AN2747 datasheet(6/44 Pages) STMICROELECTRONICS

6 / 44 page

Lamp description

AN2747

6/44

Referring to Figure 3 we can see that HID lamps have higher luminous efficacy, but some

negative aspects must be considered.

When comparing HID and fluorescent lamps, two important issues should be considered: a

greater starting voltage and the presence of acoustic resonance.

To solve the issue of higher HID ignition voltage, a sort of starting aid, called igniters, are

used to ignite the lamp.

The problem of acoustic resonance is more complex. Acoustic resonance is characteristic of

HID lamps operating at frequencies greater than 1 kHz and appears when lamp power

fluctuation exceeds a threshold value.

The current/voltage lamp waveforms and the operating frequency cannot be chosen freely,

but they are dependent on lamp type, condition and temperature. A wrong choice of

frequency and/or waveform can have a very negative effect on lamp performance and/or

lifetime and sometimes the discharge tube may be mechanically damaged.

As shown in Figure 4 and 5 for different lamps the free bands are completely different.

●

For HID lamps the solution proposed to avoid acoustic resonance uses the same

principle. The designer must avoid a combination of power fluctuation and operating

frequency . Another parameter that must be taken into account is flickering. For this

reason, the frequency used in the application is higher than 100 Hz and below 1 kHz.

For sinusoidal waveforms the instant lamp power is variable and it has twice the frequency

of the voltage and current frequency. In Figure 6 a diagram of instantaneous power for

different waveforms is shown.

Figure 4.

HID lamps operation frequency:

high pressure sodium lamps -

acoustic resonance duty frequency

map

Figure 5.

HID lamps operation frequency:

metal halide lamps - acoustic

resonance duty frequency map

Stable

Unstable

Frequency

~1MHz

~1kHz

Stable

Unstable

Frequency

~1MHz

~1kHz

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