Author Topic: Half bridge sstc help with troubleshoot  (Read 1482 times)

Offline costas_p

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Re: Half bridge sstc help with troubleshoot
« Reply #20 on: September 28, 2020, 09:16:53 PM »
Mosfets and gate resistors are dead (the reason of fuse blowing)

Vbus, voltage splitter caps, smoothing cap, bridge rectifier, mains filter seems ok.

Nothing is shorted to ground also.
Papadakis Costas

Offline davekni

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Re: Half bridge sstc help with troubleshoot
« Reply #21 on: September 28, 2020, 09:50:44 PM »
Looks like the fuse was higher-current that the holder was rated to handle, or the fuse wasn't rated for the fault current available in your outlet.

Without a variac available, another option for gentle bring-up is to wire a ballast load such as an incandescent light bulb in series with the line power.  The light bulb acts as a fuse and current limiter.  Incandescent bulbs are great because there resistance is lower when cold.  But, if you need higher current than available by paralleling available bulbs, a heating appliance such as toaster or waffle-iron or ... also works for ballast.

There are several possible causes for MOSFET frying.  It will be difficult to tell what's causing trouble without scope traces.  If you decide to replace parts and try again, you could just run at lower power with the ballast, or look to borrow or acquire a scope.
David Knierim

Offline costas_p

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Re: Half bridge sstc help with troubleshoot
« Reply #22 on: September 28, 2020, 09:56:15 PM »
I will replace the parts in the next days, and will try soft switching probably with a ballast.

When i have news, i will update the post
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Offline plasma

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Re: Half bridge sstc help with troubleshoot
« Reply #23 on: September 29, 2020, 04:13:43 AM »
Costa's
74hc14 ic is sawtooth output, maybe a schemtie hex inverter that is square wave might work better.

Offline davekni

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Re: Half bridge sstc help with troubleshoot
« Reply #24 on: September 29, 2020, 05:44:51 AM »
Costa's
74hc14 ic is sawtooth output, maybe a schemtie hex inverter that is square wave might work better.

74HC14 output is a square wave (or pulse with non-50% duty cycle).  It is often used as an oscillator, with the input having a sawtooth waveform, not the output.
David Knierim

Offline costas_p

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Re: Half bridge sstc help with troubleshoot
« Reply #25 on: September 29, 2020, 06:46:33 PM »
I was wandering, maybe apart from incandescent lamps/ballast/variac, is it correct practice to use a NTC rush current limiter, as a means of soft starting at 230v directly from the wall, so i can keep the "compactness" of my 100x100mm pcb design ?

Also i have a question: i have 2 voltage divider caps in series connecting + and - DC rail, and another one in the middle as dc block. In my understanding in each circle of the sstc the 2 caps active are the middle one and either the one on + rail, or the other in - rail depending on which mosfet is ON at the time. So in order to make calculations about F/L/Xl to find the primary peak current, should I calculate the active caps total capacitance, or calculate the 3 caps capacitance and consider it primary cap on all calculations ?

I attach 2 pictures, the one I have on my mind how a half bridge works, and a clear template for someone to correct me.
Papadakis Costas

Offline Zipdox

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Re: Half bridge sstc help with troubleshoot
« Reply #26 on: September 29, 2020, 09:24:24 PM »
The drawing is correct. But regarding the peak current calculation, the capacitance of the caps isn't really relevant. The inductance of the primary is. I have suggested before that you omit the DC blocking capacitor as the capacitive divider acts as a DC blocking capacitor and poses a risk to resonating.

Now that I mention the primary, is it in phase with the secondary? I have attached an image that shows how they should be phased. The top of the primary should be connected to the MOSFET side in the schematic, providing the primary and secondary are wound in the same direction.

Offline davekni

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Re: Half bridge sstc help with troubleshoot
« Reply #27 on: September 30, 2020, 05:39:53 AM »
Yes, as Zipdox said, an unintended primary resonance is one of many possible reasons for your FETs frying.  Didn't you fix that already per my previous suggestions?  I thought you had removed the "DC blocking" capacitor and increased the other two from 100nF to 2.2uF or 5uF or something around that.
David Knierim

Offline costas_p

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Re: Half bridge sstc help with troubleshoot
« Reply #28 on: September 30, 2020, 12:08:05 PM »
Since new parts have not arrived yet, I though I should give it a shot since the low voltage test was completed succesfully.

