Ecler APA1000, Service Manual

Page: 5 / 138

author:
Queralt
date:
000519
approved:
num:
52.0007
version:
01.00
FUNCTIONING DESCRIPTION
title:
POWER MODULE
ECLER
Moreover, as the helper-transistor's base-emitter current is temperature-dependent, the
controlling circuitry (basically the control-transistor) compensates the safe operation
area (SOA) drift due to temperature.
If the MOSFET's drain-source voltage (Vds) drops too low, a second circuitry actuates
to alter the control-transistor's triggering level, obtaining a SOA-like curve section and
a current stage, which can be adjusted adequately in order to maintain the MOSFET's
power consumption as close as possible to its SOA.
Moreover, the amplifier also includes an ANTICLIP system.
When the amplifier reaches its clipping level, the OpAmp becomes unable to keep the
system under control, and as a consequence ±V peaks appear at its output (15V
power supply). This peaks are used to be rectified and sent to an optocupler (led-
resistor) which modifies its impedance as a function of those peaks' amplitude. The
resulting impedance is part of a voltage divider, together with the amplifier's input
impedance. So, as the optocoupler increases its impedance, the amplifier's input signal
level decreases until the system becomes stable.
Also a dual-function temperature control circuitry is provided:
- Temperature-depending control of the cooling fan speed, whose voltage supply is
variable between 7 and 14 Vac.
- Amplifier shutdown when temperature exceeds approximately 90ºC.
The circuitry is formed by LM35D-type IC, which acts like a thermal probe, an
amplifier, thermal probe level comparator and a 7805-type voltage regulator.
The amplifier is responsible for the cooling fan speed control. The comparator triggers a
relay, which cuts off the MOSFETs' bias current by shunting a 22W resistance to the
BF-type transistors' load resistors. This way, the output signal of the amplifier is
effectively cutted off.
The STAND-BY circuit.
This circuit keeps the safety relay closed for about 10 seconds, thus the MOSFET's
bias current is cutted off during this period, until the whole system reaches again a
voltage-stable situation. Due to this, hearing annoying transients and noises during start
up through the loudspeakers is avoided. This delay is obtained by a RC-cell, where
R=287K, and C=47µF/50V. During start up, this RC-cell's voltage smoothly rises until
the 40106-type Trigger-Schmitt trigging level is reached, and the amplifier starts
functioning. C=47µF resets or discharges when the unit is turned off. During a short
period of time, a BC817-type transistor acts like a switch, connecting two 75W parallel
resistors to C=47µ/50V.
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