Author Topic: SSTC - blowing up transistors problem  (Read 749 times)

Offline Przemekk00

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SSTC - blowing up transistors problem
« on: September 06, 2021, 01:56:13 PM »
Hi,
I try to build Mads's Tesla coil (SSTC II), but i have a problem with it. Coil works fine at lower voltages at brigde but when i raise voltage to around 100V DC transistors Q3 and Q4 die. It happened 3 times already, always Q3 and Q4, they are cool so it's not overheating. I thought that EMI might mess with something in circuit so i made quite big metal box which i use to cover everything, even transistors. There is also a barrier inside box between transistors and rest of circuit including GDT and other parts. Box and bottom terminal of coil is grounded to metal rod outside house (it's not connected to mains, wire used to connect those points is short, it has 4-5 meters). I use a filter between mains and autotransformer. I'm sure that GDT phasing is fine, i always check it with osciloscope.  I also test my circuit using 500W heater as load instead of coil because this way EMI is much smaller and i can do some measurements without problems. I use 555 timer circuit to turn on drivers, this signal has around 120kHz and 55% duty cycle(i know it's too small frequency but i didn't have other resistors). I noticed that when i turn on my circuit without voltage on bridge the signal used to control gates of transistors is perfectly fine and square but when i start to raise voltage there appear some oscilations (on the photo). It doesn't look good in my opinion because voltage spikes are something around 18V, but it's not the worst thing. When i look at voltage on the heater it looks similar to signal on gates of transistor but for example at 100V DC those voltage spikes can reach 250V (i don't have photo of this). Despite this transistors survived 320V with heater as load and didn't blow up. If it matters i have other diodes on bridge because i didn't find those MURs, and i used "dsep15-06b". I also have 13 turs on GDT instead of 15 and 9,4 Ohm gate resistors.
I noticed that when i raise voltage using autotransformer frequency of sparks decreases  without even touching interputer's potentiometers, is it normal? For expample at 50V coil makes few sparks per second but at 80V it makes one spark for few seconds. I thought that power supply used was too weak because it was 12V 1,7A and fans need around 1A, so when i regulated potentiometers for max frequency of sparks i could hear that voltage drops because fans were spinning a bit slower. To be sure next time i will use ATX power supply but now i'm waiting for delivery of new mosfets. What do you think about it?

Photos:
https://mega.nz/folder/Niwi0Cob#0xoSJtAtqv4BYtNhkq52eQ

Offline Mads Barnkob

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Re: SSTC - blowing up transistors problem
« Reply #1 on: September 06, 2021, 03:20:30 PM »
Hi,
I try to build Mads's Tesla coil (SSTC II), but i have a problem with it. Coil works fine at lower voltages at brigde but when i raise voltage to around 100V DC transistors Q3 and Q4 die. It happened 3 times already, always Q3 and Q4, they are cool so it's not overheating. I thought that EMI might mess with something in circuit so i made quite big metal box which i use to cover everything, even transistors. There is also a barrier inside box between transistors and rest of circuit including GDT and other parts. Box and bottom terminal of coil is grounded to metal rod outside house (it's not connected to mains, wire used to connect those points is short, it has 4-5 meters). I use a filter between mains and autotransformer. I'm sure that GDT phasing is fine, i always check it with osciloscope.  I also test my circuit using 500W heater as load instead of coil because this way EMI is much smaller and i can do some measurements without problems. I use 555 timer circuit to turn on drivers, this signal has around 120kHz and 55% duty cycle(i know it's too small frequency but i didn't have other resistors).

The purpose of the 555 timer is to only give a interrupter signal at 1-500 Hz! The antenna feedback gives the secondary circuit resonant frequency to the drive frequency.

I noticed that when i turn on my circuit without voltage on bridge the signal used to control gates of transistors is perfectly fine and square but when i start to raise voltage there appear some oscilations (on the photo). It doesn't look good in my opinion because voltage spikes are something around 18V, but it's not the worst thing. When i look at voltage on the heater it looks similar to signal on gates of transistor but for example at 100V DC those voltage spikes can reach 250V (i don't have photo of this). Despite this transistors survived 320V with heater as load and didn't blow up. If it matters i have other diodes on bridge because i didn't find those MURs, and i used "dsep15-06b". I also have 13 turs on GDT instead of 15 and 9,4 Ohm gate resistors.
I noticed that when i raise voltage using autotransformer frequency of sparks decreases  without even touching interputer's potentiometers, is it normal? For expample at 50V coil makes few sparks per second but at 80V it makes one spark for few seconds. I thought that power supply used was too weak because it was 12V 1,7A and fans need around 1A, so when i regulated potentiometers for max frequency of sparks i could hear that voltage drops because fans were spinning a bit slower. To be sure next time i will use ATX power supply but now i'm waiting for delivery of new mosfets. What do you think about it?

Photos:
https://mega.nz/folder/Niwi0Cob#0xoSJtAtqv4BYtNhkq52eQ

Are your MOSFETs insulated from the heat sink? Sounds like you are short-circuiting your supply, which results in a large voltage drop.
https://kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics
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Offline Przemekk00

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Re: SSTC - blowing up transistors problem
« Reply #2 on: September 06, 2021, 03:34:31 PM »
I meant that when I test it with heater i connect enable pins to 12V DC and use astable multivibrator made using ne555 to generate antenna feedback because it needs input signal. It's just for test. I don't even use interupter when testing circuit with heater, but i checked it's output before and it looks good.
Mosfets are insulated from heatsinks, i checked it many times with multimeter.

