Effective power management of electronic systems is essential in space applications, due to the increasingly powerful onboard sensors, larger data collection expectations, and more demanding onboard processing requirements, all in a system with limited capacity.
This article is an introduction to a number of power efficiency design concepts and ideas by integrated circuit (IC) manufacturer Texas Instruments (TI), a participant in the satsearch membership program, and was produced in collaboration with the company.
Introduction
The demand for IC components for space applications is growing in both traditional and NewSpace markets. Although the required manufacturing volumes of IC components for space use may “only” be in the region of thousands per year (approximately), compared to the hundreds of millions of units developed for terrestrial use, ICs play such a wide variety of mission-critical tasks that greater attention is now being paid to their development and implementation in space hardware.
In addition, many of the newer innovations in space come with unique demands in terms of electronic power, integration, and control. This is also placing greater emphasis on the use of high-quality electronic components in orbit.
In order to be used in space, electronic components have to be tested against how various aspects of their performance can be affected by radiation.
In particular, the vulnerability of components to the total ionizing dose (TID) and single-event effects (SEE) are tested. The established quality levels used to classify components for space applications are as follows:
- Qualified Manufacturers List and Class V (QMLV and QMLV-RHA) – established industry standards, controlled by the US government, and
- Space Enhanced Plastic (Space EP) – a Texas Instruments commercial standard.
For more information on the Space EP quality level, and details of how such components perform in the space environment, please see our article on Space Enhanced Plastic.
Alongside understanding component quality levels, the size, weight, power, and cost (SWaP-C) of any system remain key design considerations for space electronics.
This article focuses on power optimization, which is traditionally achieved by improving the efficiency of all utilized sub-systems within the power tree, and essentially making power consumption a fundamental requirement for any component’s selection.
However, an additional approach, and one that can provide electronic engineers with greater component choice, is to minimize power consumption by implementing smart design architectures.
In the sections below are a number of ideas on how to optimize power consumption for a data acquisition system, without compromising signal performance goals such as signal-to-noise ratio (SNR), linearity, bandwidth, or stability.
Improve the power supply rejection ratio (PSRR)
Power supply noise can potentially compromise the integrity of the signal in a signal chain. The PSRR is an important measurement parameter used to understand the power supply sensitivity of an Analog-to-Digital Converter (ADC). The PSRR indicates the stability of the output signal from an ADC when subjected to a variation in the input voltage.
A better PSRR indicates the ability of the ADC component to more easily suppress the noise in the input power supply. Using the PSRR of an ADC, the maximum allowable ripple amplitude can be calculated with the following formula:
VPP_Source is the maximum ripple that can be present on the analog supply pin (AVDD). For further information, TI’s E2E design support resource has detailed information regarding measurement of the PSR in an ADC and calculation of the allowed supply ripple.
TI’s ADS1278-SP has a PSRR of approximately 80dB and is capable of handling a ripple of several mV at the input power supply. In these cases, low dropout regulators (LDOs), which are typically used to regulate the output voltage, can be removed from the design.
The LDO loss typically accounts for several 100s of mW that can be saved by using an ADC that has better PSRR characteristics.
Recommendation: utilize an Analog-to-Digital Converter (ADC) with a high power supply rejection ratio (PSRR). This will ensure that power supply noise, caused by variations in supply voltages, is minimized, protecting the integrity of the signal.
Use a Fully Differential Amplifier (FDA) for the filter stage and minimize supply voltages
The signal resolution during the conversion from the analog to digital form is significantly affected by the presence of signal noise.
FDAs are used to reduce the common mode noise. The FDA is an extremely flexible device that provides a purely differential output signal centered on a settable output common-mode level. The FDA and ADC are widely used in the circuitry of power-intensive data acquisition systems, so it is important they are optimized where possible.
Minimizing supply voltages as much as possible will increase the power efficiency.
For example, the supply voltage of TI’s radiation-hardened 850 MHz FDA, the LMH5485-SP, can be reduced down to a minimum voltage of 2.7V, in the case where the target effective number of bits (ENOBs) provide significant margin over the ADC resolution. For example, there are typically 18 ENOBs expected for the 5V 24-bit-ADC ADS1278-SP.
Not every application needs such a high resolution, but by reducing the input signal’s amplitude designers can optimize the FDA supply voltage down to the level where their ENOB target will still be reached.
The LMH5485-SP also has a low quiescent current, further minimizing the system’s power consumption.
Recommendation: use a fully differential amplifier (FDA) better suited to improving power efficiency in the space environment. Ideally, this should have:
- An input common-mode range below the negative rail, and rail-to-rail output
- A supply range that allows for low single-supply voltages,
- A low quiescent current
Use a suitable operational amplifier (Op Amp)
Amplifiers approaching rail supply voltages will behave non-linearly and can result in distortion to the signal. A negative voltage can be added to avoid the operational amplifier entering into the non-linear area.
