Magnetising the next generation of high-throughput satellites
With so much emphasis on the use low-cost and/or COTS components for NewSpace applications, some satellite manufacturers are designing and making proprietary isolated DC-DCs and switching point-of-loads. Power transformers and inductors are fundamental to the design of these regulators.
A block diagram of a single-channel, high-throughput, digital transponder is shown in Figure 1; simplistically, an RF carrier is received, down-converted, digitised, processed in some way, re-constructed to analogue and then up-converted for transmission back to Earth.
Figure 1 Simple block diagram of a single-channel digital transponder
Figure 2 has annotated where in the signal path and power distribution magnetic components are required, i.e. transformers, power inductors, chokes, RF inductors, and wideband baluns.
Figure 2 Annotated block diagram showing where magnetics are required
An isolated DC-DC converter is used to clean and lower the spacecraft power bus to an intermediate voltage which is used as the input rail for the various point-of-loads. Previously I discussed and compared the flyback and forward topologies both of which need a power transformer to provide galvanic isolation (Figure 3).
Figure 3 Coilcraft power transformers suitable for building isolated DC-DCs
Coilcraft Critical Products & Services offers a range of power transformers which can accept dc inputs from +14 up to +72 V with power ratings from 1.2 to 50 W. These are available in various core and coilform structures with turns ratio, inductance, power/voltage/current rating, wire size, and air gap options to help you make your desired isolated DC-DC.
High-throughput digital payloads require a broadband ADC at the receiver and a DAC at the transmitter. These mixed-signal converters have 100 Ω differential inputs and outputs respectively and a wideband balun is required to change a balanced signal to a 50 Ω single-ended one to interface to the RF down/up-conversion.
Coilcraft offers a range of 1:2 (and other) impedance ratio wideband transformer baluns suitable for ADCs and DACs. These have low insertion loss (< 1 dB) and because they are passive devices, do not generate non-linearities which could appear in-band as unwanted spurs.
Figure 4 Coilcraft wideband transformer baluns
The mixed-signal front/back-ends of high-throughput digital channelisers typically require impedance matching to maximise the power transfer to/from the RF down/up-conversion. Inductors store energy in the form of a magnetic field as long as current is flowing and I previously described various impedance-matching topologies. Multiple case size, core, SRF, and DCR options are available to help you make your desired circuit. Inductance values range from 0.67 pH up to 1 mH.
High-current, air-core inductors (springs) are also required by RF oscillators which have high Q over a large range of frequencies. The linearity of the coil's inductance is not affected by changes in current maximising tolerance and minimising the generation of unwanted harmonics.
Figure 5 Coilcraft RF inductors
Switching POLs, EMI filters, and semiconductor supply rails use power inductors, chokes, and tip/ring filters to limit ac noise on voltage rails. Switching regulators use a power inductor to store energy in a magnetic field when the FET is on, which is then released to the load when the switch is turned off. The value of inductance is chosen based on the desired level of ripple current and transient response, and the part must be able to handle the peak switching current without saturating the core, which would result in a loss of inductance. Many different values, current/power ratings, DCR, SRF, loss, EMI, package, and shielding options are available to help you make your desired circuit. Coupled inductors are also offered for SEPIC applications.
Figure 6 Coilcraft power inductors
Young electronic engineers are mesmerised by the latest FPGAs and ADC/DACs; however, these need to be powered and clocked properly to deliver their specified performance. Such parts have specific supply-rail, slew rate, and noise requirements, and the solutions discussed will enable the next generation of high-throughput satellites. I teach how to successfully design-in magnetics in my Power, Mixed-Signal, and FPGA training courses.
The magnetics presented have significant mission heritage including NASA's Joint Polar Satellite System, Parker Solar Probe, Orion, CST-100, ICESat-2, TESS, Juno, and GPSIII. The parts have EAR99 classification and the wideband transformer baluns are listed on ESA's preferred part list. The components are available in different qualification and reliability grades some of which pass NASA's low out-gassing and vibration requirements. Ultra-low temperature operation from -65ºC up to +155ºC is also offered, as well as leach resistant tin-lead terminations. COTS versions of the parts are also available and free samples can be requested to help with prototyping.
Until next month, happy coupling! The first person to tell me why the polarity of the voltage across an inductor reverses when the source is removed from a switching regulator will win a Courses for Rocket Scientists World Tour t-shirt. Congratulations to Juan, a NewSpace satellite engineer from the US, who was the first to answer the riddle from my previous post.
Rajan Bedi is CEO of Spacechips Ltd, which provides industrial R&D and space electronics design consultancy services to manufacturers of satellites and spacecraft around the world.