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Re: SSTC - blowing up transistors problem
« Reply #3 on: September 06, 2021, 06:04:02 PM »
The problem comes when you turn on the voltage slowly, you can't do that because the MOSFETS need to have the voltage kick in very hard to prevent oscillations.

I use 2 relais, one in between the transformer and the diode bridge switching on and off the AC and one relay I have in between the diode bridge and the load switching on and off the DC, first I only switched the DC side but that will kill the relais much too fast, switching off the AC side in sync with the DC side prevents the relay on the DC side from arcing too much.

I'm doing this with a ZVS driver and a flyback transformer but you can do the same thing with your system.
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Re: SSTC - blowing up transistors problem
« Reply #4 on: September 06, 2021, 07:07:55 PM »
I use relay on 12V circuit to turn on and off when it's nessesary. At first I raise voltage at capacitors to around 30-40V with autotransformer, then turn on 12V circuit and then again raise voltage to 100V, 200V etc. When i turn on it with heater instead of coil (with external signal instead of antenna feedback) it seems to work fine up to 320V except those voltage spikes, but nothing blows up. But when i use coil those transistors blow up at around 100V, there is short circuit because it trips my B16 breaker which is connected between autotransformer and diode bridge. I use breaker to protect my autotransformer, looks like energy stored in capacitors is enough to destroy them (1700 uF).

Offline davekni

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Re: SSTC - blowing up transistors problem
« Reply #5 on: September 06, 2021, 08:33:47 PM »
Quote
The problem comes when you turn on the voltage slowly, you can't do that because the MOSFETS need to have the voltage kick in very hard to prevent oscillations.
This need for rapid supply voltage applies to typical cheap ZVS circuits only.  Slow application of Vbus (0-320V) is good.

The biggest issue I see is long wires from Vbus bulk caps (C7) to H-bridge (Q1-4).  Caps need to be adjacent the H-bridge.  Many designs also include local film (snubber) caps on the bridge.

Keeping gate resistors and wires away from power connections also helps.

No need to follow this precisely, but here is an example of low-parasitic-inductance H-bridge construction:
https://highvoltageforum.net/index.php?topic=1324.msg9886#msg9886

David Knierim

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Re: SSTC - blowing up transistors problem
« Reply #6 on: September 07, 2021, 06:34:32 PM »
If you got that working well it would sort of defy any reason for people to be concerned over layout. At the very least I'd try to shorten the capacitor connections to each other with thick soldered leads instead of running a bunch of wires out to levernuts. Then you could at least just move them and the rectifier over closer to the bridge with much shorter and fewer jumpers.

Offline Przemekk00

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Re: SSTC - blowing up transistors problem
« Reply #7 on: September 08, 2021, 09:27:55 PM »
This need for rapid supply voltage applies to typical cheap ZVS circuits only.  Slow application of Vbus (0-320V) is good.

The biggest issue I see is long wires from Vbus bulk caps (C7) to H-bridge (Q1-4).  Caps need to be adjacent the H-bridge.  Many designs also include local film (snubber) caps on the bridge.

Keeping gate resistors and wires away from power connections also helps.

No need to follow this precisely, but here is an example of low-parasitic-inductance H-bridge construction:
https://highvoltageforum.net/index.php?topic=1324.msg9886#msg9886



Recently I soldered all capacitors (C7) to board, connected them with thick traces, moved it as close as possible to H bridge, added 5uF X2 snubber capacitor and voltage spikes are now much smaller but transistors blew up again this time at 160V DC, coil was making nice 20cm sparks. Now I will try to make H bridge similar to yours. Can I use 1mm thick aluminium sheet instead of copper foil and plexiglass to insulate them?

Offline davekni

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Re: SSTC - blowing up transistors problem
« Reply #8 on: September 09, 2021, 02:00:40 AM »
Quote
Can I use 1mm thick aluminium sheet instead of copper foil and plexiglass to insulate them?
Aluminum is almost impossible to solder to (due to the natural oxide layer that forms on the surface).

Glad to hear of some success.  There might be some additional issue.  If so, that will be easier to find once H-bridge parasitic inductance is low by using overlapping copper planes.
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Offline Przemekk00

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Re: SSTC - blowing up transistors problem
« Reply #9 on: September 09, 2021, 03:50:01 PM »
Okay, what about dead time of IC? Is there a threat that dead time is too small and causes both transistors to conduct at same time? Can it be increased somehow and is there any sense to do it?

Offline Felix B.

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Re: SSTC - blowing up transistors problem
« Reply #10 on: September 09, 2021, 04:48:00 PM »
You can and should add a fast diode in reverse parallel to the gate resistor so the gate discharges faster then it charges.

Offline davekni

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Re: SSTC - blowing up transistors problem
« Reply #11 on: September 10, 2021, 05:53:02 AM »
Quote
You can and should add a fast diode in reverse parallel to the gate resistor so the gate discharges faster then it charges.
With IGBTs, gate diodes are necessary.  With FETs, gate diodes are usually not needed.  With bipolar gate waveforms (anything from a GDT), FETs generally turn off faster than they turn on.  Same is not true for IGBTs.
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Re: SSTC - blowing up transistors problem
« Reply #11 on: September 10, 2021, 05:53:02 AM »

 


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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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