Switched-Mode Power Supply Ps (Ps Part); Technical Data; Functional Principle; Supply Voltage Part - Hitachi VTMX900ECT Wartungshandbuch

1. Switched-mode power supply PS (PS Part)

1.1 Technical data:

Mains voltage: 195-264 V
Maximum output: 15W / 40W (continuous / maximum output)
Operating frequency: 40 kHz
Efficiency: approx. 75 % at maximum output
Six different direct voltages are supplied on the power supply
outputs.

1.2 Functional principle:

This power supply functions in a similar way to a blocking oscillator.
In the supply voltage part [1300 to 2318], the mains voltage is
rectified and buffered in the capacitor [2318]. From this direct
voltage [2318] energy is transferred into the transformer [5301,
pins 1-3] during the conductive phase of the switching transistor
[7302] and is stored there as magnetic energy. This energy is
passed to the secondary outputs on the power supply in the in the
blocking phase of the switching transistor [7302]. With the switch-
on time of the switching transistor [7302], the energy transferred in
every cycle is regulated in such a way that the output voltages
remain constant regardless of changes in the load or input
voltages. The power transistor is activated using the integrated
switch [7303] Fig.1.

1.3 Supply voltage part

The supply voltage part extends from the mains socket [1300] to
the capacitor [2318]. Using the diodes [6310, 6311, 6312 and 6313]
the a.c. supply voltage is rectified and buffered using the capacitor
[2318]. The line reactor [5305] and capacitor [2316] create a filter to
keep interference arising in the power supply away from the mains.
Components [1302], [3326] and [3323] protect the power supply
against short-term overvoltages in the mains, e.g. caused by
indirect effects from lightning.
MC44608
8
VI
start - up
management
Vcc
management
start - up
phase
200 µA
0
1
&
stand - by
200 µA
1
2
NC
7
rms
3318
1
DEMAG
C demag
65mV/45mV
current mirror
&
stand - by
management
0 PWM
comp
leading edge
blanking
Fig. 1

1.4 Start-up with Mains-on:

Following connection to the mains, the capacitor [2310] is loaded
via the start-up resistor [3318] and a current source between pin 8
and pin 6 on the IC [7303]. Once the voltage on [2310] and
therefore the supply voltage V
approx. 13V, the IC starts up and issues pulses to its output on pin
5. These pulses are used to control the gate on the power transistor
[7302] (see Fig.2). The frequency has a fixed setting in the IC
(approx. 40 kHz). The current input on the IC is approx. 5 mA in

normal mode.

If V drops to below approx. 10V (e.g. with power limitation) or if
cc
Vcc exceeds around 15V (interruption of the control loop), the
output on the IC [7303, pin 5] is blocked. All output voltages on the
power supply, and therefore also V
dropped to below approx. 6.5V, a new start-up cycle begins. (See
also "Overload, Power Limitation, Burst Mode" section)
1.5 Normal mode:
With the power supply in normal mode, the periodic sequences in
the circuit are divided primarily into the conductive and blocking
phase of the switching transistor [7302]. During the conductive
phase of the switching transistor [7302], current flows from the
rectified mains voltage to the capacitor [2318] through the primary
coil on the transformer [5301, pins 1-3], the transistor [7302] and
resistors [3314, 3331] to earth (see Fig.1). The positive voltage on
pin 1 of the transformer [5301] can be assumed to be constant for a
switching cycle. The current in the primary coil on the transformer
[5301] increases linearly in the pattern of U=L*di/dt. A magnetic
field representing a certain volume of the primary current is formed
inside the transformer. In this phase, the voltages on the secondary
coils are polarised in such a way that the diodes [6300, 6301, 6306,
6308 and 6309] block. From the controller on [7301], a current is
supplied to the CTRL input on the IC [pin 3, 7303] via optocoupler
[7300]. Once the switch-on time for the switching transistor [7302]
has been reached, which corresponds to the current supplied on
the CTRL input, the switching transistor is switched off.
Once the switching transistor has been switched off, the blocking
phase begins. No more energy will be transferred into the
transformer. The inductivity of the transformer will still attempt to
maintain the current which has flowed through it (U=L*di/dt) at a
constant level. As the primary current circuit is interrupted by the
shut-off switching transistor [7302], the current will flow through the
secondary coils.
current mirror
current and voltage
references
200 µA
t
thermal shutdown
&
&
PWM
latch
latch OFF phase
&
stand by
regulation
block
on the IC [7303] has reached
cc
, decrease. Once V
cc
VCC
6
6307
7300
7302
5
buffer
DRIVER
4
GND
0 1
3
CTRL
2-9
has
cc
5301
+
GB