AN2242

2 Circuit Description

9/31

operation at max load and both nominal input mains voltage measured on current sense pin,

gate drive, comp and oscillator pin have been captured. It is possible to notice that the circuit

works in continuous over the entire input mains range.. Besides, the signal on pin #12 (Isen),

has a different offset and similar peak for the effect of R5 and R6. Their purpose is in fact to

compensate the over current set point variation with the input voltage, which instead has to be

almost constantover the entire input voltage mains range. Of course, the oscillator waveform

does not change in the two pictures.

In Figure 8. channel 4 shows the slope compensation signal, coming from pin #15. This pin

provides a voltage ramp during the MOSFET's ON-time, which is a repetition of the oscillator

saw tooth and is dedicated to implementing additive slope compensation on current sense.

This is needed to avoid the sub-harmonic oscillation that arises in all peak-current-mode-

controlled converters working in continuous conduction mode with a duty cycle close to or

exceeding 50%, as in this circuit. Besides, only a resistor is needed to implement slope

compensation, and since the voltage ramp is delivered during the ON-time only, the correct

operation of the control circuit is ensured even at light load. The slope compensation signal

delivered only during ON-time in fact prevents perturbations on the current sense due to the

injection of the residual part of the oscillator saw tooth that could affect the control circuit, as it

happens using the oscillator total saw tooth for slope compensation.

Figure 6.

Vin = 115 Vrms - 60Hz

Figure 7.

Vin = 220 Vrms - 50Hz