AN3027 datasheet(21/41 Pages) STMICROELECTRONICS

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AN3027

Designing a TM PFC

Doc ID 16134 Rev 4

21/41

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

the crossover frequency is fc = 11.55 Hz, the phase margin is 55°. The third harmonic

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

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

pin, the L6563S reads the instantaneous inductor current, converted to a proportional

voltage by an external sense resistor (Rs). 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 an off-state until the PWM latch is set again by the ZCD signal. The pin is

equipped with 150 ns leading-edge blanking for improved noise immunity.

The sense resistor value (Rs) can be calculated as follows. For the 100 W PFC it is:

Equation 44

where:

●

ILpk is the maximum peak current in the inductor, calculated as described in Equation 4

●

Vcsmin = 1.0 V, and is the minimum voltage admitted on the L6563S current sense (in

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 L6563S (in the datasheet):

Equation 45

The calculated ILpkx is the threshold value after which the boost inductor saturates and it is

used for calculating the inductor number of turns and air gap length. In case of boost

inductor saturation, a second comparison level at 1.7 V detects the abnormal currents and,

on this occurrence, activates a safety procedure that temporarily stops the converter and

limits the stress of the power components.

Figure 9.

Open loop tran. function bode plot

Figure 10.

Phase function

min

Vcs

Ω

296

max

pkx

Vcs

ILpkx

Ω

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