Hitachi CP2155TA, Руководство по обслуживанию

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CIRCUIT DESCRIPTION CIRCUIT DESCRIPTION
SWITCHED MODE POWER SUPPLY SWITCHED MODE POWER SUPPLY
Summary
The D4N power supply is of a discontinuous isolated flyback
design with quasi-resonant mode switching. The supply is
designed for a maximum of 84 watts output power.
Incorporated in this design is a secondary side feedback for
accurate control of the B+ output, and a standby mode which
reduces the value of all outputs but still maintains the 5 volts
output. Power consumption in this mode is less than 10
watts.
Description
The power supply is a self-oscillating, discontinuous flyback,
switching converter dependent upon the hybrid integrated
circuit IC801 (STR-F6523) for control and protection
functions. The operating frequency and pulse duty ratio vary
according to load and input voltage conditions. The
operating frequency range is 28kHz to 100kHz. IC801 also
contains a MOSFET power switching component, used for
switching the main primary winding of T801 across the
rectified AC supply voltage stored in the reservoir capacitor
CE808. AC output voltages are generated on secondary
windings and are half wave rectified and smoothed by
separate diode/capacitor networks to produce DC voltages
of 115 (120 volts on 20" models), 80, 18, 13 and 8 volts,
approximately.
Start-up voltage to IC801 is provided via R803 and R804
from the AC supply. When the PSU has started switching,
supply to pin 4 of IC801 is provided via D810 and CE809
from a primary winding on T801. This winding is also used to
provide a trigger signal for pin 1 of IC801, such that the on-
time is initiated only when the secondary voltages have
decreased and the stored energy in the transformer is zero.
Another primary winding is used to power IC801 when in
standby operation through D809 and CE812
(refer to main
panel schematic diagram)
.
The 115V H.T. supply is maintained at a constant voltage
regardless of load or input conditions (note: all other
secondary rails without secondary regulation will vary
slightly). This is achieved by comparing an attenuated
representation of the 115V rail to a reference voltage source,
IC803, and using this error signal to control IC801 on-time,
via opto-isolator IC802. As a consequence, the output
voltage is kept stable. RV820 adjusts the level of attenuated
voltage applied to IC803 and therefore, will indirectly control
the output voltage.
Protection functions are provided by IC801 and include
primary current limit, overvoltage and thermal shutdown.
Pulse by pulse primary current limit is sensed via R807 and
is set at 1.35 volts internal to IC801. If the voltage across
R807 exceeds this level the on-time is terminated. The OVP
and TSD features both enable the latch-off of IC801 and
consequently the PSU shut-down. The OVP typically
operates at 22V DC on pin 4 of IC801 and TSD at 150°C
junction temperature.
Standby Operation
When the chassis is in standby mode control of IC802 diode
current is diverted from IC803 to network D819, R822 and
TR803. This enables the standby supply circuit based
around TR802 and reduces the standby supply by regulating
the voltage across CE823 to approximately 16 volts (via the
action of ZD818 and TR803) and consequently reducing all
other voltages. This reduction in all output voltages applies
to the primary circuit where the signal for pin 1 from D811 in
now insufficient to trigger the on-time circuit and the
controller reverts to maximum off-time and minimum
frequency operation, typically 28kHz. The normal supply
winding for IC801 is now too low so the other primary
winding (mentioned above) supplies pin 1 via D809, R809
and TR801.
IMPORTANT: pins 2 and 5 are approximately -340 volts
relative to the chassis
Detailed Design Description
The input mains supply is connected to PL801 and filtered
by C801, FL801 and C802 before connection to S801.
Provision is made for a remote switch via PL803 and PL804.
The degauss coil is connected via R831, a dual thermistor
package and PL802. At switch on the positive coefficient
thermistor is low resistance and a high current flows in the
degauss coil. As the thermistor warms up its resistance
increases and the current is gradually reduced to nearly
zero. The second thermistor, across live and neutral, heats
the first thermistor helping to increase its resistance.
The filtered mains supply is full wave rectified by D801-4 and
filtered by CE808 to produce a high DC voltage. supply.
An inrush limit is provided by thermistor R802 which is a
negative coefficient thermistor.
IC801 contains an internal MOSFET and PWM controller.
When the MOSFET is switched on, current flows through the
primary of T801 (pin 13 to pin 11), through the internal
MOSFET (IC801 pin 3 to pin 2) and finally through the
current sense resistor R807. FB801 is provided only to
reduce high frequency components at switch on/off. Current
cannot flow in the secondary circuits because output diodes
now block its path.
For example: Current flow in the primary from pin 13 to pin
11 induces a voltage between pins 1 and 4 to force a current
flow out of pin 4 around the circuit and back through D817 to
pin 1. Diode D817 blocks this.
The current in the primary circuit linearly ramps up from zero
to a maximum value determined by the on-time control of the
PWM controller, IC801. The transformer is thus now
charged.
When the MOSFET switches off, this stored energy is
released into the secondaries as the induced voltage now
reverses and current flows through each of the output
rectifier diodes. During this time the voltage across T801
primary is positive on pin 11 with respect to pin 13.
Hence the voltage on the drain of IC801 equals the sum of
V
CE808
, and V
T801
typically equals about 500V at 230V AC
input.
When all the energy has been discharged the induced
voltage across T801 primary collapses and the cycle starts
again. As the voltage collapses the leakage inductance of
T801 rings with C811. This is used to implement Quasi-
resonant switching. C811 and T801 ring with a
predetermined frequency which allows the timing of the
‘switch-on’ of the next cycle to occur at the precise moment
that the voltage is at a minimum. This reduces switching
losses and also reduces EMC emissions because the
otherwise sharp edge at turn-on is removed.
Fig 2. shows the typical IC801 drain (pin 3) waveform during
normal operation at 230V AC at 40 watts load.
Note the low amplitude leakage spike at switch off and the ¼
sine wave prior to switch on as the leakage inductance rings
with C811, so that the drain voltage falls to a minimum value
prior to switch on. (Quasi-resonant switching).
Note also the ramping current typical of a flyback converter
and the zero initial current showing that the stored energy in
T801 has reset to zero defining discontinuous operation. The
initial current spike is the discharge of C811 at switch on.