Design for life—part 6

Last time we looked at spike and surge - a major cause of non-age related failure in the field. In this final episode we look at environmental affects and the various solutions available to counteract them. Around 20% of failures seen back from the field occur as a direct result of the environment they have been operated in.

                         

High ambient temperature and or poor ventilation are obvious examples but o/en the less apparent occurrences such as early morning condensation (which is commonplace inside any cold enclosure with direct atmospheric ventilation) are not always as well considered.

Somewhat counter-intuitively, relatively pure water is not particularly troublesome from an electrical leakage point of view. It is the presence of ionic substances such as fingerprints which become weakly conductive, and salts which create a thin electrolyte film that are the most problematic.

Potting or conformal coating is the industry-wide accepted solution, but few people realize that on a molecular level moisture eventually penetrates the majority of organic coatings (with the exception of Parylene). Because of this, the most crucial step is that all surface contamination is removed prior to applying the coating through vapour degreasing or semi-aqueous washing. The coatings primary role then is to prevent the subsequent deposit of ionisable contaminants.

Conformal Coating

The coating technique plays a fundamental role in its effectiveness. There are many methods available such as brush coating, spray application, conformal dipping or selective robotic process and there are pros and cons of each.

                                                                                                          

Brush coating is tends to be limited to very low volume, repair and corrective actions. Due to its highly manual nature it is o/en susceptible to air bubbles and is subjective in terms of quality. Spray application suits low to medium volume and can result in a good quality finish but is usually limited due to 3D constraints, with poor penetration under, devices.

Conformal dipping is generally highly repeatable and suited for production volume, but great care must be given to masking. Indeed the difficulty in preventing unwanted seepage means many pcb’s are unsuitable for dipping. Slumpage around sharp edges can also be a problem, but can be minimized by double dipping or supplemental spraying. If the design can accommodate it though, dipping is usually adequately effective, especially when combined with the application of a vacuum whilst the assembly is submerged in the resin which even eliminates uncoated surfaces in interior cavities.

Selective robotic process generally involves the application of atomised spray by needle injection or ultra sonic valve. The needle or valve can move around the pcb and dispense the resin very selectively and at a highly accurate thickness. The process suffers from the same 3D limitations as other spray methods and also from the same unwanted capillary effects.

It is however the technique most suited to high volume where the design of the pcb means dipping is not possible. The selection of the conformal coating material is a crucial factor that needs to be carefully considered— not only for the anticipated environment but also for the intended application technique. There are many considerations such as the atmosphere the equipment is to be protected against, temperature range, electrical, chemical and mechanical compatibility (coefficient of expansion), ease of rework, cure times and of course price. The more common materials can be summarized as follows:

Acrylic.

Pros: Ease of rework, simple film drying, fast cure time, moisture resistance, and high florescence.

Cons: High VOC (solvent evaporation), flammability, thickness dependent cure 'me, shrinkage (approx.

5%)

Epoxy.

Pros: Wide temperature range (up to approx. 155’C), abrasion resistance, coefficient of expansion (well matched to pcb substrate), dielectric properties.

Cons: Ease of rework, process intensive (maintaining viscosity, complex mix ratios), chloride contamination (solvent based), high component stress during thermal cycling

Polyurethane.

Pros: Dielectric properties, moisture resistance, solvent resistance, abrasion resistance.

Cons: Moisture effects cure rate and properties, long complete cure 'me (several days), thickness dependent cure 'me, high VOC (solvent evaporation), reacts violently with water if using heat cure.

Silicones.

Pros: Very wide temperature range (-40’C to +230’C), flexible (provides dampening and impact protection), moisture and sunlight resistance, dielectric strength, low surface energy (good penetration under components).

Cons: requires humidity to cure, solvent resistance, abrasion resistance, short pot life, and long cure times.

Potting

Potting, or encapsulation generally offers a greater degree of protection than coating as it eliminates problems such as slumpage around sharp profiles and also has major benefits in terms of mechanical shock and vibration. The main difficulty with potting is thermal management, both in terms of absolute internal component temperatures and mechanical stress due to the thermal cycling of the potting medium. Indeed it can o/en be difficult to evaluate these stresses and this can have a major impact on product longevity.

