The selected solution

AN2835

6/21

Doc ID 15073 Rev 1

Figure 4.

The timing chart

Description of the four operating states

●

–

S3 and S4 in off-state, S2 and S5 are turned on, C1 discharges through S2, L2,

Lamp and S5. Certain energy stores to L2 and C2.

●

–

S3 and S4 remain in off-state. S2 is working in high frequency in off-state, and S5

is working in low frequency and remains in on-state. The energy of L2 and C2

keeps on releasing through Lamp, S5 and the reversed body diode of S4. As S2 is

working at a higher frequency, state 1 and state 2 is repetitive until the S5 is

turned-off at t3.

●

–

S2 and S5 in off-state, S3 and S4 are turned on, C1 discharges through S3, Lamp,

L2 and S4. Certain energy stores to L2 and C2.

●

–

S2 and S5 remain in off-state. S4 is working in high frequency in off-state, and S3

is working in low frequency and remains in on-state. The energy of L2 and C2

keeps on releasing through the reversed body diode of S2, then S3 and lamp. S4

is working at a high frequency from t3 to t6, therefore state 3 and state 4 is

repetitive until S3 is turned-off at t6. One full operating cycle is completed. Starting

from t6, the behavior of t0 is repeated again. From the above analysis, we realize

the lamp current flow to this dual-buck converter, and loads to the same L2

inductor, C2 output capacitor, and HID lamp. The lamp current at state 3 and state

4 is in the opposite direction.

2.2

The power control circuit

There are two main functions of the power control circuit. One is constant current control

during warm-up and the other is constant power control during steady-state operation.

●

Constant current control

Normally, the lamp current is higher during the warm-up stage than when working at

steady-state. But a warm-up current that is too high may cause the electrode to decay

and shorten the operating life of the lamp. If warm-up current is too low, the time to

steady-state is postponed. Therefore providing a value with 20% higher than the rate of

warm-up current during warm-up time is respected. The constant current control is