Elimination of the positive zero in fixed frequency boost and flyback converters

It is shown how a fixed-frequency, leading-edge modulated pulse width modulation can eliminate the undesirable positive zero in practical boost and flyback converters. This allows a substantial improvement in the closed-loop characteristics. Several techniques are employed to predict this result. The design procedure for elimination of the positive zero is presented. Experimental verification is provided. By providing an effective feedforward of the load current, this technique ensures excellent load transient response and low output impedance. There are several advantages to this technique. Measurement of the control-to-output transfer function cannot be easily performed. Also, the positive zero elimination is dependent on the capacitor ESR value. This technique can be susceptible to noise since the output voltage switching ripple is not filtered in the compensation network.<>