Operator's Manual
Issue 14 / Mar 2016
MercuryiPS
©2016 Oxford Instruments NanoScience. All rights reserved.
Page 23
2.6.5 Magnet control board
This is the board which regulates the output current to the magnet and controls the heater for
the persistence switch. The main current output is supplied from the switch-mode power supply
and is controlled by the PWM of 3 parallel sets of push-pull FET’s switching at 200kHz, each set
120° out-of-phase with the next set, to form a Class-D amplifier output stage. The FET switch
timing is controlled by an on-board clock and FPGA giving very precise control. The push-pull
architecture allows seamless sweeping from the negative limit to the positive limit with no output
perturbation passing through zero.
Top-level output demand instructions come from the user interface via the motherboard. These
ramp instructions are converted to output demand 10 times a second by a microcontroller on
the magnet control board. The digital demand is converted to an analogue voltage signal by a
20-bit DAC. This signal is inverted and passed to an error amplifier where it is summed with the
buffered output voltage from a precision, high-power, low temperature coefficient, sense resistor
which is measuring the main output current. The sense resistor is temperature controlled at
60°C ±<2°C at all times. The output of the error amplifier is used to drive the board output so
that the output tracks the demand via a 10Hz control loop. The main output current is then
filtered to remove any PWM ripples to supply a dc current to the magnet.
In addition, the output signal from the sense resistor is measured by a 24-bit ADC and fed back
to the microcontroller as a secondary check of ramp and hold performance.
The persistence switch heater output is a configurable current output from 0.1 to 120mA with a
12V compliance. It is switchable from the main control GUI and by remote command.
2.6.6 Filter board
Final stage filter for the main current output at the back for the iPS unit where the main current
busbars exit the iPS. This board also mounts the modbus port.
2.6.7 Run-down resistors
A bank of 6 power resistors, 300W each with a resistance of 1.5 ohms mounted on a large
aluminium finned heat exchanger. They are individually FET switched and form parallel current
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