The selected solution

AN2835

8/21

Doc ID 15073 Rev 1

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Constant power control circuit

In this solution, the voltage on pin 3 of U2 (L6562A) is fixed, therefore, during the warm-

up time, pin 2 of U2 is clamped at its upper threshold, so the input peak current

detected by pin 4 is also fixed. Once the lamp power increases with the lamp voltage

increase, pin 2 decreases accordingly, the lamp works at a constant power state.

Constant power control assures the output power is constant and stabilizes lamp

luminosity without flicker. The lamp operates at the best rated lamp power. Here is an

indirect method to perform the constant power control for the lamp. As shown in

Figure 6, an Rs resistor is connected between the PFC stage and the full bridge stage.

If the output voltage of the PFC stage is constant, it means the current of Rs is

constant. With the proper controlling of the average current flow through Rs, the current

sources from the boost converter (PFC stage) become constant, and the output power

in full bridge stage is also constant, assume the power losses of the dual-buck

converter (full bridge stage) is constant. Therefore the lamp power has constant control

indirectly.

Figure 6.

Indirect constant power control circuit

The average current sense circuit is shown as Figure 7. R1 and C3 is the filter used to

obtain the average voltage on Rs. The Vf feedback signal is generated to control the on-time

of the dual-buck converter. In practice, the operating lamp voltage and current change

according to the age of the lamp, but the change in power loss to the dual-buck converter is

very small and therefore negligible. This indirect method achieves good performances in a

low power application.