SSL1623PH

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© NXP B.V. 2010. All rights reserved.

Product data sheet

Rev. 02 — 13 September 2010

6 of 18

NXP Semiconductors

SSL1623PH

SMPS IC for mains LED drivers

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 approximately equal to the value given by

Equation 1.

(1)

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.

A: Start of new cycle with valley switching.

B: Start of new cycle in a classical system.

Fig 3.

Signals for valley switching

1

2

π

L

×

-----------------------------

col007

drain

valley

secondary

ringing

secondary

stroke

primary

stroke

secondary

stroke

RC

oscillator

regulation level

A

B