RHP Zero in Boost Converter

[Abhijeet rajput]

I am working on a Boost , Voltage Mode Control.
Vin : 1.8V +/- 10%
Vout : 3.3V
ILoad Max = 300mA.

For AC Analysis I am using average loop modelling for switches.
With this model I am getting a WC phase margin of 27degrees.

But I do not see any RHP Zero which calculation shows to be at Frhp = 1/2pi (R*(1-D)^2)/L.

I am concerned about this since in transient analysis I do see some stability analysis at higher load.

Someone please let me know how to model RHP Zero for AC analysis.

Thanks in advance.

Regards,
Abhijeet

 

[E-design]

You can do a test by sweeping the duty ratio, first slowly and then increase the rate. If you notice a point where Vout and Iout drop with increasing demand, this will suggest RHPZ coming into play. Current building up in the inductor is too slow to cope with the increased demand. To prevent this, the slew rate of the duty ratio control should be prevented from changing faster than the inductor volt-seconds. Roll off your fc well below where the worst-case RHPZ will occur.

 

[Abhijeet rajput]

You can do a test by sweeping the duty ratio, first slowly and then increase the rate. If you notice a point where Vout and Iout drop with increasing demand, this will suggest RHPZ coming into play. Current building up in the inductor is too slow to cope with the increased demand. To prevent this, the slew rate of the duty ratio control should be prevented from changing faster than the inductor volt-seconds. Roll off your fc well below where the worst-case RHPZ will occur.

Thanks for your reply.
I understand the transient phenomenon and its result in terms of RHPZ coming into picture.
Issue I am facing is in AC Analysis. I wanted to have an RHPZ in my AC response so that I can take care of its stability. Otherwise I will have to presume that, we will have an RHPZ and then will have to go ahead with compensation... which will be difficult.

Regards,
Abhijeet