AN2485

Test results and significant waveforms

11/29

typically quite good, and a larger offset at high line where the energy transfer gets worse.

In order to maximize the benefit from the THD optimizer circuit, the high-frequency filter

capacitors after the bridge rectifier should be minimized, compatible with EMI filtering

needs. A large capacitance in fact introduces a conduction dead-angle of the AC input

current in itself thus reducing the effectiveness of the optimizer circuit.

2.3

Voltage feedforward

The power stage gain of PFC pre-regulators varies with the square of the RMS input voltage

as well as the crossover frequency fc of the overall open-loop gain because the gain has a

single pole characteristic. This leads to large trade-offs in 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,

which results in 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. 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 Feedforward can compensate for the gain variation with the line voltage and

provides a solution to the above-mentioned issues. It consists of deriving a voltage

corrector) and providing the resulting signal to the multiplier that generates the current

reference for the inner current control loop. In this way a change of the line voltage will

cause an inversely proportional change of the half sine amplitude at the output of the

multiplier 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 will be constant throughout the input voltage range, which improves significantly

dynamic behavior at low line and simplifies loop design.

Figure 18.

EVAL6563-400W Inductor current

ripple envelope at 230 Vac - 50 Hz -

full load

Figure 19.

EVAL6563-400W Inductor current

ripple (detail) at 230 Vac - 50 Hz -

full load

CH1: Q1/Q2 Drain voltage

CH2: MULT voltage - pin #3

CH4: L4 inductor current ripple envelope

CH1: Q1/Q2 Drain voltage

CH2: MULT voltage - pin #3

CH4: L4 inductor current ripple envelope