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CIRCUIT THEORY
A T 6 Power Supply and Charger Circuitry
Main input and unregulated DC output
Charging principle, PWM (pulse width modulation) circuit.
Overvoltage protection of the battery charging circuit output (point B on the schematic power supply
MK2-6 '13.8V Batt. charger)
Operation detection modes, battery or main (point C on the schematic power supply MK2-6 '13.8V Batt.
charger)
I.
AT-6 Mains Input Circuit
The mains supply is first connected to a line filter module including two main fuses each rated at 200
mA
(S.B.). The 110 Vac version is equipped with another main line filter which is adapted for 110 Vac use
(1 10
Vac filter P/N: 4.270002), the fuses being rated at 400 ma (S.B) each. The mains is connected to the
mains transformer over a dual-pole mains switch including a monitoring lamp (190..240Vac
=
green
pushbutton, 1 1 OVac
=
red coloured switch button P/N 4.22001 0
+
4.220050 for the cover). Mains connec-
tion is indicated via the integrated mains switch bulb LP1 which also proves that the two main fuses are
not blown. Overvoltage protection is provided by a VDR (variable differential resistor), which suppresses
possible mains spikes in order
to
protect the mains transformer. The mains transformer can be adapted
for both mains selections by means of connecting the primary windings in series for 190..240Vac and in
parallel for the 11 OVac mode.
The mains transformer secondary coils (parallel connected) generate an AC output varying between 20
and 25Vac between
3 - 4
connector pins P20 and P23 (see power supply schematic 190-240VacY50
Hz).
The unregulated output voltage (point
A)
is connected to a full-wave bridge rectifier (4 Schottky diodes,
SB 350) which generates an unregulated DC output corresponding to around 25 Vdc ( i 20
%).
The DC
current consumption over the secondary 'fuse Si2 (1.25 Adc) may change between 100 mA and 700 mA,
depending on the charging state of the battery. The standby DC current surge over Si2 will generally be
around 170 mA ( i 20
%) if
no battery
is
connected to the power supply and the AT-6 stays in standby
mode (no printing process). A 70
o / o
discharged battery will cause a current increase across Si2 up to
1.1
Adc. Once the battery has reached the final charging phase (around
90 %
of charge), the current will
decrease to the nominal standby current of
=170
mA. See point 5 for more details about calibration of the
battery charge circuit.
2.
A T 6 Battery Charger Circuit Description
The detection of the presence of the mains supply is provided by sensing the charging voltage which
corresponds to 4 4 . 5 Vdc (at 20°C) at the D4 anode side when the mains supply is ON, thus OV when the
mains is switched OFF (the battery voltage can be measured at the cathode of D4). The charging voltage
is controlled by a PWM circuit (U9,3524). The PWM circuit pumps energy (current) over the series
inductance L1 (300
pH)
by switching the FET (Tl)
ON,
thus providing the characteristic function d,
/
4.
When the FET is off, the function d ,
/
4
occurs (voltage build-up). The purpose of the PWM circuit is to
keep a constant regulated voltage (without being affected by temperature or load changes) by adapting
the duty-cycle ratio of the ON/OFF phase which drives the power MOSFET TI.
The duty-cycle ratio, which influences the output voltage, is controlled by the PWM circuit by means of
sensing the output regulator voltage output (point B on power supply circuit, right after rectifier diode D4).
The PWM differential error amplifier compares the inverting input of the voltage input sense (Pin #1) with
the internal 5V reference of the 3524. If for any reason the charging output voltage increases to 15.8 V,
the output of the voltage divider network (R12, R l O / R l l , R8, R9) connected after the non-inverting
voltage follower (U4) corresponds therefore to 5Vdc, thus switching the PWM output OFF (differential
input
=
0). Current limiting protection is performed over the current sense resistor R19 (0.124 connected
to the current sense amplifier input of the 3524 regulator.
-1-
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