Problem with output with fullbridge sstc

[Benjamin_collier]

Hello all,

I recently finished a build for a full bridge sstc. The design is pretty standard using two gdt's and secondary current feedback as well as a 555 interupter with adjustable duty cycle and frequency. Everything works fine until I start ramping up the input voltage. Obviously I expect the output to increase as I increase the input voltage and it does until I get to around 18V DC, then the output just stops increasing, even if I keep increasing the input voltage. I have increased the input up to 70V DC and still the same output as with 17V DC, maybe even a bit smaller. I don't really know why this would be happening, maybe it has something to do with the coupling of the primary and secondary? I have about 5 turns for the primary and maybe 1300 for the secondary. Any ideas what the issue may be?

[NyaaX_X]

Maybe take some photo of your circuit and post them for troubleshooting?

[Benjamin_collier]

Maybe take some photo of your circuit and post them for troubleshooting?

Yes you're right this was maybe not the best post hence no replies lol. I have since figured out the issue though. My winding ratio for my gdt's was not good -  it was about 1:1 - so I think my mosfets were going into saturation too quickly as Vgs was too low so they were not able to sink anymore current even though I was changing Vds. I have since rewound my GDT's to a 1:2 ratio and it works much better now when the gate drivers dont break..

[NyaaX_X]

That is good. But did you check the 1:1GDT waveform? The voltage amplitude was not enough to fully turn on the MOSFET? I think most of MOSFETs can accept 5V turn on signal and fully turn on at 10V. In the same time the waveform needs to switch rapidly and clearly if the amplitudes are not that high (up to 10V) to avoid much of the switching loss of mosfet.

[Benjamin_collier]

Waveforms look good now. The voltage was enough to turn the mosfet on fully however the Vgs was not large enough allow the mosfets to support the current demanded. Recall Id = unCoxW/L * (1/2) * (Vgs -Vth)^2 for a mosfet in saturation, therefore the current is directly related the voltage applied to the gate so when I increased the voltage the mosfets were able to sink more current.