LM5185-Q1: Vout is dropping at intervals with load and without load.

Hi Mohammed,

Thank you for using the LM5185.  Did you have a dummy load?  Could you share some waveforms so I can fully understand your issue?  Please label the traces and also indicate the operating conditions.

By the way, I did not see much problem in your calculator, except for the loop compensation, which can be improved for better phase margin.  

Thanks,

Youhao

Hi Youhai,

I was not able to paste images for some reason, I have edited the previous message. Thanks.

Can you tell me which trace is what signal?

Yellow: Supply current 

Blue : output voltage 

Red: transformer primary current 

What is your load condition?  Do you have a load transient that triggered the Vout drop, input and switch current rises?

Initially have disconnected all loads. It's only output.

Whatever the 0.4A load was just a resistive load. 

With load or without load, that dip in voltage is present. Initially it was a huge dip, then I tried tuning current sense resistor, and mainly it's RC filter values, then it came down to this level. 

What is dropping the voltage is a question.

In the calculator you chose the current sense resistor to be 10mOhm, however, you installed 40mOhm, why?  Could be change the CS resistor?

I am still trying to figure out why the circuit suddenly enter into such transient, if your load is steady (no load is also a steady load).  Something wrong but hidden.  

Can you monitor the following signals on the same scope picture?

• EN, VIN, COMP, SW

Thanks,

Youhao

My mistake, I had something connected to load which was causing trouble, isolated the circuit. It works now. The output ripple is very large. Any suggestions to mitigate ??

Glad to hear the updates. 

Did you measure the Vout ripple with a short ground lead probe like the one below?  I am afraid the large ripple may be unrealistic due to the use of long ground lead of the regular probe.

Thanks,

Youhao

Thanks for the assistance. 

Changed rsense to 25mOhm

Updated compensation resistor to 130K. 

Below waveforms is captured in AC coupling, ripple is around 400mV.

Is this acceptable waveform ?? What more can be done ?

Sorry for the delay due to public holidays. You scope does not seem having a stable triggering point.  Can you just do a "single" event trigger?  Also, please let me know if you used the short ground lead probe, and where did you placed it to take the measurement. Honestly, I do expect to see more rounded ripples without so many rigid turning points. 

Thanks,

Youhao

1. Thanks for the reply. Sure I will capture the waveform in single trigger and share again. Yes I did use short ground lead. With load of 50ohms resistive and some regulators down the line. Since this is power supply going to other supplies, gate driver supplies, let me know if it's ok to proceed?

2. Additionally if you can quickly advise on what more can be done to make it look more round ?

3. As  I'm working on a power board, I do not have much time to spare on this now, When the first protoboard is ready,  I can revisit this section. Thanks. 

Thank you Mohammed.  I am looking forward to your new scope pictures.  You don't need to respond to this msg.  Just get back to post the scope pictures when you have them, then I will be notified very soon.  

Good luck!