AN2242

4 Functional Checking

19/31

Q3 has been added. This circuit senses the output voltage and drive the circuit to work in hic-

cup mode if the 1.8V output voltage disappears

4.5

Over Voltage Protection

A protection that all power supplies must have is that against the failure of the feedback

circuitry. If this occurs, the SMPS output voltages can get high values, depending on the load

on each output and the transformer coupling between the windings. Consequently, the

rectifiers and the output capacitors are overstressed and they can be destroyed. To avoid this

SMPS failure, a dedicated low-cost circuit has been added that senses the auxiliary voltage,

thus providing a protection threshold not as dependent on the mains voltage or the load. This

signal is connected to the L6668, pin 7 (DIS), dedicated to a latched protection of the circuit, as

needed to properly protect the power supply from OVP or OTP. The IC pin #7 is the non-

inverting input of a comparator having the inverting input internally referenced to 2.2V (typ.). As

the voltage on the pin exceeds the threshold, the L6668 stops the operation and its

consumption is reduced to a low value. The status is latched until the Vcc goes below the

UVLO threshold. To remain in the latch status, the L6668 internal HV generator is activated

The SMPS can restart after the disconnection of the converter from the mains and the Vcc pin

decreases below the UVLO threshold. Figure 24. depicts the circuit behaviour described

above. During a feedback loop failure, the board OVP circuitry voltage intervention threshold

allows the 12V output to rise up to 16.8V at 115Vac, and doesn't change significantly with the

input voltage or the load.

CH1: DRAIN VOLTAGE

CH3: 1.8 Vout