Datasheet

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