LM5156-Q1: LM5156-Q1 DCM calculation tool

Hi Mortiz,

I am revisiting for help.

I have run the Pspice for TI using flyback for 900V input using case. And found 2 abnormal waveforms. Can you kindly provide your professional help?

1, Why red box here, will have a sharp rising of output, before SS starts?

2, Why blue box here will have hiccup gate signals? The file I used is LM5156.

BRs,

Marsh

Hi Marsh,

Can you please provide the schematic you are simulating and the specs (Vin Vout Iout)?

Thanks and best regards

Moritz

Hi Moritz,

Thanks for the reply.

1, Here is the simulation sch.

2, Can you kindly explain the red box waveform ? Why before the device startup, the output will have a voltage = Vload(target)*SS/1. We are assuming that what we are seeing at the simulation(original post) is resulted from here. What's the root cause of this voltage? How to prevent this or make this voltage small?

BRs,

Marsh

Hi Marsh,

Can you please provide the specs for your simulation file?

What is the Vin, the Vout for the different rails and the Iout?

Maybe you can also provide your simulation file.

Regarding the softstart waveform, the picture in the datasheet is for boost topology. For a flyback this waveform would be different as the output voltage will start from 0.

The ramp in the red box comes from the fact that there is a natural pass through path from the input to the output because of the rectifier diode.

Best regards

Moritz

LM5157_PSPICE_TRANS.7z

Hi Moritz,

Thank you for the reply.

Please kindly find the attachments for calculator (design requirements) and simulation file. Please kindly help to modify the simulation file to make it match the calculator. Thanks.

Support need and issue description:

1, Abnormal output (out1) ramp. I have set IC of the caps to 0. But the ramp is still there. Please kindly check the root cause.

2, RCD snubber tuning. The overshoot are too large.

3, Most important, loop tuning. Seems sub-harmonic oscillation appeared after ss.

BRs,

Marsh