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Power management forum

hou.j
Prodigy 30 points
Part Number: LM5025B

Dear！

I used LM5025B to design a forward circuit. The input voltage is 16-40V, and the output is 5V@16A. When the input is 16V, the output can only be loaded with 7A load. If it is larger, it will appear the phenomenon similar to overcurrent protection.If the TIME resistance to ground is changed from 47.5KΩ to 15KΩ, the output can be loaded with 14A when the input is 16V. In this case, I tested the gate waveforms of the switching tube and the clamping tube and the waveform of the RAMP pin (resistance is 100KΩ, capacitance is 330pF), and the duty ratio reached 67%. I wonder if this problem is related to the maximum duty ratio, and how to solve it without changing the turn ratio of the transformer.

Looking forward to your reply, thank you!

Looking forward to your reply, thank you!

  • 0
    TI__Mastermind 26295 points

    Hi,

    Thanks for connecting through E2E and I must say that your waveforms look very clean, so nice work on your PCB layout and capturing such helpful waveforms. I think your understanding is correct in that changing the Rset (TIME) resistor from 47.5kΩ to 15kΩ increased your max duty cycle clamp from ~69% to ~73%. This seemed to give the additional duty cycle needed at low line to be able to process full power. It still seems like you are at/near the max duty cycle limit and to assure you can reach full power over the entire input and temperature range, you need to verify your transformer is not going to saturate at such high duty cycle and when you are operating at higher temperature.

    Secondly, when you increased the max duty cycle clamp, you also decreased the deadtime from 158ns to 63.5ns and you should go back and see how this may have impacted the converters ability to achieve ZVS. For Rset=15kΩ, verify if there is still sufficient deadtime to allow the drain-to-source capacitance to fully resonate to zero. Minor adjustment to the transformer turns ratio might be helpful for reducing the max duty clamp and at the same time increasing the ZVS dead time. This is the skill needed to optimize the entire power stage for best performance, efficiency and ZVS over the widest line/load range possible.

    Also, please download and use the free, TI Power Stage Designer as another method for checking the transformer turns ratio in an active clamp forward converter.

    Regards,

    Steve