As all the components are essentially thermally coupled, much attention has to be paid to vulnerable items such as electrolytic capacitors which may actually run much hotter than they would in free air convection. In recent years there have been several innovations in potting compounds and there are a multitude of epoxies and silicone's available with wildly varying characteristics to suit the intended application. Attention needs to be paid to fire rating, thermal conductivity, viscosity, temperature rating, and dielectric strength and cure mechanism.

Some modern silicone's offer very low stress and very high thermal conductivity with temperature ratings well over 200’C making them ideal for many electronic applications. For high volume production however, consideration must be given to the additional processes required and potential limitations in product throughput.

It is also worth bearing in mind that the serviceability of the product is generally greatly reduced with potting - indeed rework can o/en be very 'me consuming and is usually uneconomic. It’s more important than ever to make sure the design is thoroughly proven electrically and properly thermally evaluated. Get these right however and the outcome is typically an extremely resilient product with a very long service life.

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The entire series can be found by searching and joining the “Switch Mode Power Supply Repair” group.

Advance Product Services Ltd

www.advanceproductservices.co.uk

Paul Horner is Managing Director at Advance

Product Services Ltd.

Design for life - Part 5

Last time we looked at optocoupler ageing and saw how this innocent little component constitutes such a high failure risk for power supplies.

This week we look at spike and surge - a major cause of non-age related failure in the field.

                   

Spike & Surge

The majority of engineers are aware of the catastrophic effects of high transient energies on the input and output lines of a power supply.

Indeed, voltage fluctuations on the local grid are commonplace and the variance in the quality of the

AC mains from location to location can be surprisingly large. However, a typical power supply which meets EN 61000-4-5 (basic immunity test for surge) does not guarantee low susceptibility in the field. The financial rewards of producing reliable products over and above the basic EMC standard are usually very worthwhile.

 A certain UK manufacturer saw their warranty costs fall by £2.7million per year a=er spending less than £100K on improved immunity. In the UK at least, a relatively small proportion of energy fluctuations on the grid originate from lightning strikes. Contrary to popular belief it is not a direct strike which causes the most problems, but the voltage induced on overhead lines from the magnetic field of indirect strikes. Some of the largest discharges have been confirmed at >200,000A and there are o=en several discharges per strike. You could probably even measure some transient voltage induced in a paper clip lying on your desk within 500m of a storm if you were quick enough with the scope!

Buildings in Europe whose AC power is carried by overhead wires can reckon on having 80-120 surges every year due to lightning. These are typically limited to around 6kV because the standard domestic style mains socket flashes over at the rear connectors at around this voltage and acts like a spark gap suppressor! Industrial premises with only 3ph supplies can see much more. A modest strike of 15,000A would induce around 10kV on a transmission line 150m away (even when buried in the ground).

Heavy industrial switchgear, large photocopiers & laser printers, HVAC systems, electric motors and thyristor devices are all notorious for imposing spike and surge on transmission lines, and not always at lower energies than lightning strikes. It is not widely appreciated that even if such transient energies do not cause instant catastrophic failure, repeated exposure has a proven degenerative effect, particularly with highly integrated silicon devices. Call it transient ageing if you will. It has a significant impact on long term reliability.

In all cases, a well considered surge protection stage is essential but is o=en overlooked or poorly optimised. Indeed, there are a great many variables to consider and not every engineer appreciates the subtleties of the various protection devices available.

Looking specifically at the input of an AC-DC power supply, it is desirable to place surge protection devices in both the line-to-line and line to earth positions, giving both common and differential mode protection. Metal oxide varistors (MOV’s) or VDR’s, are the most commonly used device in low-cost applications.

However, a MOV may not be able to successfully limit a very large surge from an event such as a lightning strike where the energy involved is many orders of magnitude greater than it can handle. We have seen many designs where the power supply has a scattering of varistors on the input with no sacrificial protection (e.g. a dedicated thermal fuse). The result is that the first high energy surge to arrive either causes the varistors to explode, o=en accompanied by a large plasma discharge which destroys everything else in the vicinity, or the main input fuse to blow. Either way the power supply fails and has to be returned for service the same way as if there were no protection fitted at all!

