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When shutdown has ocurred, the supply is reset by turning the mains briefly off and
on. R100 discharges C25 so that the circuit is ready for use at the next switch-on. The
special supplies generated by D13, C26, and D14, C25, are designed to discharge very
quickly at power-off, and it should not be necessary to turn the PSU off for more than
a second to reset it.
Overcurrent Protection.
The simplest form of overcurrent protection is the constant-current system; when an
attempt is made to draw excessive current, the output voltage is reduced so that no
more than a fixed maximum can be drawn. The high current capability of this PSU
means that constant current protection alone is not practical as the dissipation in the
pass devices is too high for the cooling system to deal with, and in the long term they
will overheat.
The standard answer to this problem is foldback current-limiting. Once again the
output voltage is reduced to prevent excessive current flow, but it is more severely
reduced so that the current flowing is not limited to a fixed maximum, but to a value
lower than that which triggered the protection originally. This greatly reduces the
dissipation in the supply in protection mode.
This system accomplishes its task, but can give trouble as the supply may work
perfectly into a resistive test load but not start into a real console load. The problem
is that a mixing console is essentially a constant-current load; as the supply voltage is
increased, almost the full current is drawn when only 2 or 3 volts are applied. There
are thus two stable states, with rails normal and the full current drawn, or with the
supply shut down to the few volts that will cause the foldback current to flow. When
the supply is switched on, it tends to stick in the second of these two states.
The CPS2000 avoids this problem by implementing constant-current protection that
causes complete shutdown (rather than foldback) after a fixed time delay. This is
described for the +17V supply; the -17V supply operates identically.
The first part of the problem is to measure the output current without dropping 0.6V
across a resistor to turn on a protection transistor. At 16 Amps the losses in this drop
would be unacceptable. Therefore supply current is measured by the voltage drop
across 0.05R resistors (R35,36 in parallel) which carry a quarter of the output current
and so drop 200 mV at full load. This is compared with a 200mV reference voltage
across R86, derived from bands-gap reference IC8 which sits on the output rail, by
differential amplifier TR18,19. When the input to TR19 exceeds the 200mV applied
to TR18, TR19 conducts and pulls down the internal 723 node at pin 13, reducing the
output voltage.
This causes current to cease in the mutual shutdown line, as described above, and after
a brief delay caused by C27,30, both 17V supplies are shut down.
ISSUE 8 operates exactly as above. The only difference is that the differential
amplifier TR18,19 has its tail fed from a negative subrail at approx -26V. This subrail
is generated from the AC input to the PCB by charge-pump C41,42,D22,23. The
resulting negative voltage is stabilised by 8V2 Zener DZ10, and applied to R73. This
keeps the tail current of TR18,19 more constant, and so maintains the
transconductance (current out for voltage in) at a higher level, giving closer feedback
control of current-limiting under extreme conditions. The RH PCB operates
identically.
18 Circuit Description