Accurate current sensing in spacecraft
Independent of spacecraft size, the power system challenges facing designers are similar; the aim will be to maximize 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 Tradeoffs
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).
Another important parameter of the measuring circuit is the input common mode voltage. This is particularly relevant for power rail monitoring since the measurement circuit must monitor the small differential voltage developed across the sense resistor on top of the common mode power bus being monitored. This is referred to as a high-side configuration (see Figure 1a) which is the most common configuration and most advantageous for the majority of applications.
Figure 1a. Generic High-Side Current Sensing
Ideally, the high-side current sense circuit offers flexibility such as the ability to power the circuit directly from the power rail being monitored, or to power from an independent supply that can be controlled separately. Another nice feature is the ability to supply the circuit power from either side of the sense resistor, thus allowing for the circuit’s current draw to be included or excluded from the current being monitored.
In contrast to high-side current sensing, low-side current sensing places the sense resistor in the ground return path of the load (see Figure 1b), therefore the common mode voltage is near ground and the output can be ground-referenced. The drawback with low-side sensing is that a shorted load fault will not be detected and the load is “lifted” from true ground by the voltage drop across the sense resistor.
Figure 1b. Generic Low-Side Current Sensing
IC Amplifier Alternatives
Although simple discrete transistor level implementations remain an option in spacecraft applications, the improved performance, enhanced features and compact circuit footprint of IC solutions make them attractive. Naturally there are always tradeoffs in terms of radiation performance, failure modes and flight heritage to consider.
Current sensing can be achieved with several different types of IC amplifiers:
General-purpose op amps in current sensing applications are best suited to low-side
Figure 2a. Classic Low-Side Current Sensing with General Purpose Op Amp
current sensing due to their limited common mode voltage range. Their large open loop gain requires feedback (see Figure 2a) and therefore restricts them to single-ended input signals. However, in recent years new products with high common mode input voltages, such as the LT6016, are also suitable for high-side current sensing.
Difference amplifiers are used where bidirectional current sense is required in applications such as motor control and they feature a wide common mode input range that can exceed the supply voltage to the device by a considerable margin. Difference amplifiers incorporate precision trimmed resistors that limit them to predefined fixed gain ratios.
Instrumentation amplifiers can be considered as a difference amplifier with a pre-amplifier stage (see Figure 2b) that provides more flexibility with variable gain being set by an external resistor. The pre-amplifier has very high input impedance that minimizes loading on the power bus, making it possible to measure smaller system currents than is possible with a difference amplifier. A disadvantage is that the common mode voltage is usually limited to the supply voltage
Figure 2b. Current Sensing with an Instrumentation Amplifier
Zero-drift or “chopper-stabilized” amplifiers provide the lowest input offset voltage. For example, the LT2050 is specified at 3µV, providing a very high-precision current sense. But with operation limited to 6V, this is best suited to low-side current sense applications.
Current sense amplifiers have an optimized feature set and specifications designed for the task. This can save on design time and provide a flexible solution for different current sense applications. One such current sense amplifier is the RH6105. This is a radiation hardened and MIL-PRF-38535 class V qualified version of the LT6105.
Radiation Hardened Current Sense Amplifier
The RH6105 has a unique input topology that allows the part to be used in a variety of current sensing applications including:
· High-side or low-side current sensing
· Quarter, half or full bridge inductive load driving
· Supply rail monitoring (positive and negative supplies)
· Fuse and MOSFET monitoring
The importance of a low input offset voltage has already been covered in this article. The RH6105 typically has ±100µV at 25°C pre-irradiation, facilitating the use of a small sense resistor while retaining good measurement accuracy.
As shown in Figure 3, the RH6105 retains the traditional external gain setting resistors, enabling the current sense circuit to be optimized to the input span of a downstream ADC or the specific input threshold of a comparator.
Figure 3. Typical Configuration with Gain of 50 Giving 1V/A Transfer Function
The versatility of the RH6105 comes from its unique input technology. The input common mode voltage range is –0.3V to +44V with respect to V-, independent of the V+ used to bias the device. So for example, the device could be powered from a 5V supply rail while monitoring a 28V power bus. Furthermore, the inputs of the device remain high impedance even if the V+ supply is intentionally powered down, thereby continuing to present a benign load to the system, making it ideal for redundant bus or fail-safe applications.
Figure 4. The RH6105 Can Monitor across a Fuse or Switch
Additionally, the RH6105 has a differential input voltage range of ±44V, providing the ability to monitor voltages across an open fuse or switch as shown in Figure 4. Similarly, if an open circuit develops to the load, the device will remain undamaged and the current is limited to a few milliamps.
The need for accurate current sensing in spacecraft is growing. The complexity of on-board systems and a wide range of designs are available using different classes of amplifier ICs with associated strengths and weaknesses. A dedicated current sense amplifier, such as the RH6105, offers improved performance and can simplify the design task with a feature set well suited to the requirements of high-reliability and safety-critical designs.
About the author:
Steve Munns is Mil-Aero Marketing Manager at Linear Technology Corporation.