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© Koninklijke Philips Electronics N.V. 2004. All rights reserved.

Product data sheet

Rev. 01 — 17 March 2004

5 of 16

Philips Semiconductors

TEA1620P

To ensure that the capacitor can be charged within the charge time, the value of the

oscillator capacitor should be limited to approximately 1 nF.

8.3 Duty factor control

The duty factor is controlled by the internal regulation voltage and the oscillator signal on

pin RC. The internal regulation voltage is equal to the external regulation voltage (minus

2.5 V) multiplied by the gain of the error amplifier (typical 20 dB or 10

×).

The minimum duty factor of the switched mode power supply is 0 %. The maximum duty

factor is set to 75 % (typical value at 100 kHz oscillation frequency).

8.4 Valley switching

A new cycle is started at the primary stroke when the switch is switched on (see Figure 3).

After a certain time (determined by the RC oscillator voltage and the internal regulation

level), the switch is turned off and the secondary stroke starts. The internal regulation

level is determined by the voltage on pin REG. After the secondary stroke, the drain

voltage shows an oscillation with a frequency of approximately

Where:

As soon as the oscillator voltage is high again and the secondary stroke has ended, the

circuit waits for a low drain voltage before starting a new primary stroke.

The primary stroke starts some time before the actual valley at low ringing frequencies,

and some time after the actual valley at high ringing frequencies. Figure 4 shows a typical

curve for a reflected voltage N

× V

Figure 5) transferred to the primary side of the transformer with the factor N (determined

by the turns ratio of the transformer). Figure 4 shows that the system switches exactly at

minimum drain voltage for ringing frequencies of 480 kHz, thus reducing the switch-on

losses to a minimum. At 200 kHz, the next primary stroke is started at 33

° before the

valley. The switch-on losses are still reduced significantly.

1

2

π

L

×

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