High Voltage (HV) Power Supply Board
Principles of Operation
LigaSure Vessel Sealing Generator Service Manual
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High Voltage (HV) Power Supply Board
The high voltage power supply 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 resides on
the front panel; the receptacle/filter, on the rear panel of the generator. You can
change AC line fuses, located in the receptacle/filter, 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 network (C17 and C25). In this configuration, the circuit
acts as a doubler using the right half of the bridge rectifier (CR3). For 240 Vac
operation, the triac is off and CR3 becomes as a full wave rectifier.
The control IC (U5) functions as follows: The series circuit (CR7, R48, R49, and
C28) provides power for U5. Pin 1 (Vss) is a shunt regulator that provides a –9 V
(nominal) output. The divider (R40 and R41) measures the input line voltage.
Since 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. R42 and C26 set the oscillator frequency. Pin
7connects 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 the user recycles
input power. A power dropout cannot cause the circuit to accidentally act as a
doubler when the generator receives the higher input voltage range.
AC/DC Converter
The AC/DC converter uses CR3 as either a doubler or a full wave rectifier,
depending on the input voltage. In either case, an unregulated nominal 340 Vdc is
provided to the DC/DC switching regulator. Thermistors R28 and R29 provide
inrush current limiting, and fuse F1 provides protection against faults in the DC/
DC switcher.
Capacitors C17 and C25 function as an energy storage reservoir for the DC/DC
switcher. C23 and C24 function as a high frequency bypass filter. Bleeder
resistors R46 and R47 discharge the capacitors when the user disconnects the AC
line or turns off the power switch.
Warning
Potentially lethal AC and DC voltages are present in the AC line circuitry and high
voltage DC circuitry described in this manual. Take appropriate precautions when
testing and troubleshooting this area of the generator.
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