Novel current mirrors application in high side current sensing in multichannel power supplies

L. P. DimitrovG. M. Mitev

Nuclear Electronics Lab., Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences

Reasons for high-sidecurrent measurement

• Application specific requirements

• Possibility to use “common return” load connection

• Possibility to detect output short-circuit conditions

• Possibility to measure output leakage currents

V Load

Iin

Iout

Um

Rh

V Load

Rl

Iin

Iout

Um

Problems introduced by high-sidecurrent measurement

• The measurement schematic must be capable of working under the full output voltage

• The measurement schematic must have low power consumption

V Load

Iin

Iout

Um

Rh

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 high-side 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

• Wheatstone bridge, automatically balanced by an active transistor

• Balance condition for the Wheatstone bridge Ub=0

• Assuming Ifb=0

Q2

Uin

Um

UfbIfb

Ub

I3 Io

R1R2

R3 R4

k

u

R

R

R

ui mmo

1

2

3

Types of current mirrors

• Widlar current mirror– very simple structure– handicapped by the Early

effect

– the currents differ by 2*Ib

• Wilson current mirror– relatively simple structure– very good current parity

Q1

Iin

Q2

Iout

Q1 Q2

Iin Iout

Q3 Q4

Widlar current mirrors schematic

• Strong dependence between Um and Uin

• Nonlinear for small currents

Q3

Q2 Q1

Q4

UoUin

Um

R1

R2

R3 R4

U/I

0.01

0.1

1

10

0.01 0.1 1 10Iout [mA]

Um

es

[V

]

10V

20V

30V

40V

50V

60V

70V

80V

Wilson current mirror schematic

• Minimal dependence between Um and Uin

• Almost linear in the range

Q3

Q2 Q1

Q4

UoUin

Um

R1

R2

R3 R4

Q7

Q5

Q8

Q6

U/I

0.001

0.01

0.1

1

10

0.01 0.1 1 10Iout [mA]

Um

es

[V

]

10V

20V

30V

40V

50V

60V

70V

80V

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 resultsU/I

0.00001

0.0001

0.001

0.01

0.1

1

10

0.001 0.01 0.1 1 10

Iout [mA]

Um

[V

]

10V

50V

100V

150V

200V

250V

300V

350V

Temperature responseU/I

0.0001

0.001

0.01

0.1

1

10

0.001 0.01 0.1 1 10

Iout [mA]

Um

[V

]

100(35°C)

350(35°C)

100(25°C)

350(25°C)

100(15°C)

350(15°C)

`

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 high-side 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