Circuit Description; Switched Mode Power Supply; Control Method; Output Voltage Regulation - Hitachi C1422R Service Manual

C
IRCUIT

Switched Mode Power Supply

Summary
The power supply is a self-oscillating discontinuous buck converter (step-down chopper) under peak current
control and designed for up to 65W output power at nominally 98V. The main B+ output of this converter
topology is inherently non-isolated and as such, output voltage feedback does not require an opto-coupler.
Some energy is derived in flyback mode via an isolated winding on the buck inductor to provide a low power
10V secondary supply from which are derived switched 8V (video processor) and permanent 5V (µP)
supplies.
In stand-by mode, both the B+ and 10V supplies remain close to their nominal operating levels but with
virtually zero load, the PSU enters a burst-mode whereby typically 180 in 200 cycles are skipped. In this way,
the stand-by power consumption (including degaussing circuit) is less than 4W.
Description of Operation of the Buck Converter
Figure 4 shows the main components of the buck converter. The output voltage may be controlled between
zero and the input voltage by varying the on-time of Q801. During this period, Vin-Vout is applied across the
buck inductor (T802 winding pins 7 & 8) and the current in it ramps up linearly. When Q801 is turned off, -
Vout is applied across the buck inductor and energy is delivered to the load and C809 combination.
V
IN
Start-Up
At start-up, the overwind output voltage (T802 winding pins 7 & 8) is not present so a start-up bias circuit is
required. This consists of R806, D802 and R809. Approximately 16V is produced at the junction of R806 and
D802 which is fed via R809 to turn on Q801 for the first time.

Control Method

During the on-time of Q801, the buck inductor voltage flows through the current sense resistor R814 forming
an analogue of the buck inductor current. This is fed to the base of Q802 along with a DC bias current from
the error amplifier (via R813) such that once a certain buck inductor current level is reached, Q802 turns on.
When this happens Q801 is turned off and the inductor current free-wheels through D804. This is peak
current-mode control.

Output Voltage Regulation

The non-isolated B+ rail means that no opto-isolator is
required for direct Vo control. It can be seen from Figure 5 that
Vo minus a small zener voltage drives the emitter of this
transistor (Vo-Vz) whilst a resistive potential divider feeds the
base (kVo, k<1). If, for instance, the output voltage tends to
rise, Ve rises by ∆Vo whereas Vb only rises by ∆kVo. The net
result is an increase in negative bias and an increase in
collector (control) current. R818 in the potential divider
provides an element of control over the B+ voltage.
D
ESCRIPTION
280 - 390
Q801
Fig 4 The Buck Converter Topology
T802 (pins 7 & 8)
C809
D804
Vz
R815
10
(100V)
V
OUT
D806
Ve
Vb
Control Current
Fig 5 The Error Amplifier
R817
R818
kVo
R820