Modularized DC-DCs to power low-voltage, high-current spacecraft avionics

Article By : Rajan Bedi

Modularized DC-DCs provide efficient power solutions for spacecraft avionics needing to exploit the processing benefits of the latest, ultra deep-submicron FPGAs/ASICs.

Manufacturers of satellites are increasingly using on-board digital processors containing ultra deep-submicron FPGAs and ASICs to deliver space-based telecommunication, Earth observation, M2M, and internet services.

OEMs of launchers and spacecraft are more and more using the latest FPGAs in safety-critical avionics. These device have low-voltage, high-current supply requirements and traditional switching POLs become duty-cycle limited if required to generate core voltages <= 1V directly from the spacecraft bus, or do not have the necessary specification to power a 20 A load when supplied by an intermediate isolated rail. Some applications, like telecommunication payloads, need multiple low-voltage, high-current semiconductors necessitating 100 A power-distribution solutions.

The conventional solution to powering space-grade FPGAs and ASICs has been to interleave individual POLs to increase the total effective current available to supply these semiconductors (Figure 1). This approach results in a larger physical footprint, a bigger BOM, and a greater distance (impedance) between the regulators and the FPGA/ASIC, adding distribution losses that impact efficiency and the transient response.

switching regulator parallelisationFigure 1 FPGA/ASIC demands require regulators placed close together to minimise distribution losses.

The increase in the dynamic-load requirements of FPGAs/ASICs demands that their regulators are placed as close as physically possible to minimise distribution losses (I2R) due to large currents and to maximise the transient response.

A modularized DC-DC topology keeps the voltage high and current low as close to the load as possible to reduce distribution and conversion losses. The traditional architecture is ‘modularized’ into regulation followed by voltage transformation. A DC-DC topology has optimum efficiency when VIN=VOUT, which degrades as the input-to-output ratio increases. For example, with an unregulated input between 36 and 60V, the nominal bus voltage would typically be 48V instead of the legacy 12V intermediate rail. For the same power level, 48V needs one quarter the current and distribution losses (I2R) are 16 times lower. Placing the regulator first to produce this ‘modularized bus’ will achieve the highest efficiency. As its input can be lower or higher, the DC-DC requires a buck-boost topology. Following regulation, the next step is conversion from 48V to <= 1V using a transformer that multiplies the load current by the same ratio.

A modularized architecture uses high-voltage regulation followed by current multiplication to provide more efficient, smaller, faster, high-density and scalable power solutions for avionics, exploiting the processing benefits of the latest ultra deep-submicron FPGAs and ASICs.

Cobham Semiconductor offers the only space-grade, modularized power solution comprising an input regulator module (IRM) and an isolated point of load (iPOL) (Figure 2).

RM iPOL system architectureFigure 2 This is an illustration of an IRM/iPOL system architecture.

The output voltage from the iPOL is a fixed step-down ratio of its input, i.e. VOUT = VIN / K and multiple iPOLs can be connected to one IRM to increase the output current further as illustrated in Figure 3.

IRM iPOL current sharingFigure 3 This illustration shows IRM/iPOL current sharing.

An adaptive loop feature compensates for output voltage changes as the load current varies: as more current is drawn, the voltage at the FPGA/ASIC drops because of the iPOL’s internal output resistance and PCB trace/plane resistance. This increase is reflected at the iPOL input, raising the current drawn from the IRM, which in turn increases its output voltage slightly. This boost to the iPOL input is transferred (fixed step-down ratio) to its output, returning the load to its nominal value. Since the output resistance of the iPOL will change as its temperature varies, each iPOL includes a thermistor which can be connected to the IRM to account for its temperature variations when compensating for its resistive drop.

The IRM is a highly-efficient, ZVS/ZCS, buck-boost regulator available in +28, +70, and +100V input options, which can also be used as a stand-alone, high-power, non-isolated, step-up/step-down DC-DC. The desired, modularized-bus voltage is preset using one external resistor. The IRM has a specified temperature range from −40 to +85°C, a total-dose hardness of 50 krad(Si) and latch-up immunity > 80 MeV-cm2/mg.

The iPOL uses a proprietary ZCS/ZVS architecture that uses a high-frequency, spectrally-pure sine wave generating less harmonic content than the typical PWM signal. Operating at the resonant frequency results in a very low, non-inductive output impedance allowing the iPOL to respond almost instantaneously (<1 µs) to step changes in the load current. Its high bandwidth obsoletes the need for large point-of-load capacitance. Even without any external output capacitors, the output of the iPOL exhibits a limited voltage perturbation in response to a sudden power surge! Minimal external bypass capacitance (in the form of low ESR/ESL ceramic capacitors) is sufficient to reduce ripple and eliminate any transient voltage overshoots such as SETs.

The topology by virtue of employing a fixed-division ratio (K-Factor) converter does not impose the bandwidth limitations of an internal control loop trying to maintain regulation.

The iPOL modularized DC-DC solution has a specified temperature range from −40 to +125°C, a total-dose hardness of 100 krad(Si), and latch-up immunity > 80 MeV-cm2/mg. A range of iPOLs are available with different division ratios and power ratings as listed below.

Table 1 iPOL parts offered
iPOL parts Cobham

The IRM and iPOL devices can be procured in gulled-wing surface-mount package types (Figure 4).

iPOL packaged partsFigure 4 Here are some IRM/iPOL packaged parts from Cobham.

Three IRM options are available to address the trend of using higher, unregulated bus voltages as listed below.

Table 2 IRM parts offered
input regulator modules Cobham

To date, over 1000 IRM/iPOL modules have been produced and the technology has achieved flight heritage with the first parts launched in early 2016. Two evaluation boards are available populated with a K=1/40 iPOL: one contains a 100V input IRM and the other a 28V device.

PDM eval board CobhamFigure 5 Available IRM/iPOL evaluation boards contain 28 and 100V input IRM.

Modularized DC-DCs provide more-efficient, smaller, faster, higher-density, and scalable power solutions for spacecraft avionics needing to exploit the processing benefits of the latest, ultra deep-submicron FPGAs/ASICs. Three IRM options are available and 13 different iPOLs are offered, each with unique step-down (K-Factor) ratios.

Spacechips teaches, demonstrates and compares the IRM and iPOLs with other space-grade power solutions on its space electronics training courses taught around the world and further information can be viewed on the Cobham product page.

Dr. Rajan Bedi is the CEO and founder of Spacechips, which provides ultra high-throughput on-board processing and transponder products, design consultancy in space electronics, training, technical-marketing and business-intelligence services.

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