DC-DC Converters for New Space Applications

Figure 1: Comparison of FPA™ and traditional middleware architecture.

Vicor’s Factorized Power Architecture (FPA) uses a modular approach to minimize I²R distribution losses, increase efficiency and improve transient response. The FPA consists of two stages: regulation and transformation. First, a 48V intermediate voltage rail is generated from an external power source using a buck-boost topology, which is much higher than the lower bus voltage typically input to the POL. For example, the current required for a 48V output bus is 4 times lower than a 12V intermediate bus (P = VI) for the same power, and the PDN losses are the square of this current (P = I²R), which reduces losses by 16 times. The highest efficiency will be achieved by configuring the regulator first and regulating to a 48V output.

Figure 2: Full-Bridge SAC™ Series Resonant Topology

The second stage of the FPA™ uses a transformer to convert the 48V intermediate voltage rail to the required load voltage. The output voltage is a fixed ratio (K factor) of the input voltage, determined by the turns ratio. As the voltage is reduced, the current is increased accordingly, for example, an input current of 1A may be doubled to an output current of 48A:

The FPA is formed by combining the current multiplier of the Pre-Regulator Module (PRM™) and the Voltage Transformer Module (VTM™) . These two devices work together to complete the DC-DC conversion. The PRM converts the unregulated input power into a regulated “factored bus”, and the VTM then converts (steps down) the 48V to the required load voltage.

The high bandwidth of the VTM avoids the need for large point-of-load capacitors. Even without any external output capacitors, the output of the VTM will experience only limited voltage perturbations in response to sudden power surges. A small amount of external bypass capacitance (using low ESR/ESL ceramic capacitors) is sufficient to eliminate any transient voltage overshoots. The VTM provides a unique capacitance multiplication capability without the bandwidth limitations imposed by the internal control loop’s efforts to maintain regulation. For example, when a K factor of 1/48 is used, the effective parallel output capacitance is 2304 times the input capacitance, or C _SEC = C _PRI * K² . This means that significantly less decoupling capacitance is required downstream of the VTM, and only a small amount of capacitance is needed at its input to achieve the same energy storage effect as the large tantalum capacitor typically added to the 1V output of a traditional buck module, as shown below. Low impedance is a key requirement for efficiently powering low voltage, high current loads, and using a VTM also reduces the effective resistance seen by the secondary side by a factor of K². This allows the VTM to be placed close to the load, both laterally and vertically, making the power distribution network (PDN) lower loss. The low current, high voltage intermediate bus of the FPA means that the PRM can be physically far away from the VTM without affecting efficiency. This provides greater flexibility in deciding where to place the PRM, reduces concerns about congestion in the load area, and provides more freedom in the design of the power plane to achieve maximum current density. This floorplanning is very different from the traditional brick approach, which requires the isolated DC-DC and POL to be placed close together to minimize I²R distribution losses.

Current space-grade isolated DC-DC converters and buck POLs are PWM-based devices, with output power proportional to the duty cycle of the switching frequency. These hard-switched converters use square waves to drive an inductor or transformer, and the MOSFET loses energy when switching. Square waves contain a large number of harmonics that must be filtered or they will affect the entire system through conduction or radiation. VTM's topology uses sinusoidal currents in the primary winding, producing a cleaner output noise spectrum and requiring less filtering. Existing space-grade buck regulators and forward/flyback DC-DC converters have efficiencies ranging from 67% to 95% and 47% to 87%, respectively .

To meet the power distribution and low voltage, high current requirements of future new space application constellations, Vicor is seeking space-grade certification for its Sine Amplitude Converter (SAC) topology. This zero-current switching/zero-voltage switching (ZCS/ZVS) technology offers higher efficiency, greater power density, and lower electromagnetic interference (EMI) than existing space-grade DC-DC converters. SAC is a transformer-based series resonant forward architecture that operates at a fixed frequency that is the same as the primary resonant circuit resonant frequency, as shown below:

The field effect transistor (FET) on the primary side is locked to the natural resonant frequency of the series resonant circuit and switches at the zero voltage crossing point, eliminating power losses and improving efficiency. In the resonant state, the reactions of the inductor and capacitor cancel each other, minimizing the output impedance and making it a pure resistor, thereby reducing voltage drop. The resulting very low output impedance enables the VTM to respond almost instantaneously (<1μs) to step changes in the load. The current flowing through the resonant circuit is sinusoidal with less harmonic content, resulting in a cleaner output noise spectrum and requiring less filtering of the load voltage.

SACs use a forward topology where input energy is transferred directly to the output. Leakage inductance on the primary side is minimized since it is not a critical energy storage element. The unique operation of the SAC forward topology enables higher switching frequencies, using smaller magnetic components with lower inherent losses. The resulting efficiency gains mean less power is wasted in the energy conversion process, simplifying thermal management and allowing for higher output currents and higher power densities in smaller packages. Faster operating frequencies allow energy to be transferred to the output more frequently, improving transient response to dynamic load changes in just a few cycles.

Vicor's DC-DC components have been risk-assessed by Boeing and are designed for use in O3b satellites that will provide space-grade internet services. Initially, Vicor will offer four radiation-tolerant DC-DC converters:

Figure 3: New BCM ^® , PRM™ and VTM™ radiation-hardened DC-DC converters.

The four DC-DC converters feature a redundant system architecture with two identical parallel power systems and fault-tolerant control to meet single-event effect (SEE) requirements. To reduce manufacturing costs, the parts are packaged in a plated epoxy molded BGA package with excellent thermal conductivity, named SM-ChiP™, and are compatible with standard surface mount, pick-&-place, and reflow soldering processes. These DC-DC converters fall under the EAR99 regulatory category, have an operating temperature range of -40 to 125°C, and offer a variety of overvoltage, short-circuit current, undervoltage, and thermal protection features. The target total dose radiation tolerance is 50kRad (Si); SEE and other reliability data will be released later this year.

To highlight the superior power density of these new radiation-tolerant DC-DC converters, their relative size is compared to existing space-grade switching POLs and isolated DC-DC converters in Figures 4 and 5, respectively. The power density (W/in³), efficiency (%), and current density (A/in²) of each converter are highlighted in blue, orange, and red, respectively. A range of efficiency values ​​are typically specified for different load conditions.

Figure 4: Comparison of space-grade switching POLs and the VTM2919 series.

Figure 5: Comparison of space-grade isolated DC-DC with BCM ^® and PRM™.

The new radiation-hardened commercial off-the-shelf SAC™ DC-DC converters offer a significant increase in output power, density and efficiency in a smaller footprint and form factor than existing qualified converters. The regulated voltage is cleaner and requires fewer bulk decoupling capacitors. These parts will begin to gain operational experience next year and evaluation boards are available now.

Figure 6: Modular 100V power distribution solution for spacecraft avionics.