Mindray Passport 5-Lead, Service Manual

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Block Diagrams Theory of Operation
2 - 20 0070-00-0420 Passport 5-Lead, 5L, LT, XG Service Manual
FIGURE 2-11 Power Supply Block Diagram
Power Supply
The Frontend Module is essentially an electrically isolated data acquisition system. The only
portion of this module that is directly connected to the host system is the non-isolated part of
the optoisolators (U15, U17, and U18) used for communications, and the switch mode,
forward converter, isolated power supply.
The isolated power supply (Fig. 2-11) is built around a switch mode converter integrated
circuit, U11. This device outputs two pulse width modulated signals (at pins 11 and 14) to
drive the power transformer T1 with FETs Q1 and Q2. Each of the pulses occurs during its
own respective half cycle of one full period. The half cycle timing is set by R178 and C83.
The modulation of the FET drive pulses is executed by comparing the bulk power voltage
VDC, with a percentage (set by R138 and 179) of the reference voltage on U11 pin 16. In
short, the modulator outputs wide ( 83% of a half cycle) pulses for low VDC values, and
narrow (35% of each half cycle) pulses for high values of VDC. The range of VDC is
specified at 9 volts minimum to 18 volts maximum. The voltage being switched across the
primary of T1 (pins 1, 2, and 3) is transferred to the secondary windings (pins 4, 5, and 6),
and filtered to a DC voltage by components CR30,31,33,36, L1 and L2, and C169 and
C171. The isolated DC voltage will vary between approximately 7.1 volts and 9.3 volts,
depending on VDC and the secondary load current requirement.
VDC
VDC
L1
L2
T1
+7.5I
-7.5I
Q9
Q8
U47
+7.5I
+5D
+5V
U49
U50A
U48
+5T
U50B
+2.5v
Pulse
Width
Modulator
Regulator
Tracking Regulator
Isolation Power Supply
PGND
Non-isolated