AN3372 datasheet(27/37 Pages) STMICROELECTRONICS

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AN3372

Half bridge operation

Doc ID 018625 Rev 1

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Half bridge operation

Init phase

After the startup of the auxiliary power supply gives the right voltage at the L6382D5, the

auxiliary supply voltage for the microcontroller is generated. After this sequence the

microcontroller gives the right startup signals at the driver.

Ignition phase

The ignition voltage is generated by the high voltage transformer. On primary winding a

voltage is generated by means of a capacitive discharge. The differential of potential is

transferred (high voltage) at secondary winding causing lamp ignition. The voltage level

across the lamp is about 3 kV.

Warm-up phase

During this phase a relative high current must be supplied to avoid the lamp extinguishing. In

this case the maximum lamp current is limited at 30% higher than the nominal value.

Burn phase

In this phase the lamp power must be controlled regulating the lamp current. To avoid

acoustic resonance the lamp is driven with low frequency square wave current lamp current.

Current sensing

The current is monitored measuring the voltage across an Rsense obtained by a current

transformer. This signal is filtered in order to obtain information on the real average current

that flows in the lamp. This signal is the feedback used by the MCU.

Voltage sensing

The lamp voltage is obtained measuring the voltage across the lamp, filtering this signal,

and sending this signal to ADC of the microcontroller. In this way the lamp voltage is

controlled directly by the MCU.

Lamp power management

The lamp power is obtained multiplying the lamp voltage signal for the lamp current.

The lamp voltage is sensed directly across the lamp. The lamp current is obtained

considering the feedback signal coming from the current sense circuit.

The MCU calculates the lamp power multiplying the value of lamp current for the lamp

voltage.

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