Author Topic: Steve ward sstc 5 - Antenna distance vs voltage spikes  (Read 889 times)

Offline iWei

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Steve ward sstc 5 - Antenna distance vs voltage spikes
« on: August 15, 2021, 02:53:00 PM »
I’ve built Steve wards’s sstc 5 design, and all seems fine and dandy except for the Vds voltage spikes. After frying 2 ixfh22n65’s(I blame the Vds spikes), I switched to the current low inductance half bridge design.

Now the weird thing is that the Vds spikes could vary from nil to almost 4x the input voltage(in the video it’s 12v), depending on how close the antenna is. The only thing different is:

1. I used a cd40106 @12v(perhaps a different triggering level?)
2. MOSFETs with lower gate charge (faster transitions could be the reason for the spikes?)
3. Wired up a Schmidt trigger oscillator(~100khz) in the feedback path to help get things started

How should I change this to not have to get lucky with antenna position at full mains? Should I increase gate resistor values(although I only have either 10 or 100 ohms, maybe bodge a few in parallel...)

My circuit is essentially this
https://circuitsaladdotcom.files.wordpress.com/2021/05/pcb-tesla-coil.jpg
Minus the CT and rc phase shift network

Video -
https://drive.google.com/open?id=1BHUYEaLnInuGtSTaVINaNJUv0bO7pIjG

Edit: now that I think back about it peak current might also be the culprit since I only have 4 turns wrapped around a 7cm diameter coil

I might have found the culprit, an online calculator gives an impedance of ~ 5 Ohm @ 800Khz which is certainly too low. With a new 20Ohm impedance primary and the low inductance half bridge the fets might have a better time

Also melted a ground probe from using daisy chained server PSUs, must have failed to float the secondary on one of them
« Last Edit: August 15, 2021, 05:36:30 PM by iWei »

Offline AstRii

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Re: Steve ward sstc 5 - Antenna distance vs voltage spikes
« Reply #1 on: August 15, 2021, 09:36:15 PM »
Are you sure 100kHz is the correct f0? The coil seems to be rather small for 100kHz, you're maybe exciting some harmonics only
Marek Novotny
Czech Republic, Czech Technical University in Prague
www.uhvlab.org

Offline iWei

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Re: Steve ward sstc 5 - Antenna distance vs voltage spikes
« Reply #2 on: August 16, 2021, 02:00:16 AM »
Are you sure 100kHz is the correct f0? The coil seems to be rather small for 100kHz, you're maybe exciting some harmonics only

The Schmitt trigger oscillates at 100khz with 22k resistor and whatever parasitic capacitance is present on the input, when I bring the antenna near the topload the feedback overrides the oscillator and it jumps to the f0 of the coil which is about 750khz

Offline davekni

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Re: Steve ward sstc 5 - Antenna distance vs voltage spikes
« Reply #3 on: August 16, 2021, 04:19:29 AM »
The two copper planes for VBus+/- (Q1 drain and Q2 source) are great.  Two additions will make it much better and help with your Vds spikes.  First, add a copper plane on the back side of that board connected to the common (left in your image) sides of C2 and C3 (PR12 in the schematic).  Second, add capacitor(s) on the front side bridging VBus+/-.

It will also help to get bulk capacitance (C11 and C12) as close as possible to the Q1/Q2 board.  If it isn't possible to get them very close, then use twisted-pair wiring from C11 and C12 to the Q1/Q2 board.

The added 22k feedback resistor for self-oscillation is great.  I'd recommend that for general use.  If 750kHz is the correct frequency for your secondary, then I'd lower that resistor more until you are close to 750kHz initially.  750kHz is rather high, so may cause heating issues on the gate driver and/or gate series resistors.  On the other hand, your low-turn-count primary may be fine at that high frequency.  (Perhaps frying is due to the initial 100kHz oscillation on that low-inductance primary.)

If there are still issues, posting images of scope traces would be helpful (with description saying what node is being probed).  Are you getting Vgs overshoot or undershoot with your 10-ohm // diode configuration?  If so, especially if undershoot is bad, adding another 10-ohm resistor in series may help.  This resistor would not be bypassed by the diode, so slow both rising and falling edges of Vgs.
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Re: Steve ward sstc 5 - Antenna distance vs voltage spikes
« Reply #4 on: August 17, 2021, 12:41:32 PM »
The two copper planes for VBus+/- (Q1 drain and Q2 source) are great.  Two additions will make it much better and help with your Vds spikes.  First, add a copper plane on the back side of that board connected to the common (left in your image) sides of C2 and C3 (PR12 in the schematic).  Second, add capacitor(s) on the front side bridging VBus+/-.

