Power Supply/RF Board
Principles of Operation
Force FX Electrosurgical Generator C Service Manual 4-21
High Voltage Power Supply
The high voltage power supply section contains the power entry circuitry, auto mains
switching circuitry, AC/DC conversion circuitry, and a DC/DC switching regulator.
Power Entry Circuit
The power entry circuit consists of an integral three wire power cord receptacle, fuse
drawer, EMI filter, and a separate power switch. The power switch is mounted on the
front panel. The receptacle/filter is mounted on the rear panel of the generator. AC line
fuses are changeable from the rear of the generator.
Auto Mains Switching Circuitry
The auto mains switching circuit detects the AC line voltage level and controls the triac
(D1). This triac controls the topology of the AC/DC converter.
For 120 Vac operation, the triac is on, which connects the AC neutral to the center of the
AC/DC converter capacitor bank (C3, C10, C11, and C22). In this configuration, the
circuit acts as a doubler using the right hand half of the bridge rectifier (CR80).
For 240 Vac operation the triac is off and CR80 is used as a full wave rectifier.
The control IC (U1) functions as follows: The series circuit (CR1, R1, R2, and C9) provides
power for U1. Pin 1 (Vss) is a shunt regulator that provides a -9 V (nominal) output.
The divider (R3 and R4) measures the input line voltage. Because the voltage at pin 8
varies with the line, it can sense the line voltage zero crossing as well as the peak voltage.
Pins 2 and 3 are inputs to an oscillator used for triac triggering timing. R7 and C1 set the
oscillator frequency.
Pin 7 is tied to Vss, which places the circuit in the fail-safe mode. Thus, once the circuit
enters full bridge mode, it remains in that mode until input power is recycled. A power
dropout cannot cause the circuit to accidentally act as a doubler when the higher input
voltage range is used.
AC/DC Converter
The AC/DC converter uses CR80 as either a doubler or full wave rectifier, depending on
the input voltage. In either case, an unregulated nominal 300 Vdc is provided to the DC/
DC switching regulator. Thermistors R32 and R33 provide inrush current limiting, and
fuse F4 provides protection against faults in the DC/DC switcher.
Warning
Potentially lethal AC and DC voltages are present in the AC line circuitry, high voltage DC circuitry,
and associated mounting and heat sink hardware described in this manual. They are not isolated
from the AC line. Take appropriate precautions when testing and troubleshooting this area of the
generator.
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