Re: Flyback charge pump for TC PSU

["Malcolm Watts" <M.J.Watts-at-massey.ac.nz> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi Chris,

On 26 Jul 00, at 12:17, Tesla list wrote:

> Original poster: "LOCKWOOD, Chris J." <clockwood-at-orbeng-dot-com.au> 
> 
> Hi,
> 
> Does anyone have any experience with flyback converters as the main PSU for
> small/medium power TC?

Yes - they're great. Obviously, coil output is going to be a 
function of the power the flyback can deliver. Flybacks in the 
TV/ignition coil range are great for small tabletop coils. For 
serious work have a look at the DC charger Marco Denicolai has 
built - not a flyback but a quasi-resonant topology.
 
> My initial impression is that this would be a fantastic way of delivering
> adjustable power to a coil, without the dramas of big heavy xfmrs /
> autoxfmrs... 
> 
> I built a coil using 3 car ignition coils, running off a 30V 15A PSU in
> flyback configuration- through a simple cct using IGBTs. The secondaries
> pumped up the tank cap through a series of 50 1kV 200ns diodes (resistored
> etc). This was decoupled through a ~10mH inductor, and 1nf ceramic cap stack
> to save the diodes (from excess dV/dT), and prevent power being pulled back
> out of the tank.
> 
> The idea was to get a higher peak power (for longer sparks?) with a lower
> rep rate by using a big cap.
> 
> The results aren't anywhere as impressive as I would have hoped. My tank cap
> is 100nF (quite big)- an oil-filled type scored from a university physics
> dept throw-away. I can only get 15-16" at around 20 zaps/sec, when the coil
> is tuned to ~150kHz (static gap, ~30cm diam primary, tapped at 4 turns
> (+-1/8 turn) for best results)...
> 
> I'm thinking that perhaps the cap is crap. After discharging from 10KV
> (shorted for a second) the voltage rises again, to approx 1kV. Is this
> normal for a HV oil cap???

It is a sign that the dielectric suffers from hysteresis 
losses and is not the best for TC applications. 

> Does the rep rate affect the spark length significantly?

Absolutely.

Regards,
malcolm