Power Supply/RF Board
5-24
Force EZ-C Series Service Manual
The high 2 (fulgurate) setting works the same as the high 1 (fulgurate) setting,
except the T_ON\signal is a continuous pulse train with a pulse width of 1.69 µs
and a frequency of 30 kHz.
In the low 1 coag setting, the K1 setting allows these conditions:
•
diode CR7 is in parallel with the MOSFET body drain diode (like the cut
modes)
•
K15 is open
The T_ON\ signal is a continuous pulse train with a pulse width of 2 µs and a
frequency of 39 kHz. The output resonates with a nominal frequency of 290 Khz.
The microcontroller treats the low 2 and low 3 coag settings as feedback
controlled cut modes. Its operation is the same as pure cut described above,
except the power curve is different.
Spark Control Circuit
The spark control circuit uses the voltage sense circuit to monitor the output
voltage. It interrupts the delivery of power if the output voltage exceeds a preset
threshold. This greatly reduces sparking when removing an activated accessory
from tissue. The sparking occurs because the RF stage tuning results in a higher
natural gain at light loads than at heavy loads. Thus, during sudden transitions
from heavy to light loads, the output voltage rises faster than the microcontroller
can respond. This analog circuit works outside the microcontroller loop at a much
greater speed.
The rectified but unfiltered waveform from the output voltage sense circuit goes
into a peak detector (U13A, CR8, and C76). This input signal is called
VSEN_SCC. A high impedance buffer (U13B) maintains the integrity of the peak
detected signal. The output of this buffer divides and goes to a comparator. The
other input to the comparator is an analog threshold level (VMAX_CLP) that the
feedback microcontroller on the Control board sets: This input depends on the
mode and power setting.
When the peak detected sample of the output voltage exceeds the threshold, the
condition fires one-shot U15A, which generates a 3 ms pulse (SPARK_CON) that
goes to the T_ON ASIC on the Control board. the T_ON ASIC ignores this pulse
if it occurs during the first 0.2 seconds of activation. Otherwise, SPARK_CON
causes the T_ON\ signal to stop. The feedback microcontroller on the Control
board senses this and realizes that the circuit has suppressed a spark. The
feedback microcontroller waits either 10 ms in low 2 or low 3 coag or 100 ms in
pure cut, then re-initiates T_ON\ with a frequency of 470 kHz. The frequency
returns to 393 kHz after one second of continuous activation or when the
generator is reactivated.
