AN3027 datasheet(24/41 Pages) STMICROELECTRONICS

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Designing a TM PFC

AN3027

24/41

Doc ID 16134 Rev 4

the design. For example, setting the gain of the error amplifier to get fc = 20 Hz at 264 Vac

means having fc = 4 Hz at 88 Vac, resulting in a sluggish control dynamics. Additionally, the

slow control loop causes large transient current flow during rapid line or load changes that

are limited by the dynamics of the multiplier output. This limit is considered when selecting

the sense resistor to let the full load power pass under minimum line voltage conditions, with

some margin. But a fixed current limit allows excessive power input at high line, whereas a

fixed power limit requires the current limit to vary inversely with the line voltage. Voltage

feed-forward can compensate for the gain variation with the line voltage and allow

overcoming all of the above-mentioned issues. It consists of deriving a voltage proportional

to the input RMS voltage, feeding this voltage into a square/divider circuit (1/V2 corrector)

and providing the resulting signal to the multiplier that generates the current reference for

the inner current control loop (

Figure 13). In this way a change of the line voltage causes an

inversely proportional change of the half sine amplitude at the output of the multiplier (if the

line voltage doubles, the amplitude of the multiplier output is halved and vice-versa) so that

the current reference is adapted to the new operating conditions with (ideally) no need for

invoking the slow dynamics of the error amplifier. Additionally, the loop gain is constant

throughout the input voltage range, which improves significantly dynamic behavior at low

line and simplifies loop design. Actually, deriving a voltage proportional to the RMS line

voltage implies a form of integration, which has its own time constant. If it is too small the

voltage generated is affected by a considerable amount of ripple at twice the mains

frequency that causes distortion of the current reference (resulting in high THD and poor

PF); if it is too large there is a considerable delay in setting the right amount of feedforward,

resulting in excessive overshoot and undershoot of the pre-regulator's output voltage in

response to large line voltage changes. Clearly a trade-off is required. The device

implements voltage feed-forward with a technique that makes use of just two external parts

and that limits the feed-forward time constant trade-off issue to only one direction.

Figure 13.

Mains detector and discharge resistor allow fast response to sudden line

drops not depending on the external RC

A capacitor CFF and a resistor RFF, both connected from the VFF (pin #5) pin to ground,

complete an internal peak-holding circuit that provides a DC voltage equal to the peak of the

rectified sine wave applied on pin MULT (pin #3). In case the VFF pin is connected directly to

the RUN pin, the following values are suggested as a compromise between the response

time in case of mains transients and input current distortion:

(16)

CFF

Ω

= M

RFF

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