How to select a space-grade switching regulator
Given today's time-to-market pressures and the need to deliver right-first-time hardware yesterday, selecting the wrong regulator could prove costly and result in your product getting to market late.
Selecting the most appropriate switching 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 compensation parts? What is your thermal management strategy? PCB and/or 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? Maybe for political or geographical return reasons, you might have to procure parts from a specific continent.
The packages of all space-grade, radiation-hardened, non-isolated switching POLs are illustrated below. These have been scaled equally to allow you to compare their relative sizes as well as types and the number of connections. When possible, I have selected the surface-mount versions of regulators, however, several vendors only offer through-hole parts and/or flanged chassis packages.
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. The table lists the specified values for the input and output voltage ranges, the maximum load current and power rating for each device, peak efficiency, 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, regulator 8 contains two integrated N-channel FETs but needs 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 3 and 9 require only five, but part 11 has 50 pins. 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 30, 40, or 50 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 pins to facilitate current sharing when paralleling multiple devices. Some devices also require direct access to the error-amplifier control loop.
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 parts 1 and 5 provide a good summary, whereas those for regulators 3, 4, 6, and 7 contain a very general statement on SEEs.
Can we assume that 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 elsewhere.
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. If you need some guidance on testing power MOSFETs during irradiation and how to check for SEB and SEGR, email me.
Next year will see the release of a new switching POL regulating from a maximum DC input of +12V, outputting an adjustable supply voltage from 0.8V to 0.85% of Vin at 7A, a peak efficiency of 95%, with configurable soft start, current-sharing, and placed in a 28-pin flat package.
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? Can you assemble and mount the device? Procurement and political factors may also influence your decision.
In my power course, I continue the discussion and compare efficiencies, SEE sensitivities, PCB layouts, explain the voltage ranges as well as revealing the identity of the parts. I 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 presented 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 CubeSats 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.
I'm off to the lab to drool over my 17 regulators. Until next month, are you a type 1, 2 or 3?
P.S. The first person to tell me how this last sentence fits with this post will get a free Courses for Rocket Scientists pen. Congratulations to Ciara from Ireland, the first to answer the riddle from my previous article.
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