The Vz Supply; Vz Switch Control; Vz Supply In Parallel Mode; Vz Supply In Series Mode - Crown Macro-Tech MA-3600VZ Service Manual

130366-1 Rev. A
According to Ohm's Law, the bigger the power
supply, the more heat the power transistors must
dissipate. Also, the lower the resistance of the power
transistors, the more voltage you can deliver to
the load. But at the same time that you lower the
resistance of the transistors, you increase the current
passing through them, and again increase the amount
of heat they must dissipate.

3.7.2 The VZ supply

An articulated power supply, like VZ, can circumvent
much of this problem by reducing the voltage applied
to the transistors when less voltage is required.
Reducing the voltage reduces the heat. Since the
amplifier runs cooler, you can safely pack more
power into the chassis.
The VZ supply is divided into segments to better
match the voltage and current requirements of the
power transistors. Remember that audio signals like
music are complex waveforms.
For music the average level is always much less than
the peak level. This means a power supply does not
need to produce full voltage all the time.
The VZ supply is divided into two parts. When the
voltage requirements are not high, it operates in
a parallel mode to produce less voltage and more
current (Figure 3.3). In this mode the power transistors
stay cooler and are not forced to needlessly dissipate
heat. This is the normal operating mode of the VZ
power supply.
POWER
TRANSISTOR
SPEAKER
LOAD
POWER
TRANSISTOR
Figure 3.3 VZ Supply in Parallel Mode
When the voltage requirements are high the VZ supply
switches to a series mode to produce the higher
voltage and less current (Figure 3.4). The amplifi ed
output signal never misses a beat and gets full voltage
when it needs it—not when it doesn't need it.
©2000 Crown International, Inc.
VZ POWER SUPPLY
+ +
VZ VZ
STAGE STAGE
POWER
TRANSISTOR
SPEAKER
LOAD
POWER
TRANSISTOR
Figure 3.4 VZ Supply in Series Mode
Sensing circuitry watches the voltage of the signal
to determine when to switch modes. The switching
circuitry is designed to prevent audible switching
distortion to yield the highest dynamic transfer
function—you hear only the music and not the
amplifi er. You get not only the maximum power with
the maximum safety, you also get the best power
matching to your load.

3.7.3 VZ Switch Control

The two halves of U03 form identical comparators
that monitor the available voltage of DC supply V2
and compare it to the output voltage of the amplifi er.
When a positive going output voltage exceeds a
predetermined ratio of the available supply voltage,
U03 pin 1 produces a low voltage triggering U04.
When triggered, the "Q" output of U04 changes from
low to high driving the gates of FET's Q00, Q01, and
Q02. The other half of U03 (pin 7) reacts to negative
going output voltage. Both halves of U03 receive V2
and amplifi er output voltage differentially.
The time constant set by C18 and R16 on the input
of U04 sets the maximum switch frequency of the
supply. This time constant forces the supply to stay in
the series mode regardless of amplifi er condition for
200 ms. The reset pin of U04 (pin 4) forces the output
of U04 low when FET damage conditions exist.
C16 and C17 provide hysteresis around the compara-
tors of U03 to insure stable operation.
VZ Protection Circuit
Protecting high current transistors can be troublesome
in circuits that do not provide convenient current
sample points. FETs Q00-Q02 fall into this class of
problems, but protection has been designed based
on the following two conditions being present at
the same time:
MA-3600VZ Service Manual
VZ POWER SUPPLY
+
VZ
STAGE
+
VZ
STAGE
Theory of Operation 3-5