HP 70427A User Manual page 509

Block Diagram
Controller, I/O Interface and Power Supply Assembly A6
Power supply efficiency is maximized by minimizing the voltage drop
across the output regulators. This can only be accomplished if the voltage
supplied to the output regulators is reasonably constant. There are several
design considerations which help make this possible. First the power
supplied by the MMS mainframe is regulated to within ±1%. The mainframe
supply operates at 40 kHz output. This is done to make power transformers
and filter capacitors small enough to mount on PCBs.
The design of the custom power transformers makes allowances for the
following design considerations:
1. resistive voltage drop in the transformer's winding
2. voltage loss in the rectifier diodes
3. waveform of the voltage supplied by the MMS mainframe
4. effect of the voltage transit snubber circuits on the transformer and
rectifier outputs
5. value and efficiency of the output filter capacitors
6. the amount of the expected load
The result is an unregulated supply voltage which can be predicted and will
remain within 0.1 to 0.3 volts at maximum load, and will only rise 2 to 3
volts under minimum load for each of the eight supply voltages.
The un-regulated +50 V and +25 V supplies operate from a single
center-taped winding on power transformer T100. The 30.7 V clamp
produced by VR2 and Q111 limits the maximum voltage for the two
supplies to +61.4 V and +30.7 V with no load. The inductors L9 and L8 help
to reduce 40 kHz spurs. The +50 V supply provides power for the YTF
tuning coil. The +25 V supply provides power to the YTF heater and the
YTF voltage to current converter.
The six regulated supplies are all very similar in circuit topology. They are
all low noise, high current, DC coupled inverting amplifiers. Each supply
also has over-current protection, and an output current monitor which can be
read through the diagnostic multiplexer system.
The OP-77s U102 and U109 used in the power supply reference dominates
the noise of the power supplies at frequencies less than 10 Hz. The supply
output noise is set by the supply voltage gain times the noise of the
reference. The DC voltage gains are –1.5 for the ±15 V supplies, –1.0 for the
±10 V supplies, and –0.516 for the ±5.15 V supplies.
The OP-27 op-amp used in the power supply regulators dominates their
noise performance at frequencies greater then 10 Hz. The regulators are
designed as a high current inverting amplifier, with a 10μF capacitor placed
across the feedback resistor in each of the supplies. This capacitor places a
pole in the frequency response of the regulator between 1 Hz and 3 Hz. As
frequency increases, the regulator's rejection to reference noise increases at
13-74 HP 70427A/HP 70428A User's Guide