Today there are 19 space-grade, non-isolated switching regulators offered by 12 suppliers. Which part is right for your project? Most vendors will say theirs is the best DC-DC to power your load.
Given today’s time-to-market pressures and the need to deliver right-first-time hardware, selecting the wrong regulator could prove costly and result in your product getting to orbit late.
Selecting the most appropriate switching point of load (PoL) for your project requires the consideration of many factors: first and foremost, what are the specific voltage and current requirements? Then, how much budget do you have? How much PCB area is there? Is the part fully integrated or will you have to buy and add an external inductor, capacitor, output sense resistors or a feedback-compensation network? What is your thermal management strategy? PCB and/or a physical heatsink?
Do you need an Enable input or a Power Good output? Is soft-start required and/or do you need to parallel POLs to increase the effective load current? How critical is output current and voltage monitoring for your application to protect the regulator and the load if a fault is detected?
Other considerations include: does the part have the necessary reliability and radiation hardness? Is the vendor an approved supplier? Does their lead time fit within your project timescale? Our industry is currently facing supply-chain challenges! Maybe for political or geographical return reasons, you might have to procure parts from a specific continent?
The packages of the 19 space-grade, radiation-hardened, switching PoLs are illustrated below (Figure 1): these have been scaled equally to allow you to compare their relative sizes, the number of connections as well as power and current densities. When possible, I have selected the surface-mount versions of regulators, however, several vendors only offer through-hole parts and/or flanged chassis packages.
Figure 1 Top view of space-grade, non-isolated, switching POLs.
Table 1 lists and compares the above POLs and as I do not want to be seen to endorse or criticise any specific supplier, I have deliberately not revealed the identity of vendors or part numbers.
Table 1 Space-grade, non-isolated switching regulators.
Table 1 lists the specified values for the input and output voltage ranges, the maximum load current, whether a device is integrated or additional external components are required, the number of pins/leads and the package size. For each part, the specified recommended values are included and not absolute maximums.
Integration means something different to the various vendors: a green tick denotes that the topological inductor and capacitor as well as the control loop are included within the package. A red cross signifies that at a minimum, an external inductor and capacitor are required to use the device. As an example, regulators 1 and 5 contains two integrated N-channel FETs but need an external inductor, capacitor and output sense resistors. Think about overall PCB area including thermal management when selecting a regulator!
All the regulators use current-mode control of the feedback loop because of superior line and load transient responses, more accurate and faster current limiting, simpler loop compensation and the ease of load sharing. However, the supplier of regulator 1 also offers 6 and 9A versions with voltage-mode control where the input ranges from 3 to 13.2V, the output from 0.6V to 90% of Vin, placed in a 64-pin package. This topology introduces a double pole at the resonant frequency of the LC filter and a zero due to the ESR of the output capacitor. All types of external compensation can be used to provide a stable loop gain with appropriate phase margin. The transfer function of a current-controlled buck is a single-pole system which simplifies compensation.
In terms of the number of physical connections, regulators 4 and 6 require only five, but part 1 has 44 pins. Given that the performance and capability of all the PoLs is similar, why is there such a difference? The former have Vin, Vout, GND, Enable and an Adjust to allow you to vary the magnitude of the output voltage. Parts with thirty, forty or fifty pins have multiple Vin and Vout connections to reflect a higher power rating, many GNDs, with some devices distinguishing between Power and small-signal returns, Enable, Power Good, soft-start control and I/O to facilitate current sharing when paralleling multiple devices. Some devices also require direct access to the error-amplifier control loop. Another reason for an overly large package is that the supplier has previously qualified it for another product and its more economical to re-use this for their latest PoL. Part 13 is available mounted on a small PCB with a fixed output voltage or the controller IC can be procured separately to allow you to add external passives and create a custom 4 A buck converter.
The specified efficiency can be misleading and varies with output voltage and load current. Many users get mesmerised by the headline number stated on the front page of datasheets only to discover that it’s a lot lower for their application.
In terms of radiation hardness, some vendors include SEL, SEB, SEGR, SEFI and SET data as well as total-dose levels at the standard rate and also for ELDRS. The datasheets for regulators 5 and 8 provide a comprehensive summary of tolerance levels whereas parts 6 and 7 contain a very general top-level statement on SEEs. Should we assume their heavy-ion or proton testing did not detect any of the above effects? A polite request to suppliers: as a user and buyer of space-grade switching POLs, I would like to see some consistency and for you to list sensitivities for all of the above effects, or at least tell us that they were checked for and not observed. We really do want to use your POL and you could make it easier for us to select your part – it would be tragic if your DC-DC is rejected because the SEE information in the datasheet is incomplete and the design/procurement engineer is simply too busy to hunt for it! Compared to small-signal transistors, power MOSFETs are larger (bigger device volume) and have a lower doping profile making them more susceptible to radiation damage. Likewise, to help us meet our time-to-market needs, I would like to see schematic symbols, PCB footprints and 3D models available for download in an EDA neutral format.
This post has started a comparison of qualified, space-grade, switching POLs: selecting the most appropriate regulator for your project depends of many factors including your specific voltage and current requirements, how much budget your project has, how much PCB area you have, i.e., is the part fully integrated or will you have to buy and add an external inductor, capacitor, output sense resistors or feedback-loop compensation components. Do you need control inputs/outputs and/or thermal protection? Procurement and political factors may also influence your decision!
In our Power course, we continue the discussion and compare efficiencies, SEE sensitivities, PCB layouts as well as revealing the identity of the parts. We also compare all space-grade linear regulators as well as isolated switching DC-DCs which convert the bus voltage to an intermediate rail. The suitability of GaN and SiC for space is also taught and the overall objective is to help you to make informed technology selections so you can deliver your space electronics right-first-time!
This post has focused on qualified, switching, non-isolated regulators for traditional satellites. Some vendors also offer specific POLs for small satellites with less screening for lower-cost avionics or reduced levels of total-dose tolerance to reflect shorter mission lengths. These parts are not covered in this article! One supplier offers a 2 A switching PoL that has not been included because there are many cheaper and smaller, space-grade 3 A linear regulators available.
I’m off to the lab to drool over my nineteen regulators. Until next month, are you a type 1, 2 or 3? The first person to tell me how this last sentence fits with this post will win a Courses for Rocket Scientists World Tour tee-shirt. Congratulations to George from Switzerland, the first to answer the riddle from my previous article.
Dr. Rajan Bedi is the CEO and founder of Spacechips, which designs and builds a range of advanced, L to K-band, ultra high-throughput on-board processors, transponders and Edge-based OBCs for telecommunication, Earth-Observation, navigation, internet and M2M/IoT satellites. The company also offers Space-Electronics Design-Consultancy, Avionics Testing, Technical-Marketing, Business-Intelligence and Training Services. (www.spacechips.co.uk). Rajan can also be contacted on Twitter to discuss your space-electronics’ needs: https://twitter.com/DrRajanBedi
Spacechips’ Design-Consultancy Services develop bespoke satellite and spacecraft sub-systems, as well as advising customers how to use, select and procure the right components, how to design, test, assemble and manufacture space electronics.
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