ConMed sabre 180, Service Manual

Page: 35 / 72

3-9
will draw collector current in proportion to its
base current, which in turn is controlled by
VBASE and Rb. At sufficiently high base current,
Q1 saturates (collector-base voltage nearly zero),
transferring maximum available power to Z1
while Q1 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 Q1s
emitter from the circuit. Collector current then
flows out of Q1s base pin into Cb until all of the
charge stored in Q1 during turn-on is washed
out. Then Q1 completely shuts off, ceasing power
transfer to Z1.
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 Q2s 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 (>60W) Cut. During the latter mode,
power increases take place because the dynamics
of Z1 allow Q1 to saturate over a greater propor-
tion of the conduction cycle as VBASE is
increased. Once saturation occurs, excess stored
charge accumulates, extending the time required
to remove the charge on turn-off and effectively
increasing the duty cycle of the amplifier. At
lower Cut power settings, VBASE is too low to
allow saturation, so Q1 conducts only partially,
absorbing some of the power that could other-
wise be supplied to Z1. This mode is not as effi-
cient as saturated operation because Q1 sees
simultaneous voltage and current. However, the
current at this point is low enough to limit Q1's
dissipation to that which can be dissipated by the
heatsink without excessive junction temperature
rise. When VBASE drops to below about 0.6 V,
no power is delivered to the load since this volt-
age is too low to cause base current to flow.
The PA consists of a single power MOSFET dri-
ving the emitters of three bipolar transistors. Each
bipolar base has its own bypass capacitor and is
driven from a common VBASE supply. Each col-
lector and each base is separately fused, allowing a
failed part to disconnect itself from the circuit
without seriously affecting performance. Failure
of a single bipolar transistor will reduce the RF
power available by about half, since the other
devices will continue to operate.
Voltage snubbing networks protect GATE,
VBASE, and the power MOSFET drain from
being damaged in the event of any transistor fail-
ure. This limits the extent of failure damage. Each
collector is equipped with a diode which allows
the voltage on the output bus to swing negative
with respect to ground, as it does in all monopo-
lar modes of operation at sufficiently high power
and load resistance.
In Cut modes, GATE is a fixed frequency rectan-
gular pulse and VBASE is varied from about 0.3
to +8.5 Vdc to control output power. The same
is true in Blend except that GATE is further mod-
ulated to produce dead time with no output. In
Monopolar Coag, VBASE is fixed while GATE is
varied in length. This variation in conduction
time controls the amount of energy stored in the
inductive part of Z1 every cycle and therefore
varies the output power level. Bipolar Coag Mode
uses a fixed VBASE and varies the number of
pulses per cycle period to control output power.
See Figure 3.5 for representative GATE drive
waveforms.