How Current-Sense Amplifiers Monitor Satellite Health
How do we know how a satellite in space is doing from here on Earth? Learn how current-sense amplifiers or CSAs are a crucial part of several satellite monitoring systems.
Several commercial satellite companies have entered the space sector with a major impact, revolutionizing this once largely government-funded activity. These companies, along with many others, are developing telecommunication mega-constellations, robust radar networks, and enhanced optical imaging platforms for low Earth orbit, medium Earth orbit, and geostationary equatorial orbit.
These missions have led many designers to pivot from basing satellite designs on simple discrete components such as operational amplifiers (op-amps) or transistors in favor of more highly integrated microcircuits, which helps save time with design effort, assembly, and test.
In this article, we will discuss how CSAs can monitor the health and functionality of satellite power distribution systems and various other electrical components by implementing features such as power-rail current monitoring, point-of-load detection, and motor-drive control. Current-sense amplifiers (CSAs) are a good fit in a wide variety of applications throughout a satellite’s electronic systems.
Basics of CSAs
A CSA enables both high- and low-side sensing designs; you can configure the system to have a shunt resistor before or after the load (as shown in Figure 1) to monitor for anomalies in the expected delivered load current such as an overcurrent event.
Figure 1. High- and low-side implementations
Table 1 summarizes the trade-offs of high- and low-side implementations. Both configurations have their advantages and disadvantages, depending on what the system designer is looking to accomplish with the CSA.
|High Side||Low Side|
|Implementation||Differential input||Single or differential input|
|Susceptible to ground disturbance||No||Yes|
|Common voltage||Close to supply||Close to ground|
|Common-mode rejection ratio requirements||Higher||Lower|
|Load short detection||Yes||No|
Table 1. High-side vs. low-side sensing
One of the most common use cases for CSAs in a satellite is to monitor the main power-rail input current to detect single-event transients. A CSA’s ability can handle the application of voltages greater than the supply voltage to its input pins offers more design flexibility than traditional op-amps or other discrete approaches, where the common-mode input pin voltage is bound by the supply voltages of the op-amp. When using a CSA to monitor the main power rail, you can place a shunt resistor on the high or low side of the load. The high side is typically the preferred setup when monitoring the main power rail, so you can leverage the CSA to detect load shorts for system protection and help avoid complete system failures.
It’s possible to leverage a CSA to perform point-of-load detection for overcurrent protection, system optimization, or closed-loop feedback, which are all useful ways to collect data on vital system components and determine the health or power consumption of particular system loads. Using data from the CSA, the system can make data-driven decisions such as self-calibration, short detection, or throttling current flow to load components such as power amplifiers (PAs) and other various electronic systems and ensure proper operation. A CSA’s accuracy, high voltage range, and supply-voltage-independent common-mode range make it possible to more easily monitor mission-critical components and help ensure mission success.
Figure 2 shows a common discrete setup of a CSA coupled with a comparator, using a defined reference voltage to set the trip level. In this configuration, the CSA is being used on the high side and measuring the differential voltage developed across the sense resistor. The CSA sends the output to both the comparator input and analog-to-digital converter. With this configuration, the system can continuously monitor the current to the load; if an unexpected event occurs, the fast comparator will trigger and make a data-driven decision to throttle or shut down the system to avoid complete failure.
Figure 2. Discrete overcurrent protection
The INA901-SP from Texas Instruments is a Qualified Manufacturers List (QML) Class V space-grade CSA capable of both high- and low-side sensing, with an input voltage ranging from –15 V to 65 V, a 50-krad (Si) radiation-hardened-assured (RHA) specification at a low dose rate, and single-event latch-up (SEL) immunity up to a LETEFF = 75 MeV-cm2/mg SEL. The INA901-SP helps minimize the number of devices required to monitor supply-rail health and protect satellite systems from an overcurrent event.
Radio-frequency Communication Applications
Communication systems are a common application for point-of-load detection, where CSAs play a vital role in controlling the operation of the PA over its lifetime. When a satellite’s communication equipment is broadcasting radio waves, adjusting the gate voltage for the specific bias point of the transistor in the PA controls the current being delivered to help improve system efficiency. There are two methods to control current flow through the PA. The first method, an open-loop concept, has a few drawbacks including a fixed-control voltage for the bias, which disregards the impact of supply variations, device aging, and fluctuations caused by temperature swings. The second method is a closed feedback concept leveraging a CSA and several other components, which enables dynamic control of the PA transistor’s bias points but results in a larger printed circuit board footprint.
Figure 3 is an example of a closed-looped system monitoring current flow through the drain of the PA, monitoring VDD with a bus monitor and overcurrent protection with a comparator. Depending on your constraints regarding board space, cost, precision, or the number of antennas, the optimal method for dynamic control may vary. Most approaches include a CSA to serve as part of the feedback chain to adjust the bias and improve efficiency.
Figure 3. Bus voltage, current, and overcurrent feedback
In motor-drive applications, the motor-driver circuitry generates pulse-width modulated (PWM) signals to precisely control a motor’s operation. These modulated signals are subject to the monitoring circuitry placed in line with each motor phase, which delivers feedback information for the control circuit. Because real-world amplifiers (as opposed to theoretical amplifiers) are less than perfect, the amplifier’s failure to adequately reject the large PWM-driven input voltage steps of the common-mode voltage can affect the output. Real-world amplifiers do not have infinite common-mode rejection, and undesirable fluctuations appear at the amplifier output corresponding to each input voltage step.
Figure 4 shows an example of a CSA in a motor-drive application. The red amplifier indicates where to place an inline CSA in the system. Figure 5 shows the outputs of a competing device, while Figure 6 shows the output of the INA240-SEP.
Figure 4. Inline implementation of CSAs (only one phase shown)
Figure 5. Competing device output vs. PWM input
Figure 6. INA240-SEP output vs. PWM input
These output fluctuations can be fairly large, and depending on the characteristics of the amplifier, can take significant time to settle following the input transition. Leveraging the enhanced PWM rejection technology in the INA240-SEP helps provide high levels of suppression for large common-mode transients (ΔV/Δt) in systems that use PWM signals, which is especially useful in motor-drive and solenoid applications. This feature enables accurate current measurements with reduced transients and associated recovery ripple on the output voltage.
The INA240-SEP from Texas Instruments is an ultra-precise device that is capable of a –4-V to 80-V common-mode voltage with a gain error of 0.2%, a gain drift of 2.5 ppm/°C, and an offset voltage of ±25 μV. The device is part of TI’s Space-Enhanced Plastic (Space EP) radiation-tolerant portfolio to 30-krad(Si), with SEL immunity up to 43 MeV-cm2/mg at 125°C, targeting low Earth orbit applications.
Current sensing provides many benefits to a system, including optimized performance, improved reliability, and condition monitoring to protect system vitals. Because space-grade CSAs enable direct measurements with highly accurate results, they help systems perform correctly for many years in the harshest environments. For more Texas Instruments space products, see www.ti.com/applications/industrial/aerospace-defense/overview.html#.
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