XTi Tracker Operation
Ben Carroll
8/21/2006
What is a tracker?
• A tracker connects between the power supply and
the amplifier stage.
• It continuously adjusts the rail voltage to the
amplifier output stage to the minimum needed.
• Creates a more efficient system than two step
supplies such as VZ, H, or G
• Is not as efficient a comparable BCA design.
Why use a tracker ?
• Improved electrical efficiency
– Increases the overall amplifier output power capability
from the output semiconductors
– For a given output power it reduces the current drawn
from the wall, which leads to a smaller power supply
and line cord.
– For a given output power, the main amplifier and power
supply heatsinks can be significantly smaller
• Ability to limit the maximum voltage across the
output semiconductors
Prior Art - tracker
Half Bridge
Full Bridge
Output
Output
Prior Art – Class H
Output
Xti implementation
Two inductors added to traditional Class H design
Output
Tracker at Work
• Black trace is
the tracker
output
• Upper blue trace
is the amplifier
output
• Lower blue trace
is the tracker’s
error amp output
Unique properties of this
implementation
• Lower amplifier rails supply both the output
stage and the control supply
• Tracker power stage is not switching at idle
• Partial power processing
– Further increases efficiency
– Reduces voltage stress on MOSFET and Diode
XTi4000 –7 schematic
Xti4000 –8 schematic
Circuit commonality
• Four nearly identical circuits
– Look at blocks for only one Channel
Functional Blocks
•Each one is comprised of:
–Precision half wave rectifier
–PWM level translator
and offset generation
–Gate Driver
–Error Amplifier
–Power Stage
–Error Signal translator
Error signal translator
TL494
Control IC
–TL494 control IC
Error
Amp
Precision half wave …
PWM level
translator
Power
stage
Gate
Driver
Precision half wave rectifier
• D60 diode pair form a
precision half wave rectifier
• Inverting stage
• D50 clamps the maximum
drive signal. R714 sets the
scaling.
• R397 sets the static offset
• C277 and R392 are a
feedforward compensation
network
• C223 locally compensates the
op-amp
• C174 is a peak hold cap
Error Amplifier
• Inverting error amplifier, with
inverting terminal at a virtual
ground.
• R701 and R699 form the DC
feedback path. C255, R742,
R94 form the AC path
• D73 forms an error amp clamp
• The drive signal at C174 is
inverted
• The error amp out signal is not
inverted
Error signal translator
(Old)
• Translates the error amp
out signal to one that
matches the input voltage
range of the TL494 control
IC
• 4V = minimum duty cycle
• 0.5V= maximum duty cycle
• Translation is dependant
on the accuracy of the +/15V rails
Error signal translator
(New)
• Revised circuit used in –8 and
later PWA
• Uses TL494 Vref supply
instead of +/-15V supply
• Significant portion of Vref
tolerance is canceled out
Vref
Vref
TL494 Control IC
• Internal Oscillator
running at about
500kHz
• R324 increases the
maximum duty cycle,
D33 is needed when
C71 is discharged
• Translated error amp
signal is injected on
pin #3
• Inverted PWM output
can be seen on pins 8
and 11
• Vref is 5V
PWM level translator
• Similar to circuit used
on Itech and drive
pack
• Consists of two
grounded base stages.
• R77 and R734 sets the
translator current
• R734 removes charge
from Q55 base, to
speed the part up at
turn off
• Q46 is never active,
its parasitic
capacitance cancels
out capacitance in
Q62
Gate Driver
• IR2106 (U26) translates
the high side (positive)
gate drive signal and
buffers the low side
(negative) signal
• Q33 and Q42 form a
buffer to increase the
drive current capability
• D79 and D77 protect the
IR2106 from
overvoltage transients
• Power for high side gate
drive is fed from two
source:
– Current source
– Bootstrap (D32)
Power Stage
• Each channel requires two
power stages, each with one
MOSFET and Diode
• D176 and D177 are flyback
diodes
Questions?