AN3329 datasheet(27/47 Pages) STMICROELECTRONICS

27 / 47 page

AN3329

Functional check

Doc ID 18376 Rev 1

27/47

Overvoltage protection

Figure 54 and 55 show the OVP response, observed opening the resonant stage feedback

loop. Upon an overvoltage condition of either 24 V or 12 V output, the Zener diodes D16 and

D17 are reverse-biased and conduct respectively, forcing Q10 on by the resulting base

current, which causes Q9 to conduct shorting to ground the anode of the U5 optocoupler

and the disappearance of V_C44. Therefore, the L6564 and the L6599A are not powered.

The two transistors mentioned form a pnp-npn SCR structure that force the PFC and

resonant converter to be latched off until the 5 V standby is removed by a mains voltage

recycling. In the case of an open loop event, the 12 V and 24 V outputs reach 14.16 V and

28.26 V respectively.

Figure 52.

Startup by on/off signal at full load

and 115 Vac - 60 Hz O/P voltage

rising

Figure 53.

Turn-off at full load and 115 Vac - 60

Hz by on/off signal

CH3: 24 V output

CH1: On/off signal

CH2: CSS (pin #1)

CH4: 12 V output

CH3: V_C44

CH1: HB voltage

CH2: PFC drain

CH4: On/off signal

Figure 54.

OVP at full load and 115 Vac - 60 Hz

on 12 V - 24 V

Figure 55.

OVP at full load and 115 Vac - 60 Hz

on 12 V - 24 V - outputs detail

CH3: V_C53

CH1: HB voltage

CH2: PFC drain

CH4: V_C44

CH3: V_C53

CH1: 24 V output

CH2: 12 V output

CH4: V_C44

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