6-12
from 10V to -1.4V, and when the output of the
differential amplifier is negative (-4V), the RAMP
charges from -1.4V to 10V.
The output of the comparator U5-1 provides the
trigger for the gated Flip-Flop (U11A, U11B,
U12A, U12B). As the RAMP charges in the
positive direction and exceeds the 10V on U5-2,
the output (U5-1) switches high, which causes a
trigger pulse for the Flip-Flop (U11-2 & U12).
The Flip-Flop outputs (U12-3 & U12-4) toggle
causing the RAMP to discharge, and as the
RAMP drops to less than 10V, the output of the
comparator (U5-1) switches back low. Using an
oscilloscope on U5-1 and the RAMP, the output
switches high for a short duration at the “peak”
of each positive RAMP cycle, providing a clock
pulse for the flip-flop circuit and each clock pulse
causes the RAMP to switch between charging and
discharging.
6.5.2 HV Regulation Control Loop
The High Voltage Power Supply ( HVDC) and
Control Circuits are part of the power control
loop. This discussion will not go into the power
control loop, but instead it will focus on how the
power control loop works with this circuit. Let us
start the discussion at the inputs labeled HV_SNS
(high voltage sense), J1-1.
The HV_SNS connects to the high voltage power
supply (HVPS) and is a ratio of the HVDC.
The HV_SNS comes into this circuit on J1-1 &
J1-2 as a 40KHz square wave that proportionally
corresponds to the HVDC. The HV_SNS signal
is filtered and then converted to a DC voltage
with a precision rectifier (U4C, U4D & associated
components). To calibrate the HVDC requires
only a single adjustment to the “loop gain” and
this is done at RA4 (labeled HV ADJ). The
amplifier U4B is a non-inverting amplifier (U4-7)
with an output that is proportional to the HVDC
at 45mV/V. (1V/.045V = 22 HVDC.) This sig
-
nal will be referred to as the “measured HVDC”.
A differential amplifier (U4A) is used for the dif
-
ference between the measured voltage and request
-
ed voltage (V
CON is referred to as requested
HVDC). When the output U4-1 is positive, the
measured voltage exceeds the requested voltage
and causes the high voltage to be reduced. When
U4-1 is negative, then the requested voltage is
greater than the measured voltage and the HVDC
is increased.
VCON is driven by a DAC that is located on the
Display Panel and has a range of 0V to 9V. In
all modes except ABC™, V
CON increases as the
power setting is increased. In ABC™, V
CON is
fixed at 9V, where 9V of V
CON results in 200V on
the high voltage power supply.
The input signal labeled P
ERR (NOTE: PERR orig-
inates on the Power Control Assembly - product
term of the output voltage and current) is used
to control the HVDC for power regulation and it
will override V
CON, but only to reduce HVDC.
PERR cannot increase HVDC greater than the
requested HVDC by V
CON. PERR is active for
Cut, Blend, Bipolar & Pinpoint Coag only and it
is part of the power regulation loop. The analog
switch U9B switches P
ERR into the circuit when-
ever one of the four listed modes is activated. The
output of U9 (P
ERR) ties into the voltage control
loop using a precision clamp (U3B), which will
pass on to the anode of the output diode (D2),
the highest voltage of the two inputs.
When the output power exceeds the requested
power, P
ERR is positive and will be passed
through D2, which puts a positive voltage on
the input to the integrator U3C. The same
applies when the measured HVDC is greater than
requested HVDC at the Differential Amp (U4A)
output. A positive voltage on the input of an
integrator will result in a negative integrator out
-
put, which reduces the HVDC.
The output of the integrator (U3C) is a positive
voltage and is used as a reference on a compara-
tor U5D. On the non-inverting input (U5-12) of
the comparator is the RAMP that is synchronized
to the AC Line. Each time the RAMP on U5-12
exceeds the reference voltage on U5-13, it causes
the output of the comparator to switch from low
to high.
The purpose for the NAND function (U12C)
insures the triac trigger cannot occur before the
ramp transitions downward, thereby limiting the
triac triggering range to the values of 80 to 170
degrees. U12-10 triggers U14-4 on the rising
edge which results in a pulse of approximately
100uS on U14-6. U14-6 triggers the selected
triac driver according to the mode activated.
The high voltage triac (HVTR) is selected for
Pinpoint, Spray, and ABC™, while the low volt
-
age (LVTR) is selected for Cut, Blend, & Bipolar
modes. These two signals are active LOW.
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