Selecting a space-grade isolated DC-DC

-January 25, 2017

Today, there are six major suppliers of qualified, radiation-hardened, isolated DC-DCs which convert the power bus to an intermediate voltage for input to POLs. Which part is right for your project?

Most vendors will say theirs is the most suitable and the best regulator to supply your space electronics. All of the manufacturers offer a range of components each with their own package with multiple voltages and different power ratings, and several suppliers let you choose the desired output voltage(s) to optimise the efficiency of the overall distribution. Given today's time-to-market pressures and the need to deliver right-first-time hardware yesterday, choosing the wrong DC-DC could prove costly and result in your product getting to market late.  

Selecting the most appropriate DC-DC for your project requires the consideration of many factors: first and foremost, what are the specific power, voltage, and current requirements? Then, how much budget do you have? How many rails do you need and must all of these be individually regulated? Is a negative supply or an integrated EMI filter required, e.g. MIL-STD-461? Is trim adjustment needed? Are enable inputs or remote-sense outputs necessary? How much PCB area is there and what is your thermal-management strategy?

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. Some satellite manufacturers can make their own DC-DCs and there are equally good technical and commercial arguments for making or buying.

The power ratings of the isolated, space-grade DC-DCs currently being offered by semiconductor vendors range from 1.5 to 120 W, with the forward topology being the most popular. This is based on the standard buck regulator with the addition of a transformer to provide galvanic isolation and to increase or decrease the output voltage depending on the turns ratio as shown below.


Figure 1 Single and two-switch forward converter topologies

A forward converter does not store energy during the switching, conduction phase of the low-side FET, but passes it directly to the load via the transformer action. The output voltage is set by the turns ratio and the PWM duty cycle as follows:  



The operational duty cycle is usually limited to less than 50% to reset the transformer's core during each switching period. Typically a third winding is used to realise flux resetting.

A single-switch forward converter is normally used when the power level is below 200W: a major disadvantage of this topology is that the voltage stress on the FET is the sum of the input voltage, the reflected transformer voltage and the overvoltage turn-off spike caused by leakage inductance. A transistor with a voltage rating greater than twice the input is required and passive-snubber or active-clamp primary circuits are also used to limit the voltage rise on the drain during core reset.

To reduce the voltage stress on the low-side FET, a second transistor is often added which clamps the voltage on each switch to the input power bus as shown above. The high-side FET also limits the leakage inductance by returning its energy to the input through diodes D3 and D4. This reduces system loss, improves efficiency, minimises EMI, and avoids the need for a snubbing circuit.

During operation, both transistors are turned on and off simultaneously: when the FETs are conducting, power is delivered to the load through the transformer and the output filter. With the switches off, there is no input power, however, current flowing in the magnetizing inductance will cause the voltage on the primary winding to reverse as shown below:


Figure 2 Current paths when FETs are on and off

Some suppliers of isolated, space-grade DC-DCs offer a fly-back topology available with either a single low-side FET or two transistors to avail of the same benefits as described above. A fly-back (buck-boost) converter stores energy in the transformer's magnetic field when the switch conducts, which is then transferred to the load when the FET is off. A fly-back converter can be viewed as two inductors sharing a common core with opposite polarity windings as shown below:


Figure 3 Single and two-switch fly-back converter topologies

The six major manufacturers offer a range of space-grade, isolated DC-DCs each with their own packages with multiple voltages and different power ratings. Some of the case styles offered by one of the vendors are illustrated below and these have all been scaled by 1:2 to allow you to compare their relative sizes as well as the types and number of connections.  


Figure 4 Hermetically-sealed packages offered by a space-grade provider

Table 1 lists and compares the above DC-DCs and as I do not want to be seen to endorse or criticise any specific vendor, I have deliberately not revealed their identity or part numbers.  


Table 1 Space-grade, isolated DC-DCs offered by one supplier

This post has started a comparison of qualified, space-grade, isolated DC-DCs which convert the input power bus to an intermediate voltage. In my Power course, I continue the discussion and compare all space-grade, isolated DC DCs, their efficiencies, topologies, SEE sensitivities, as well as revealing their identity. The overall objective is to help you to make informed selections so you can deliver your space electronics right-first-time!  

The dates and cities for my 2017 courses in Space Electronics in the US, Asia, and Europe have now been published and places can now be booked by emailing info@courses-for-rocket-scientists.com. Until next month, don't get snubbed!

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 Thomas from Germany, the first to answer the riddle from my previous article.

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