SSL1523P datasheet(7/17 Pages) NXP

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SSL152X_2

Product data sheet

Rev. 02 — 10 March 2009

7 of 17

NXP Semiconductors

SSL152x

SMPS ICs for low power systems

Figure 5 shows a typical curve for a reﬂected output voltage N

× V

o of 80 V. This voltage is

the output voltage Vo (see Figure 6) transferred to the primary side of the transformer with

the factor N (determined by the turns ratio of the transformer). Figure 5 shows that the

system switches at the 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.

8.5 Demagnetization

The system operates in discontinuous conduction mode all the time. As long as the

secondary stroke has not ended, the oscillator will not start a new primary stroke. During

the ﬁrst tsup(xfmr_ring) seconds, demagnetization recognition is suppressed. This

suppression may be necessary in applications where the transformer has a large leakage

inductance and at low output voltages.

8.6 Minimum and maximum duty factor

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).

8.7 OverCurrent Protection (OCP)

The cycle-by-cycle peak drain current limit circuit uses the external source resistor RI to

measure the current. The circuit is activated after the leading edge blanking time tleb. The

protection circuit limits the source voltage to VSOURCE(max) and thus limits the primary

peak current.

8.8 Short circuit winding protection

The short circuit winding protection circuit is also activated after the leading edge blanking

time. If the source voltage exceeds the short circuit winding protection voltage Vswp, the IC

stops switching. Only a power-on reset will restart normal operation. The short circuit

winding protection also protects in case of a secondary diode short circuit.

Fig 5.

Typical phase of drain ringing at switch-on (at N

× Vo =80V)

200

400

800

f (kHz)

phase

(

°)

−20

−40

mgt424

600

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