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DRX-2

100

3.2 Power Supply Unit

Functional principle:
This power supply works in the way of a flyback converter.
In the mains input part [1931 to 2309], the mains voltage is
rectified and buffered in the capacitor [2309]. From this
direct voltage at [2309] energy is transferred into the
transformer [5300, pins 7-5] during the conductive phase
of the switching transistor [7307] and is stored there as
magnetic energy. This energy is passed to the secondary
outputs of the power supply in the blocking phase of the
switching transistor [7307].With the switch-on time of the
switching transistor [7307], 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
mains voltage. The power transistor is driven by the
integrated circuit [7313].

Mains input part:
The mains input part extends from the mains socket [1931]
to the capacitor [2309]. The diodes [6301, 6302, 6305 and
6306] rectify the AC supply voltage, which is then buffered
by the capacitor [2309]. The common mode coil [5302]
and capacitor [2302] work as a filter to block interference
arising in the power supply from the mains. Components
[1302], [3306] and [3304] protect the power supply against
short-term over voltages in the mains, e.g. caused by
indirect lightning.

Start-up with Mains-on:
After connecting the power cord to the mains, the
capacitor [2325] is loaded via a current source between
pin 8 and pin 1 in the IC [7313]. Once the voltage on [2325]
and therefore the supply voltage Vcc of the IC [7313] has
reached approx. 11V, the IC starts up and provides pulses
at its output pin 5. These pulses are used to drive the gate
of the power transistor [7307].

The frequency of these pulses is depending on load and
mains voltage. The current consumption of the IC is
approx. 5 mA at Vcc in normal mode.

If Vcc drops to below approx. 9V (e.g. with power
limitation) or if Vac exceeds approximately 16V (e.g.
interruption of the control loop), the output of the IC [7313,
pin 5] is blocked and a new start-up cycle begins. (See
also “Overload, Power Limitation, Burst Mode“ section)

Normal operation:
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
[7307]. During the conductive phase of the switching
transistor [7307], current flows from the rectified mains
voltage at capacitor [2309] through the primary coil of the
transformer [5300, pins 7-5], the transistor [7307] and
resistors [3321, 3352] to ground. The positive voltage on
pin 7 of the transformer [5300] can be assumed as
constant for a switching cycle. The current in the primary
coil of the transformer [5300] increases linearly. A

magnetic field representing a certain value of the primary
current is formed inside the transformer. In this phase, the
voltages on the secondary coils are polarized such that
the diodes [6300, 6303, 6307, 6308, 6310, 6313, 6317 and
6319] block. From the controller [7315] a current is
supplied into the CTRL input on the IC [pin 3, 7313] via
optocoupler [7314]. Once the switch on time of
theswitchingtransistor[7307] - that corresponds to the
current supplied into the CTRL input - has been reached,
the switching transistor [7307] is switched off.

When 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 keep the current flowing at a constant level
(U=L*di/ dt). Switching off transistor [7307] interrupts the
primary current circuit. The polarity of the voltages on the
transformer is reversed, which means that the diodes
[6300, 6303, 6307, 6308, 6310, 6313, 6317 and 6319]
become conductive and current flows into the capacitors
[2305, 2312, 2319, 2322, 2326 and 2328] and the load.
This current is also ramp-shaped (di/dt negative, therefore
decreasing).

The feedback control for the switched-mode power supply
is done by changing the conductive phase of the switching
transistor so that either more or less energy is transferred
from the rectified mains voltage at [2309] into the
transformer. The regulation information is provided by
voltage reference [7315].

This element compares the 5V-output voltage via voltage
divider [3332, 3333, 3334] with an internal 2.5V reference
voltage. The output voltage of [7315] passes via an
optocoupler [7314] for insulation of primary and secondary
parts as a current value into pin 3 on the IC [7313]. The
switch-on time of the transistor [7307] is inversely
proportional to the value of this current.

Overload, power limitation, burst mode:
With increasing load on one or more of the power supply
outputs, the switch-on time for the power transistor [7307]
increases, and thus also the peak value of the delta-
shaped current through this power transistor. The
equivalent voltage of this current profile is passed from
resistors [3321] and [3352] via [3365] to pin 5 of the IC
[7313]. If the voltage on pin 2 reaches approx. 0.4V in one
switching cycle, the conductive phase of the switching
transistor is ended immediately. The check is done in each
individual switching cycle. This process ensures that no
more than approx. 60W can be taken out from the mains (
= power limitation ).

If the power supply reaches the power limit, the output
voltages and the supply voltage Vcc on pin 1 of the IC
[7313] will be reduced following further loading. If Vcc is
less than approx. 9V at any point during this process, the
output of the IC [7313, pin 6] is blocked. All output voltages
and Vcc decrease and a new start-up cycle begins. If the
overload status or short-circuit remains, the power
limitation will be activated immediately and the voltages