ZVS Flyback driver with disappointing results

[Gisho]

I bought a ZVS driver from China which was rated for 30V 10A. I  hooked it up to a flyback transformer with 5+5 turns on the ferrite core. I was expecting some powerful white hot arcs to fly out but all I got was some thin purple arcs.I tried different turns but it didn't help.I'm using a SMPS @14V 10A but when the circuit was switched on it drew only 3A.
PLS HELP ME !!!

[Mads Barnkob]

Hi Gisho and welcome to HVF

ZVS drivers like these are power hungry. They have no limitations, but the power supply voltage. The higher voltage, the higher current draw. You need to feed them their maximum specified voltage to get the specified power output.

[Gisho]

Mads Barnkob!!! Hi I've seen you in a lot of forums.Thanks for the reply. I tried another 12V supply in series but didn't improve performance. Is there something wrong related to the SMPS? I have no other power supply but this. I'm hungry for big arcs though

[John123]

Have you measured the output voltage of your power supply when its powering an arc?

Show pic of your PSU and the actual ZVS driver board (the larger ones are more suited for induction heating), sometimes you can fiddle with it to get more power at lower voltages. ZVS doesn't really start to shine until about 24v with the bog standard driver, what your getting there is about what I got with 12v IIRC.

You could try putting your power supplies in series providing both of them are not referenced to earth ground at their output.

I think for 12v you have to decrease the primary turns (try 4+4 and maybe 3+3) and add more tank capacitance, the idea is to decrease the primary impedance whilst keeping the frequency the same. Those film capacitors you often find switched mode power supplies will do for short runs (they often eventually overheat), just solder them across the terminals of the tank capacitors already there on the bottom of the board.

Also any idea what the value of your input inductor is? Been a while since I played with a ZVS driver but altering the inductance might help for 12v. davekni is the man to talk to about this as he knows a lot about the ZVS driver.

But even with these changes you won't get a whole lot more out of it at these input voltages.

[Gisho]

When drawing arcs the voltage is about 14.6V. but the current still remains at around 2.7A- 3A .I've attached pics of the PSU & driver board.
Will there be any risk of damaging the driver board by attaching my own capacitors?
The voltage rating of the driver board as per the description on Ebay was 12-30V. So I'm hesitating to increase the voltage

[John123]

When drawing arcs the voltage is about 14.6V. but the current still remains at around 2.7A- 3A .I've attached pics of the PSU & driver board.
Will there be any risk of damaging the driver board by attaching my own capacitors?
The voltage rating of the driver board as per the description on Ebay was 12-30V. So I'm hesitating to increase the voltage

The difference between the ZVS driver on 12v and 24v should be night and day, but if you've tried putting power supplies in series and it didn't help then something else must be going on (you sure the power supply output side isn't grounded to mains earth ground internally? That could short out the PSU when series operation is attempted and trigger it to limit its output). Is it a flyback transformer out of a CRT TV or monitor or something else like a photocopier etc.

Just out of interest any idea what the resistance of the current sense resistor is inside that voltage/current meter is you've added? Sometimes they can drop a couple of volts when more than a few amps pass through them and this will further limit your power.

It looks like you've twisted some of the wires together, this can also cause unwanted voltage drops at higher currents if they're not tight.

For making the mods it won't damage the driver as long as the added capacitor doesn't fail short from overheating or over-voltage, I used to make ZVS drivers out of any old parts and would use those mains rated film capacitors you find in power supplies. They'd run warm and start bulging eventually but for short runs they are fine, just solder it across any of the two black capacitors on the PCB and keep an eye on its internal heating.

Back in my day ready made drivers like this didn't exist 

Aim for around 0.68-1uF for now and see how you get on, you can keep adding capacitors in parallel and they add up. You currently have 0.54uF. Their voltage rating needs to be at-least 3.14 times the power supply voltage. If in doubt post a pic of any spare capacitors you've got and I'll pick some out for you.

Can you post a picture of the flyback transformer you're using including the windings you've made and also the underside of the ZVS driver? Just want to check that there's no cracked solder joints.

Edit: Do the MOSFETs get warm? They could be fake if they're heating much up at these power levels.

[Gisho]

@John123 I took your advice and tried 3+3 turns. I Also removed the V/A meter to avoid complications Now there is a significant increase in performance  I have these caps both 0.68uF and will solder them. The mosfets are barely warm after 5 minute runs. Any ways I'm going to use this for a small SGTC. Is there any protection circuit that I can add to protect this driver from the RF spikes?

[John123]

Much better! You probably don't even need those extra caps now but from the length of those capacitor legs you could just screw them at the circuit board across the outer legs of the primary coil without soldering (blue wires from your other pic).

If the extra capacitance is too big and makes the power supply shut off or the flyback screech but you still want more power then try putting the new capacitors in series, this will reduce the overall capacitance value of the new capacitor (it will act like a single smaller capacitor). Again just watch their temperature as they are not really rated for this kind of circuit, but short runs are ok.

You might find this video useful of someone swapping ZVS capacitors out in real time (music is a bit loud). It's not my video but shows you what you can expect. Not only is it dropping the frequency but it's allowing more tank current to circulate.

As for protections I think maybe putting the spark gap of the tesla coil in parallel to the flybacks output rather than in series with the tesla coil primary will provide some basic overvoltage protection.

Like this.

[Gisho]

I added the caps in parallel, no screeching and the arcs got a little better,hotter & even changed colour (I accidentally burned the wood frame in the pic ). But now there's another problem. I ran it for a minute and the mosfets got really hot. I removed the caps because those mosfets are kinda precious to me , I can't buy any spares in my area   

[John123]

Wow that's a lot of arc power for 14v! Please be careful around that high voltage as it indicates many tens of milliamps of arc current which is enough to be lethal.

What MOSFETs are they? Fake components sometimes end up in items such as this, and fake ones will probably have a higher on resistance. The on resistive losses can still add up even in an efficient circuit like the ZVS driver, won't take more than a couple watts of losses to bring those small heatsink up in temperature.

Without a scope its hard to see what's really going on but you could try using 4+4 and 5+5 turns with just one extra cap and see how it goes. You can also reduce the capacitance of those new capacitors by putting them in series eg: two 680nF capacitors in series will become one 340nF capacitor, it will also cut down on capacitor heating.



This is one of the disadvantages of lower supply voltages, higher currents are needed for the same power levels compared to higher voltages, this causes more I^R losses.

I think it's just going to be a case of finding which combination of primary turns and tank capacitance result in the most output power for acceptable MOSFET heating levels.

Alternatively if it was working fine before the extra capacitance was added and you're happy with the results then you could just call it a day until you've found a power supply with a higher supply voltage. Does the power supply you're using now have any output voltage adjustment pots near its output terminals?

[Gisho]

Yes, maybe I'll have to get another power supply for better results but I'm thinking that this would be enough for a small Tesla coil. @John123 Thank you very much for taking your time to help me  , if I need any help during the build I'll post it on this forum.