Advertisement

Blog

Don’t Let Lightning-Induced Surges Shock You (or Your Spacecraft)

Three, two, one, blast-off . As the spacecraft shoots off toward space, you're anticipating the discoveries of the latest mission and wondering what the crew members will be experiencing during the flight. The last thing you want to worry about is if your design engineers used the right circuit protection technology. And you shouldn't be worried if your engineering team took the right precautions during the design phase.

Why is circuit protection so important when designing circuits for space applications?

  1. Lives are at stake. Space vehicles and their supporting systems must be designed to protect the lives of the crew members who will be subjected to the dangers and vulnerabilities of space travel.
  2. Technology is becoming more complex as circuits get smaller. A large spacecraft is really just a sum of all the little circuits and components that work together to make the vehicle operate. So it's vital to protect each delicate electrical component and digital circuit from hazards like electrostatic discharges and lightning-induced surges. No matter how small, a damaged component or circuit could mean that an entire system will fail, compromising the safety of the entire mission.

A blast from the past
The Apollo 12 mission, launched on Nov. 14, 1969, is an excellent example of how simple incidents can interfere with the highly complex set of electrical systems that control a spacecraft.

At 36 seconds after launch, lightning struck Apollo 12 and caused a massive current to travel through the outer skin of the spacecraft, down to the launch vehicle, and through the rocket flumes to the ground (Figure 1). As a result, launch controllers lost telemetry contact with the crew. The strike triggered the overload circuits, which tripped up the silicon-controlled rectifiers (SCRs) in a box down in the service module where the fuel cells (the spacecraft's primary DC power source) were located. All at once, all three fuel cells were disconnected. At the same time, the momentary low voltage input to the DC-to-AC inverter tripped the AC undervoltage sensor. This caused the AC Bus 1 and 2 fail lights to illuminate. The transient that affected the silicon-controlled rectifiers in the fuel cell disconnect circuitry affected SCRs in the AC overload circuits in the same way.

Figure 1

Lightning struck both the Saturn V rocket and the launch tower shortly after Apollo 12 lifted off.

Lightning struck both the Saturn V rocket and the launch tower shortly after Apollo 12 lifted off.

As if one lightning strike weren't bad enough, a second one occurred 52 seconds after liftoff. This strike tumbled the craft's inertial measurement unit gyroscopes and caused minor measurement instrumentation failures. Because the craft's battery-powered emergency bus continued to operate, the crew was able to reset critical systems and continue with their mission safely.

What prevented major permanent damage to the systems and the space vehicle? It was the quality of the structural electrical bonding between the launch escape system, command module, service module, spacecraft-lunar module adapter and Saturn V inertial unit. The inertial unit computer in the launch vehicle assumed ascent guidance and control, because the command module computer, which would normally have provided backup for ascent, went offline after the first lightning strike. Having the redundant computer for ascent guidance and control located in the launch vehicle, rather than the command module, prevented the need to abort the flight.

Since the Apollo days, NASA has implemented extensive monitoring equipment to ensure that space vehicles aren't launched when there is a chance of lightning. Today, Launch Pad 39B at NASA's Kennedy Space Center has been modified to include a sophisticated lightning protection system of several large towers.

Today's strategies for circuit protection
Fortunately for today's circuit designers, designing protection against lightning-induced surges doesn't usually involve the construction of huge towers. In fact, circuit protection devices are getting smaller and smaller to fit within the miniaturized footprints of their modern applications.

For several decades, Littelfuse has have been providing circuit protection components that have been a critical part of spacecraft and ground control systems. In the 1960s, Littelfuse developed MICRO and PICO® subminiature fuses for NASA, which were used for the Gemini and Apollo space programs. Consider some of the other circuit protection devices provided by Littelfuse today.

High-reliability micro fuses
Littelfuse supplies NASA, other space agencies, and military contractors with 262/268/269 series fast-acting, high-reliability micro fuses with high breaking capacity. They are designed for the protection of electrical, electronic, and communication equipment with printed circuit boards that are used in DC circuits and AC circuits up to 400 Hz, in the most extreme conditions.

