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First SSTC design - Need some critique

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DerStrom8

Mon Aug 18 2014, 01:22PM

Registered Member #3704 Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92

Thanks guys, excellent points!

@Dexter, good catch on the RC values. I've been to Steve's page several times but somehow missed that point about the R and C values. My coil will be operating at around 212kHz. Therefore, if I stick with the 1nF capacitor, I'll need around 7045 ohms. Thinking I might just use 6.8k. I've updated the schematic to reflect this. Thanks again for pointing that out! Right on about the prototyping on a breadboard. I already have half the circuit set up.

@zzz_julian_zzz thanks for the info regarding the CM300s. I didn't bother with phase lead on this particular design because the IGBTs I currently have, while small, are reasonably fast. If I end up using brick type IGBTs though, I'm sure I'll have to look into phase lead a bit further. I've seen some designs that simply use an inductor and a resistor in series, placed across the secondary of the feedback CT. Is it really that simple? As for your note about the 7404 vs. 7414, that's a great idea. I'm not sure why I chose the '04 in the schematic, because originally I was going to use the '14 for everything.

I have applied the recommended changes, so here's the current schematic:

I'll see about doing some more breadboarding and testing today. Unfortunately I need to wait for the function generator I ordered to arrive (I only have vintage tube-type ones that only go up to around 100kHz and are horribly inaccurate). I'll post back when I've done some testing.

Thanks again,
Matt

DerStrom8

Sat Aug 23 2014, 04:33PM

Registered Member #3704 Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92

Hi guys,

I've made a few more changes to my setup. After some discussion with other experienced DR builders, I have added a second topload which is removable in case it does cause too much loading on the secondary. My main reason for adding a second topload was to bring down the resonant frequency. 212kHz was quite high for the CM300s that I bought, and the second topload brings my Fres down to around 185kHz. With the added phase lead adjust this should be easier on the IGBTs.

I also re-wound my GDT to avoid excessive leakage inductance. Instead of winding the coils individually, I'm just using Steve's method of wrapping the entire CAT5 cable around and paralleling all of the striped conductors to use as the primary:

I fed a ~200kHz square wave into the primary to see how the secondary waveform compared, and it's pretty darn close. The yellow trace is the primary, blue is the secondary:

The droop is due to the 50-ohm output from my function generator, and is not a fault of the transformer construction.

Finally, I connected my GDT to the output of the UCC27425 pair, left the enable pins high, and fed in a ~185kHz signal to simulate feedback into the input pins. The secondary waveform (unloaded) is seen here:

There's some overshoot and some ringing, but that should be fixable once it's actually connected to my bridge, and I can adjust the damping resistor if necessary.

I've done a rough-fit of my bridge:

I'll be doing some work on the heat sink this afternoon (the IGBTs don't quite fit, I need to grind down some ridges).

Once again, here's the updated schematic:

My question for today: Where might I find the type of connector to fit the Gate/Emitter tabs on the IGBTs? Closest I found were 3/16" straight female spade connectors, but they're a bit large, and I wold prefer right-angle ones. Any recommendations?

Thanks,
Matt

DerStrom8

Tue Aug 26 2014, 02:35AM

Registered Member #3704 Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92

Hi guys,

Not sure if anyone's still watching this thread, but I've completely redesigned my secondary coil due to some limiting factors I ran into (no usable 240v or high-current source, etc). My new coil is 4.5" x 12" with a 4.5" x 15.75" topload, wound with #34 AWG wire. However, according to JavaTC, my total energy transfer time is almost 20uS! Is that normal? Most coils I've seen have a transfer time of <10uS. Also, I can't seem to find the math that determines the transfer time, so I'm not sure what I would need to change. If you guys could point me in the right direction here, it'd really be appreciated!

JavaTC input:

JavaTC output:

Uspring

Tue Aug 26 2014, 10:55AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Transfer time is roughly equal to 1/(2*k*f). It's not a very important parameter for DRSSTCs. More interesting is the coupling k, which seems to be about 0.2, which looks ok to me.

Mads Barnkob

Tue Aug 26 2014, 11:40AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

DerStrom8 wrote ...

My question for today: Where might I find the type of connector to fit the Gate/Emitter tabs on the IGBTs? Closest I found were 3/16" straight female spade connectors, but they're a bit large, and I wold prefer right-angle ones. Any recommendations?

This is the standard size cable shoe that fits a CM300:

DerStrom8

Sat Sept 13 2014, 04:33PM

Registered Member #3704 Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92

Hi all, I know it's been a while since I've posted. I made a lot of progress and several major changes.

First of all, I'll be driving this DR with two paralleled bridges made up of FGH60N60UFD IGBTs instead of the CM300s. The expected operating frequency is around 120kHz with the new secondary. I designed the board layout for the bridges myself:

Top:

Bottom:

My latest schematic is shown here:

I am wiring up the flip-flops/drivers at the moment, and testing it by using my function generator to simulate a 120kHz feedback signal.

EDIT: Question removed because I found the problem

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