AN4350 datasheet(8/34 Pages) STMICROELECTRONICS

8 / 34 page

Circuit description and design guidelines

AN4350

8/34

DocID025181 Rev 1

2.3

Output voltage control - primary side regulation

The IC is specifically designed to work in a primary side regulation (PSR) and the output

voltage is sensed through a voltage partition of the auxiliary winding, as shown in Figure 3.

The signal on the DMG pin is sampled-and-held at the end of transformer demagnetization

to get an accurate image of the output voltage (VOUT) and it is compared with the internal

error amplifier reference voltage VREF (2.51 V typ.).

During the MOSFET's OFF-time the leakage inductance resonates with the drain

capacitance and a damped oscillation is superimposed on the reflected voltage. The internal

S/H logic is able to discriminate such oscillations from the real transformer demagnetization.

When the DMG logic detects the transformer demagnetization, the sampling process stops,

the information is frozen and compared with the error amplifier internal reference.

The internal error amplifier is a transconductance type and delivers on the COMP pin

a current proportional to the voltage unbalance of the two inputs: the COMP pin generates

the control voltage that is compared with the voltage across the sense resistor, thus

modulating the cycle-by-cycle peak drain current to regulate the desiderated output voltage.

Figure 3. Voltage control principle: internal schematic

The COMP pin is used for the frequency compensation: typically an RC - CC network, which

stabilizes the overall voltage control loop, is connected between this pin and ground.

When implementing the high power factor (HPF - see Section 2.4: High power factor

implementation), the output voltage control loop must have frequency bandwidth much

lower than the “control current loop” (100/120 Hz, that is the double of line input frequency).

As a consequence the output voltage control loop has to be designed in order to have

bandwidth much lower than 100/120 Hz (i.e. BWCV < 5 - 10 Hz).

AN4350 Datasheet(PDF) 8 Page - STMicroelectronics