TVS diodes
The AK series of TVS diodes (shown in Figure 2) were designed for high-energy transient voltage protection applications. These axial-leaded, high-power transient suppressors are optimized for use in AC line protection, as well as demanding AC or DC applications. Using Littelfuse's foldback technology, they offer clamping performance superior to conventional approaches like silicon avalanche diode (SAD) technologies. They provide a clamping voltage that's lower than the avalanche voltage but higher than the rated working voltage. So any voltage rise due to increased current conduction will be kept to a minimum to ensure the best possible level of protection.

AK series TVS diodes also offer the advantages of a no-wearout mechanism, lower leakage rates, and a fast response to surges. And their compact design simplifies board layout. They can be connected in series and/or parallel to create very high-capacity protection solutions.

Figure 2

AK series TVS diodes recover fully, unlike traditional surge protection device modules that begin to degrade after each surge event.

AK series TVS diodes recover fully, unlike traditional surge protection device modules that begin to degrade after each surge event.

Varistors
Littelfuse's LSP10 series surge protection modules (shown in Figure 3) provide transient overvoltage protection for outdoor and commercial LED lighting fixtures. Constructed with Littelfuse-designed thermally protected varistors, they provide robust surge current handling capabilities. A built-in thermal disconnect function provides additional protection from catastrophic failures and fire hazards, even under extreme circumstances (e.g., varistor end-of-life or sustained overvoltage conditions). The LSP10 series is available with either parallel or series connections. With the parallel-connected version, an indicator wire can activate an LED to notify maintenance personnel when to replace the SPD to ensure that the luminaire remains protected. The series-connected version cuts luminaire power off to provide a clearly visible indication that the SPD should be replaced.

Figure 3

One version of the LSP10 series surge protection modules.

One version of the LSP10 series surge protection modules.

Want to learn more about protecting sensitive electronics from lightning-induced surge currents? Visit Littelfuse.com to learn more about TVS diodes and varistors.

31 comments on “Don’t Let Lightning-Induced Surges Shock You (or Your Spacecraft)

  1. vasanjk
    October 25, 2014

    Bharat Nice post. Circuit protection science fascinates me(of course without its associated equations). TVS diodes and Varistors – They are becoming tinier by the day. I remember seeing some 0402 packages, if I remember right. So much power handling capacity in miniscule packages.I would like to know how to choose between a varistor and TVS diode if I have both with similar rated voltages.

  2. bshenoy
    October 25, 2014

    @vasanjk – yes, both varistors (we call these smaller versions of varistors Multi-layer Varistors or MLVs) and TVS diodes are becoming smaller as the mobile device industry evolves. We are seeing 0201 and even 01005 size parts being placed on boards and used in smartphones mainly. So, how do you choose between TVS and MLV (varistor) is a good question. The big difference between the two is the clamping efficiency or clamping speed. The TVS diode will clamp instantaneously meaning it has a lower dynamic resistance compared to a varistor which will take longer to fully conduct and clamp. So this caused more of the ESD surge energy to be “let-through” into the circuit downstream which can cause issues and even damage to the ICs. On the other hand, MLVs in there larger sizes such as 1206 and 1812 have greater surge handling capabilty as compared to TVS of the same size ; they can also be much  more cost effective in these sizes. So, we are talking about very high ESD surges or fast transient surges, load dump automotive transients, etc.

    Bharat

  3. vasanjk
    October 25, 2014

    Bharat A local circuit protection Components vendor told me, as a thumb rule, it is good to use TVS diodes where human body interaction is more such as keypads etc while Varistors could be used at board to board interfaces , sensor inputs, etc. Pls share your views on this.

  4. bshenoy
    October 25, 2014

    @vasanjk – it more has to do with how sensitive the downstream circuit is to surges. Human interaction and board-board may have different ESD surge characteristics but key aspects to select a part technology is related to surge levels, and clamping efficiency. Capacitance also comes into the equation ; for high speed signal circuits, you want the device to be lower in capacitance so it does not distort the signal. Low capacitance TVS diodes have much better clamping performance compared to low capacitance varistors.

