Full bridge for Tesla coil blowing mosfets need help on circuit

Thread Starter

nissan20det

Joined Apr 3, 2015
69
I will get more in depth on this tomorrow when I have more time to wright. I have a full bridge setup I just made, I have ran it on a couple small power supplies. 20v 90W laptop charger, 28v 2a transformer. It works quite well. Now I don't have a variac to step up mains slowly(USA 120vac). What I came up with was a CFL LED 150watt dimmer. I first just tested its output voltage after rectifying and using a light bulb for load. I got it down to 18v Sweet!!! So I went ahead and tested it. First try, I got light up right away . Of course me being to excited I started to turn it up and up. Totally spaced I had a 120watt halogen for load...oops... I meant to lower the crap out of that once I established my first light. So It fried the dimmer along with two Mosfets(1 High and low side)...

Well I fixed that, So second try... I get it to light up but only for 5-6 seconds and it dies, dimmer starts to hum. I Look all through my circuit and realized when I changed my lower current fast switching diodes for higher current ones I plugged one of the diodes and drain output one bread board pin off(I did this before the first test) So My first light was on a screwed up half bridge. Plugged that in correctly, replaced the Mosfets on the other sides high and low side. So third try, This time still with 120w bulb just to start, I turn it on and wow way better then the first light, probably 4 inch streamers with dimmer all the way down.(Full bridge definitely working now) I shut it off and try a forth time, Lights up again... Sweet!!! so before anything else happens i turn it off grab my phone for video, set it up, turn it on ....nothing, I also hear a hum. Its the dimmer, popped a high and low side Fet again!!!!

So the dimmer fried most likely do to the 120watt halogen, wile trying to up the current. the fets must be getting a surge when I power off. Can someone help me with this. I drew a quick diagram of my half bridge. Could this be a voltage spike causing gates to pop over 20v? Do I need zeners, if So does 2 in series double? i have a bunch of 15v's so two of those in series, would that be 15v or 30?. Is this a situation where TVS diodes would help?

Thanks guys

sstc (4).png
 

Thread Starter

nissan20det

Joined Apr 3, 2015
69
okay I did add two 15v zeners in series, anode to anode, gate to source. blew another two mosfets... UGHH.... I should have my freewheeling current covered with my MUR460's, voltage spikes with zeners, and dead time with 56ohm and 1n4148 on gate. Im so lost any help would be great. Ill check what my actual dead time is on my scope tomorrow. The only thing I can think of is the tesla coil is about 4ft from the driver circuit on the same table, maybe the output from TC is affecting the Gate Drive Transformer through RF? Maybe causing it to turn the other "OFF" fet ON.

I do see now that Its always a high side and low side that pop. but ones that are turn on opposite of each other. so say always the high and low on the same side of primary(oppositely switching). So it makes since that I am turning one ON as the other isnt fully OFF. Now with my dead time theoretically being tackled, thats why I lean towards interference. What do you guys think?

To help you understand my driver circuit, here is another schematic, Im using his fet driver but to a 4 fet GDT. and the modulator is a multi vibrator in my situation. Overvolt protection on driver circuit is not used and his feedback transformer i could not get working. so I am using a antenna in its place. The H bridge I have is my first post

LoneOceans SSTC.jpg
 

JUNELER

Joined Jul 13, 2015
183
Hi,
I dont understand your aim of doing the circuit. what is your main objective of the circuit to be use for.

no2 why do you need a variac to slow down the mains 120vac to what level....

WHAT is your load to use and for what purpose. your circuit is not define properly,where the output is to be driven .

Lastly why antenna is use for feedback.. its completely wrong . find the right circuit for your needs..
 

Thread Starter

nissan20det

Joined Apr 3, 2015
69
The purpose is for a solid state tesla coil which is stated in thread name.

Everyone uses a variac with teslas. going directly to full current is asking for failure.

the load is my primary, purpose it to power the TC, My circuit shows the output just fine and is labeled as PRIMARY.

The antenna is for closed loop, So your TC is always tuned to resonance. Not only is that widely use in the coiling population it works quite well.

Im sorry, what I was asking is for help, not to be shut down. This circuit is just fine. I guess your so confused because you dont know what your looking at. Please save your criticism for something you understand because it is no help to me.. Thanks
 
Last edited:

ronv

Joined Nov 12, 2008
3,770
I will get more in depth on this tomorrow when I have more time to wright. I have a full bridge setup I just made, I have ran it on a couple small power supplies. 20v 90W laptop charger, 28v 2a transformer. It works quite well. Now I don't have a variac to step up mains slowly(USA 120vac). What I came up with was a CFL LED 150watt dimmer. I first just tested its output voltage after rectifying and using a light bulb for load. I got it down to 18v Sweet!!! So I went ahead and tested it. First try, I got light up right away . Of course me being to excited I started to turn it up and up. Totally spaced I had a 120watt halogen for load...oops... I meant to lower the crap out of that once I established my first light. So It fried the dimmer along with two Mosfets(1 High and low side)...

Well I fixed that, So second try... I get it to light up but only for 5-6 seconds and it dies, dimmer starts to hum. I Look all through my circuit and realized when I changed my lower current fast switching diodes for higher current ones I plugged one of the diodes and drain output one bread board pin off(I did this before the first test) So My first light was on a screwed up half bridge. Plugged that in correctly, replaced the Mosfets on the other sides high and low side. So third try, This time still with 120w bulb just to start, I turn it on and wow way better then the first light, probably 4 inch streamers with dimmer all the way down.(Full bridge definitely working now) I shut it off and try a forth time, Lights up again... Sweet!!! so before anything else happens i turn it off grab my phone for video, set it up, turn it on ....nothing, I also hear a hum. Its the dimmer, popped a high and low side Fet again!!!!

So the dimmer fried most likely do to the 120watt halogen, wile trying to up the current. the fets must be getting a surge when I power off. Can someone help me with this. I drew a quick diagram of my half bridge. Could this be a voltage spike causing gates to pop over 20v? Do I need zeners, if So does 2 in series double? i have a bunch of 15v's so two of those in series, would that be 15v or 30?. Is this a situation where TVS diodes would help?

Thanks guys

View attachment 127442
I can't quite read the FET types, but my guess is 100 ohms in the gate is not going to give enough delay.
Of course any noise that turns them both on at the same time is a disaster. Can you scope them?
 

Thread Starter

nissan20det

Joined Apr 3, 2015
69
Absolutely. I'll post some pics up here in a sec, I was ganna test may actual dead time as well. The fets are ixtq50n25. Ya wasn't sure on value most guys do under 20ohms so I just went double to be safe.

And ya before my next test I was going to ground a cage over my circuit to keep Tesla frequencies out.

I did notice a second frequency in it when it ran. So I could hear my modulation frequency but then there was another -50 Hertz pulse. It would kind of very when I came close to the antenna. This has never happened before. But I'm also so lost because when I first started this up on the first dimmer it worked f****** great. Like 10 mins.So I have a feeling i fried something else when I popped that first dimmer. So I'm going back through everything with the scope. I found a very weird situation here.

With the antenna in it and say the Tesla is actually running on low power I can see the waveform coming out of the gdt and it is fairly good. Right now I am testing basics, I have my multivibrator set at about 200khz I'm getting a pretty damn clean Square wave out of it. I can also see after my inverter and out of the mosfet driver looks clean with no load. But it is only clean out of the mosfet driver when it's not connected to the gdt. When connected, I am getting a very f*****-up signal and that signal of course is coming out of the gdt. Before I tested it this exact way and it was super clean now I can't even get a signal out of my gdt unless I let it self resonate.

So I'm thinking the fet driver popped when I fried the dimmer. Why it continues to work on low power... No clue..... But I get a output with feed back but none if I make my own. Which is not right. So I'll hop on the circuit here in a hour or so and scope some stuff as well as change that driver.
 
