Novel current mirrors application
in high side current sensing in
multichannel power supplies
L. P. Dimitrov
G. M. Mitev
Nuclear Electronics Lab., Institute for Nuclear Research
and Nuclear Energy, Bulgarian Academy of Sciences
Reasons for high-side
current measurement
Rh
Iout
V
Load
Um
Iin
Iout
V
Load
Um
Iin
Rl
• Application specific
requirements
• Possibility to use
“common return” load
connection
• Possibility to detect output
short-circuit conditions
• Possibility to measure
output leakage currents
Problems introduced by high-side
current measurement
Rh
Iout
V
Load
Um
Iin
• The measurement
schematic must be
capable of working under
the full output voltage
• The measurement
schematic must have low
power consumption
Present high-side current
measurement solutions
• Complex differential amplifier and level
shifter circuits
– excellent measurement characteristics
– require separate high-voltage power supply,
usually drawn from the output
• Specialized ICs for current sensing in
industrial applications
– well suited for measurement of larger currents
– poor power efficiency in the sub-mA range
Goals and tasks
• Find a simple and cheap approach for highside current monitoring
– evaluate the specifics of using current mirrors
for high-side current measurement in detector
power supplies
– research and analyze suitable schematics
– build a test circuit and measure its
characteristics
Principles of measurement
Uin
R2
R1
Ub
Q2
Ifb Ufb
Um
R3
R4
I3
Io
• Wheatstone bridge,
automatically balanced
by an active transistor
• Balance condition for the
Wheatstone bridge Ub=0
• Assuming Ifb=0
um R 2 um
io 

R3 R1 k
Types of current mirrors
Iin
Q1
Iout
Iin
• Widlar current mirror
Iout
Q3
Q4
Q2 Q1
Q2
– very simple structure
– handicapped by the Early
effect
– the currents differ by 2*Ib
• Wilson current mirror
– relatively simple structure
– very good current parity
Widlar current mirrors schematic
R1
Uin
Uo
R2
Q2
Q1
Q3
Q4
R3
R4
Um
• Strong dependence
between Um and Uin
• Nonlinear for small currents
R1
Uin
Uo
R2
Q2
Q1
Q6
Q5
Q7
Q8
Q3
Q4
R3
R4
Um
Wilson current mirror
schematic
• Minimal dependence
between Um and Uin
• Almost linear in the range
Simulation setup
• Wheatstone bridge
– R1=100Ω, R2=15kΩ
– R2/R1=150
– R3=63kΩ
– k=(R1.R3)/R2=420
• Current mirrors
– high-side mirror - BC556 transistor pairs
– low-side mirror – BC546 transistor pairs
– R4=R3
Test board setup
• Wheatstone bridge
– R1=100Ω, R2=15kΩ
– R2/R1=150
– R3=63kΩ
– k=(R1.R3)/R2=420
• Wilson current mirrors
– high-side mirror – FMMT558 transistor pairs
– low-side mirror – FMMT458 transistor pairs
– R4=R3
Experimental results
U/I
10
10V
1
50V
Um [V]
0.1
100V
0.01
150V
200V
0.001
250V
0.0001
300V
350V
0.00001
0.001
0.01
0.1
Iout [mA]
1
10
Temperature response
U/I
10
100(35°C)
1
Um [V]
350(35°C)
0.1
100(25°C)
`
0.01
350(25°C)
100(15°C)
0.001
350(15°C)
0.0001
0.001
0.01
0.1
Iout [mA]
1
10
Results analisys
• The results clearly show that the Wilson
current mirror based schematic is well
suited for current measurements in a
dynamic range of 2.5 decades
• The thermal response over the working
range is negligible
• The power consumption of the circuit is
very small, determined by the R2/R1 ratio
Conclusion
• The presented circuit is suitable for highside current monitoring in detector power
supplies
• It has the potential to reduce the
component count, board space and
manufacturing costs of power supply units
• It provides for increased power efficiency,
with little or no sacrifice of measurement
accuracy