  5. geek
    October 26, 2014

    @Bharat: Interesting post as always on circuit protection and its importance. I wanted to know if there's any significant overhead that's added to the circuit when circuit protection is used. In other words, is there a downside of using it?

  6. Netcrawl
    October 26, 2014

    @bshenoy I agree with you, ESD events are caused by the rapid discharge of electrostatically charged bodies or devices, these are quick events with relatively high peak currents and the entire event is in nanoseconds. ESD parts are commonly characterized to three defined models–Human Body Model (HBM), Machine Model (MM) and Charged Device Model (CDM).

  7. bshenoy
    October 26, 2014

    @tzubair – the overhead you talk about is the capacitance concerns I mentioned below. For ESD protection devices used on high speed signal/comm lines, capacitance can cause signal distortion. So you want TVS device with low cap. We also make Polymer ESD protector which is ultra low cap 0.07 pf. For fuses and PTCs, the overhead is the series resistance, voltage drop.

  8. samicksha
    October 28, 2014

    it is good to use TVS diodes where human body interaction is more such as keypads etc while Varistors could be used at board to board interfaces , sensor inputs, etc.

    A TVS diode can respond to over voltages faster than for varistors or gas discharge tubes.

  9. geek
    October 28, 2014

    “the overhead you talk about is the capacitance concerns I mentioned below”

    @bshenoy: I understand the technical overhead in this case which is the capacitance. My question was more from a commercial perspective. What is it that a circuit manufacturing company may have to spend extra to implement circuit protection. I guess most commercial decisions are taken on the basis of cost vs benefit analysis and this has to be taken into account.

  10. bshenoy
    October 28, 2014

    @tzubair – Very general question about overhead. Most designer think about circuit protection as overhead and try to determine what “penalties” are paid in either cost, space, or circuit performance in order to utilize the circuit protection devices. This is normal. Only adbice we can offer is that this process start early in the design process where there is more flexibility to make changes, compromises, or even devleop custom solutions to fit needs of customer. More often than not, customers contact us too late and there is little help we can offfer since board is passed layout.

  11. vasanjk
    October 28, 2014

    BHARAT, I agree completely. Circuit protection implementation must start very early in design process.while optimizing the main circuit, circuit protection area can also be optimized from any redundancy and over kill. Thus, costs can be minimized. Also it pays in the longer run in terms of product longevity.

  12. ue2014
    October 29, 2014

    vasanjk – I also agree with you. If we could always start thinking about the circuit protection from early stages, we could minimize the cost to a great extent. Also when considering the commercial factor too in term of price, I think customers would never mind paying little extra for a product that last longer than others.

  13. vasanjk
    October 29, 2014

    @ue2014 Longevity may be important but robustness is much more vital. The product must work in stringent conditions based on its target application. Proper protection circuitry ensures performance at all conditions.

  14. chirshadblog
    October 30, 2014

    @vasanjk: Exactly, the product should be compatible and should work according to match the requirements. 

  15. chirshadblog
    October 30, 2014

    @UE2014: I think it's better not to target the cost itself. Targeting cost might affect the quality so its better to be on the watchful

  16. geek
    October 31, 2014

    “this process start early in the design process where there is more flexibility to make changes, compromises, or even devleop custom solutions to fit needs of customer”

    @bshenoy: One of the ways to ensure this would be to have a checklist whereby no design can go into production unless the circuit protection is adequate and that there's a go-ahead from the member of the design team who looks after circuit protection. I don't think a lot of companies are following this practice as yet.

  17. yalanand
    October 31, 2014

    One of the ways to ensure this would be to have a checklist whereby no design can go into production unless the circuit protection is adequate and that there's a go-ahead from the member of the design team who looks after circuit protection.

