Covidien Force FX, Service Manual

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Power Supply/RF Board
4-20
Force FX Electrosurgical Generator C Service Manual
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.
Capacitors C3, C10, C11, and C22 are an energy storage reservoir for the DC/DC
switcher. C29 is a high frequency bypass filter. Bleeder resistors R5 and R6 discharge the
capacitors when the AC line is disconnected or the power switch is turned off.
DC/DC Switching Regulator
The DC/DC switching regulator is a buck derived, pulse width modulated (PWM)
transformer. It is an isolated, fixed frequency, full bridge converter. The PWM IC (U5) is
used in the voltage mode. The output of the regulator is adjustable from approximately
zero (0) to 180 Vdc.
The full bridge consists of four power MOSFETs (Q1, Q3, Q4, and Q5) that operate at AC
line potential. Transistors Q3 and Q5 are on while Q1 and Q4 are off, and the reverse. In
this manner, power signals to the power transformer are bidirectional, or push-pull.
This allows full utilization of the transformer core magnetization capability. Regulation is
achieved by modulating the time that each MOSFET pair is on. Capacitor C32 in series
with the power transformer T3 primary prevents DC flux imbalance.
A snubber circuit (C27 and R31) absorbs leakage energy spikes. Another snubber circuit
(C49 and R51) reduces spikes due to reverse recovery of the output bridge rectifier.
The gate driver circuitry for each MOSFET is transformer-coupled through T1 to provide
AC line isolation. It consists of a dual MOSFET driver (U3) and various damping resistors.
Resistors R12, R18, R21, and R26 minimize turn-off oscillations. Resistors R22 and R23
damp ringing due to parasitic inductances in T1. Blocking capacitors C24 and C25
prevent DC flux imbalance in T1.
Note:
T1 consists of two transformers electrically and magnetically isolated from each
other but assembled into the same package. T1A and T1B form one transformer; T1C