About DCDC buck Compensation Types

GARAM AN

Intellectual 1990 points

Hello,

I'm looking buck compensation, and each of control types(current, voltage) has their own compensation type.

For example, Voltage mode is usually using type III, Current mode is usually using type II.

I could find the difference between type II and type III(difference of number of poles), and want to know type II is used for current mode and type III is used for voltage mode.

Thank you

over 5 years ago

0 Andy Chen over 5 years ago

TI__Mastermind 34785 points

Hi Garam,

Pls specify what's your concern.

Voltage mode has double poles formed by L/C, Type III compensation is more commonly used in voltage mode as it has two zero to cancel the double pole, but if output capacitor has large ESR and the ESR zero is close to the pole freq, then TYPE II also can be used in voltage mode.

B R

Andy

0 GARAM AN over 5 years ago in reply to Andy Chen

TI__Intellectual 1990 points

Thank you Andy,

I perfectly answered my question.

How much is the large enough ESR to be used as an ESR zero?

Is this just theoretical or do you have real case?

If you have seen real case, can you tell me whether they use tantal or ceramic, how much capacitance and ESR?

Thank you!!

0 Andy Chen over 5 years ago in reply to GARAM AN

TI__Mastermind 34785 points

Hi Garam,

The output capacitor ESR zero freq is Fz_esr=1/(2*Pi*ESR*Cout), Fp is the double pole freq formed by inductor and Cout, TypeII is ok to use in voltage mode if the Fz_esr <2*Fp.

For example:

L=10uH, Cout=100uF, ESR=150 mohm.

Fp=5.04KHz, Fz_esr=10.62KHz.

Type II compensation should be Ok to use in this case.

B R

Andy

0 GARAM AN over 5 years ago in reply to Andy Chen

TI__Intellectual 1990 points

Thank you!

On the picture, the phase is positive 95 degree at 0dB.

The phase margin is the difference between -180 and the phase at 0dB.

On this picture, is the phase margin -275(-180-95) degree?

0 Andy Chen over 5 years ago in reply to GARAM AN

TI__Mastermind 34785 points

Hi Garam,

Phase margin is normally the loop phase at gain cross 0dB, on above curve, the phase margin is 95 degree at gain cross zero at 3.6KHz.

B R

Andy

0 GARAM AN over 5 years ago in reply to Andy Chen

TI__Intellectual 1990 points

Hi Andy,

On the picture, phase is 95 degree and cross frequency is 280 Hz when gain is 0dB.

Fc=280Hz, correct?

Phase Margin = (0 - 95) or (-180 - 95) ?

0 Andy Chen over 5 years ago in reply to GARAM AN

TI__Mastermind 34785 points

Hi Garam,

Sorry, the Fc is 280Hz as you point out.

The phase at 50Hz should be near -90 degree for -20dB/Oct gain roll down, consider -180 degree phase inverting by error amplifier. then total phase shift is -90-180=-270 degree, which is same thing as 360-270=90.

B R

Andy

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About DCDC buck Compensation Types