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 Application specific requirements Possibility to use “common return” load connection Possibility to detect output short-circuit conditions Possibility to measure output leakage currents VLoad Iin Iout Um Rh VLoad Rl Iin Iout Um

Problems introduced by high-side current measurement The measurement schematic must be capable of working under the full output voltage The measurement schematic must have low power consumption VLoad 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 U b =0 Assuming I fb =0 Q2 Uin Um UfbIfb Ub I3Io R1R2 R3R4

Types of current mirrors Widlar current mirror –very simple structure –handicapped by the Early effect –the currents differ by 2 * I b Wilson current mirror –relatively simple structure –very good current parity Q1 Iin Q2 Iout Q1Q2 IinIout Q3Q4

Widlar current mirrors schematic Strong dependence between U m and U in Nonlinear for small currents

Wilson current mirror schematic Minimal dependence between U m and U in Almost linear in the range

Simulation setup Wheatstone bridge –R 1 =100Ω, R 2 =15kΩ –R 2 /R 1 =150 –R 3 =63kΩ –k=(R 1.R 3 )/R 2 =420 Current mirrors –high-side mirror - BC556 transistor pairs –low-side mirror – BC546 transistor pairs –R 4 =R 3

Test board setup Wheatstone bridge –R 1 =100Ω, R 2 =15kΩ –R 2 /R 1 =150 –R 3 =63kΩ –k=(R 1.R 3 )/R 2 =420 Wilson current mirrors –high-side mirror – FMMT558 transistor pairs –low-side mirror – FMMT458 transistor pairs –R 4 =R 3

Experimental results

Temperature response

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 R 2 /R 1 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