[SOLVED] H bridge inverter temperature issues

[shazan]

Hi
I am in the process of completing a low frequency bi-directional H bridge Sine wave inverter/charger, one that works as a bridge-less boost in the charging mode, and as a standard H bridge in the inverter mode. Switching freq is 5KHz. I have a system working with three 55NF06 MOSFETs parallel in each leg of the bridge. No issues on the inverter side. The temperature goes up to 125C with large loads of up to 55 Amps, without a fan. At a load of 10Amps, the temp is constant at 40C.
No issues till now. Yes, there is a fan, its not on for this test.
In the charging mode, I am limiting the current to 10A. The temperature rises to 110C within about half hour, though the current is shared in the 3 switches, with heat sink.
I thought my switching wasn't taking place fast enough, but then I should have the same rate of rise in temp in the inverter mode also. I know since this is CC charging, the voltage needed to keep the charging current at 10A will require increased pulse widths, but at roughly 3amps per switch with the 55NF06 having a Rj-c of 1.36C per watt and Rds of 0E018, the calculated average I2R wattage per switch comes to 0.2watts, so where is the rise in temp coming from? what am I missing in the bridge-less boost for it to get so hot, so fast?
Points to note during the charging mode:
1. The charging does not follow a sine PWM, but is static at the required width (for 10A) for roughly 80% of the i/p sine wave, midway in the half cycle.
2. During charging, both the lower legs of the bridge are switched on simultaneously in both half cycles, rather than alternating between switching and utilising the body diode for alternate half cycles.
Your suggestions and advices are eagerly awaited.

 

[FvM]

For clarification, you should show a circuit diagarm and scheme of the boost charging mode.

As far as I understand, it uses transformer leakage inductance as boost inductor. And I fear, it might drive the transistors into avalanche breakdown, causing high losses.

 

[shazan]

For clarification, you should show a circuit diagarm and scheme of the boost charging mode.

As far as I understand, it uses transformer leakage inductance as boost inductor. And I fear, it might drive the transistors into avalanche breakdown, causing high losses.

Thank you FvM for your input. Please explain how the system will be driven into avalanche breakdown?
Here is the h bridge in both modes:


I am unable to show the PWM for the charging side, but it is a fixed width for 5 seconds, to keep the current at 10Amps into the battery. Since the switching frequency is 5KHz, the switches are kept on for the required pulse width from the 5th count of the 200uS interrupt after zero cross is detected to the 45th count. The current is measured every 2 seconds, and pulse width is varied based on value obtained after every 5 seconds.
On the inverter side, the SPWM is at 5KHz, with 50 steps per half cycle.

 

[FvM]

O.K., I was thinking of a push-pull inverter and overlooked the reference to H bridge. Inductive overvoltages should be safely clamped to battery.

It would be best to acquire voltage and current waveform to clarify how the losses are generated. The current during on-time must be much higher than the average charging current, I presume.

I would expect that the bootstrap mode uses short pulses (constant on time) over the full sine wave to get a halfway sinoidal input current.

 

[shazan]

Thank you FvM, your pointer showed me the way
Problem solved.