I designed a new pcb layout for 2.2uF caps and removed completely the "dc block" cap, as davekni and Zipdox correctly suggested. (the explosion video is with the first setup 100/150nf)

Will using an NTC as inrush limiting, be able to soft start the charge to capacitor? Or should I use another NTC into the + DC rail after the smoothing cap in order to feed the mosfets "softer" current? Or by just adding the larger caps this problem will be solved if nothing else is wrong)

I am looking into a solution where i can run the coil directly from the wall, as a show piece, without having to use variac/balasts/incadencent lamps all the time (of course i understand those extra components play a critical role in tuning and troubleshooting the sstc)

Another question I have is, the 2 cap voltage divider supplies the primary with only half of the rectified voltage, in my peak current calculations, i should use 320v or 170v?

Forgive my questions, I am a HVAC techician, with limited knowledge regarding LCR circuits.

EDIT: the new pcb layout has oval slots for caps, so i can easily choose caps and not have to design and print again and
         again pcbs
« Last Edit: September 30, 2020, 12:09:46 PM by costas_p »
Papadakis Costas

Offline klugesmith

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Re: Half bridge sstc help with troubleshoot
« Reply #29 on: September 30, 2020, 04:26:02 PM »
>> Or should I use another NTC into the + DC rail after the smoothing cap in order to feed the mosfets "softer" current?

No. DC rail is not a good place to add resistance on purpose.

>>I am looking into a solution where i can run the coil directly from the wall, as a show piece, without having to use variac/balasts/incadencent lamps all the time...

Wish there were a picture handy of my Ballast Box.   Electric utility box with two ordinary 120V receptacles that are wired in series, clearly marked as such on the outside.  For example, a toaster or lamp plugged into one outlet serves as a ballast for anything plugged into the other outlet.

>>Another question I have is, the 2 cap voltage divider supplies the primary with only half of the rectified voltage, in my peak current calculations, i should use 320v or 170v?

If the capacitance is much larger than resonant value, primary coil sees + or - 170 volts.
I found current calculations and measurements in this well written document by Loneoceans:  https://loneoceans.com/labs/sstc2/

« Last Edit: September 30, 2020, 04:37:45 PM by klugesmith »

Offline davekni

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Re: Half bridge sstc help with troubleshoot
« Reply #30 on: September 30, 2020, 07:28:50 PM »
Yes, everything kludgesmith said.  An NTC inrush-current-limiter is a good idea, in series with the bridge rectifier.  It can be in one of the AC input legs or in one of the DC outputs of the rectifier.  Before the bulk cap, not after.  The NTC will avoid burning switch contacts or plug prongs when turning your coil on during normal use.

On the bright side, knowing now that you hadn't changed caps yet, that is likely why FETs fried.  So there's more chance that your next try will be successful.

I agree that ballast isn't desirable for any final design.  Ballast is very helpful during bring-up.  Tektronix used to make a variac/ballast box with internal incandescent bulbs and switches to select the amount of ballast (100W to 600W total):


Besides that old ballast/variac box (with unfortunately non-functioning meters), I also have a couple series-connected outlet setups as kludgesmith does.  Very useful.

Good luck!
David Knierim

Offline costas_p

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Re: Half bridge sstc help with troubleshoot
« Reply #31 on: September 30, 2020, 08:09:38 PM »
Thank you all for your replies and suggestions, currently I will be waiting around a month for the new pcb to arrive from china in order to proceed.

I will update this post once the new tests will be done.

 
Papadakis Costas

Offline Zipdox

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Re: Half bridge sstc help with troubleshoot
« Reply #32 on: September 30, 2020, 11:47:21 PM »
For the inrush limiting you could use an NTC thermistor, but you have to make sure it's rated for the current it'll be passing without overheating. But a better solution would be using a power resistor and relay to bypass it once the cap charges.

Offline davekni

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Re: Half bridge sstc help with troubleshoot
« Reply #33 on: October 01, 2020, 06:33:03 AM »
For the inrush limiting you could use an NTC thermistor, but you have to make sure it's rated for the current it'll be passing without overheating. But a better solution would be using a power resistor and relay to bypass it once the cap charges.

What I find in most larger power supplies is an NTC thermistor bypassed by a relay once DC power comes up.  I use that combination in several of my projects too.  The low resistance of the NTC after power-up limits relay inrush current.
David Knierim

Offline costas_p

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Re: Half bridge sstc help with troubleshoot
« Reply #34 on: November 01, 2020, 06:09:19 PM »
New (correct value parts) are arriving to my home, and in the next days I will replace the capacitors of the bridge.

Also I finally found a good offer for a 3kVA variac, because the 1kVA i am using is blowing its fuse when i apply more than 100V.

Meanwhile a video with 85V ac test:
Papadakis Costas

High Voltage Forum

Re: Half bridge sstc help with troubleshoot
« Reply #34 on: November 01, 2020, 06:09:19 PM »

 


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