AN4007

Protection features

Doc ID 022455 Rev 2

25/33

2.4

Brownout protection

Brownout protection is basically an unlatched device shutdown functionality whose typical

use is to sense mains undervoltage or unplugged mains. The VIPER37LE has a BR pin

dedicated to this function which must be connected to the DC HV bus through a voltage

divider.

If the protection is not required, it can be disabled by connecting the pin to ground. In the

presented converter, brownout protection is implemented but can be disabled by changing

the jumper JMP.

The converter's shutdown is accomplished by means of an internal comparator internally

referenced to 450 mV that disables the PWM if the voltage applied at the BR pin is below the

internal reference.

PWM operation is re-enabled as the BR pin voltage is more than 450 mV plus 50 mV of

voltage hysteresis that ensures noise immunity. The brownout comparator is also provided

with current hysteresis. An internal 10 A current generator is ON as long as the voltage

applied at the BROWNOUT pin is below 450 mV and is OFF if the voltage exceeds 450 mV

plus the voltage hysteresis.

In

Figure 27 the converter’s power-down is shown: once the main is disconnected and the

bulk capacitor is discharged, the IC stops switching when the DC bus voltage falls below

78 V. This reduces the RMS input current and ensures monotonic output voltage decay.

Figure 28 and 29 show brownout protection during the wake-up phase: once the DC bus

Figure 27.

Brownout protection: converter’s power-down phase

AM11366v1

V2: 459 mV

M: 1.0 s/div

V1: 78 V