Mindray Passport 5-Lead, Service Manual

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Block Diagrams Theory of Operation
2 - 54 0070-00-0420 Passport 5-Lead, 5L, LT, XG Service Manual
2.2.6 +12 / -23.5 Volt Converter Board
FIGURE 2-23 Internal Power Supply Block Diagram
Detail
This flyback converter runs at 100 KHz and gen12 volts (to power loads with large
step load changes) and -23.5 volts (a low current supply for the display). A 2843 PWM (U1)
drives a power fet (Q1) which switches the primary of the flyback transformer (T1). C9 and
C10 are low esr aluminum electrolytics which provide the instantaneous pulsitile power
required locally by the input of the converter. L1, L2 and C8 filter converter high frequency
(100 KHz) noise from the input power line. CR1,C11 and R10 snub energy stored in the
primary leakage inductance of T1. L5 and L6 are lossy ferites which damp high frequency
(10’s of MHz) oscillations in the primary switching loop. CR6 does not function in normal
coarse of operation of the converter, but clamps voltage spikes which might occur at the
drain of Q1 for short circuit conditions of the converter’s main output. Likewise, CR5 does not
function in the course of normal operation, but clamps any abnormal voltage spikes which
might couple thru Q1’s drain gate capacitance. CR4 decouples Q1’s gate from the
capacitance of CR5 so as not to slow down the switching time of the fet. R9 keeps the fet
turned off for times when U1-6 is not active. C18 and R8 provide a high speed, yet damped,
switching path between U1-6 and the gate of Q1. R3 and R4 form a feedback voltage
divider from the 12 volt output back to the inverting input (U1-2) of the error amp in the pwm.
R5, C14, and C13 provide compensation for the voltage feedback loop. C12 and C16
locally decouple Vcc and Vref of the pwm respectively. R6 and C15 are the timing
components for the internal oscillator of the pwm. T2 provides primary current mode
feedback to the pwm. The positive going current ramps are passed by CR2 and scaled by T2
and R7 to give a corresponding voltage waveform of roughly .12 volts / amp in the primary.
C17 filters leading edge spikes from the reconstructed current waveform due primarily to
discharge of Q1’s drain capacitance. CR3 allows enough voltage backswng to reset T2’s
core and yet clamps the voltage so as not to destroy Q1.
LIN E /B A T T E R Y P C B
0 6 70-00-04 49
+1 2/-23 .5V C O N VE R T E R P C B
06 70-00-045 0
S W IT C H
FU S E
IN P U T
F IL T
F E T
sw itch
Q 3
is o
g a te
d rive
lo w
b a t
cu to ff
U 1
J2 -12 V D C
J2 -13
J2 - 14
J2 - 7,8
J2 - 9
J2 - 1,2
J2 - 11
B LE N
-2 3 .5 V
+ 12 V
+ 15 V
-5 V
-1 5 V
d io d e s
J2 - 14
J2 - 9,1 0
J2 - 12
C V 1.3
C V 1.6
C V 1.5
+ 15 V
+ 5V
-1 5 V
C V 1.1
F E T
sw itch
F E T
sw itch
R E G
R E C T
&
F IL T
I
S EN S E I
S EN S E
P W M
& O .C .
p ro te ct F U S E S
F 1 -3 D IO D E
O R
J3 -1
J3 - 2
+ 17 V
B at B
B at A
C h g A
C h g B
J3 -3
J1 -2
J1 -1
J3 - 5
J3 - 6
cr 1 1,1 2
C R 8 -1 0
C 5 ,6
to
S ID E
P A N E L
R E C T
&
F IL T
F .B .
xfm r
F 1
Q 1
U 1
U 2
J4 - 1- 4
JP 4 -
1 -4
U 3
C R 1 0
C 3 ,4
C R 9
C 1 ,2
C 8 -1 0
R 1 1
T 1
Q 3
T 2
to
C P U B D