Device description

HVLED807PF

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Doc ID 023464 Rev 5

Equation 6

external parameters defining the output current are the transformer ratio n and the sense

The previous formula (Equation

6) is valid for both standard and high power factor

implementation.

4.6

Constant voltage operation

The IC is specifically designed to work in primary regulation and the output voltage is

sensed through a voltage partition of the auxiliary winding, just before the auxiliary rectifier

diode.

Figure 18 shows the internal schematic of the constant voltage mode and the external

connections.

Due to the parasitic wires resistance, the auxiliary voltage is representative of the output just

when the secondary current becomes zero. For this purpose, the signal on DMG pin is

sampled-and-held at the end of transformer's demagnetization to get an accurate image of

(2.51 V typ - see

Table 5: Electrical characteristics).

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 S/H

logic is able to discriminate such oscillations from the real transformer's demagnetization.

When the DMG logic detects the transformer's 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 an output current

proportional to the voltage unbalance of the two outputs: the output generates the control

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

cycle-by-cycle peak drain current.

The COMP pin is used for the frequency compensation: usually, an RC network, which

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

Equation 7

Section 4.7: Voltage feed-forward block ).