primary/secondary interaction

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Jason Judd" <JJudd@xxxxxxxxxxxxxxxx>

Hello list,



during the last year or so that I have been working on DRSSTCs I have 
noticed that the secondary coil seems to pull the primary frequency around 
quite a bit. With a VCO driving the inverter this frequency change causes 
non zero current or hard switching of the IGBTs. The amount of frequency 
shift seems to depend on the Primary impedance. My first DRSSTC primary had 
an impedance of around 2 Ohms and seemed not to drift around very much. My 
current primary is around 25 Ohms and is pulled all over the place.



I don't really have enough maths knowledge here to work out what is really 
happening so I am hopping someone can explain what is happening and what 
the relationships are so I can predict and simulate future behaviour and 
optimise primary and secondary design for stable operation.



Thanks,



Jason.