AN3009 datasheet(23/36 Pages) STMICROELECTRONICS

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AN3009

Designing a transition mode PFC

Doc ID 16032 Rev 4

23/36

The two bode plot charts refer to the PFC operating at 265 Vac and full load. In these

conditions, the crossover frequency is 11.55 Hz and the phase margin is 55 °C. The third

harmonic distortion introduced by the E/A 100 Hz residual ripple is below 3%.

Pin 4 (CS): this pin is the inverting input of the current sense comparator. The L6564

receives the information on the instantaneous inductor current through this pin, provided by

an external sense resistor (Rs) via an RC filter. As this signal crosses the threshold set by

the multiplier output, the PWM latch is reset and the power MOSFET is turned off. The

MOSFET stays in the off state until the PWM latch is set again by the ZCD signal. The pin is

equipped with 150 ns leading-edge blanking to improve noise immunity.

For a 100 W PFC, the sense resistor value (Rs) can be calculated as follows.

Equation 44

Where:

●

ILpk is the maximum peak current in the inductor, calculated as shown in

Equation 4.

●

Vcsmin equals 1.0 V and is the minimum voltage admitted on the L6564’s current

sense (as per the datasheet).

Because the internal current sense clamping sets the maximum current that can flow in the

inductor, the maximum peak of the inductor current is calculated considering the maximum

voltage Vcsmax admitted on the L6564 (as per the datasheet).

Equation 45

The calculated ILpkx is the value at which the boost inductor is not saturated and is used to

calculate the inductor’s number of turns and air gap length.

Figure 9.

Open-loop transfer function bode

plot

Figure 10.

Phase

min

Vcs

Ω

296

max

pkx

Vcs

ILpkx

Ω

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