It will also help to get bulk capacitance (C11 and C12) as close as possible to the Q1/Q2 board.  If it isn't possible to get them very close, then use twisted-pair wiring from C11 and C12 to the Q1/Q2 board.

The added 22k feedback resistor for self-oscillation is great.  I'd recommend that for general use.  If 750kHz is the correct frequency for your secondary, then I'd lower that resistor more until you are close to 750kHz initially.  750kHz is rather high, so may cause heating issues on the gate driver and/or gate series resistors.  On the other hand, your low-turn-count primary may be fine at that high frequency.  (Perhaps frying is due to the initial 100kHz oscillation on that low-inductance primary.)

If there are still issues, posting images of scope traces would be helpful (with description saying what node is being probed).  Are you getting Vgs overshoot or undershoot with your 10-ohm // diode configuration?  If so, especially if undershoot is bad, adding another 10-ohm resistor in series may help.  This resistor would not be bypassed by the diode, so slow both rising and falling edges of Vgs.

That half bridge design is from you  :D it was really easy to make too

I omitted the bulk bus cap because I wanted to run it from unfiltered mains for lower heating (and haven’t built the interrupter yet)

In hindsight I was powering it from a rewound MOT isolation transformer(2 primary coils of similar dimensions slapped together and the iron core clamped back together)
and then a triac to control the output voltage(by cutting off before the peaks giving lower max voltage)

BUT later I found out it puts out spikes that charges the 3uf caps to 400v, from initial inrush? Or from the triac turn off spikes? (i was hoping that loading it and the film bus caps will lower voltage spikes oh well best not to risk it anymore)

Perhaps I might as well run it straight from the isolation transformer, or even direct rectified mains since I don’t have a variac. If the fets are gonna die, they are going to die either ways from the (nasty?) triac output albeit less explosively than directly connected to mains


Anyways with the new half bridge and a 6.5 turn primary, I got much better looking waveforms (although isn’t Vds spikes supposed to increase proportionately lesser with higher Vbus?) And I’m not sure why the drain waveform is curving in instead of being flat.

1st pic has Vbus @24V, the sine wave is primary current and the squarey wave is the Vds of the low side mosfet

2nd pic is with Vbus at 38V, same modes being probed. (The drain spikes got bigger?)

3rd pic is the gate waveform 100khz, with primary virtually unloaded

At 38V some of the switching transients gets into the gate voltage but it doesn’t dip past the threshold level of the fets(at least not what I can see with my 20mhz scope)
« Last Edit: August 17, 2021, 03:31:48 PM by iWei »

Offline iWei

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Re: Steve ward sstc 5 - Antenna distance vs voltage spikes
« Reply #5 on: August 17, 2021, 02:23:14 PM »
Tested at ~160v full wave rectified mains(my multimeter ain’t true rms so it might be lying to me)

The Vds spikes looks really scary unless my shortened-but-still-significantly-long ground leads about 3 inches could cause my oscilloscope to lie to me too(probe in photo)

The previous scope shots at 24V and 36V is with those extra short spring ground probes but I didn’t dare to put my hand near metal objects operating at higher voltages

Offline davekni

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Re: Steve ward sstc 5 - Antenna distance vs voltage spikes
« Reply #6 on: August 18, 2021, 06:32:19 AM »
Quote
That half bridge design is from you  :D it was really easy to make too
I thought of that after posting my last reply - that there likely were perpendicular planes on the bottom side not visible in the pictures.

Quote
I omitted the bulk bus cap because I wanted to run it from unfiltered mains for lower heating (and haven’t built the interrupter yet)
That is fine.  It is the way my SSTC works, no bulk caps.  VBus tracks rectified line voltage.  However, I'd suggest additional film or ceramic capacitors across VBus (as I have).  Reason is mostly to reduce inductance of VBus capacitance.  SMD parts bridging the copper plane gap are ideal for low inductance.

Quote
In hindsight I was powering it from a rewound MOT isolation transformer(2 primary coils of similar dimensions slapped together and the iron core clamped back together)
and then a triac to control the output voltage(by cutting off before the peaks giving lower max voltage)
TRIACs can be used to turn on after line voltage crests.  TRIACs will not turn off once conducting until current reverses.  Cutting off before peaks will not work.

Quote
Perhaps I might as well run it straight from the isolation transformer, or even direct rectified mains since I don’t have a variac. If the fets are gonna die, they are going to die either ways from the (nasty?) triac output albeit less explosively than directly connected to mains
For initial more-gentle testing, put an incandescent light bulb in series with the incoming line power.

