AN966 Datasheet, PDF (4/21 Pages) STMicroelectronics – The front-end stage of conventional off-line converters

AN966 APPLICATION NOTE
Figure 3. Error Amplifier and Overvoltage Detector Block.
+Vo
R1
R2
Ccomp.
∆I
1
2
-
E/A
X
+
2.5V
-
+
2.25V
∆I
PWM
DRIVER
D97IN591
40µA
When the current in excess reaches about 37µA, the output voltage of the multiplier is forced to de-
crease thus the energy drawn from the mains is reduced. This slows down the rate of rise of the output
voltage. In some cases, this "soft braking" action is able to prevent the output voltage from exceeding
the regulated value too much.
If the output voltage further increases despite the soft braking, so that the current entering the E/A
reaches 40µA, a "sharp braking" takes place. The output of the multiplier is pulled to ground, thus turn-
ing off the output stage and the external MOSFET. Also the internal starter is switched off. The internal
current comparator is provided with hysteresis, thus the pull-down will be released and the output stage
re-enabled as the current entering the E/A falls approximately below 10µA.
Figure 4. Dynamic and Static OVP operation.
OVER VOLTAGE
V_{OUT nominal}
40µA
I_SC ^10µA
E/A OUTPUT
2.25V
4/21
DYNAMIC OVP
STATIC OVP
D97IN592A