4 RHPZ effect and solution

The loop compensation for charge-mode control is simple when the PFC operates in DCM. Loop compensation becomes a challenge, however, because a right-half-plane zero (RHPZ) appears in the control loop when the boost converter operates in CCM [3]. The RHPZ induces a phase drop that negatively impacts the potential phase margin of the control loop. Equation 7 expresses the small-signal model for the control loop as:

where R_LOAD is the output load of PFC, D is the pulse-width-modulation duty cycle, ${\mathrm{\omega }}_0=\mathrm{ }\frac{{\mathrm{V}}_{\mathrm{O}\mathrm{U}\mathrm{T}}\mathrm{T}\left(1-\mathrm{D}\right)}{\mathrm{s}\mathrm{L}\mathrm{C}}$ and ${\mathrm{\omega }}_{\mathrm{z}}=\mathrm{ }\frac{{\mathrm{R}}_{\mathrm{L}\mathrm{O}\mathrm{A}\mathrm{D}}\mathrm{T}{\left(1-\mathrm{D}\right)}^2}{\mathrm{L}}$ .

Equation 7 clearly shows the RHPZ ω_Z. Its frequency varies with load, boost inductance and D (D varies with the input and output voltage), which makes loop compensation very difficult.

To eliminate the RHPZ, Equation 8 modifies the feedback signal:

Figure 7 modifies the control law, where you can see that I_REF is now modulated by V_IN, not by V_IN².

With this modification, Equation 9 expresses the small-signal model of the control loop as:

The RHPZ disappears and the system becomes a first-order system, which is very easy to compensate.

Figure 8 illustrates the verification of the new control algorithm through simulation, achieving a sinusoidal input current waveform.