GE Cardiocap 5 Service Manual

							
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5.2.2 Main structure 
The general schematic structure of the SR92A720 power supply is shown below.  
RFIINPUTFILTER
L
N
PE
100...240V~
V7R16
V1
C9
V8
L1V26V12T1
L2
15.7V/4A
C18
X12-4
+
X12-5
X12-1
X12-2
-
X12-3
X12-6
Externalshutdownsignal
feedbackExternal
signal
GatedriveGatedrive
Inrushresistor
 
Figure 5-2. General schematic structure of SR92A720 
This AC/DC converter contains two converter stages: a preconverter and a postconverter.  The 
preconverter is an AC/DC converter which together with input low-pass filtering draws almost a 
sinusoidal current from the mains.  As a result, input line current harmonics are reduced and the power 
factor is improved, therefore, this converter stage is typically also called a power factor correction (PFC) 
converter.  This PFC converter is based on boost topology and its output voltage of about 370 V is fed 
to the subsequent DC/DC converter stage. The DC/DC converter uses a forward-type topology to 
convert the intermediate voltage of 370 V to an output voltage of 15.7 V (typical value).   
5.2.3 PFC converter 
The duty of the PFC converter is to reduce input line current harmonics and to improve the power factor 
so that the power supply meets the standards related to input current harmonics (IEC 1000-3-2 / EN 
61000-3-2).  The operational mode of the PFC converter is a boundary current mode.  When the 
inductor (L1) current reaches zero level, the FET (V1) is switched on, and, therefore, the name 
“boundary current mode.”  Respectively, when the inductor current reaches double the input line 
current value, the FET is switched off.  The switching frequency is varying because the instantaneous 
rectified input line current alternates (as illustrated in the following figure). 
INDUCTORCURRENT
AVERAGE
PEAK
On
OffMOSFETV1
o
 
Figure 5-3. Inductor current waveform 
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The PFC converter controls the intermediate voltage.  The intermediate voltage has been set to about 
370 V.  In addition, there is a small (
						

							
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Output voltage is controlled by a two-loop system.  This means that there is a fast inner-voltage-
control loop and a slow outer-voltage-control loop.  The slow-voltage control is an external voltage 
signal, which, by setting the offset to the inner-control loop, determines the level of the output voltage.  
If the external feedback signal is not connected to pin 6 of connector X12, the output voltage 
measured from connector X12 settles to approximately 10 V.  The external feedback signal cannot 
draw the output voltage lower than this 10 V level.  The maximum output voltage level that the external 
voltage control signal can cause is set by trimmer resistor R52.  Typically, the external control signal 
sets the output voltage to 15.7 V. 
5.2.7 Over-voltage protection 
The over-voltage detection circuit is not placed in the AC/DC power supply PCB (A722x).  The active-
low signal (open collector) is fed to pin 3 on connector X12.  When this pin is pulled down, the power 
supply will be shut down.  If the external shutdown signal is released, the SR92A720 unit will not start 
again even if the input line voltage is connected.  Shutdown mode can be reset by removing the mains 
plug and then reconnecting it after at least 30 seconds. 
5.2.8 Current limit 
Output current limit is set to a current level of about 6 A.  In case of over current, the power supply 
operates in a “hiccup” mode.  In this mode, due to over current, supply voltage from control circuit N2 
is switched off by means of transistor V25 causing a discharge of bulk capacitor C20.  Capacitor C20 
will be recharged by the current through resistors R80–R897.  However, because the recharging time 
is longer than the discharging time, output current rms value is low, allowing safe operation in case of 
over current.  The same current limiting feature protects the function and limits the output current 
when the output is short circuited. 
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5.3 DC/DC board functional description 
5.3.1 Introduction 
The DC/DC board converts the output voltage of the AC/DC power supply or the battery voltage to 
various supply voltages for the electronics of Cardiocap/5 monitor. Another main task of the board is 
battery charging.  
 
Figure 5-5. DC/DC board  
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5.3.2 DC/DC board block diagram 
The input voltages of the board are VDD (from the AC/DC power supply) and BAT (the battery voltage). 
The DC/DC board trims the level of VDD. Switching power supplies convert VDD or BAT to supply 
voltages for monitor electronics. The board output voltages are +3.3V, +5V, +12V, +15V, -15V and 
+15VD.  
The battery is charged when VDD is present. During mains dropouts, the monitor is powered by a 12V, 
sealed, lead-acid type battery.  
There is no CPU chip on the DC/DC board. The main processor on the CPU board controls the power 
supplies’ sequencing as well as switching off. An 11-channel, 12-bit A/D converter is connected to 
the CPU board via a slow serial-data bus.  
 
