Guru
Joined: 09/08/2007
Location: AustraliaPosts: 4406
| Posted: 09:39am 05 May 2017 |
There will also be a constant added loss due to magnetizing and eddy current losses in the transformer toroidal core. That could be quite substantial, probably accounting for most of the missing 22 watts of loss. That should also show up as as parasitic idling power under no load.
The peak to peak PWM current will always be inversely proportional to the inductance of the primary choke, but this inductance is not going to stay constant.
The ferrite will very quickly saturate at only a very few amps. It should work well to reduce mosfet switching losses under no load idling conditions, but will do nothing at all to reduce the PWM current peaks at full output power.
A tape wound toroidal choke will take about four times the magnetic flux as ferrite before it saturates, and as it probably also has a much larger cross sectional area than the ferrite E cores, it adds even more to that. But it too will eventually saturate.
The way to do this is to use a laminated steel core, but with a large air gap. A grid tie choke will give us exactly that, a pair of C cores with an introduced air gap between the two halves. But do not expect a choke out of a 1.5 Kw grid tie to work in an 8Kw inverter at full flat out power. We are going to either have to string several re wound grid tie chokes together, or go to something similar but larger.
There is also no reason why we cannot use both a small ferrite choke in series with a much larger steel cored choke, although if the steel choke is well up to the job, it will probably be found to be all you need.
Conclusion.
Adding a choke to the primary is a very good way to reduce peak PWM current which will both reduce mosfet switching losses and improve reliability by reducing the stresses on the mosfets.
Any choke will eventually saturate if the ampere turns are too high for it.
More turns creates more inductance, but lowers the saturation threshold.
Once it has saturated, the inductance effectively falls to near zero.
Definitely not what you want to happen at peak output power.
There will be an optimum number of turns which gives the highest inductance, where it still has some safe margin below final saturation.
Cheers, Tony.