High Voltage (HV) Power Supply Board
5-16
LigaSure Vessel Sealing Generator Service Manual
DC/DC Switching Regulator
The DC/DC switching regulator is a buck derived, pulse width modulated (PWM)
system. It is an isolated, fixed frequency, full bridge forward converter. The PWM
IC (U4) functions in the voltage mode. The output of the regulator is adjustable
from approximately 0 to 120 Vdc.
The regulated power supply incorporates a full H bridge and consists of four
power MOSFETs (Q3, Q4, Q5, and Q6) that operate at AC line potential.
Transistors Q3 and Q4 are on while Q5 and Q6 are off, and the reverse. In this
manner, power signals to the power transformer are bidirectional, or push-pull.
This allows full use of the transformer core magnetization capability. Modulating
the time that each MOSFET pair is on achieves regulation. Capacitor C35 in
series with the power transformer T3 primary prevents DC flux imbalance. A
snubber circuit (C22 and R35) absorbs resonant harmonic energy spikes. Another
snubber circuit (C32, R50 and C33, R51) reduces spikes due to reverse recovery
of the output rectifier, CR8. Bidirectional transient suppressor, TS1, limits high
voltage spikes by detuning the snubber circuit and protecting CR8 from reverse
excess PIV transients.
The gate driver circuitry for each MOSFET, transformer-coupled through T2 and
T4, provides AC line isolation. It consists of a dual MOSFET driver (U1) and
various damping resistors. Resistors R16, R26, R44, and R53 minimize turn on/
off delay and damp on/off ringing. Blocking capacitors C6 and C34 prevent DC
flux imbalance in T2 and T4. Resistors R15 and R52 limit the turn-on surge of the
H bridge MOSFET components.
A high voltage diode, CR8, provides full wave center tap rectification for the
output of the power transformer. L1, C30, and C31 filter the rectified power
signal. The regulated HVDC output from this supply is the input power to the RF
stage of the generator.
The SYS_ECON signal from the microcontroller controls the output voltage
level. This 0 to 5 Vdc signal sets the reference for the PWM control loop. An
external op-amp (U2) buffers the SYS_ECON signal and provides error amp
control of the PWM switcher for stable closed loop performance. Closed loop
control results from comparing the feedback voltage from the output divider (R2
and R3) to the ECON input signal.
The output of U4 is a pair of complementary signals that are pulse width
modulated by comparing ECON with the internal oscillator ramp waveform. At
the start of an oscillator cycle, the U4 output turns on. It turns off when the ramp
voltage crosses the ECON level. The two output signals from U4 (pins 11 and 14)
feed the MOSFET drivers (U1A and U1B).
R20 and C12 set the U4 oscillator frequency to approximately 82 kHz. C13
controls the pulse width duty at power on for slow start control. Transformer T1
monitors the power transformer primary current, protecting against faults in the
DC/DC switcher power stage and faults in the output load circuitry. CR1, CR2,
CR5, and CR6 rectify the output of T1; R24 and C21 filter it. This filtered signal
goes to the current limit pin (pin 9 of U4). During an overcurrent condition the U4
current limit function resets the HVDC output to a low voltage level (around
10Vdc) until the current falls. Pin 9 of U4 also allows remote shut down of the
DC/DC switcher through Q1 and CR4. The shut down signal, labeled
HV_ENABLE, comes from the main microcontroller on the control board.
The resistor divider on the high voltage DC output formed by R5 and R37
provides dosage error sensing, identified as HV_SEN, to the control board.
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com