Pins and their functions

AN2934

12/27

Doc ID 15394 Rev 1

4

Pins and their functions

4.1

Brownout and power surge features

The VIPer17HN has a dedicated pin–called the BR pin– for the brownout and power surge

features. This pin (pin 5) has its comparator set for a 0.45 V input. The unit’s shutdown and

start-up can be set to the desired line voltages by attaching a resistor divider from the bulk to

this pin. If this feature is not required, the pin can be grounded. The J3 jumper is used for

this purpose. When the jumper is installed on the left-two pins marked "N", the brownout is

grounded or defeated. With the unit set to half the output power, it shuts down at 32 Vac and

restarts at 53 Vac. When the jumper is installed on the right-two pins marked "BR", the

brownout feature is active and the unit shuts down at 60 Vac and restarts at 70 Vac.

In today's appliances, such as washing machines, dryers, dishwashers and the like,

mechanical timers have been replaced with electronics. When the AC line drops for a short

duration, the appliance must ride through its cycle without going back to the beginning. If the

AC line drops for a longer period, information about where the appliance is in the cycle must

be saved. When the AC line comes back, the appliance has to continue from where it left off

and not start over again. This is known as the "hold-up time".

Ride-through is defined as the input line voltage dropping for a number of cycles and the unit

under test continuing to operate correctly without the microprocessor being reset. Typically,

tests are done at a nominal 115 Vac. The line is sensed and when missing pulses are

detected, the unit starts shedding loads, saves the settings and tries to "ride through" the

line dropout. If the input line does not come back in time, the power supply has to maintain

an output voltage until all the information has been saved. When the line does come back,

from the starting input voltage to the point where the PWM stops is the time the unit runs for.

The brownout pin turns off the unit at a certain voltage, reducing stress on the components

but affecting the hold-up time. C12 may be increased to delay the turn-off, forming an RC

time constant and therefore achieving a good hold-up, but turning off the unit if the voltage

dwells at a low line. The customer must evaluate the stresses on the components with

regard to the hold-up required.

The plots in Figure 4 and Figure 5 show the difference between the two settings of J3 on the

single reference board. The strap can be set to ground (N) or to (BR) with brownout active.

With the strap in the (N) position, the input capacitor can discharge to ~45 Vdc before a

glitch becomes noticeable on the main output. With the strap in the (BR) position, the

brownout is active and stops the PWM from switching when the voltage on the input

capacitor reaches a predetermined voltage set by the brownout divider. Vout is equal to 0.45

Vdc.

Equation 1

Vin

Vout R1

R2

+

()

R2

-----------------------------------------

=