2000 Amp 20 volt precision MRI magnet power supply with very stable current regulation and very low noise.

1. High Precision High Current
Power Supply
• Requirements
• For MRI magnet
• Supply IM = 2000 Amperes DC to magnet
• Magnet current regulation: +- 1.0*10-8 per degree C of the control electronics
• Magnet current noise: less than 2.5*10-7 RMS of supply current RMS
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Larry Miller
www.linkedin.com/in/lrmiller
miller@elect-design.com
Copyright 2014 Larry Miller

2. High Precision High Current Power Supply
Block Diagram
+28v
supply
0.001 ohm
LM=
12 mHy
RM=
2 ohm
CM 100 uF
Filter
Chopper-stabilized differential
preamp OPA333 G=gain
.025 ppm/oC
.01-10Hz .13 ppm RMS
>10 Hz 0.02 ppm/ 𝐻𝑧
Low noise
voltage ref
filtered
LTZ1000A
.05ppm/oC
.05ppm RMS
@.1-10Hz
Buffer
x200
x200
x20
IO
ES = 0.001 * IO
ED
IO / ED = P. P ≈ (2000 A/V) * ωT
2 / (s + ωT)2
ωT = 2 * 105
C ≡ ED / IO C is shunt,preamp,filter,buffer cascaded
IL = IO ωT
2 / (s2 + 2 ζ ωT + ωT
2)
IL
EI
-
+
Plant P
Controller C
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LarryMillerPhDmiller@elect-design.com

3. High Precision High Current Power Supply
Control system abstraction
• The feedback control system acts to minimize the sensitivity of the
output y to the disturbance d, producing an output that approximates
reference input r (in this case the voltage ref voltage)
• A sensor detects the output y to send back as feedback. Any detector
adds noise. This is represented by n.
• The Plant produces the needed output y (Io in this case). The plant
here is the power transistor bank.
• The Controller drives the Plant with u based on the error input e.
• y = (Pd + CPr + CPn) / (1 + CPF)
P
Plant
C
Controller
Reference
(desired
output)
r
n
Measurement noise
d Disturbance
y
Output
-
+ Σ
Σ
Σ
e u
F Filter
c
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LarryMillerPhDmiller@elect-design.com

4. High Precision High Current Power Supply
Noise and Disturbance Parameters
• Disturbance d is produced within Plant, and consists of the noise and
drift of the power transistors. Based on typical power transistors, and
expected thermal fluctuations in the water cooled transistor bank, d
produces a disturbance in Io (without feedback) having approximate
spectral density as a fraction of Io at frequency f of (0.5 * 10-4 / (1 + f) )
per 𝐻𝑧. Io is 2000 amps.
• From data sheets, n has estimated noise density as fraction of Io of
1.6*10-7 per 𝐻𝑧 at 1 Hz and 2*10-8 per 𝐻𝑧 above 10 Hz.
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LarryMillerPhDmiller@elect-design.com

5. High Precision High Current Power Supply
Primary goals of Controller
• Stable system
• even when various system parameters vary over ranges of expected variation.
• Minimum effect of disturbance d
• Minimize ∂y/ ∂d, the contribution of the disturbance to the output: ∂y/∂d = P /
(1 + CPF)
• Minimum effect of sensor noise n
• Avoid passing noise n to Plant output at frequencies where the noise will add
error
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LarryMillerPhDmiller@elect-design.com

6. High Precision High Current Power Supply
Error 9*10-6 after 80 us.
160 us
Overshoot
22%
45o phase
margin
18 db
gain
margin
10
kHz
1
kHz
100
Hz
∂y/∂d
Frequency unit=10 kHz

7. High Precision High Current Power Supply
Performance
• ∂y/∂d is 0.01 at 150 Hz.
• At 150 Hz, d has noise 0.5 * 10-4 / 150 = 0.33 * 10-6 per 𝐻𝑧.
• ∂y/∂d is 0.01 so noise at y is 0.33 * 10-8 per 𝐻𝑧.
• Q of magnet-filter capacitor circuit LM,CM is 3 (dissipation is eddy
currents and hysteresis in the magnet steel). Thus resonance is 30 Hz
wide and the noise therein is 0.33 * 10-8 * 30 = 1.8 * 10-8 RMS
relative to Io. The effect on the magnet current is larger by Q:
• 0.54 * 10-7 RMS
• Contribution of disturbance d at other frequencies is smaller
• The LM,CM filters out frequencies above 150 Hz. ∂y/∂n is
approximately 1.0 below this frequency, so the contribution of n to y is
2.5*10-8 per oC, and 2.5*10-7 of Io RMS noise, up to 150 Hz.
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LarryMillerPhDmiller@elect-design.com