6-9
with a light load. The degree of dampening is
used to determine if a load is present or not.
Refer to Figure C-6 and locate the input labeled
+ASEN. This signal is a ratio of the primary
current on the ABC™ output transformer. ASEN
is compared to a reference voltage at U15-3 and
each peak of the damped waveform that exceeds
this reference level will cause the output of U15
to switch high at the same frequency as the output
(570KHz). If the output of U15 has four pulses
for one cycle (1 cycle is 35uS active/60uS target),
the logic within U7 assumes a load is not pres
-
ent and the system remains in the Target mode.
However, if less than four pulses are detected for
one cycle, the logic within U7 assumes a load is
present and switches to the active mode.
Staying with the signal ASEN, it is also connected
to a comparator (U16). This comparator is
used for ABC™ over-voltage detection. Without
going into significant detail, if the output of U16
pulses 192 consecutive cycles, the logic within U7
assumes the system has locked up in the active
mode and the RF is inhibited. Note that the
inverting input of this comparator has a potenti
-
ometer (RA9). This potentiometer allows calibra
-
tion of the over voltage limit, and should ABC™
fail to initiate, it may be necessary to refer to the
calibration section for this adjustment.
The target and booster pulses are a means of lim
-
iting RF leakage by controlling both the output
voltage and repetition rate when the output is not
loaded. Both modes are fixed outputs, meaning
the power setting has no influence on the output
voltage. The resistor ladder (R24, RA4, R17,
& R19) sets the limits for both the target and
booster modes. Target has 1032 pulses with a
peak voltage of approximately 1800Vpk, and then
the booster is switched in. The booster mode has
32 pulses with a peak voltage of 6000Vpk and
these pulses are used to ionize the argon gas to
assist with initiation. When a load is not detected
during the booster pulses, then the target/booster
cycle is repeated until a load is sensed. The volt
-
age level of each mode is selected by U4 with
BM_TAR for target and BM_BST for booster.
These two signals should toggle on and off when
ABC™ is activated without a load on the output.
When a load is sensed, SE_P
CON is set and now
the power setting determines the pulse width and
the logic reduces the repetition rate from 60uS to
35uS. Output power is controlled by pulse width
and the pulse width controller is U5 and Q2. For
the start of each cycle of ABC™, Q2 is turned off
and this allows C10 to charge linearly. When the
charge of C10 exceeds the P
CON voltage on U5-3,
the output of U5 switches high and terminates the
pulse time.
Now is a good time to bring up Spray. Spray
Coagulation and ABC™ are quite similar, only
Spray is lower power and does not have the target
modes. The output power in spray is controlled
by HVDC, and the pulse width in spray is fixed
at about 1.3uS. Calibration of Spray is performed
by adjusting RA3, which simply scales P
CON
down for a pulse width that will correct the out
-
put power to match the dial setting.
6.4 High Voltage Power Supply [A7]
The High Voltage Power Supply (HVPS) pro-
vides variable regulated DC voltage and current
to the RF amplifiers which converts this energy
into high frequency surgical current. The HVPS
is contained on two separate circuit board assem
-
blies.
The High Voltage output assembly contains the
power devices and capacitive filters which provide
the high voltage output. The HV/Flow Control
assembly generates the control signals required
to regulate the high voltage output and performs
other high voltage related tasks. This section will
be specific to the high voltage assembly and the
control circuit for the high voltage will follow.
6.4.1 Power Supply Topology
The HVPS is a phase controlled type power sup
-
ply. With this topology, output voltage is con
-
trolled by varying the phase angle at which the
AC mains sinusoidal waveform is permitted to
conduct. Typically, a triac or SCR in series with
the incoming AC line is off during the rise of the
mains sinusoidal waveform. Following the peak
of the waveform, a trigger is asserted at the gate
allowing the triac or SCR to turn on and the
line voltage present at the triggered phase angle
is available to charge filter capacitors commonly
used with these topologies. The phase angle trig
-
gering sequence occurs for each subsequent half
cycle of the sinusoidal waveform.
6.4.2 Phase Control Output
Referring to the High Voltage Output schematic
(Figure C-12), the line isolated AC voltage enters
the printed circuit assembly at J1-1 (AC Hi), J1-3
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