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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi all,
designing my first real DRSSTC. For the calculations of MMC I would like to know some real life values of measured peak primary tank circuit Q-factor WITHOUT OCD limiting (so "natural" Q). Am I right thinking it will be around 10? I want to operate the coil in the "natural Q" mode, with OC limiting only as a safety measure. When I design the circuits so that the maximum allowable Q will be 15, I guess it's fine, or is it actually overkill? The MMC turns out pretty large (22kVDC rated for 600Vbus fullbridge with 11.5kV peak on it).
Many of the well working coils, e.g. the Steves, Eric and Mads seem to have been designed around Q=10, judging from the max currents, bus voltages and primary specs. I'd advise that only for k>=0.2. If you want to be safe you might want to use 15, since you can always get primary current up by detuning.
I've once suggested using a large primary inductance to store up energy, which then is discharged into the secondary, when it gets into tune. This might get you the biggest arcs for a given set of IGBTs. The drawbacks are, that the sudden discharge might cause secondary flashovers, so decreasing the ratio of arc length to necessary coil size. Costs of the MMC are also higher due to their bigger voltage requirements.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Thanks a lot for the reply, Uspring. I guess i will design the coil for a maximum Q of 15 then, as I think I will be using coupling below 0.2.
This means the MMC turns out larger than what most people use (voltage-wise). Either a lot of coilers push the MMC really hard, or they rely on the OC limiting to trip the pulse; in this case the coil cannot operate optimally.
I've measured a Q of about 10 on my ~1kW coil _after_ the initial current peak. Q was quite a bit higher before it is pulled into tune. k is only about 0.14. I've tried to avoid the primary current peak by not tuning the primary too low. That reduced the current peak but then the primary current started to beat and spark output got less. The beating is still a mystery too me, since I would expect one of the modes to die out eventually under arc load.
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DRSSTC primary tank Q-factor
