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DRSSTC design questions

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LAN_403

RateReducer

Tue Dec 18 2012, 09:30PM

Registered Member #4603 Joined: Wed Apr 25 2012, 07:33PM
Location: Austria
Posts: 159

Hi. I amplanning to build a half bridge DRSSTC with this IGBT:

I have a few questions:

-> I want to use a snubber capacitor. How much ÂµF should i use?
-> if i run the coil on 325V DC would a 800V TVS string out of 8 piece 100V 1,5kW fit for each IGBT?
-> the datasheet says max. gate voltage +-20V. I want to use a GDT. How much voltage should i use on the gates? Maybe 18V+-? Zener diode values?
-> will a pair of TC4452

+ GDT be strong enough to drive the IGBT @ around 60kHz?

Sorry for all the questions... ^^
Thanks in advance!

Best regards, Daniel

zrg

Wed Dec 19 2012, 12:52AM

Registered Member #4762 Joined: Sun May 06 2012, 05:59PM
Location: Russia
Posts: 93

1. A snubber capacitor (what for? it is totally not required in DRSSTC's (half)bridge) or a DC film capacitor parallel to electrolytics? if the least, as much as possible, but 4-10 uF should be enough.
2. I don't think E-K TVS is ever required for DRSSTCs, never used them and never seen any schematic that includes them.
3. As seen from Ic/Vge desaturation graph in the datasheet, this brick is able to hold 500A at 11V on the gate, and the line rises at ~200A per 1V. If you're not planning more than 2kA primary current, 18V G-E voltage should be enough. Use 20-22V zeners.
4. I would recommend a discrete driver like in Steve's UD2, but looks like these drivers can easily charge the 20nF gates of the brick. Try them out, it would probably work quite fine.

RateReducer

Wed Dec 19 2012, 01:14AM

Registered Member #4603 Joined: Wed Apr 25 2012, 07:33PM
Location: Austria
Posts: 159

Thanks for the answers!

1. Ah Okay. Then i will use a filter capacitor.
2. In Steve Wards DRSSTC guide, he said "Use Transient Voltage Suppressors (TVS) from collector to emitter of each IGBT....."

They should catch the fast transients and save the IGBT from overvoltage. They are not too expensive - i think i will use them in my bridge...
3. Alright 18V. But... 2kA!? How does the IGBT (600A pulse) handle 2kA? I donÂ´t understand that.
4. I already have the TC4452 on hand. I will try to drive the brick with them. If it does not work i will use the discrete driver.

Regards

zrg

Wed Dec 19 2012, 08:39AM

Registered Member #4762 Joined: Sun May 06 2012, 05:59PM
Location: Russia
Posts: 93

Ah, okay, let's trust Steve. But in well designed DRSSTC there are no overvoltage spikes (I think Steve added that recommendation because of some recent to the date of the guide creation failure with IGBT overvoltages, which happened because of some inner resonance of the inductance of the bridge layout and DC filter caps, which matched the actual coil's resonance and produced the overvoltages. I think Steve can tell better himself). At least TVSes won't hurt :)

For the 2kA, look here -

As to my experiments, generally bricks are capable to withstand a 10x of rated continuos current in DRSSTC. Their pulsed current rating is usually rated for 1-10 milliseconds, while in the coil they actually operate for 50-100 microseconds.

Dr. ISOTOP

Wed Dec 19 2012, 09:23AM

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

With a good low-inductance bus you should be fine without TVSes or snubbers. However, you should put some film caps in parallel with the lytics to neutralize the ESR of the electrolytics at high frequencies.

Kizmo

Wed Dec 19 2012, 12:14PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

TVS diodes over C-E and gate are really good idea to have!

In normal operation they dont do much BUT if something goes wrong (for example primary strike or flash over) there can and will be seriously nasty overvoltage conditions and thats where TVS diode really shines due to their speed and high current capability.

So at least i would recommend TVS diodes everywhere along with other protection methods.

RateReducer

Wed Dec 19 2012, 01:01PM

Registered Member #4603 Joined: Wed Apr 25 2012, 07:33PM
Location: Austria
Posts: 159

@ zrg: 10x rated continuous current - evil!

Then i will set the OCD to about 1500A ...should be save?
Your link just brings me back in my thread ^^

@Dr. ISOTOP: I installed them yesterday :)

@Kizmo: That makes sense! TVS diodes are on order!

I want to put the DRSSTC driver into a metal housing and connect it to the ground of the secondary, strike rail and the heatsink. Is that a good or a bad idea?

Physics Junkie

Tue Dec 25 2012, 09:19PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

RateReducer wrote ...

I want to put the DRSSTC driver into a metal housing and connect it to the ground of the secondary, strike rail and the heatsink. Is that a good or a bad idea?

I believe that should work just fine. I remember reading an article on proper grounding and ideally the strike rail, secondary, and any other metal objects that have a good chance of suffering a strike should be grounded to an rf ground together, while the heatsinks and any other sensitive metal objects apart of your electronics (such as aluminum enclosure/metal housing around sensitive circuit boards or step down transformers and things of that sort) should be grounded to your mains ground. But I dont think this is a "must do" since people ground everything altogether and I've never seen/heard of it not working this way. Somebody else might be able to elaborate a bit better on that as proper grounding is very important.. and if I remember correctly, the article I read might have been referring to sgtc's but it may not make a difference. Hope that helps a bit

Edit: I found the article

Steve Conner

Wed Dec 26 2012, 03:34PM

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

The heatsink should be connected to the same ground as the driver circuitry. The negative terminal of the DC bus should be connected to this same ground via a small class Y capacitor.

If you don't do this, enough RF voltage can appear across the IGBT insulators and gate drive transformers to arc them over, with spectacular results.

This is the only thing that really matters in DRSSTC grounding.

RateReducer

Wed Dec 26 2012, 03:57PM

Registered Member #4603 Joined: Wed Apr 25 2012, 07:33PM
Location: Austria
Posts: 159

Thx for the article Physics Junkie. Very informative.

Hi, Steve.
Heatsink connected to low voltage driver ground and DC bus ground connected via capacitor. OK.
Where should i ground the strike rail and the secondary base?

Steve Wards shematic looks like this:

...what will be the better concept?

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