TEA152X

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© NXP B.V. 2010. All rights reserved.

Product data sheet

Rev. 04 — 14 September 2010

6 of 20

NXP Semiconductors

TEA152x

SMPS ICs for low-power systems

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.

Figure 4 shows the drain voltage together with the valley signal, the signal indicating the

secondary stroke and the RC voltage.

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 5 shows a typical curve for a reflected output voltage N

× V

the factor N (determined by the turns ratio of the transformer). Figure 5 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 PWM system

Fig 4.

Signals for valley switching

mgt423

drain

valley

secondary

ringing

secondary

stroke

primary

stroke

secondary

stroke

RC

oscillator

regulation level

A

B