buck which a chopper module is designed for. But for tesla coils, this becomes a big problem because at switching cycles, if there is any dead time the diodes can turn on for very brief periods of time. Even during ring down when the diodes carry lots of power, at 60khz, this is less than 10uS half-wave length.

I've had the coil blow up after about 10 minutes twice, taking out the reverse recovery diode of one leg both times.

Does anyone know the physical reason for why a diode would have this rating? It doesn't seem that most diodes have it. Do all bricks have this (like the trusted CM600)? Or is it something specific to the diodes they chose to use in these modules I have?

Steve Conner

Wed Apr 18 2012, 07:01AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Diodes take time to turn on and off. They also have switching losses as a result.

If you try to drive a diode faster than it was designed for, it'll spend much of the time stuck halfway on, with a high voltage across it and a high current flowing through it. It will overheat and blow up.

Diode speed is usually specified in terms of forward and reverse recovery times. I've never seen "minimum on-time" before. A pessimistic interpretation might be the sum of the forward and reverse recovery times.

dude_500

Wed Apr 18 2012, 10:30PM

Registered Member #2288 Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179

Steve Conner wrote ...

Diodes take time to turn on and off. They also have switching losses as a result.

If you try to drive a diode faster than it was designed for, it'll spend much of the time stuck halfway on, with a high voltage across it and a high current flowing through it. It will overheat and blow up.

Diode speed is usually specified in terms of forward and reverse recovery times. I've never seen "minimum on-time" before. A pessimistic interpretation might be the sum of the forward and reverse recovery times.

My interpretation based on a similar situation roughly described a while back in a lecture, is that a single large pulse of current is bad if it stays on for less than 10uS, but is fine if it stays on for more than 10uS. This is not describing a thermal issue as far as I interpreted it, rather some semiconductor physics where damage is done if current stops too soon. I can't think of a good reason for that, though.

Steve Ward

Thu Apr 19 2012, 06:38AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Can we see the datasheet? Maybe the 10uS was just the pulse length they chose to characterize the diode with.

I always thought diodes forward recovery time was just really fast, and so they never bothered posting it in most datasheets. 10uS sounds impossibly slow for any diode worth putting in a IGBT module. If you told me it took 500nS to turn on completely, id much sooner believe that, though still be surprised at how slow it is.

As to your instructors pulse power example, are you sure he meant the same current in both cases, or was it just the same amp-seconds?

Dr. ISOTOP

Thu Apr 19 2012, 04:30PM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

Datasheet attached.
]ds_fd600r17kf6c_b2_2_2.pdf[/file]

Sulaiman

Thu Apr 19 2012, 05:01PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Seems strange to have a 10us diode compared to the igbt times.

E.TexasTesla

Thu Apr 19 2012, 09:19PM

Registered Member #4362 Joined: Sat Jan 21 2012, 03:44AM
Location: Texas
Posts: 98

That technical sheet says "diode-inverter" at the top.
Is that a language translation title or is this diode really part of an inverter circuit internal to igbit?

dude_500

Thu Apr 19 2012, 09:31PM

Registered Member #2288 Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179

The second diode is just a diode. It's an IGBT with a freewheeling diode in the package, with an additional diode in the package isolated, identical to the freewheeling diode.

I believe the brick is intended for buck-boost use. I have used it in a 10kW boost and it works great, that's never burned out.

In case it's confusing, bwang and I are friends and have the same pile of bricks, which is why he had the datasheet.

teravolt

Fri Apr 20 2012, 04:32AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

the data sheet states that he recovery time trr is 250ns. if it used in a bridge the diode capacitance is charged when the other fet is turned on so is it a problem like Steve Conner sais if the frequency is to high

dude_500

Fri Apr 20 2012, 10:10PM

Registered Member #2288 Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179

teravolt wrote ...

the data sheet states that he recovery time trr is 250ns. if it used in a bridge the diode capacitance is charged when the other fet is turned on so is it a problem like Steve Conner sais if the frequency is to high

That does not explain why the diode would have a minimum on time, that would explain a maximum number of cycles per second as a function of voltage, or something along those lines.

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