    @tzubair, most of the companies already follow this methodology. But  sometimes these checks also fail to identify some issues which may be new to the design being implemented. So its very important to learn from previous issues.

  18. yalanand
    October 31, 2014

    Targeting cost might affect the quality so its better to be on the watchful

    @chirshadblog, but its always good idea to keep an eye on the cost factor because sometimes cost factor might escalate. No doubt quality is important but cost factor is also important.

  19. yalanand
    October 31, 2014

    The product must work in stringent conditions based on its target application.

    @vasanjk, I agree with you. I think that is the reason most of the products are tested in varying condition from low temperature to high temperature, from low voltate to hight voltage. But sometimes it would be difficult to mimic the realtime conditions.

  20. yalanand
    October 31, 2014

    My question was more from a commercial perspective. What is it that a circuit manufacturing company may have to spend extra to implement circuit protection.

    @tzubair, I dont think its good idea to save some cost by not having circuit protection circuit. Its always better to provide circuit protection since it improves the reliabiilty of the product.

  21. yalanand
    October 31, 2014

    A TVS diode can respond to over voltages faster than for varistors or gas discharge tubes.

    @samicksha, true. Its also important to know how much of energy TVS diode can absorb. I think most of the energy is stored as heat because transients are brief.

  22. yalanand
    October 31, 2014

    Human Body Model (HBM), Machine Model (MM) and Charged Device Model (CDM).

    @Netcrawl, Human body is modeled by a 100 pF capacitor and a 1500 ohm discharging resistance. I am curious to know how did we arrive at those numbers ?

  23. vasanjk
    October 31, 2014

    TVS diodes, Ferrite beads, Varistors etc are manufactured so as to protect electronic components in extreme conditions. This means the designer, if takes care to employ these protection components, by default, empowers the circuitry to perform under such conditions.

  24. PCR
    October 31, 2014

    Exactly, this will impact the final price of the product and for sure there will be a market for the quality product always. 

  25. samicksha
    October 31, 2014

    TVS will fail if they are subjected to conditions beyond those that the particular product was designed to accommodate.

  26. geek
    October 31, 2014

    “I dont think its good idea to save some cost by not having circuit protection circuit. Its always better to provide circuit protection since it improves the reliabiilty of the product.”

    @yalanand: This logic may work if you're dealing with one, two or a handful of cicruits. When you're producing them commercially then cost becomes important and each addition to the design adds to the total cost. This is why I mentioned that it is important to consider the costs sometimes.

  27. geek
    October 31, 2014

    @yalanand: This may be the case in other developed countries but the kind of companies that I have worked with, they have yet to make circuit protection a necessity. For them it's something they're still considering a luxury to have in a circuit.

  28. Sachin
    October 31, 2014

    This means the designer, if takes care to employ these protection components, by default, empowers the circuitry to perform under such conditions.

    @vasanjk, I agree with you. I think all this info is mentioned in the datasheet and hence we should refer to the datasheet before we choose the appropriate component.

  29. Sachin
    October 31, 2014

    When you're producing them commercially then cost becomes important and each addition to the design adds to the total cost. 

    @tzubair, I agree with you. Cost is important parameter as well and hence we should always keep in mind about cost and make sure that we provide reliable product at reasonable price.

  30. Sachin
    October 31, 2014

    This may be the case in other developed countries but the kind of companies that I have worked with, they have yet to make circuit protection a necessity.

    @tzubair, circuit protection circuit is an optional circuit. If you have it in your product your product will be more reliable. Finally i think it all depends on the price, if the price of the product is more you would want to provide extra reliability by providing such circuits.

  31. Sachin
    October 31, 2014

    One of the ways to ensure this would be to have a checklist whereby no design can go into production unless the circuit protection is adequate and that there's a go-ahead from the member of the design team who looks after circuit protection.

    @tzubair, I totally agree with you. If we implement measures such as checklist then I am sure we can make sure that no design goes into production without the circuit protection. I think different companies have different approach towards this and very few companies follow checklist method to qualify the product.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.