Poorer man's DRSSTC

[TDAF]

So, I've been working on a drsstc for the better part of 6 months now.
Here are some pictures.

I've tried low voltage testing it with 30V on the bus
but no matter what I do I can't get any output

I apologize for the shit photos and the shit build quality

What might the culprit likely be??

[Mads Barnkob]

Secondary coil and topload looks okay, the topload might be a little too  small, but it should work okay!

The primary coil is not useable at all, coupling is way too high and you got no way of tuning the primary circuit to the secondary circuit, the most important part of a DUAL RESONANT SSTC, this is pure essence of it.

You need to make a flat copper spiral primary coil and start with a coupling of around 0.15 to 0.2, check your design with JavaTC calculator.

Do you have a oscilloscope to check the primary current with a homemade current transformer?

The reason there is no output is definately because it is nowhere near in tune.

Tell us more about the calculated resonance frequencies of your primary and secondary circuits, what inductances/capacitances is inthere?

[TDAF]

Well the coupling is around .1

The primary and secondary are in tune
As ive calculated the frequencies with javatc beforehand
Also, I'll post the javatc screenshot soon

The primary capacitor is around .136uf

[TDAF]

The thing is that the primary starts and ends way before the secondary winding even begins

So, on camera, it looks like a large coupling but in actuality, it's pretty low.
I tried making a helical coil using copper pipe but that was a miserable failure
So, now I've got $20 worth of scrap copper

I realize that the toroid is too small
It will soon be upgraded to 4"x20" toroid or maybe a 3"x21" toroid we'll see

The driver used is a Yunbao Chinese DRSSTC driver

the interrupter is 555 based
with burst-mode, BPS, and On-time adjustments

The switching transistors are FGA60N65SMD
In a full bridge configuration

The Bus capacitor is about 1000uf
bus voltages will eventually be around 320V

the GDT core is an FT140-43 with CAT5 cable wound around it
The feedback and OCD CT's are wound on generic ferrite cores

The tank capacitor is 2x942C .068uF 1600V capacitors in parallel

The rectifier is a KBPC2510

The toroid is of the ugly drain tube with aluminum tape type

[TDAF]

There you go

[oneKone]

have you tried swapping the connections on the feedback ct? (change phasing). My input for no breakout is that with 30v input if it is far out of tune nothing will happen. i found this out first hand when i made a primary with no tapping points, it can be a tedious job adjusting the turns to find the correct tuning. 

I've tried the exact same driver as you and found it was doa. It ended being a dead tc4423 and the controller ic. If you have access to an oscilloscope it would greatly improve fault finding. 

[TDAF]

Yeah the driver works

I've tried changing the phase of the cts to no avail

Might the problem be due to low voltage testing??

[Mads Barnkob]

Yeah the driver works

I've tried changing the phase of the cts to no avail

Might the problem be due to low voltage testing??

If its tuned really bad, you will get no breakout at 30V, so yes low voltage testing COULD be your problem. Even if it is badly out of tune, you should have some oscillations going on and they should be powerful enough to light up a CCFL bulb held closely to the secondary coil, did you try that?

[Teravolt]

hi TDAF, please send more pictures and make Shure they are in focus before you send them. we know how the ud1.3 is suppose to work so some close ups of the fets and gate transformers and feedback transformers. this can help the community to see what you are doing. do you have any test equipment? this may not be a walk in the park to get it going and it will require some detective work.

[TDAF]

I have access to a great lab
Where I'll be going soon for testing the drsstc out

I'll post pictures whenever that happens

[TDAF]

If its tuned really bad, you will get no breakout at 30V, so yes low voltage testing COULD be your problem. Even if it is badly out of tune, you should have some oscillations going on and they should be powerful enough to light up a CCFL bulb held closely to the secondary coil, did you try that?

But the javatc numbers state that the coil is reasonably tuned right?

[TDAF]

So, I found the problem .

It was two blown IGBTs

One halfbridge was completely blown out for some reason

now this will be a halfbridge DRSSTC

[Mads Barnkob]

So, I found the problem .

It was two blown IGBTs

One halfbridge was completely blown out for some reason

now this will be a halfbridge DRSSTC

Be sure to check the phasing of your gate drive transformer, that you are not driving them on one leg at a time and thus short circuit the DC bus through the two IGBTs, instead of across the primary circuit.

Do you have a oscilloscope to do some measurements of this?

[TDAF]

Yes I do

I've checked the phasing of the gdt
And it's turned out of be perfectly fine

The failed igbts are shorted all across the three terminals

For a halfbridge
How does one choose the capacitor values of the voltage dividing leg ie the two capacitors in series in a half bridge

[Mads Barnkob]

Be sure to also check the gate resistors, diodes, zvs diodes etc. that sits on the secondary side of the GDT, they can also get damaged in a inverter failure.

Voltage divider capacitors in a half-bridge is a matter of losses, there is quite a difference from electrolytic to film capacitors depending on the switching frequency. The capacitance value needs to be large enough to keep the center point stable, with low ESR and ESL also being important. Under some transient conditions the center point is fluctuating, so it is important to derate the capacitors voltage wise and 50% voltage derating is well within reason.

The half-bridge topology ideally requires only one-half the voltage ratings for its input capacitors when compared to a full bridge. However, during an extended overload condition, with traditional cycle-by-cycle current limiting, the center point of the half-bridge capacitive divider drifts either towards the input voltage or the ground. This leads to the saturation of the power transformer and requires the input capacitors to be rated to at least the input voltage.

I also updated my article on DC bus capacitors with your question on half-bridges http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/dc-bus-capacitor/

[TDAF]

So, the coil isn't working no matter what I do

all I get is a low-frequency hum from the GDT
which according to me might be from the fact that the igbts aren't switching!!

but, when taken out and separately tested the IGBTs work perfectly fine

I've checked all the traces on the PCB and have come up with nothing at all....

[TDAF]

So, after a fair share of frustration and contemplation of seppuku here I am with a redesigned half bridges I pulled out all the components from my own and decided to go the low inductance ratsnest route

Now the drsstc just pops the breaker

But the heatsink warm up a little so I guess with a better outlet I might just get some sparks outta it

[oneKone]

I'd try having the primary on a slightly larger diameter form, and also trying different amount of primary winds. On both my drsstc the primary wind tap/endpoint has been different to what it was calculated to be.