6-16
Refer to the schematic (Figure C-14). Each drive
cycle starts with 01 and the pulse width of this
signal is approximately 1µS. The signal is ampli
-
fied from 5V to 12V at U1, turning on the driver
FET Q2. The driver Q2 drain is pulled to ground
allowing current flow from the center tap of T1.
The secondary of T1 enables the power devices
Q8 and Q17, allowing current flow from the HV
Power Supply (+HV input). When Q17 switches
on, it allows current flow from the +HV, through
Q8, through C6, and the primary of the output
transformer (connected to J2-1 & J2-2 - located
on the RF Output Assembly ), and to -HV. This
completes one-half cycle of a drive waveform.
When 01 turns off, /01 and 02 switch on. The
/01 “on-time” is only 200nS, long enough to
switch on Q1 and pull the charge out of T1. The
02, however, works the same as 01, only it switch
-
es on the power devices Q9 & Q16, allowing
current to flow from +HV, through Q9, and in
the opposite direction of the output transformer
(reversing the polarity) and then finally through
Q16. This completes one full phase of RF drive
on this amplifier.
The switching action of alternately turning on 01
and 02 cause the primary of the output transform
-
er to have a positive and negative polarity. On the
RF Output Assembly is the filter that converts the
square wave generated by the amplifier to a sine
wave that is delivered to the patient. The drive
frequency is 461KHz, and the repetition rate of
each cycle is 54µS. (See Figure 6.1.)
6.8 Single-Ended Amplifier [A9]
See Figure C-16 for this section. The Single
Ended Amplifier assembly houses the power
switching devices (MOSFETs) that amplify the
drive waveforms for Spray and ABC™ modes of
operation. The drive signals for this amplifier
originate on the Power Control Assembly. The
signal labeled SE is the drive signal and the one
labeled /SE is only a reset pulse to reset the mag
-
netics of T1 & T2, and it has a pulse duration of
about 200nS.
The input signals are amplified by U1 from 5V
to 12V. When SE switches from low to high, Q4
and Q2 are both switched on, and then when SE
switches back to low, the /SE signal switches high
to switch on Q3 and Q1. Each drive pulse (SE)
enables all the power devices (Q9, Q17, Q8, &
Q16) at the same time.
When the power devices switch on, viewed as
a closed switch, the current flows from +HV,
through the output transformer primary (+tank
to -tank) and is passed to -HV. The current
through the transformer primary is developing a
charge that is stored in the primary of the trans
-
former. When the driver is turned off, viewing
the power devices as open switches now, the
charge stored in the primary is reversed and flows
in the direction of +HV. It is during this time
that the energy is delivered across the barrier to
the secondary of the output transformer. The
components C6 and primary of the output trans
-
former make up a “tank” circuit that can store
a charge. When a load is not connected to the
system output, then the current alternately flows
from one side of C6 to the other until all the
energy is dissipated.
The repetition rate for a drive cycle is 35µS for
Spray and the pulse width is fixed at about 1.3µS
typically. (See Figure 6.1) The output power
for the Spray mode is controlled by varying the
HVDC. In ABC™, the power is controlled by
varying the pulse width, and the HVDC is fixed
at 200V. The pulse duration in ABC™ active is
from 200nS to approximately 3µS.
ABC™ has three separate modes that will be
discussed in this section. The modes are Target,
Booster, and Active. The modes Target and
Booster alternately are active whenever a load is
not present at the tip of the ABC™ handpiece.
The Target mode is basically 1032 pulses at a
400nS pulse duration, and then the next 32 pulses
will have a pulse duration of 600nS which is
called the Booster mode. Following the Booster
mode, the cycle starts all over again until a load is
sensed at the ABC™ handpiece, and then the unit
switches over to the Active mode.
The repetition rate is also different for the ABC™
modes. Target and Booster have a drive pulse
every 60µS, but in the active mode it occurs every
35µS. The purpose for the Target and Booster
mode is to reduce the RMS voltage when a load is
not present, and this results in lower RF Leakage.
Once a load is present, which means the Beam is
Active, RF Leakage is no longer an issue and the
RMS voltage is increased to enhance performance.
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