While this is easier in commercial applications through the addition of buck converters or negative LDOs, it is more difficult in space due to the limited power budget and choice of components. Therefore, a rail-to-rail operational amplifier is recommended.
An Op Amp with rail-to-rail output stages is capable of generating output signals up to the supply rail. A maximum output signal swing can be achieved in a system with low single-supply voltage using the rail-to-rail output stage of an operational amplifier.
The common-mode input range is increased with the use of a rail-to-rail input stage for the Op Amp; however, it is not required in every application.
An example of an Op Amp with rail-to-rail functionality, that is suitable for space applications, is the Texas Instruments LMP7704-SP. This component is a low-input bias, rail-to-rail input-output (RRIO) component with a wide supply range. The RRIO Op Amp has a supply operation as low as +2.7V and a typical input bias current of ±500 fA.
The lower the bias current of an amplifier, the lower the voltage drop across the source resistance, and hence the lower the input current noise. The LMP7704-SP is an example of an amplifier that is capable of maintaining a linear behavior with respect to the ideal output voltage, due to its low input bias current, even with a signal source that has high output impedance.
Typically, most operational amplifiers used in space missions (and related applications) are made using Bipolar Junction Transistor (BJT) technology, and are usually radiation-resistant. However, BJT components have higher input bias currents than Complementary Metal Oxide Semiconductor (CMOS) components, which can reduce system accuracy.
TI’s LMP7704-SP is a CMOS-based product developed to withstand single-event effects (SEE) experienced in space – designed to bring the benefits of more power-efficient CMOS technology while still protecting against radiation.
“The LMP7704-SP is unique in the market due to the CMOS architecture. Radiation-hardened Bipolar amplifiers historically have had better radiation tolerance they also have a higher input bias current. The ultra-low Ib of the LMP7704-SP is an advantage for our customers to be able to connect them to a wide variety of sensors.” – Evan Sawyer, systems engineer, precision amplifier products at Texas Instruments.
Recommendation: use an operational amplifier (Op Amp) better suited to improving power efficiency in the space environment. Ideally, this should have:
- Rail-to-rail input-output (RRIO) stages; to achieve maximum output signal swing and a wide supply range, even with low single-supply voltages,
- A low input bias current, so that the input current noise, resistance, and voltage drop across the source are all minimized, and
- Complementary Metal Oxide Semiconductor (CMOS) architecture, as opposed to Bipolar Junction Transistor (BJT) architecture, as it is more power-efficient, provided the component is radiation-hardened.
Ensure effective Pulse Width Modulation (PWM) control in DC-DC converter design
The increase in typical load currents in today’s satellite power architectures require more advanced modulation and control. Pulse Width Modulation (PWM) is a technique used for power control and regulation from the power source to load.
For example, the TI TPS7H5001-SP PWM controller supports non-isolated (e.g. buck and boost) and isolated (flyback, forward, active clamp, push-pull, and half/full-bridge) topologies. The TPS7H5001-SP PWM controller has a configurable switching frequency from 100 kHz to 2 MHz, an external synchronization using an SYNC pin, and synchronous rectification outputs.
Synchronous rectification (SR), or active rectification, is used to eliminate the voltage drop across the diodes and to increase the power efficiency. SR also improves power density, efficiency, manufacturability, thermal performance, and reliability, ultimately decreasing the overall cost of power supply systems.
Although SR is an industrial standard for a variety of terrestrial commercial applications, it is increasingly regarded as an important consideration for space applications due to the stringent power budgets available.
The PWM controller allows usage of an external gate driver to support silicon (Si) metal-oxide semiconductor field-effect transistors (MOSFETs) and Gallium Nitride (GaN) field-effect transistors (FETs). GaN FETs are quickly being implemented in space-grade power systems and can enable improved power density. GaN allows for much faster switching resulting in higher frequency and smaller magnetics.
The PWM controller TPS7H500x family are designed to provide high-efficiency DC/DC conversion across the entire satellite power architecture – from high-voltage solar-panels to distribution voltages and low-voltage point-of-load power, with and without isolation.
Recommendation: utilize a PWM controller with an active rectification feature, preferably supporting the latest GaN FETs as well as Si MOSFETs.
Conclusion
Optimizing electronic circuit power architectures in order to develop a more efficient, and cost-effective, electronic system is an important task for an engineer looking to maximize their SWAP-C budget.
In this article we have presented a number of clear recommendations on how this can be achieved. In summary these are;
- Utilize components with a high power supply rejection ratio (PSRR) to allow for removal of the LDO.
- Use a fully differential Amplifier (FDA) for the filter stage for the best noise performance that could be “traded off” for a reduction in supply voltages.
- Use an operational amplifier (Op Amp) better suited to improving power efficiency in the space environment.
- Utilize a PWM controller with an active rectification feature, preferably supporting the latest GaN FETs as well as Si MOSFETs.
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