Philips 27PS60S321 manual Protection Events, Demagnetization sense

SPMS Page 22 of 32

isdischarged to V UVLO .The output voltage will decrease depending on the load.

zt3: Charge ofV CC when gate drive is inactive Thethird interval starts when the UVLO is reached. The internal currentsource charges the V CC capacitor(also the soft start capacitor is recharged). Once the V CC capacitor is chargedto the start-up voltage, the driver is activated and a new burstcycle is started.

Figure:

Protection Events

The SMPS IC 7520 has the following protectionfeatures:

Demagnetization sense

This feature guarantees discontinuous conductionmode operation in every situation. The oscillator will not starta new primary stroke until the secondary stroke has ended. Thisis to ensure that FET 7521 will not turn on until the demagnetization oftransformer 5520 is complete.The function is an additional protectionfeature against:

zsaturationof the transformer,

zdamage of the components during initial start-up,

zan overload of the output.

The demag(netization) sense is realized by an internalcircuit that guards the voltage (Vdemag) at pin 4 that is connectedto V CC winding byresistor R 1 (R3522). The Figure below shows the circuit and the idealized waveformsacross this winding.

7/8/2004