I don't really know how to title this issue. Basically, I am designing a soft-switched zero-voltage transition boost converter, I will attach a basic schematic. When running the converter and assessing the waveforms, at steady-state, I achieve the response that I desire. However, at start-up, there is a huge in-rush of current, which peaks at 1600A or so. I will include an image of the waveform also.

I am unsure what the source of this large current is. The general idea of this ZVT converter is to have a small auxiliary circuit running with a small duty cycle (10% or so) before the main switch opens, so that a small current flows through the auxiliary diode, switch, and inductor to allow the main switch to softly commutate. When I change the output capacitor to lower values, the peak value tends to decrease - could this be intefering in some way and causing the peak?
I have another issue with my design. The main switch runs at a 40% duty cycle to get 400V from a 240V input, and the auxiliary switch runs at 5-10%. However, the current in the auxiliary circuit is not much smaller than the current flowing through the main diode - which obviously is the opposite of what I want with this topology. I thought it may be because the inductor value is too small, however I used the same values that were outlined in the paper in which I discovered this topology, and at the same frequency etc.

Any help would be appreciated for these two issues.

Please see attached photos.

Thanks!

UPDATE: added a zoomed in view of the MOSFET current upon start-up. As you can see the right shape is obtained but when the switch turns ON there is a huge spike in current. Further along the waveform when steady state is reached, this no longer happens.

 

Welcome to AAC!

For what duration. I often see up to 1000% of steady state current or more for the first 2-3 ms using LTspice. Others here can give some input on dampening it I'm sure.

 

Hi,

In another simulator: 0 Ohm source resistance is one cause of (what I suspect are often improbably) large start-up current spikes; and regarding capacitors charging at start-up - the larger the capacitor, the larger the spike.

You can test the theory by placing a series resistor in front of the capacitor(s) and see the new start-up current waveform, that or making the load resistance much larger just to troubleshoot what's causing it and eliminate possible suspects.

Anywhere to strategically place a resistor, or switch in a series resistor with the capacitor and then switch it out again, using two MOSFETs with alternate logic level high and low signals?

 

Hello Daniel, Sam,

Thanks for your help. I have non-idealized the source with a small R and L, and also placed some small resistors in series with the inductors (0.1) and capacitors (15e-3) to represent an ESR.

I have also placed a larger 10 ohm resistor (just a random value) in series with the resonant capacitor which I believe is causing the issue and the current spikes have reduced massively, peaking at 130A instead, this whole issue lasts around 0.01 seconds. At around 0.015 seconds the response is closer to what I am looking for. Maybe I will try switch a larger resistor in for the first 0.015 seconds then take it out?

Thanks guys,
J

 

Okay so I just placed a really large resistor in series with the resonant capacitor so that it doesn't have much current flow and the MOSFET still exhibits a weird peak response. The output capacitor also exhibits weird spikes in the first 0.015s then settles: