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is my power supply set up suitable for zvs use?

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Davebmx

Wed Feb 29 2012, 09:24AM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
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Posts: 40

Ok cool well what i'm going to do is resolder everything onto a pcb tonight to try and clean the whole thing up. I will also try get a hold of a large cap today. I actually built my tank circuit in parallel i just got my words mixed haha! About the two small diodes, when i downloaded the data sheet it said the were 'fast response time' is this the same as the glassy zeners? or do you think i should replace them? I think i might also pick up some brand new mosfets today just to make sure.

Forty

Wed Feb 29 2012, 09:45PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

zener diodes operate differently than standard diodes. a zener will allow current to flow in the forward direction (as a normal diode) but if the reverse voltage exceeds the zener voltage then it will conduct in the reverse direction too.
The zeners are there to protect the gates from voltage spikes above 12v (or whatever voltage zeners you have.)
The fast diodes are there to discharge the gate of one mosfet when the other one is conducting. "fast response time" probably just means that the diode can change nonconducting to conducting and from forward conducting to reverse blocking quickly (which is good.)
look for mosfets with 100-200v rating, avalanche rated, highest current and lowest "Rds on" you can find. If you can't find something like that at the store, then the IRFP250 is probably what you want to order (it's the most widely used fet for the zvs I believe. I haven't used them though)
The large filter cap isn't physically going to be that large, so look for one in salvage. A higher voltage rating won't hurt and you can parallel up as many as you want (mismatched won't matter, e.g. 35v 680uF + 25v 1000uf = 25v 1680uF)

Davebmx

Thu Mar 01 2012, 09:42AM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
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Posts: 40

thanks for the info forty. i went and bought all new components and arranged them on a prototype board. on powering it up however i think i fried one of the fets because my heat sink wasnt on properly and then this caused the negative power terminal to get fried? apart from these problems do you think i have the circuit correct or are there other reasons why this might not have worked. are my toroids too close together? are the copper tracts to small on that board to handle the current? mosfets too small? the datasheet did say they could handle 100v but they seem very small. I have also included a pic of the three 25v 2200uf caps i have used for my smoothing. As i havent done any electronics since school and am learning stuff all the time i was surprised to see how long the caps kept current in the circuit after the main power was turned off. still power according to my multimeter a few minutes after power off! the last pic shows where the circuit board fried.

Forty

Thu Mar 01 2012, 05:27PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

I've got mosfets that small that can handle a lot more than 100v. Do you have a part number for them so I can look up the datasheet? the other three parameters I mentioned are very important too. If the circuits not running properly then that'll leave the capacitors charged for a while. You could put a 10k resistor across the capacitors to bleed off the charge a little faster (~3min to discharge completely when circuit's off, resistor dissipates 0.04 W when circuit's on) or you could just short it with a screwdriver for a little spark (there's only 1.32 J stored)

The inductors should be alright like that as long as the wires from one and protruding into the windings of the other. the magnetic field is theoretically only inside of the toroid material.
The last pic definitely looks like the trace couldn't handle the current, but all is not lost. for the short track lengths you can bead up some solder to improve current handling (which you've mostly done already) and for the long lengths like where it blew, you can tin a straight length of wire and solder it along the trace to fix it right up. Afterward, make sure there's no solder bridges where you don't want them, and that flux hasn't gotten between the traces (looking at the last pic again I can see quite a bit) Isopropyl alcohol and a rag will clean it up, just make sure it dries before you apply power!
Also in the last pic i notice the black striped wire's connection at the lower left looks awfully close to the trace along the edge so double check that that's ok.

