AN2640 datasheet(22/36 Pages) STMICROELECTRONICS

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Recognition technique

AN2640

22/36

Recognition technique

To identify the connected lamp, the power must be evaluated by measuring both the lamp

voltage and current.

In this way, by multiplying these measurements, it is possible to obtain the lamp power:

Equation 13

With the evaluation board based on STMicroelectronics' ST7FLIT19BF1 MCU and L6382D5

driver, the lamp power measurements can be easily calculated. Our proposal is based on a

patented method that evaluates the PFC TON. The PFC is a boost converter working in

transition mode (TM). In the transition mode operation the boost converter works with a fixed

switch conduction time, TON, and variable frequency.

To measure the lamp power the constant TON is evaluated and moreover the TON is

proportional at the load power as shown in the following relationship:

Equation 14

Where:

●

TON is the PFC switch conduction time

●

L is the PFC inductor value

●

Vinrms is the RMS AC input voltage

●

Po is the load power, that is, the lamp power

This technique provides a key advantage of obtaining the lamp power information by directly

reading the PFC conduction time without multiplier evaluations in the board.

When the mains is switched ON the microcontroller performs a measurement on the AC

input voltage. After this phase it starts the half-bridge. The PFC is activated during this initial

phase to distinguish the family type and a cathode resistance measurement is performed to

select the lamp type.

After this selection, the preheating phase is performed until the ignition phase turns the

lamp on.

After the ignition the connected lamp is recognized and starts the run phase.

Using the described technique it is simple to calculate the lamp power. Experimental results

have confirmed this data.

lamp

⋅

2LP

⋅⋅

inr

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