AN3286 Datasheet, PDF (31/42 Pages) STMicroelectronics – This document describes

AN3286
Testing the board
The OVP protection is in auto-restart mode; after the shutdown for the protection tripping,
the V_DD recycles and, if the overvoltage is still present, the protection is tripped again,
indefinitely, until the cause of the fault is removed, at which point the converter resumes
normal operation, as shown in Figure 39.
Figure 39. Auto-restart mode of the
overvoltage protection
Figure 40. Auto-restart mode of the
overvoltage protection - zoom
2.10
Brownout protection
Brownout protection is basically a not-latched device shutdown function activated when the
mains voltage falls below the minimum specification of normal operation. There are several
reasons why it may be desirable to shut down a converter during a brownout condition.
Firstly, to avoid overheating of the primary power section due to an excess of RMS current.
Secondly, to avoid spurious restarts during converter power-down, which may cause the
output voltage to not decay to zero monotonically. A typical example of this happens when
the mains is unplugged in a converter designed for the European range (230 V_AC). The
converter is shut down due to the overload protection (due to the low input voltage the
converter is not able to supply the full power) but the voltage on the bulk capacitor is still
higher than V_{DRAIN RESTART}, so the device starts again and the output voltage rises again.
This situation may be dangerous for some loads and in many applications it is advisable to
avoid it.
If this protection is required, the BR pin of VIPer25 must be connected to the DC HV bus
through a voltage divider. Otherwise the pin is to be connected to GND (the function is
disabled until V_BR is lower than V_DIS, which is 50 mV typ., as shown in Figure 41). In the
presented converter both settings are possible acting on the jumper J (see schematic in
Figure 2).
The BR pin is the inverting input of a comparator whose non-inverting input is internally
referenced to V_BRth (0.45 V typ.). The PWM is disabled if the voltage applied at the BR pin is
below V_BRth, and is enabled when the BR pin voltage is above V_BRth + V_BRhyst, where
V_BRhyst is a voltage hysteresis (50 mV, typ. value) that ensures noise immunity.
The brownout comparator is also provided with current hysteresis; an internal current
generator, I_BRhyst (9.5 µA typ.), is ON as long as the voltage applied at the brownout pin is
below V_BRth and is OFF if the voltage is above V_BRth + V_BRhyst.
Doc ID 17998 Rev 1
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