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MKP-MKT capacitors

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Steve Conner

Tue Jun 19 2012, 10:14AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Tank capacitor can refer to any capacitor in a resonant circuit. Any resonant circuit can be called a tank circuit.

The metaphor is on two levels: the resonant circuit is a sort of container for storing energy, and the first radio transmitters used tuning capacitors made from metal plates immersed in an oil tank.

This is not a DRSSTC, the capacitors are just DC block caps, so MKT should be all right. For the tank capacitor in a DRSSTC, polypropylene film/foil (FKP) is strongly recommended. But since the peak current in a DRSSTC is accurately controlled, you can get away with MKP if you know what you're doing.

brandon3055

Tue Jun 19 2012, 01:51PM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

zzz_julian_zzz wrote ...

Are you going to build an SSTC or DRSSTC? I dont think that the capacitor on the primary side is there for resonance tuning of the system, so you probably go for an SSTC.?. (also, the guide you posted is an SSTC but with Capacitor on prim) MKP stands for metallized polypropelene film, MKT is for metallized polyester film. Now, for your application with high current flowing across caps, you might wanna a higher current handling capability capacitors(MKP) , polyester can work also, in performance output they not too much differential, but the polyester will suffer more because they are not normally designed for higher current pulsed apps like DRSSTC. -since its an SSTC, the current is not that high(400+ Amps), the current depends on the Fres, and ON-time from the interrupter.

I personally think you can make it with MKT(with right capacitance value), but you have to at least measure the current on the prim which flows on ur caps, and then check on the Datasheet if it's covered on the rating and also check for thermal rise.

EDIT: Also, i re-checked the bridge schematics and there seem to be a lacking component on it which is very very critical (especially when it is ur first time to build) The snubber capacitor. This capacitor catches the transient voltages made form the inductance on your layout on the bridge,, You can have 5 uf /400V MKT connected on the + and - of the C7 on this schematic

But put it as close as you can on the Irfp460 mosfets. You might want to also add a little more resistance on the gate of the mosfets, to 10 ohms, this will add clearance from the OFF time form the previous alter pulse from other side mosfet pair before the other pair turns On and conduct. THanks.

@ Unleashed, What do you mean by not tanking and only for resonant caps? THanks

I noticed that circuit dos not use schottky's (mine dose) how important are they for irfp460's ?

Ben Solon

Tue Jun 19 2012, 08:33PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

Ash Small wrote ...

It looks to me like the 0.68uF caps C8 and C9 are 'series resonant' caps. 'Parallel resonant' caps are 'tank caps' (as far as I'm aware), because, with the inductor (primary) they form a 'Tesla tank circuit'. If I'm not mistaken, the current is much higher in a tank circuit (parallel resonant) than in a series resonant circuit, because the voltage is higher (the voltage rises in a tank circuit).

(Someone please correct me if I've made a mistake)

series and parallel resonant circuits are just about the same thing, just energy is added to the tank in different ways.

in a parallel tank, the cap and inductor are directly in parallel, and input current is from elswhere. all the high resonant current is directly in between the two.

but in a series tank, the resonant current has to travel through the switching device.

in the diagrams below, you can see that in the parallel tank, the current input is from the sine source, but resonance is between the reactive components. but in the series tank, the current is also supplied by the sine wave, but the high resonant current is passed through the sine wave source.

i have done a horrible job explaining this, but you should get the picture. read up, there are better sources of info than me. and i think it would be easyer to understand if applied to something more basic: the topologies of induction heaters- lclr and series resonant.

Ash Small

Tue Jun 19 2012, 11:51PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Yes Ben, I'm aware of the differences.

I've only ever come accross the parallel circuit being called a tank circuit in the past.

The point I was making is that the current 'can' rise much more in a parallel tank cuicuit that a series circuit, because it 'sloshes' backwards and forwards, while more current is added (given the correct conditions).

This is the principle upon which SGTC circuits, etc. work. (with 'loose coupling' between primary and secondary.

