Stability analysis for Flyback Power Supply

[chaitanyab]

Hi,

I am designing a Flyback converter. I want to understand the inference of stability analysis related to power supply.
1.What is the meaning of a stable system?
2.Is it for a certain load step it won't oscillate? how do we get that step frequency from Bode plots?
3.Is crossover frequency is the maximum step load frequency we can apply?
4.Sufficient Gain and phase margin, how to relate them in power supply terminologies?
5.How do we relate or get the Line and Load regulation from the Bode plots of Gain and Phase?

Regards,

Chaitanya

 

[D.A.(Tony)Stewart]

Hi,

I am designing a Flyback converter. I want to understand the inference of stability analysis related to power supply.
1.What is the meaning of a stable system?

Stable means a step load won't cause lots of ringing or oscillation from a non-linear or capacitive load or any desired load

2.Is it for a certain load step it won't oscillate? how do we get that step frequency from Bode plots?

Phase margin of 60 degrees or 20 dB Gain margin implies good dampening. 0 margin implies oscillation or positive feedback of at least unity gain and 180 phase shift from inverter.

Minimum tolerable is usually 30 degrees and 10 dB if gain does not drift. ( ballpark)

3.Is crossover frequency is the maximum step load frequency we can apply?

Step Ringing occurs at 0dB gain transition which is maximum bandwidth at unity gain.

4.Sufficient Gain and phase margin, how to relate them in power supply terminologies?

Power Supplies often have conditions for stable operation such as minimum range of ESR caps and complex feedback for PID loop or simple current feedback ( 1st order) Depending on the application, some require a minimum pre-load to improve stability.

Specs I suggested above might be a starting place but you will find margins change widely depending on step size (Before after) and direction of step. Often good Mfg of power supplies will give specs for 50~100 step noise or some variation of this. This obvious adders to error voltage with ripple, temperature drift and is usually lumped under LOAD REGULATION Specs.

5.How do we relate or get the Line and Load regulation from the Bode plots of Gain and Phase?

The Zero crossing for each determines margin but does not determine line or load regulation.

Load regulation is basically ratio of source to load impedance when the load is linear and unless specified does not include ringing from step. This a 10V supply with 10 milliohm source and 10A load or 1 Ohm will result in 1% load regulation (0.01/1)

Bode Analysis is best tool for improving SMPS stability by design of compensation feedback.

Tony Stewart