AN3286 datasheet(31/42 Pages) STMICROELECTRONICS

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AN3286

Testing the board

Doc ID 17998 Rev 1

31/42

The OVP protection is in auto-restart mode; after the shutdown for the protection tripping,

the VDD 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.

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 VAC). 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 VDRAIN 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 VBR is lower than VDIS, 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 VBRth (0.45 V typ.). The PWM is disabled if the voltage applied at the BR pin is

below VBRth, and is enabled when the BR pin voltage is above VBRth + VBRhyst, where

VBRhyst 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, IBRhyst (9.5 µA typ.), is ON as long as the voltage applied at the brownout pin is

below VBRth and is OFF if the voltage is above VBRth + VBRhyst.

Figure 39.

Auto-restart mode of the

overvoltage protection

Figure 40.

Auto-restart mode of the

overvoltage protection - zoom

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