Quote
Anyways with the new half bridge and a 6.5 turn primary, I got much better looking waveforms (although isn’t Vds spikes supposed to increase proportionately lesser with higher Vbus?) And I’m not sure why the drain waveform is curving in instead of being flat.
Generally true - that spikes do not increase too much with higher Vbus.  Often do increase with increased current, however.
I'm guessing that the curved drain waveform is due to inductive coupling of primary current into scope ground lead loop.  (If the curve showed up on only the top half of Vds waveform, then charging/discharging of the 1uF Vbus caps would be a possibility.)

Quote
The Vds spikes looks really scary unless my shortened-but-still-significantly-long ground leads about 3 inches could cause my oscilloscope to lie to me too(probe in photo)
Are you floating your scope or isolating mains?  Presuming a good scope ground connection (3 inches should be fine for 20MHz bandwidth), the spikes do look large.  Have you scoped Vgs at lower Vbus?  That would provide useful information.  My guess is that the diodes across 10-ohm gate resistors are causing FETs to turn off too fast.  (Fast is good if inductance is really low, such as a row of SMD ceramic caps bridging the Vbus plane gap.)  Most FET designs don't include turn-off speed-up diodes.  IGBTs need them due to slower turn-off times.  If you do leave the diodes in place, I'd add another 10 ohms in series with the gates after the existing diode//resistor parts.  Vgs scoping will help here.
« Last Edit: August 18, 2021, 06:52:20 AM by davekni »
David Knierim

Offline iWei

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Re: Steve ward sstc 5 - Antenna distance vs voltage spikes
« Reply #7 on: August 19, 2021, 05:54:56 PM »
I’ve been spamming film caps on Vbus and the capacitor divider, adding gate resistors when I turned the knob from 5V/div to 2V/div and the Vbus waveform magically transformed

Turns out the scope might actually be lying to me, but honestly that isn’t too far out since it’s a 40Yr old scope with 4 electrolytics that vented already(and I only replaced those that vented)

Abit of shame I couldn’t see the difference the modifications made but Vds waveforms looks pretty decent imma leave them in



Thanks for your help guys couldn’t have done it without your guidance

Offline davekni

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Re: Steve ward sstc 5 - Antenna distance vs voltage spikes
« Reply #8 on: August 20, 2021, 05:57:10 AM »
Great to see success!

Quote
Turns out the scope might actually be lying to me,
That sounds likely since changing volts/div made the spikes mostly disappear.  Not too surprising to have some faulty contacts in the attenuator switch.  You implemented a low-parasitic-inductance layout quite well.  I had been disappointed thinking that someone could get such high spikes with a clean layout.  Nice to see that it is working well, not some issue with extremely-fast FET switching as I'd been thinking.

Thank you too for sharing your project with us.
David Knierim

Offline iWei

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Re: Steve ward sstc 5 - Antenna distance vs voltage spikes
« Reply #9 on: August 20, 2021, 02:04:32 PM »
I pretty much bent the antenna backwards (further away from the coil) and spark length doubled.. Looks like i really should add feedback phase adjustment.

Hyperspace pirate has a dead easy but brilliant way of adding phase lead, could this also be used in DRSSTC phase lead adjustments? Although given the nonlinear rc charge/discharge and variable threshold of the schmitt trigger i wonder if exact 50% duty cycle is doable?
« Last Edit: August 20, 2021, 05:16:13 PM by iWei »

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Re: Steve ward sstc 5 - Antenna distance vs voltage spikes
« Reply #9 on: August 20, 2021, 02:04:32 PM »

 


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post Re: Hi! 811 A tesa coil
[Vacuum Tube Tesla Coils (VTTC)]
304er
January 07, 2022, 05:56:28 PM
post Re: First SSTC based on profdc9 PCB Pack
[Solid State Tesla Coils (SSTC)]
davekni
January 07, 2022, 05:15:38 AM
post Re: DRSSTC tuning
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
January 07, 2022, 05:01:21 AM
post Re: How to prevent secondary to secondary to primary arcs?
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
January 07, 2022, 04:59:59 AM
post Tophat QCW 1
[Dual Resonant Solid State Tesla coils (DRSSTC)]
dbach
January 07, 2022, 02:27:35 AM
post How to prevent secondary to secondary to primary arcs?
[Dual Resonant Solid State Tesla coils (DRSSTC)]
dbach
January 07, 2022, 02:19:40 AM
post Re: General Questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Mads Barnkob
January 06, 2022, 09:33:06 PM
post Re: First SSTC based on profdc9 PCB Pack
[Solid State Tesla Coils (SSTC)]
alan sailer
January 06, 2022, 06:09:39 PM
post Re: DRSSTC tuning
[Dual Resonant Solid State Tesla coils (DRSSTC)]
kubajed
January 06, 2022, 04:48:22 PM

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