Fresenius Medical Care 4008 E Technical Manual page 449

+24-V regulator (circuit diagram 1/2)
The input voltage for the +24-V regulator is applied across P5 and P3. Supplied with clock pulses,
this input voltage is connected to the storage reactor L1 via the transistor T3. With T3 being
closed, the difference between input voltage and output voltage is above L1 (P8 more positive
than P9). This fact causes the current in the reactor to increase linearly. The freewheeling diode
D4 is inhibited during this phase.
As soon as T3 opens, D4 becomes conductive. The output voltage plus the on-state voltage of D4
is now applied across the reactor (P9 more positive than P8). The current in the coil, which now
also flows via D4, decreases linearly until switch T3 closes again. The output voltage for the
duration of the individual switching cycles is held constant by means of C20.
The output voltage is regulated by IC 1, which is provided as pulse width modulator. This
modulator is supplied with pulses at a frequency of approx. 70 kHz (determined by R3 and C4).
For regulation, the pulse width of the output signal (IC 1, pins 12 and 13) is modified.
In order to make a regulation possible, the output voltage is fed back to IC 1, pin 1, via R9 and R2.
Pin 2 of IC 1 is connected to the reference voltage of the module (IC 1, pin 16). IC 1 regulates the
pulse width such that the difference between the voltages of IC 1, pin 1, and IC 1, pin 2, becomes
zero. Via an inverter stage (R31, R32, and T5), the pulse-width-modulated signal switches
transistor T2 which, in turn, is used to activate the transmitter UT1. At the output of UT1, diode D6
cuts off the negative portions of the transmitter voltage. Via D5 and R26, the signal reaches the
gate of the main switching transistor T3. Transistor T4 is used to short-circuit the gate-source
capacity of T3 at its cutoff moment. This leads to the achievement of as short a switching time as
possible and a considerable reduction in switch dissipations.
The current which flows through T3 is constantly monitored via a current multiplier (between P4
and P7). To this end, the voltage at the current multiplier is amplified via IC 3, pin 6, R10, and R13
and delivered to a comparator (IC 3, pin 3). The amplification factor has been selected such that
the comparator output (IC 3, pin 1) changes its state at a current of approx. 15 A. The signal of the
comparator output is directed to IC 1, pin 4, thus causing the current clock pulse to be interrupted
immediately and the pulse width to be limited to a lesser degree (quasi-soft start) upon the next
clock pulse due to the connection to IC 1, pin 9 (T6, R4, and C3). As a consequence, the output
voltage (+24 V) drops.
In order to protect the following circuits and solder tracks, the +24-V voltage is monitored via IC 5.
Should the output voltage fall below +22 V (R20, R22) for more than approx. 200 ms (determined
by C17), IC 5 cuts PWM IC 1 off via the latter's pin 10 (low-voltage recognition). The regulator
then starts again only after the machine has been switched off and on.
Should an excess voltage develop at the output (> +26 V, R12 and R21), IC 5 activates the triac
TR1, which short-circuits the output voltage, thus releasing the fuse SI1 (at P5).
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Fresenius Medical Care 4008 4/09.03 (TM)