This circuit has what others have described as a 'DC blocking capacitor', which could also be described as a 'series resonant capacitor'. The current will not be as high as it would if it was a parallel circuit, given the right conditions.

The voltage also rises in a parallel resonant circuit much more than in a series resonant circuit, as far as I'm aware. (Ohm's law says the current can't rise unless the voltage also rises).

This is what I always considered a 'Tesla tank circuit' to be, but I stand corrected.

(I understood the parallel tank circuit ten years ago, or more, but I only really understood the series resonant circuit over the past year or so, since I became a 'regular' on this forum.)

I do accept, however, that, due to the inductor, the cap in a series resonant circuit 'can' charge to a higher voltage than it would without the inductor being present.

(I've probably not done a much better job of explaining this than you have

)

Ben Solon

Wed Jun 20 2012, 03:01AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

ok, yea, i just tend to call any inductor and capacitor operating together at a resonant frequency a "tank". and i dont think its an easy concept to explain XD.

anyways, just because it has a series cap doesn't make it a series resonant circuit. it also has to be operating on that special frequency. the purpose of a non resonant series capacitor in this case is to literally to block dc current. if you get an enormous on time on one leg of the bridge for some reason, only so much current will be pulled through the primary until the cap charges up to its fullest and no more current can flow. if you are familiar with the term "flux walking" which i just discovered days ago reading push-pull smps threads on audio forums. basically, if the the push is slightly greater than the pull or vise-versa, the avarege energy passed through the circuit sums up as a non 0 number, giving a dc value.

Ash Small

Wed Jun 20 2012, 02:04PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

ben123324 wrote ...

ok, yea, i just tend to call any inductor and capacitor operating together at a resonant frequency a "tank". and i dont think its an easy concept to explain XD.

anyways, just because it has a series cap doesn't make it a series resonant circuit. it also has to be operating on that special frequency. the purpose of a non resonant series capacitor in this case is to literally to block dc current. if you get an enormous on time on one leg of the bridge for some reason, only so much current will be pulled through the primary until the cap charges up to its fullest and no more current can flow. if you are familiar with the term "flux walking" which i just discovered days ago reading push-pull smps threads on audio forums. basically, if the the push is slightly greater than the pull or vise-versa, the avarege energy passed through the circuit sums up as a non 0 number, giving a dc value.

I agree, Ben, but surely the cap value is chosen to allow the inductor to operate efficiently at the specified frequency? Surely the most efficient cap value would be the 'resonant value'?

Maybe someone with more experience in this subject than you or I could clarify this, as this still puzzles me a bit?

I can see, however, that if the frequency is not constant, that the capacitor would no longer be resonant.

Ben Solon

Wed Jun 20 2012, 05:45PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

as you point out: sstc locks its primary frequency to the resonant frequency of its secondary, so yes, it changes.

and it would have to be a value that would NOT resonate, because while resonance is efficient, you don't want that kind of current in a comparatively small sstc. as long as the cap isn't dampening the switching, it will work. and that is where i stop, because that'sm what confuses me: why an lc circuit operating at its non resonant frequency wont actually dampen switching efforts.

Steve Conner

Wed Jun 20 2012, 08:48PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

If the capacitor is much larger than the resonant value, it looks like a short circuit at the resonant frequency. In other words, a sufficiently large capacitor won't "dampen the switching": it'll behave the same as if there was no capacitor, except for the desired DC blocking effect.

Ash Small

Wed Jun 20 2012, 10:25PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve Conner wrote ...

If the capacitor is much larger than the resonant value, it looks like a short circuit at the resonant frequency. In other words, a sufficiently large capacitor won't "dampen the switching": it'll behave the same as if there was no capacitor, except for the desired DC blocking effect.

The DC blocking effect prevents 'flux walking'.....Is that all it does?

Or does it also control things getting 'out of hand' when the (presumably) frequency drops during a ground strike, or whatever?

brandon3055

Thu Jun 21 2012, 01:07AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

I would emagin it would also lower the primary current

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