AN2383 datasheet(19/39 Pages) STMICROELECTRONICS

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AN2383

Measurements at 50 Hz

Doc ID 12433 Rev 3

19/39

5.5

Working level 9

Figure 11.

System working at level 9

At level 9, the system delivers the maximum output power. I-CTRL rises accordingly.

The waveform shown in Figure 11 was taken while a 22 cm-diameter iron pot filled with

water was placed on the plate.

5.6

Real-time current adjustment

As seen, the induction cooking system works on the principle of a series L-C resonant

circuit. When the size of L and C are set, the resonant frequency is set as well.

Unfortunately, this value does not depend only on the resonant tank. In fact, the size and

material of the pot affect the resonant frequency too. This causes the system to have an

oscillating resonant frequency strongly dependent on the type of pot placed on the plate at

different times.

Therefore the 9 working levels cannot be based on constant frequency levels. The PWM

frequency must be adjusted to the selected level in order to work with the pot placed on the

plate at that moment.

So each working level does not work on a constant PWM frequency, but a constant current.

By reading the I-CTRL feedback signal, the MCU smoothly adjusts the PWM frequency in

order to keep the current constant for the selected working level. Each level has a

corresponding constant value of current.

MAINS

VLINK

ZVS

I - CTRL

AN2383 Datasheet(PDF) 19 Page - STMicroelectronics