Invivo Millennia 3155A, Руководство по обслуживанию

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1-6
The description of the 5V supply will also suffice for the 3.3V section, with exceptions as noted.
The input power to U14 is RAWPWR, and it is applied all the time that the power supply has an
input voltage. Turn-on of U14 is done via the /SHDN pin brought HIGH. When U14 becomes
active, the gate drive to transistor Q26 turns it ON. Transistor Q26 switches RAWPWR into the
energy-storage inductor L10, causing the inductor current to ramp up, delivering current to the 5V
devices on the 5V bus. After a period of time, transistor Q26 is turned OFF, and the stored energy
in inductor L10 maintains its current as a decaying current ramp, with transistor Q17 supplying the
current. Diode D37 passes current if Q17 is not ON when the transistor Q26 is turned OFF.
After some time, transistor Q17 is turned OFF, and transistor Q26 is turned ON to repeat the cycle.
U14 adjusts the duty cycle of the transistors so that the output voltage is regulated. Because the gate
of Q26 must swing volts above its source, which swings the full amount of the input power, U14
provides a bootstrapping action via pin 18 capacitively sensing the swing of the inductor through pin
17. Diode D38 acts as a rectifier pumping the Q17-drive voltage supply up to approximately five
volts above the peak of the inductor swing. The drive from pin 16 then tracks the inductor waveform
about five volts above it, allowing transistor Q26 to stay firmly ON as needed.
Current limiting is provided by controlling the voltage drop across R97, a low-value resistor, and the
voltage-sensing for the regulation is into U14, pin 21.
The 3.3-V supply is similar, except that it has its own circuit section, including transistors Q24, Q18,
and inductor L9. A similar bootstrapping is provided for this side, also. Current-limiting is done
with resistor R96, and the voltage sense is into pin 1.
Diode D46's voltage drop provides a slightly higher output voltage for the five volt supply than U14
normally supplies, to overcome wiring voltage drops.
Capacitors C70 and C85 allow a soft start of U14; capacitor C27 bypasses the internal reference of
U14; diode D14 clamps the highest turn-on voltage to a safe level for U14. Transistor Q30 is the
local five-volt source for the bootstraps. Note that transistor Q30 is OFF if U14 is OFF.
The resistor R103 may be removed to put the three-volt section of U14 on standby, if there is no use
for the three-volt supply. The three-volt supply will be used for motherboard power needs whenever
three-volt processors are used.
1.2.5 +12 and -12 Vdc Supplies. (Please refer to 185C187, sheet 2, and 185C197.) A flyback
transformer (T1, on AB69A) supplies voltages used for analog functions. The operation cycle starts
when power-control IC U11 is turned ON by mono U1A (pin 10 goes LOW). U11 delivers a drive
pulse to the gate of transistor Q6 (on AB69A) that turns it ON; current from RAWPWR begins to
flow in the transformer primary, storing magnetic energy in transformer T1's primary. At this time,
none of the output rectifier diodes are conducting.
After a period of time, the drive to the gate of transistor Q6 is shut OFF, and it ceases conducting.
A voltage spike occurs on transistor Q6's drain at this time (arising from the energy discharge of the
leakage inductance of T1's primary). The rest of transformer T1's stored energy is available to the
loads, and the rectifier diodes all turn on, delivering this energy to their various output capacitors
until either all the stored energy in transformer T1 is dissipated, or transistor Q6 is turned on again.
After a period of time, transistor Q6 is turned ON again by U11, and the cycle repeats. Voltage
feedback is provided from the +12V output through resistors R75 and R76 to regulate the +12V
output; the other outputs approximate the 12V output according to their relative transformer turns
ratios. Current limiting is provided by sensing the drop across resistor R22 (on AB69A) and
comparing it with a voltage determined by resistors R39, R78, and R79 (all on AB69). Resistor R39
causes the in-limit current to reduce as the output loading increases further (limiting internal
dissipation in the event of a dead short). This is called "fold-back" current limiting.