Davebmx

Fri Mar 02 2012, 08:50AM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
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Posts: 40

On close inspection i think i made a pretty big school boy error! i hadnt broke the contacts between the poles of the 470 resistors so they were just being bypassed. the fets were irf540a heres the data sheet

does say they are capable of 28a but without the 470 resistors im assuming they were overloaded? I have ordered some new board, all new diodes, resistors of different current handlings and some irfp250 mosfets. Last night i checked most of the components and they were just showing conductance both ways with the multimeter so it must have fried pretty much all of them. I desoldered them and stuck in some other diodes and resistors i had lying around and i seperated the toroid and moved them away from each other. stuck in a pair of random mosfets i had and powered it up. I got it up to 19.75v without any smoke and for a short while i had 19v coming out of it but then when i increased the power a bit more i blew the fuse in variac. can i test the driver circuit by attaching a bulb to the output or a motor or something to see that it actually produces power? as i dont get any voltage in my tank circuit but that could just be due to something wrong with that. i was thinking about seeing if i could borrow a oscilloscope. What i did notice is that the caps are discharging almost instantaneously after power off so im hoping thats pointing to me getting closer to a working circuit. Like you said though the problems im experiencing might be more to do with post construction cleanliness than anything else so as soon as the new stuff arrives i will crack on with the rebuild and keep you posted!

Davebmx

Sun Mar 04 2012, 11:01PM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
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Posts: 40

ok i put together a new circuit on some fresh strip board. I tried to build it cleanly and thought i had done an ok job haha. well after i firing it up a few times i realised ihad missed some connections and also wiried one of the irfp250's the wrong way round. I thought i had it sorted but there was no voltage in the connections to resonant tank capacitor, i decided to try it at first with the just the very small induction coil and 3uf cap to see if it was a problem with my series circuit. anyhow i then switched my multimter down to millivolts and there was the power. I had been expecting though for the 12v dc going into the driver circuit to come out at 12v dc? then the circuit started smoking very quickly after that and after turning off the mosfet furthest away from the smoothing caps was red hot. im really quite stumped! the diodes are in a stupid postion becuase i had put in some massive 50a diodes when i started and thought they might be the problem so quickly swapped them out.

Davebmx

Mon Mar 05 2012, 12:13AM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
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Posts: 40

i quickly built the circuit below with all brand new spares i had and similar results. both mosfets got extremely hot around 10 volts and there was only 50 millivolts showing in the tank circuit. im wondering if its the mot. i wound it with a two pieces of enamelled 2mm solid core wire side by side. theres approx 20 windings so thats 10 windings per piece of wire. they are then soldered at both ends so i guess both pieces are in parallel. as im using a 1 amp variac i could still only see this producing around 20 amps though

Newton Brawn

Mon Mar 05 2012, 12:40AM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Hi Davebmx!

Do you mind to show the circuit diagram of your device ?

Thanks

Newton

Forty

Mon Mar 05 2012, 05:54AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

So brand new parts and it's still not working? I trust that you have it wired properly since you've redone it several times by now.
when you turn it on are you slowly ramping the voltage up from zero or just having it set for ten volts and then turning it on? Do the latter.
Also try starting it up with a piece of metal already in the work coil (should increase inductance, decrease frequency, and might make it start up easier. I'm not sure but it's worth a shot.
I'm sure your mot is fine because you said you were getting the desired voltage out of it.
I think the problem is that your fets aren't switching properly and one of them is getting stuck "on."

Davebmx

Mon Mar 05 2012, 10:34AM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
Location:
Posts: 40

Hi Newton i'm using the diagram for the modified mazilli driver found in the projects area of the site looks like this!

thanks for the reply Forty. I was wondering about ramping up the power because the diagram for the circuit says an input of 15v to 60v not 0v to 60v so that makes me think the circuit will need a minimum of 15v to start up? Do you think my heat sinks are too small also? When i was testing i had a little steel pin in the work coil so that there was a load but i might try a bolt instead. one or both of the mosfets appear to be turning all the power into heat. definately feel a little closer to getting it to work but no cigar yet! should the driver circuit deliver the exact same voltage out as it does in bar a few mv for conductance and heat loses?

many thanks!!

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