Valleylab Force FX-8C, Service Manual

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Power Supply/RF Board
Principles of Operation
Force FX-8C Service Manual 4-19
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. 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 and T1D form the other.
The output of the power transformer is full wave rectified by a high
voltage diode bridge (CR10, CR13, CR19, and CR23). L1, C33, and C35
filter the rectified power signal. The regulated DC output from this supply
is the input to the RF stage of the generator.