Performing accurate current sensing in spacecraft

Accurate current sensing and monitoring is necessary in control loops where load current information is needed for power supply monitoring, motor and actuator control, current limiting, fault protection, line compensation and many other functions. This article looks at direct current sensing using a shunt resistor, as well as alternative solutions and design trade-offs. Although the focus is on satellite power supply applications, much of the discussion can be applied more generally to other current sensing applications.

Requirements
Independent of spacecraft size, the power system challenges facing designers are similar; the aim will be to maximise performance while finding the right balance between weight, reliability and efficiency so that the mission requirements are achieved at the lowest overall cost and with minimum risk to the program.

Driven by a demand for greater capacity, more functionality, higher resolution and more instruments, the trend is one of increasing power system complexity and higher energy consumption. At the circuit level this translates to:
 • An increasing number of supply rails with tight power supply tolerances and careful power up sequencing to support increasingly complicated electronics such as FPGAs.
 • The use of more robust, fully electronic protection systems to replace relays and to handle the associated management of redundant power busses.
 • A greater emphasis on efficiency through the use of higher voltage power systems, sophisticated battery management and advanced energy control.

Designing a power system with these features requires many current monitoring circuits throughout the payload and ancillary circuits. The ideal solution combines efficiency, accuracy and a small footprint. Add in radiation-induced effects and the limited choice of space qualified, radiation hardened or suitably capable COTS (commercial off the shelf) components, and the task becomes even more difficult.

Power supply current limiting in spacecraft
To protect power supplies from output short circuit faults or transient abnormalities beyond the safe operating conditions, a Latching Current Limiter (LCL) can be used that acts as a resettable electronic fuse. Fold-back Current Limiters (FCL) are more sophisticated and seek to maintain safe power supply operation by setting an upper operating current limit for the circuit.

Typical constant current protection circuits reduce the output voltage to control the maximum power delivered as the load resistance decreases. A disadvantage to this approach is the larger input to output voltage differential, which results in increased dissipation in the control element (PD = [VIN – VOUT] x IOUT). To address this, a further refinement is to reduce the output current with the output voltage. This avoids excessive power dissipation in the event of a severe fault condition and thermal damage. Incidentally, some linear power regulator ICs include a similar chip-level current limiting mechanism, which prevents thermal runaway and eventual destruction of the device outside of its safe operating area.

Current sensing trade-offs
Direct current sensing measurements are invasive to the circuit as a sense resistor is placed in series with the load to create a voltage drop proportional to the load current per Ohms Law. The selection of the sense resistor is a trade off between power dissipation in the resistor and current measurement accuracy. To avoid excessive power dissipation in the sense resistor, it must be as small as possible whilst still able to resolve a minimum current signal. The minimum accurately reproduced signal is limited mainly by the DC input offset of the measuring circuit (see graph).