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Thread Starter

nissan20det

Joined Apr 3, 2015
69
Okay all of driver circuit was okay. So I'm still stuck. Maybe u can help but to measure my dead time I am having troubles.The two low side fets source's are connected to ground. The two high side fets source is connected to the primary (OUTPUT). My scope is portable, ds202. The problem I'm having is to see my GDT Waveform correctly I need to ground on one output and probe the other on each separate coil. Now the low sides are tide together at ground (low side fet source). High side GDT's outputs connect to gate and source of the high side fets. So I can't see a high and low gate waveform at the same time. I want to so I can see my dead time. I some how need to ground my scope on the low side fets source (ground) and the high side fets source (primary out) without shorting it.

Could I use two caps in series from high source to low source and ground the scope in the middle of the two caps?
 

ronv

Joined Nov 12, 2008
3,770
Okay all of driver circuit was okay. So I'm still stuck. Maybe u can help but to measure my dead time I am having troubles.The two low side fets source's are connected to ground. The two high side fets source is connected to the primary (OUTPUT). My scope is portable, ds202. The problem I'm having is to see my GDT Waveform correctly I need to ground on one output and probe the other on each separate coil. Now the low sides are tide together at ground (low side fet source). High side GDT's outputs connect to gate and source of the high side fets. So I can't see a high and low gate waveform at the same time. I want to so I can see my dead time. I some how need to ground my scope on the low side fets source (ground) and the high side fets source (primary out) without shorting it.

Could I use two caps in series from high source to low source and ground the scope in the middle of the two caps?
Well, that's a pain.:(
You need a differential probe.
How high is the voltage?
Maybe a small resistor in the source of the low side and look at the current?
 

Thread Starter

nissan20det

Joined Apr 3, 2015
69
Voltage when working and testing is always 12V on driver circuit and 20-28v on low working voltage and at main with dimmer down, probly around 80v is when the just stop working.

Well I just looked at the two low side fets since those do oppositely switch. Im not actually seeing the high low transfer but it should be close enough. So looking at it like that it doesn't seem to have any dead time. As I increase the gates resistor much above 100ohms I only see the front portion of the square wave round out. the back end is a nice straight switch down but adding resistance seems to just round the front not really add delay. Im not even getting to the point of heat yet do I doubt this is an issue yet but I can see this "ON" rounding effect cause a problem. It would make the fets slowly turn on cause all that power dissipate as heat in the fets with them resisting that current during the slow on transition. So I also think to much resistance there would also be bad. So Is it the fact im looking at the low to low transfer not high to low? theoretically the should be the same since the GDT windings are identical right? so why am I not seeing an off period between pulses? They only overlap at the bottom end of the square, maybe 2-6%.

I also had a thought or theory rather, that I am popping two due to one fet shorting out, say the high side first causing positive to be constantly seen at one end of the primary, on that same side of the primary the low side fet then switches on therefor shorting the low side from pos rail through the high then through the low fet to ground. The other side of the primary, 2 fets, Stays intact due to the DC blocking cap. so theres a thought that it could be one failing not two at the same time.

But over all I think it could be a few things,

1. H Bridge impedance causing voltage spike- I doubled up all jumpers
2. Voltage spike at gate- Zeners should protect that right? should they be after or before gate resistor? stay with 2 in reverse series or just 1?
3. Shoot through not enough dead time- Why am i not seeing a delay as I add resistance?
4. interference from tesla running making mosfets switch at wrong times- will cage driver on next test

Also I cannot for the life of me get that guys feedback transformer to work. its simple 50turns with secondary ground ran through the toroid ring. I tried fliping connections and way the wire went thinking phase was off. but I cannot get a signal when on transformer leg is grounded. If I leave one end open and other as feedback to inverters it works but isnt that just a wound antenna then? I tried a number of different cores and just cant get it. its like when I ground one of the two pins I just am pulling the inverter to ground so then not pulsing at all then WTF??? any thoughts on that?
 
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