AN3212 Datasheet, PDF (7/36 Pages) STMicroelectronics – The driving idea behind this application note is to exploit the possibility

AN3212
Circuit description
Care should be taken in designing the EMI filter due to the constraints indicated above. In
Section 4: Measurements two versions are presented, with their different responses.
The main drawback to this configuration is that it lacks a bulk capacitor which stores energy
on the primary side, and then the output current is affected by a high ripple, unless a large
electrolytic capacitor is used on the secondary side.
The second point is the true way to get a good power factor.
Referring to the VIPer17/27; Off-line high voltage converters datasheets, the cont pin is the
control that allows reducing the MOS peak current setting from the internally fixed point to
about 1/10 of that value. This can be accomplished by means of a resistor Rlim connected
between this pin and ground. Figure 4 and 5 represent the current ratio iDlim/(iDlim @ 100k)
as a function of Rlim. As can be seen, changing Rlim from 100 Kohm to a few kohms
progressively limits the corresponding MOSFET peak current.
Figure 4. ID_LIM vs R_LIM - VIPer17
Figure 5. ID_LIM vs R_LIM - VIPer27
An equivalent function can be implemented connecting the cont pin to a variable voltage
through a fixed resistor; in this way the peak current can be modulated simply varying the
control voltage: reducing the voltage, lowers the current.
Then, if the rectified mains voltage is scaled and applied to the cont input, the resulting
MOSFET peak current, and also the corresponding average input current, are shaped just
like Vin, obtaining the required high power factor.
The resistor array made up of R2, R6 and R13 implements this function, where R13 is the
lower practical value that fixes the minimum peak current, and R2 + R6 come out as a
consequence to guarantee a sufficient power transfer to the output (the lower the value, the
higher the output power).
On the other hand, to maintain a constant (average) output current, some kind of regulation
is required and for this reason, on the secondary side, there is an error amplifier which
senses the LED current and drives an optocoupler (see Section 2.2). On the primary section
the corresponding phototransistor is connected to the “FB” pin, and through this input the
voltage of the VIPer's PWM comparator is modulated.
In this way, the MOSFET peak current envelope follows the shape of Vin until it is somehow
limited by the clipping action of the feedback.
It is worth noting that the bandwidth of this loop must be very low, otherwise it would
counteract the Vin modulation.
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