Figure 5-6. DC/DC board block diagram 
5.3.3 Structure of the power supply section 
The input voltages of the switching power supplies and the battery charger are connected by n-channel 
FETs, which are controlled by high-side driver circuits. These input switches reduce leakage current from 
the battery and make it possible for the CPU or control electronics to shut down a faulty power supply.  
Power-on sequencing (rising order of the voltages) is controlled by the main processor. The  +3.3V (the 
supply voltage for the CPU) is switched on first by the control electronics. +5V rises after +3.3V without 
CPU control. After that come other voltages. VCC_INT, +5V_INT and +2.5VREF are internal voltages of 
the DC/DC board.  
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Figure 5-7. DC/DC board power supplies 
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5.4 Detailed description of the power supplies 
Synchronous switching is used in all step-down (buck) type switching power supplies in order to 
improve the efficiency. In synchronous switching (or synchronous rectification), the normal schottky 
diode of a buck converter is bypassed by a FET conducting at the same time as the diode. This reduces 
diode loss remarkably. 
All regulated output voltages of the DC/DC board are short-circuit protected. Protection is based on 
current sensing and limiting by a switcher circuit except +15VD, which is protected by a separate 
circuit breaker. 
5.4.1 +3.3V power supply 
The +3.3V switcher is a synchronous switching step-down (buck) converter. Its input voltage is taken 
either from VDD or battery via a diode selector. The input voltage is 10 to 16V. 
The output voltage is +3.20 to +3.46V. Maximum output current is 1.8A. 
5.4.2 +5V power supply 
The +5V switcher is a synchronous switching step-down (buck) converter. Its input voltage is taken 
either from VDD or battery via a diode selector. The input voltage is 10 to 16V. 
The output voltage is +4.85 to +5.25V. Maximum output current is 2.8A. 
5.4.3 +12V power supply 
The +12V switcher is a synchronous switching step-down (buck) converter. Its input voltage is +15VB, 
which is taken from VDD via a diode or directly from the boost converter. The input voltage range is 14 
to 16V.  
+12V switcher output voltage is +11.4 to +12.6V. Maximum output current is 1.4A. 
5.4.4 +/-15V power supply 
The +/-15V switcher circuit is a flyback regulator.  
Input voltage for +/-15V switcher is VDD/BAT, which comes via a diode selector either from VDD or 
from battery. The input voltage range is 10 to 16V.  
The output voltages are +14.4 to +15.6V and -15.6 to -14.4V. Both outputs can be loaded at 0.2A 
max. 
+15V and -15V supply the analog electronics. 
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5.4.5 +15VD power supply 
+15VD is used as a “dirty” +15V supply in the monitor. It powers the thermal printer, DC motors, 
pneumatic valves, and isolated switching power supplies.  
On the DC/DC board, the regulated voltage is actually +15VB, which is named +15VD after circuit 
breaker electronics. During mains usage, +15VB is taken from VDD via a diode. When the mains 
voltage drops, the monitor continues operation on battery. In battery use +15VB is converted from the 
battery voltage by a boost type switcher. The boost converter and VDD have common control 
electronics. The operation is described further in the next sections.  
+15VB boost converter 
The input voltage range of the boost converter is 10 to 14V.  
The boost converter output voltage +15VB is +14.5 to +15.4V. Maximum output current is 3.5A. 
A boost converter cannot limit its short-circuit output current. To prevent the components from 
excessive heating in an overcurrent situation, a signal OC_15VB shuts +15VD consumption. It is also 
connected to the CPU to inform the CPU of exceeding the input current limit of the boost switcher. 
OC_15VB trip point is 6.8 to 7.9A. The output voltage will drop at a lower input-current level than that. 
This is meant to protect the output FET in fault condition.  
VDD control 
The same control circuit that adjusts +15VB boost converter also adjusts VDD when the monitor is in 
mains use. VDD is adjusted so that +15VB is at a little higher level than the boost regulator would 
regulate it at. When mains and VDD drop, +15VB falls into the lower level where the boost converter 
continues its regulation. To achieve adequate accuracy and to prevent the two adjustments from 
overlapping, the control circuits use a common reference voltage as well as a common resistor divider.  
A signal TRIM_VDD is connected to AC/DC power supply to adjust VDD. VDD is increasing when 
TRIM_VDD is increasing. The difference between the two adjustments of +15VB is 0.2 to 0.45V, which 
