Whether the gain margin of the DCDC power system should be negative or positive？

Hello,

The concept of gain margin and phase margin have to do with feedback and stability. 

If your loop is 180 degree out of phase, your feedback network will go from negative feedback to positive feedback and the design will be unstable and oscillate. This is equivalent to swapping the polarities of the operational amplifier in a DC stable system.  The system just loses control.

To prevent loop instability most designers will try to get the phase margin greater than 45 degrees. This is to keep the design away from the 180 degree points where the loop is unstable.

I have seen PFC loops that have a phase margin of 25 to 30 degrees that do not oscillate.  However, the closer the control loops phase margin gets to 0 degrees the more oscillatory the circuit becomes.  You actually can verify this with a bench setup.  Adjust the loop to 60 degrees, 45 degrees, and 20 degrees.  You run a load transient test on the output of 25% and different phase margins and observe the behavior.  During the load step the output's ringing during the transient will change depending on the phase margin.

When it comes to gain margin this is when your loop gain is 180 degrees out of phase.  This is unstable so you want your gain as low as possible, so any large signal transients will be attenuated and not amplified.  This is why the loop gain should be less than -2 to 10 dB to ensure transients at this frequency are not amplified but are attenuated.

Please note gain margin and phase margin are generally chosen by the power supply designer and/or company they work for.  This is to increase reliability of the design and avoid oscillatory behaivor.  So these specifications can vary from designer to designer, and company to company.

I hope this help resolved your question.  If it did, please hit resolved at the end of the thread.

Regards,

Hello,

To you ensure that you do not have instability your loop should have an attenuated gain at 180 degree phase shift.  This should be a negative loop gain and not positive.

Generally when talking about gain margin engineers generally talk in magnitude and do not pay attention to the negative sign.

In regards to the LM5155.  If you are having an issue with this device, please repost in the e2e with the LM5155 in the thread title.  This is so the appropriate applications engineer can help you with your design.

Regards,