FARMTM

Upon application of input power, the output bus capacitors begin to charge. The FARMTM thermistor limits the charge current, and the exponential time constant is determined by the hold-up capacitor value and the thermistor cold resistance. The slope (dv/dt) of the capacitor voltage versus time approaches zero as the capacitors become charged to the peak of the AC line voltage.

The FARMTM switch that bypasses the inrush limiting PTC (positive temperature coefficient) thermistor is open when power is applied, as is the switch that engages the strap for voltage doubling. (see Fig. 7). In addition, the converter modules are disabled via the Enable (EN) line, and Bus-OK (BOK) is high.

If the bus voltage is less than 200V as the slope nears zero, the voltage doubler is activated, and the bus voltage climbs exponentially to twice the peak line voltage. If the bus voltage is greater than 200V, the doubler is not activated.

If the FARMTM bus voltage is greater than 235V as the slope approaches zero, the inrush limiting thermistor is bypassed. Below 235V, it is not bypassed.

The FARMTM converters are enabled 50 milliseconds after the thermistor bypass switch is closed.

Bus-OK is asserted after an additional 50 millisecond delay to allow the converter outputs to settle within specification.