means that +15VB is adjusted to 14.7 to 15.85V when VDD is present.  
+15VD circuit breaker 
To make the output voltage of the boost converter short-circuit protected, a separate circuit breaker is 
added on the DC/DC board. The circuit breaker consists of an n-channel FET switch, a high-side driver, 
and a current-measuring power resistor. When the set current limit is exceeded, the FET switches the 
load off. After a short period of time the driver tries to reconnect the load. If overload still exists the 
sequence is repeated.  
The current limit is of fold-back type, which means that the trip level of the current is lower when the 
output voltage is near to ground. In other words, the load current must be reduced to a lower level than 
it was when switched off to enable reconnection. The current limit trip level is 5.1 to 7.2A. Signal 
OC_15VD/ becomes active when the current limit is tripped.  
The FET and current measuring resistor cause a voltage drop between +15VB and +15VD. This voltage 
drop is allowed to be maximum 0.25V at 4A load current. +15VD voltage range is 14.25 to 15.85V at 
load currents 0 to 4A.  
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CPU power connector (X6) 
PIN  SIGNAL/VOLTAGE I/O DESCRIPTION 
X6/1 GND   
X6/2 GND   
X6/3 +3.3V out To CPU and 3.3V LCD display 
X6/4 +3.3V out  
X6/5 +3.3V out  
X6/6 +3.3V out  
X6/7 GND   
X6/8 GND   
X6/9 +5V out To CPU and 5V LCD display 
X6/10 +5V out  
X6/11 GND   
X6/12 GND   
X6/13 +12V out To LCD backlight inverter 
X6/14 GND   
Mother board power connector (X2) 
PIN  SIGNAL/VOLTAGE I/O DESCRIPTION 
X2/1 VDD/BAT out Not utilized 
X2/2 GND   
X2/3 +15VD out To parameter boards 
X2/4 +15VD out  
X2/5 GND   
X2/6 GND   
X2/7 +15V out To parameter boards 
X2/8 -15V out To parameter boards 
X2/9 GND   
X2/10 GND   
X2/11 +5V out To parameter boards 
X2/12 +5V out  
X2/13 +5V out  
X2/14 +5V out  
X2/15 GND   
X2/16 GND   
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AC/DC power supply connector (X8) 
PIN  SIGNAL/VOLTAGE I/O DESCRIPTION 
X8/1 VDD_SHUTDOWN/ out Shut AC/DC power s. 
X8/2 GND   
X8/3 GND   
X8/4 TRIM_VDD out Adjust VDD 
X8/5 VDD in AC/DC output voltage 
X8/6 VDD in  
5.4.6 Fan control 
The supply voltage VFAN is linearly regulated from +15VD. The operating voltage range of the fan used 
is 10.2 to 13.8V. The fan is CPU controlled by signal FAN_ON. In the circuit used, VFAN drops to the 
level of 1.2 to 1.3V when FAN_ON is inactive. 
Fan connector (X3) 
PIN  SIGNAL/VOLTAGE I/O DESCRIPTION 
X3/1 VFAN out Fan supply 
X3/2 GND   
5.4.7 Battery charger operating principle 
A 12V 2.6Ah sealed, lead-acid battery, with a lifetime of approximately 5 years, is used as a back-up 
supply.  It will run the monitor for 15 minutes in case of mains dropout. It is not possible to start the 
monitor on battery. The battery is charged whenever VDD is present. The charger will operate without 
CPU control, because the CPU is out of use during monitor standby.  
VDD is used as charger input voltage. 
The control circuit is Unitrode UC3906, which is optimized for lead-acid batteries. The configuration 
used is a dual-level float charger with three charge states: a constant current bulk-charge state, a 
constant voltage over-charge state, and a float-charge state. A cycle begins with the charger in the 
bulk-charge state, sourcing the battery with a constant current Imax. When the battery reaches the 
voltage level Voc, the charger goes to over-charge state. The battery voltage is kept at a constant 
voltage Voc until the charge current has reduced to a level of appr. Imax/10 (Ioct). The charger enters 
the float-charge state, where a lower constant voltage Vfloat is supplied to the battery.  
The float voltage maintains the capacity of the battery and it can be fed to the battery indefinitely. The 
lifetime of a lead-acid battery greatly depends on the float-voltage level. The correct level is, in turn, 
temperature dependent. A proper charging method requires temperature compensation of Vfloat. On 
the DC/DC board, the internal-reference voltage of the battery-charger circuit tracks the temperature 
characteristics of lead-acid cells.  
If the battery voltage is below the threshold voltage (Vt) the charger enters a trickle-charge mode, 
where the charge current is reduced to value It. An overdischarged battery is charged with the trickle 
current (It) until the battery voltage reaches the level Vt above which the bulk-charge begins. The 
charger output short-circuit current (Isc) is limited by the same resistors as the trickle current. 
Maximum charge current for the 12V 2,6Ah battery is 0.78A. 
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