Isolated buck H bridge control system design

Any references on how to go about modelling an transformer isolated H bridge buck converter in s or z domain?. I am planning to design a compensator for better closed loop response and stability. All the resources online point to modelling buck converter with a single switch and in non isolated configuration.

Currently for compensator design, I look out for the frequency region where the LC low filter cutoff happens and at that region, the phase response drops almost to -180 degrees. Then I would design a phase lead lag compensator with some phase being added in the frequency region where the phase response drops to -180 degrees. Then I use the same compensator in the SMPS design without taking into account other components present.

It would be great some of you guys can shed some light if approximation made is correct and also provide some resources to gain insight into modelling converters.

renga92 said:

H bridge buck converter

Click to expand...

Similar to a configuration that turns 170 VDC into 120 VAC. Often used in DC-to-AC power inverters.

The H-bridge creates a buck converter first in one direction, then a buck converter in the other direction. Sine-PWM is applied which varies output voltage.

LC low filter cutoff

Click to expand...

The L value should not be so great that it reduces your desired Ampere level to the load.
Select the C value so it combines with L to create a cutoff frequency 1.5 to 2 times your output frequency.

@BradtheRad : H bridge configuration of AC to DC converter converting 230VAC to 200VDC with 2A rating. Thanks for your practical inputs on how to go about choosing the LC values. We are designing a compensator to overcome the -180 phase shift introducted by the LC filter. Can we just increase the phase margin in the range of frequency where -180 phase shift occurs assuming that other components in the converter doesn't contribute much to the phase change ?. Is it this assumption valid to simplify the compensator design or is it absolutely necessary to factor all the components by creating a model of whole converter ?.

While you're at this experimental stage is will help if you perform 'studies' of the LC filter by itself, in order to observe its effects. Try various LC ratios, various frequencies, various load Amperes, etc.

Eventually you'll want to observe the overall combination of all sections, in case there are additional causes of phase shift. These might serve to reduce the phase shift you are concerned about already, or they might add a further phase shift.

You also may find a particular combination of transformer specs that produces a waveform that does not need so much filtering as you originally expected.