True Bridgeless PFC Converter Achieves Over 98% Efficiency, 0.999 Power Factor

Jul 1, 2010 12:00 PM
Dr. Slobodan Cuk, President, TESLAco, Irvine, CA

The resonant circuit for positive input voltage in Figure 10a had only one capacitor, resonant capacitor Cr. On the other hand, the resonant circuit for negative input voltage has two capacitors in series as in Figure 10b. However, because of the much larger output capacitor, both reduce effectively to the resonant circuit shown in Figure 10c. Moreover, due to the automatic changeover of the roles of the two current rectifiers from positive polarity input voltage to negative polarity input voltage, the resonant circuit of Figure 10c is applicable to the same ON-time interval for either positive or negative polarity of input voltage. The resonant circuit will therefore result in resonant capacitor voltage as in Figure 8b and in the resonant inductor current as in Figure 9a as was obtained before for positive input voltage.

The resonant inductor, Lr, must be once again fully flux-balanced during the same ON-time interval, DTS, only, which results from switched network in Figure 7c and circuit model in Figure 10c:

VCr = V (9)

as the resonant inductor cannot support any net DC during this ON-time interval.

Note that the steady state DC voltage on the resonant capacitor has changed substantially from VCr = 0 for positive input voltage to VCr = V for negative input voltage.

Replacing Equation (9) into Equation (8) we get the dc conversion ratio as:

V/Vg = 1/(1-D) (10)

which is the same as Equation (7) for positive input dc voltage.

Therefore, despite different dc voltages on the resonant capacitors for positive input voltage (zero dc voltage) and for negative input voltage (output dc voltage V), the dc conversion ratios are identical for positive and negative polarity input voltages as shown by Equations (7) and (10).

As before, the resonant discharge current ir of capacitor Cr is limited to only a positive cycle of resonant current as current rectifier CR2 now permits conduction in only one direction as in Figure 7c. As the resonant current starts at zero level, this effectively restricts the resonant discharge interval once again to exactly one-half of the resonant period, the same as before.

The waveforms over the complete period for resonant inductor current ir(t) and resonant capacitor voltage vCr(t) are then illustrated in Figure 9a and Figure 9b. Note how the continuity of the voltage on resonant capacitor results in the same ac ripple voltage Δvr at the transition between two intervals. Once again, the resonant capacitor dc voltage is not zero, but equal to output dc voltage V. Thus, the same dc conversion gain function is obtained despite drastically different steady-state values of dc voltage on capacitor Cr equal to zero for positive input, and equal to output dc voltage V for negative input.

The new PFC converter has an energy transferring capacitor, Cr, which during the OFF-time interval TOFF charges and at the same time passes the input charging current to the load. Then during the ON-time interval, TON, this capacitor forms a resonant circuit with the resonant inductor Lr and exchanges the energy stored in previous OFF-time interval with resonant inductor. The resonant inductor is much smaller than PWM inductor L. As a result, it stores much less inductive energy than the PWM inductor L. Nevertheless, the current direction in this resonant inductor changes from one direction in OFF-time interval to another direction in the ON-time interval. This change of the direction of resonant inductor current during the short transition could cause the voltage spikes on the switch S. The faster the change, the bigger the voltage spike would be. However, due to small energy stored in this small inductor, this spike can be effectively suppressed by use of Zener diodes.

RESONANT CIRCUIT ANALYSIS

As seen above, operation of the converter from positive input voltage and negative input voltage, results in the resonant circuit models, which can be both described by the two first order differential equations whose solutions are:

ir(t) = IP sin(ωrt) (11)

vCr (t) = Δvr cos(ωrt) (12)

Δvr = IP RN (13)

where:

RN = Natural resistance

fr = Resonant frequency

ωr = Radial frequency

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