Vz Power Supply; Vz Supply Operation; Simplified Vz Supply - Crown Macro-Tech MA-5002VZ Service Manual

MA-5002VZ Service Manual

4.3 VZ Power Supply

VZ means Variable Impedance and is the name of
Crown's patented articulated power supply technology.
It enables Crown to pack tremendous power into just
5.25 inches of vertical rack space.
A power supply must be large enough to handle the
maximum voltage and current necessary for the ampli-
fier to drive its maximum rated power into a specified
load. In the process of fulfilling this requirement, con-
ventional power supply designs produce lots of heat,
are heavy, and take up precious real estate. And it's no
secret that heat is one of a power amplifiers worst en-
emies.
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 transis-
tors, you increase the current passing through them,
and again increase the amount of heat they must dissi-
pate.

4.3.1 VZ Supply Operation

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. Reduc-
ing the voltage reduces the heat. Since the amplifier
runs cooler, you can safely pack more power into the
chassis.
Toroid Bridge 1
+
–
Bridge 2
(MOSFETs)
– +
Figure 4.2 Simplified VZ Supply
4-4 Circuit Theory
+VCC Buss
D 810
HI V HI I
D 811
-VCC Buss
The VZ supply is divided into segments to better match
the voltage and current requirements of the power tran-
sistors. Remember that audio signals like music are
complex waveforms. Refer to Figures 4.2 and 4.3.
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 volt-
age requirements are not high, it operates in a parallel
mode to produce less voltage and more current.
The power transistors stay cooler and are not forced to
needlessly dissipate heat. This is the normal operating
mode of the VZ power supply.
When the voltage requirements are high, VZ switches
to a series mode to produce higher voltage and less
current. The amplified output signal never misses a beat
and gets full voltage only when it needs it.
Sensing circuitry watches the voltage of the output sig-
nal to determine when to switch VZ modes. The switch-
ing circuitry is designed to prevent audible switching
distortion to yield the highest dynamic transfer function
— you hear only the music and not the amplifier. You
get not only the maximum power with the maximum
safety, you also get the best power matching to your
load.
In Figure 4.2, the individual components are
shown. Upstream of the toroid transformer,
though not shown, is where shutdown protec-
tion and soft-start circuitry taps in to control AC
mains input to the power supply. The VZ Control
circuitry senses audio level and switches the ar-
ticulating VZ supplies to either parallel (high cur-
rent) mode for lower level audio, or series (high
voltage) mode for high program peaks.
Figure 4.3 shows current flow with power supply
and grounded bridge operating to-
gether. Notice that the ungrounded
VZ supply operates much like a
VZ Control
battery.
Circuitry
ing DC supply made up of two in-
ternal batteries which operate in
either series or parallel.
In both examples it can be seen that when the
MOSFET switch is off, the dual supplies are
forced to operate in a parallel mode. Audio level
is sensed via a line tapping off the NFb loop.
When audio level is rising and at about 80% of
the parallel mode supply voltage, the MOSFETs
(the switch is actually a three-device compos-
130446-1 Rev. A
More exactly, it is a float-
©2000 Crown International, Inc.