Fresenius Medical Care 4008 4/09.03 (TM) 8-151
8.17 P.C.B. LP 638 Power supply
8.17.1 Description
● General information
Three regulated voltages are available at the outputs of P.C.B. LP 638:
– +5 V/2.5 A
– +12 V/1.5 A
– +24 V/15 A
One switching regulator is provided to generate each of these voltages.
Due to its higher capacity, the +24-V regulator is made out of discrete components (switching
transistor, current limitation).
One integrated type-L-296 switching regulator is used to generate the +5-V voltage and one to
generate the +12-V voltage.
● +5-V regulator (circuit diagram 2/2)
Via points PGL1 and PGL2, the AC voltage of the 20-V winding of the main transformer is directed
onto the P.C.B. After having been rectified via GL1 and smoothed by means of C26, a non-
regulated voltage of approx. 26 V is available as input voltage for the regulator. High-frequency
disturbances are filtered via the input filter comprising DR100 and C101. Capacitors C100 and
C102 are mainly used for additional smoothing of the input voltage, which is applied to IC 100, pin
3, of the switching regulator. The storage reactor DR101, the freewheeling diode D100 as well as
output capacitors C106 and C107 are applied to the output of the regulator IC 100, pin 2.
Capacitor C108 is used to suppress high-frequency spikes, which might be caused by the
switching procedure. The maximum current is limited to approx. 2.5 A by resistor R101. The
switching frequency of the regulator is determined by C110 and R103. It amounts to approx. 50
kHz. To be regulated, the output voltage is fed back via IC 100, pin 10. Via input IC 100, pin 1, any
overvoltage is recognized at output IC 10, pin 2 (e.g. caused by a defect of the L-296 switching
transistor). Via IC 100, pin 15, the thyristor TR100 is energized and the output, thus, short-
circuited. Due to the resultant high current, fuse SI2 is released.
In addition, IC 100 is used to generate the power reset (e.g. upon start of the machine). The power
reset is available at IC 100, pin 14. Resistor R102 is provided as pull-up resistor to +5 V.
● +12-V regulator (circuit diagram 2/2)
The circuit is designed in the same manner as that of the +5-V regulator. However, pin 14 of IC
200 is not utilized. In addition, only a part of the output voltage is fed back via voltage dividers
R203 and R204, in order to generate the +12-V voltage.
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