ConMed ExcaliburPLUS PC - RF and Output Systems

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rows, prevent the cross connection of the multi-
plexed signal whenever one of the keys is
depressed. If one of these transistors shorts out,
one of the 7-segment digits (I1 - 13) is written to
continuously causing it to be overdriven and glow
brighter than normal. Display I9 is lit through
driver U7 when U2 SL3 is high and U4 is dis-
abled by Q. The internal circuitry of U2 reads
each digit data stored in internal RAM and drives
the segment output (BO - B3 and A0 - A3) in
sync with the scan lines. Current is then supplied
to the correct digit, displaying the appropriate
number. The segment outputs of U2 are con-
nected to the current driver (U1) which delivers
approximately 40 mA to each segment. The dis-
plays are common cathode variety so that the cur-
rent is supplied to the anode and pulled to
ground at the common cathode connection by
the scanned open collector driver.
The keyboard (Schematic 4.2) is part of the front
panel assembly and is connected to the Display
PWB through connector A2J3. There are also
ground connections from the front panel electro-
static shield which is incorporated into the mem-
brane switches. These grounds straps are con-
nected directly to the chassis and prevent EMI
(ElectroMagnetic Interference) and ESD
(ElectroStatic Discharge) from being conducted
into the logic circuitry via the keyboard.
3.6 Power Amplifier
The Power Amplifier (PA) is a hybrid cascode
amplifier made up of high voltage bipolar tran-
sistors and low voltage power MOSFETs. See
Schematic 4.5. Figure 3.3 shows the basic hybrid
cascode amplifier configuration. The combina-
tion of O1 and O2 make up a fast, high-voltage
amplifier that can be controlled by the combina-
tion of the dc voltage VBASE, and the fixed
amplitude, variable pulse width signal, VGATE.
In the OFF condition, VGATE is near ground,
turning off O2 so that no drain current can flow.
Thus no base or emitter current can flow in the
bipolar transistor. Since V+ is always greater
than VBASE, the collector base junction is
reverse-biased, so no collector current will flow
and no power is delivered to the load. Turn-on
commences with VGATE rising rapidly to about
+10V. This results in a large pulse of base current
flowing in Q] from Cb, quickly turning Q1 on
3-12
which delivers power to Z1. After the turn-on
transient, Q2 will be conducting hard and O1
will draw collector current in proportion to its
base current, which in turn is controlled by
VBASE and Rb. At sufficiently high base cur-
rent, Q] saturates (collector-base voltage nearly
zero), transferring maximum available power to
Z1 while QI and Q2 dissipate little power due to
the low voltage across them. Turn-off commences
with VGATE quickly dropping to nearly 0 V,
shutting off Q2 and effectively disconnecting
Q1's emitter from the circuit. Collector current
then flows out of Ols base pin into Cb until all
of the charge stored in Ol during turn-on is
washed out. Then O1 completely shuts off, ceas-
ing power transfer to Zl.
I“
Z1
DI
Pb Qi
VBASE cb
て Re
o
VGATE
Figure 3.3 Basic Hybrid
Cascode Configuration
The collector voltage may rise to many times the
value of V+ after turn off. Since the emitter is
now disconnected, the collector-base voltage can
take on the highest value which that junction will
sustain with little chance of second breakdown.
The base bypass capacitor, Cb, is sized to ensure
that it can absorb all of the stored turn-off charge
without allowing Q2’s drain voltage to approach
its breakdown limit. Further, this charge is now
available to charge the base on the next cycle,
thereby significantly reducing the net current
drain from the VBASE supply.
The saturated operation described above takes
place in both Monopolar Coag and during high-
power(>150 W) Cut. During the latter mode,
power increases take place because the dynamics