Author Topic: Problems with building Steve Ward's SSTC 5  (Read 781 times)

Offline martin-robomaze

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Problems with building Steve Ward's SSTC 5
« on: August 08, 2021, 03:12:15 AM »
Hi, I'm trying to build my first SSTC based on Steve Ward's SSTC 5, but I encountered some problems with it. First of all, when I power it up, it is pretty hard to find a position, where the antenna feedback would start working, and even when the circuit starts to oscillate, the frequency of oscillations is from 1.5 to around 2.5 MHz, even though the calculated resonant frequency of the secondary is 320 kHz(diameter 5cm, length 30cm, 1500 turns). The coil lights up CFLs and creates tiny arcs to my hand at 16 volts, so it seems like it oscillates on its higher resonant mode. How can make the circuit oscillate on the secondary resonant frequency? Also, when I powered the circuit with 16 volts DC, it drew 2 amps, which seems to be a little high, because I have seen people that had this current consumption when powering the coil from mains. Could this be because of the high switching frequency? I am attaching my schematics(I have made 2 separate pcbs, one for the driver and one for the half bridge).
Thank you for your replies.
EDIT: I am using IRFP460 mosfets instead of IRFP240 that are in the schematic
« Last Edit: August 08, 2021, 05:02:16 AM by martin-robomaze »

Offline davekni

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Re: Problems with building Steve Ward's SSTC 5
« Reply #1 on: August 08, 2021, 06:43:56 AM »
First thing I would try is reversing polarity (reverse GDT primary wires or Tesla primary coil leads).  I've seen at least one other case where backwards polarity oscillated at high frequency rather than not-at-all.
David Knierim

Offline martin-robomaze

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Re: Problems with building Steve Ward's SSTC 5
« Reply #2 on: August 12, 2021, 03:20:21 PM »
Hi, sorry for the late reply, I wasn't able to test the coil until now. I reversed the primary polarity and powered the coil with 60 VDC. It started to resonate at 340khz and it lighted up CFLs. It was even arcing to my hand. but after 15 seconds or so, the MOSFETS blew up... I had them heatsinked with an old CPU cooler, is it enough? Could they blow up because of a voltage spike when one of the transistors turns off? Shouldn't the diodes(D1, D2) prevent the spikes? If the reason for the mosfets blowing up was the high voltage spike, would RC snubbers help even if the diodes are already there? I don't know if images of my coil would help, but I attached them just in case.
Thanks for any reply.

Offline davekni

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Re: Problems with building Steve Ward's SSTC 5
« Reply #3 on: August 13, 2021, 05:45:59 AM »
Do you have an oscilloscope?  Scope traces are the best way to find problems.

Quote
Could they blow up because of a voltage spike when one of the transistors turns off? Shouldn't the diodes(D1, D2) prevent the spikes?
The FETs have internal body diodes too.  D1 and D2 may help if they are faster than the internal diodes.  However, the diodes clamp spikes only if trace inductance to capacitors (C3,4,5,6) is low enough.  Without scope measurement of bridge output, I cannot tell if parasitic inductance is your issue.

Quote
I had them heatsinked with an old CPU cooler, is it enough?
That is likely sufficient if the circuit is working correctly.

Is 340kHz about right for your coil?  If you haven't already, I suggest using JavaTC to model your coil and see what frequency should be expected.
David Knierim

Offline AstRii

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Re: Problems with building Steve Ward's SSTC 5
« Reply #4 on: August 13, 2021, 11:52:47 PM »
I don't think R4 and R5 are necessary. This way you're limiting the current to only 12V/20R = 0.6A, which means 0.3A for each FET gate. The fall time may not be quick enough to avoid cross-conduction if you only have 0.3A current sinking capabilities. C6 at 340kHz also adds additional 4.8ohms of impedance, which is quite a lot for a DC blocking capacitor, I would use at least 1uF. Also get rid of those large white resistors on your bridge! Those are wire wound very high inductance resistors, they will only make a mess.

If everything is working as it should and the transistors still blow up, I would be suspicious about:

1.) Cross-conduction
     Are you using a R+D circuit on the gates? If not, you should definitely have some. Try increasing the gate charge resistance to add some deadtime.
2.) High voltage transients
     Use RC snubbers on the FETs and try to decrease parasitic inductance as much as possible (use twisted cables, make short traces).
3.) Noise on the gates of the FETs
     Shield your driver (metal grounded box for the driver perhaps).

Also your coil seems to me as just a little too big for 340kHz, are you sure your driver is locking to a correct resonance frequency?

« Last Edit: August 14, 2021, 12:01:04 AM by AstRii »
Marek Novotny
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Offline davekni

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Re: Problems with building Steve Ward's SSTC 5
« Reply #5 on: August 14, 2021, 05:38:44 AM »
Quote
I don't think R4 and R5 are necessary.
Good point.  I hadn't noticed those resistors in the GDT primary circuit.  Especially at 340kHz, switching will be too slow.

Quote
C6 at 340kHz also adds additional 4.8ohms of impedance, which is quite a lot for a DC blocking capacitor, I would use at least 1uF.
Yes, C6 is a bit low.  It is the value in some of the standard SSTC designs, and generally workable.  Appropriate GDT primary coupling capacitance does not depend on frequency (unless GDT parallel inductance is low, ie. wound on a powder core rather than ferrite).  The driver conducts current only during switching.  Worst-case, IRFP460 total gate charge is 210nC at 10Vgs, for 420nC of two FETs.  420nC discharged from 0.1uF (100nF) is a 4.2V drop.  That makes Vgs +-7.8V instead of +-12V.

Quote
Those are wire wound very high inductance resistors, they will only make a mess.
In general I'd agree, non-wire-wound resistors are better.  However, the added ~1uH is likely not an issue here with the relatively-slow switching.

Quote
Are you using a R+D circuit on the gates?
Gate diodes are generally not needed for FETs.  Gate waveform slew rate and threshold voltage is usually sufficient.  IGBTs almost always do need diodes, as turn-off time for IGBTs is usually well more than turn-on time due to minority carrier storage time.

Quote
Also your coil seems to me as just a little too big for 340kHz, are you sure your driver is locking to a correct resonance frequency?
Yes, I had the same thought.  Why I suggest using JavaTC to verify.

One final thought on FET frying:  The market is flooded with Chinese cheap fake parts, with IRFP460 being notorious for fakes.  Actual die size (and current capability) may be half of real IRFP460s.
David Knierim

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Re: Problems with building Steve Ward's SSTC 5
« Reply #5 on: August 14, 2021, 05:38:44 AM »

 


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