Crown XTi 1000, Руководство по обслуживанию

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Tracker Theory of Operations
Ben Carroll
8/4/2006
Rev 1 – initial version
The tracker operation in the XTi products and its derivatives is an extension of the
classic class H supply (used in the XS amplifier) and is also similar to the VZ supply
used in the MA3600 and MA5000. The main power supply provides two distinct rail
voltages that supply power to the tracker’s power stage, which in turn provides the rails
to the amplifier’s output stage. The higher rail voltage is model dependant (approx 120-
140V), and the lower rail, which is about 30 V independent of model. This lower rail
also supplies the +/-15V, and 3.3V regulators. The trackers are comprised of four power
stages that individually feed: channel #1 positive, channel #1 negative, channel #2
positive, and channel #2 negative. Each of these power stages is a buck regulator
connected in series with a 30V supply. This concept is called partial power processing,
as the buck regulator does not process 100% of the power flowing through the power
stage.
With no or only low-level signals present, the buck regulators are not active and
the lower rails feed the amplifier output stage ( thru diodes D35, D36, D42, and D43 ).
This rail voltage is about the same as a D75. This makes the system very low loss at idle.
The heating due to the bias current in the pre-drivers and drivers is reduced dramatically
compared to what it would be if fed off of the higher voltage rails. Second the switching
MOSFETS
(Q47, Q48, Q60, and Q61) are not active, thus dissipate no power.
As the signal level increases the buck power stages become active. When a
positive signal is demanded of channel #1, that respective trackers control loop responds
keeping the instantaneous voltage on the output’s collector terminal approximately 20V
above its base terminal. The control loop works to keep this voltage constant, until the
buck regulator reaches its maximum duty cycle or the amplifiers rail voltage reaches a
preset maximum level. This maximum level limits the voltage across the output stage
during high line conditions. Then as the voltage begins to reduce, the buck regulator
reduces its duty cycle to maintain a constant 20V across the collector to base junction
until the rail voltage drops back down to 30V. Then the channel #1 negative side tracker
becomes active in a similar manor. Channel #2 works similar to channel #1, both
channels are completely independent of each other. Because of this four completely
independent power stages, and control loops are needed.
The following paragraphs will walk through the detailed operation of a trackers
control loop. Each paragraph will explain the operator of an individual block. During the
design process each block was carefully scrutinized to ensure that the maximum
performance was attained from a minimum component count. Therefore some blocks
contain several functions.
Absolute value detector
U23B is an inverting precision absolute value function block that also contains
phase lead compensation (C277 & R392), voltage offset generation (R397), maximum
voltage clamp (D50), and a peak hold function (C174).
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