ZOLL Base Powercharger 4x4, Service Manual

Page: 31 / 75

FUNCTIONAL DESCRIPTION
IV-10
Switchmode Power Supply Operation
At the heart of each compartment is a switchmode power supply which establishes
each of the charging configurations for the charging compartment. When a battery is
inserted and charging is initiated, the switchmode power supply provides a constant
voltage to charge the battery. As the battery charges up and the input current into the
battery falls below a predetermined level, the battery charging mode is changed by the
microprocessor to a constant current charge until the battery is fully charged. Upon
completion of the constant current charge sequence, a timed overcharge is applied to
insure that the battery has been fully recharged.
U103 is a fixed frequency pulse width modulation control IC that contains the building
blocks for the power supply. As shown in Figure 1, U103 is configured in a "buck"
orientation with power FET Q103 being the switch shown in the diagram.
30VDC+
GND
BAT+
BAT-
D113
L101
C124
Figure 1 Buck Mode Switching Regulator
Q103
When the switch is closed, current builds through inductor L101 inducing a magnetic
field. The amount of energy stored in the inductor depends upon how long the switch
is closed. When the switch is opened, current starts to decrease through the inductor.
The voltage at the cathode of D113 clamps to -0.4V as the current through the
inductor decreases (V = L*di/dt). With catch diode D113 now forward biased, there is
a path for the energy stored in the inductor to be transferred to the load. The cycle
repeats itself with the "ON" and "OFF" time of the switch determined by a feedback
control loop.
The PWM control device U103 operates at a 100KHz switching frequency which is
determined by the C107 and R118 R/C network. An internal open collector transistor
"switch" at U103.9 turns Q103 on through a series gate resistor R138 by pulling one
end of R137 to ground. D109 clamps the gate-source voltage of Q103 to acceptable
levels and the network consisting of Q105, D108 and R139 speeds up the turn-off
time of Q103. There are three error feedback amplifiers that control the "ON" and
"OFF" time of the internal switch; two of which are internal to the PWM control device
and one control amplifier (U102A) that is external. All three amplifiers are OR'ed
together at pin 3 of U103. The internal amplifiers are tied together and brought out to
pin 3 of U103 and the external amplifier U102A is diode OR'ed through D101 such
that the amplifier that demands minimum "ON" time, dominates control of the loop.
Power to the regulator IC is filtered via R115 and C117.