RF Board
5-20
LigaSure Vessel Sealing Generator Service Manual
RF Driver
The RF driver circuit processes a T_ON\ gating signal which enables the RF
generator within the LigaSure unit. The T_ON\ signal, a TTL level pulse train
signal, originates in the microcontroller. T_ON\ has a 60% duty cycle active high
pulse waveshape. When activated, T_ON\ starts RF energy development with
either handswitch or footswitch requests.
On the RF board, integrated circuit chips U2 and U11 process the T_ON\ signal.
U2 is a quad NAND gate chip configured as a tri-state buffer that allows one of
two actions:
•
Process the T_ON\ signal to start an RF output.
•
Inhibit the T_ON\ signal to disable the RF output if the system detects a single
fault failure in the RF output.
U11 is a power MOSFET driver chip that processes T_ON\ by converting the
TTL level signal to a 12 V pp signal. This 12 volt signal drives Q5, the power
MOSFET RF generating component. TP12 provides a test point inverted T_ON\
signal at the gate of Q5 with a 40% duty activation. U11 receives power from
U10, which has an output permanently set high for all RF activation modes.
RF Output
The RF output circuit develops the energy for all operative modes of the LigaSure
generator. The circuit consists of a resonant tank network: T7, L1, L2, C57, C63,
C58, C62, C59, C65, C66, C60, and power MOSFET Q5. RF energy results when
the T_ON\ signal, with a high voltage DC (HVDC) source present at test point
TP13, switches device Q5 on and off. HVDC, originated by the high voltage
power supply, provides source power for the RF output stage. When switched on/
off to develop RF, Q5 creates a signal that exhibits a half-sine waveshape of
approximately 350 volts peak amplitude. This signal represents the result of
resonant action that occurs between the inductive and capacitive components in
the RF output circuit. The half-sine period timing is typically 800 ns for unloaded
generator output. Under load, when the generator applies RF energy to tissue, the
half-sine period timing increases to a maximum of 1.25 µs. The half-sine
waveshape is periodic, repeating every 2.115 µs with the on/off switching of Q5.
The half-sine signal appears at test point TP11. Resonant action between
inductive and capacitive components converts the half-sine signal to a continuous
sinusoid RF output.
Components TS1, TS2, and TS3 provide hardware overvoltage limiting to the RF
output. These components turn on and off automatically as the output peak
voltage exceeds the maximum output operating voltage of 203 Vrms. TS1, TS2,
and TS3 are bidirectional transient suppressor devices that limit overvoltage to
approximately 245 Vrms. To arrive at the 245 Vrms limit, the system sums TS1,
TS2, and TS3.
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