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Terry's DRSSTC - Starting to come together...

Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi All,

My second try at the protection card works perfectly ;-))




I had a tense moment when I could not find two stupid 47K 1/4 watt resistors in the house to complete it!! =:O But in the oldest deepest darkest bag of resistors, I finally found to of them! I will order about 10,000 extra from DigiKey Monday :o))

This card reads the primary current and has an adjustable trip point from 0 to 500 peak amps. If the current gets too high, it drops out the H-drive for 500mS. The card also monitors the IGBT heat sink temperature and is adjustable from 0C to 150C to stop the H-bridge if the IGBTs get too hot.

I sort of dry fit the stuff in the box so far:


I probably should have tried to squeeze the input filter and cap board down in size a little... The H-bridge almost gets lost since it is very small. I will either have a thinner fan from the top or mount the board on it's side. I have not "tested" the bridge yet, but I really like it!!


The bridge has four IRG4PF50WD IGBTS with built in reverse diodes as my OLTC used. They do about 750 amps and were some of the first high power fast semiconductor IGBTs to use copper plated conductors on the die surfaces to really get current out (not a secret anymore). I worked on the plating power supplies that make the semiconductor die for them ;-)) (in fact, I guess I worked on "all" the plasma supplies that make them...)

The TO-247 packs are mounted directly on the isolated heat sinks without any "silly pads". Electrical insulating pads are super bad for thermal transfer!! The packs are also mounted with spring "Bellville" washers which are really super important to "squeeze" the thermal grease down to minimal thickness over time. So the "die" and the "heatsinks" are really thermally connected well!! The thermal protection and nice fan take over from there. The reason for using small IGBTs and very small heatsinks is that this whole card has only $75 worth of parts on it! All the traces are super short and the thing is very small which helps speed and reduces all that "parasitic" crap. Sort of a repairable/workable "brick". It can be happily run from GDTs or opto drivers. For Fo bypass, it has two 4.7uF 600V CD 940 poly caps right under it across the buss. Not much will get past them to the electrolytics ;-)) I also have a 470V 6500A MOV across the output to "catch" the primary loop high voltage incase the IGBTs blow themselves away. I "think" the MOV's capacitance (~400pF) will also help "tame" the switching a little. Big 30V transorbs very directly from emitter to gate... The heat sinks have mounting pins but I tear two of them out (got in the way...) and don't solder them incase I have to pull off an IGBT/heatsink combo. There is not too much on the board so "IF" an IGBT blows, it should be cheap and easy to repair. The IGBTs are $7.11 in QTY 10 and $5.33 in QTY 100 (sold out from DigiKey tonight >:-)) All the connections to the board are big bolt on or screw clamps for easy hook up. The 5.1 ohm resistors may change in value at a whim... I might have added 10K little resistors across the gate just to pull down the IGBTS for testing, but that is trivial. Maybe should put a drain resistor across the caps (0.27Joule)... The board can be "played" with without fear of ESD damage... So while Jimmy, Dan, Steve, and Steve may have already figured everything out, maybe there is something a little "new" here ;-))

The controller is looking very much like a digital computer (old style) these days... The big aluminum box is real nice since it houses the mess very nicely!! I worry about propagation errors... I know a guy that builds chess playing CPUs/computers out of plain TTL gates for "fun".... Might have to get his advice... It would be easy if it did not have to work so darn fast...