Crown MARCO-TECH 5000VZ User Manual page 13

Notes
level has dropped sufficiently. Capacitors in the U705 circuitry control
the speed of the down-shift. In the Bi-ODEP mode, the switch opérales
as it would in AUTO mode unless ODEP limiting ¡s in progress. When
ODER ümiting occurs optic coupler U704 shorts a negative potential (-
VCC) to the 555 reset control to turn the MOSFET switches off and keep
them off (low voltage/high current mode) until the ODEP limiting
condition clears.
Upstream of the toroids are the soft-start and protection mechanisms
used to power down the amplifier. See Protection Systems.
The low voltage power supply utüizes a sepárate transformer. The front
panel power switch and a 1A fuse (F702) are the only components
upstream of this transformer. The output of the transformer produces
±24VDC unregulated. U715 and U716 produce regulated ±15VDC
respectively. A sepáratefullwave rectif¡er produces pulsed DC for Over-
voltage sense and Soft-start control.
ODEPTHEORY: MA-5000VZ
To protect the output stages from adverse thermal conditions, a
specially developed "ODEP" (Output Device Emulation Protection)
circuit is used. It produces a complex analog output signal proportional
to the always changing safe operating área (SOÁ) margin of the output
transistors. This output signal controls the Voltage Translatorstage and
Low Side output stage bias. This action removes only the drive that may
exceed the safe-operating-area of the output stage.
Thermal sensors give the ODEP circuitry vital ¡nformation on the
operating temperature of the heat sinks on which the output devices are
mounted. This temperature signal combines with the complex ODEP
signal to form the heart of the patented ODEP protection scheme. Ref er
to block diagram on the next page for a discussion of the basic operation
of the ODEP system.
The ODEP circuitry actually comes ¡n two parts, one positive and the
other negative. For the purposes of discussion, only channel 1 ODEP
circuitry is covered here, and focus will primarily be on the positive half.
An LM-334Z thermal sensor provides a calibrated output from the output
modules. At 25°C its output is 2.98V, with a 10mV increase per every
1°C rise in heatsink temperature.
This thermal sensor output, from the positive sensor, goes to three
destinations. First is a buffer which drives the calibrated temperature
test point at pin 7 of TP1/TP2. Second ¡s an over-temperature limit trip.
This will cause both the positive and the negative ODEP circuit to go into
and remain in hard ODEP until the heatsinks cool. Third, it goes down
into a circuit which combines thermal and output power Information for
proportional fan control and, whenneeded, limiting. The thermal sensor
from the negative output module only performs this last function.
A pair of sense lines from the Low Side emitter resistors provide current
information. Combined with VCC information actual instantaneous
power ¡s calculated. A combining circuit determines the net thermal
12-MA-5000VZ