AN2835

Experimental results

Doc ID 15073 Rev 1

17/21

From Section 2.3, we know that high voltage pulse generates continuously before it steps

into steady-state. Once the HID lamp is in open-circuit or absent from the system board,

there is no chance to step into steady-state. In such a condition, the ignition circuit is not

only continuously generating high pulse voltage, but also the full bridge circuit is working at

low frequency (about 200 Hz) as the output of U2 stays high before pin 4 of U2 detects a

current signal.

To avoid a hazard from 3~5 kV on the system board while the HID lamp is in open-circuit or

the HID lamp is absent from the system, the building of a timer to abort this high voltage

pulse generation may be important.

If it is necessary to abort the high voltage pulse generation, a NE555 timer (see Figure 20)

is used to set up a limited time (normally 5 minutes) to turn-off the circuit at the scheduled

time, or apply an MCU in digital solutions. The microcontroller gives more flexible and

precise time control compared to one with a simple hardware solution.

Figure 20.

The timer circuit

Table 3.

Test results of power factor and efficiency

Conditions

Pin

Pout

Efficiency

(Pout/Pin)

PF

Watts

Watts

%

∼

At 110 Vac

77.8

68.9

88.5

0.99

At 220 Vac

76.9

68.9

89.6

0.98