Valleylab LigaSure - Single Fault Protection Circuit

RF Board
5-24 LigaSure Vessel Sealing Generator Service Manual
Single Fault Protection Circuit
This circuit provides protective mitigation to shut down the RF output if a fault
condition occurs in which both the primary V_Sen and the redundant VR_Sen
signals go to zero. This avoids a dosage error condition by not allowing the
delivery of RF energy without the primary, redundant, or both sense signals
providing monitoring control through the microcontroller.
Single fault control shuts down the RF output by deactivating both the RF
generator and the high voltage power supply simultaneously.
• It deactivates RF output by inhibiting the T_ON\ signal from pulsing the RF
generator power FET Q5. The inhibit action occurs in the logic gating
provided by chip U2, configured as a tri-state buffer with inhibit control.
• It shuts down the high voltage power supply using a crowbar action on the
SYS_ECON signal, which generates the high voltage DC power sourced to
the RF generator. The crowbar action occurs when the Q1 FET turns on. IC
chips U4A and U4B monitor the primary V_Sen and redundant VR_Sen
levels, comparing these signals against a fixed reference bias voltage to
determine a simultaneous mutual fault or zero volt condition. The outputs of
U4A, U4B, and the Q3 drain connect together such that the corresponding
inputs simulate a wired NOR logic. This implementation is necessary because
a zero volt sense signal level, without RF activation, does not constitute a
fault.
The single fault protection circuit, therefore, actively evaluates a single fault
condition only when the RF generator produces RF energy. Two other conditions
disable the single fault protection circuit:
• RF startup with footswitch or handswitch requests
• Pulse operation in which seal algorithm control shuts off RF energy
The RF_ACT\ command drives Q3 with active low logic switching, a function of
the RF activation requests from either footswitch or handswitch. The circuit
components U1, CR1 through CR3, and Q2 provide the necessary signal interface
processing of the footswitch and handswitch commands to generate RF_ACT\. In
addition to supporting the NOR function logic with U4, the RF_ACT\ command
also starts the startup single fault disable by providing an input trigger to the
monostable one shot device, U3. With each activation request, U3 actively
disables single fault sensing for approximately 360 ms to allow sufficient time for
the system to develop RF energy and for the primary and redundant voltage sense
circuits to sense the developed energy. During seal mode operation, the control
signal VSEN_MON_RE_TRIG, supplied by the microcontroller, also triggers U3.
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