An important characteristic to consider with MOV’s is that they degrade when exposed to a few large transients, or many smaller ones. As they degrade, their trigger voltage falls lower and lower, ultimately leading to thermal runaway of that particular device. Therefore to ensure good long term reliability, correct voltage rating is essential. It is also worth noting that selecting a device with a higher energy (joule) rating typically increases the life expectancy exponentially.

                                                

It is common to see multiple MOV’s in parallel to increase the overall joule rating of the network, however unless specifically matched sets are used, each MOV will have a slightly different non-linear response when exposed to the same overvoltage. This invariably leads to current hogging and premature failure of the individual device. Thus the ‘effective’ surge energy of the network is dependent on the MOV with the lowest clamping voltage, and the additional parallel MOV’s do not provide any benefit.

Furthermore, because each MOV has a relatively high leakage current (typically around 0.5mA at working voltage for a 20mm device), using many devices in parallel can lead to unacceptably high earth leakage currents. The other two devices commonly used in protection networks are transient voltage suppressor diodes (commonly referred to as Transorbs and also sold under the name Transil) and gas filled discharge tubes (GDT’s)

Whereas the practical response times of MOV’s are in the 40-60ns range, suppressor diodes respond to spikes within 1 - 10 pico-seconds, mostly limited by the inductance of the connecting circuitry. This makes diodes ideal for suppressing sub-nanosecond spikes generated by the many thyristor controlled devices sat on the mains supply. Sub-nanosecond spikes show up, do their damage, and are gone before MOV’s even notice.

Diodes also have the added benefit that they do not degrade with repeated surges which means they can be selected with clamping voltages much nearer to the AC working voltage than with MOV’s. The disadvantage of suppressor diodes is that they offer a lower ‘cost/energy handling’ ratio in comparison to other devices and they tend to be physically larger for the same energy rating. However, if space and cost are not critical, they are one of the most effective devices available for suppressing fast energy transients.

Gas discharge tubes consist of two electrodes surrounded by a special gas mixture in a sealed glass or ceramic enclosure. The gas is ionized by a high voltage spike which causes an arc to form between the electrodes and current to flow. GDT’s can conduct more current for their size compared to diodes and MOV’s but are crucially different in that they continue to conduct until the source voltage has dropped close to zero.

This has huge implications for DC and indeed has to be considered carefully for AC whereby it is quite possible to have a full half cycle of mains energy to absorb in addition to the initial spike or surge energy. It is critical therefore that this follow-on current is controlled. Like MOV’s, gas discharge tubes have a finite life and can only handle a few very large transients. The typical failure mode is a modified trigger voltage or, if subject to very high energies, a dead short. GDT’s take a relatively long 'me to trigger, 100nS pulses 500v above rated voltage will o=en be completely unsuppressed. However, gas discharge tubes offer the highest energy handling capabilities of all protection devices and have exceptionally low capacitance.

By far the most effective suppression networks utilise a combination of components to give high energy, high current capability with a very fast response 'me. Parallel devices are to be avoided unless using specifically matched sets and thermally vulnerable devices must be protected by dedicated components. Any design which neglects a well optimised surge and spike suppression network can expect substantially increased failures rates in the field.

Next _me we will look at ways to improve circuits from environmental factors such as moisture, electrolytes and contamination and why conformal coating is not as simple as it first appears

Advance Product Services Ltd

www.advanceproductservices.co.uk

Paul Horner is Managing

APS repairs it's 10,000th power supply!

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Design for Life - Part 4

Last time we looked at Power MOSFETs and uncovered some surprising factors which have a big impact on long term reliability. This week we look at Optocouplers and see why this innocent little component constitutes such a high failure risk for power supplies and what you can do about it.

Optocoupler Ageing

Most designers have a good appreciation of electrolytic capacitor ageing, but we also see many age related failures due to optocouplers. Generally this manifests itself as a reduction in the effective current transfer ratio (CTR) over 'me. This doesn’t sound too serious until you recognise that optos are commonly used to enable the converter stage of a power supply across a primary to secondary isolation barrier.

A degraded opto can and o2en does render the entire power supply inoperable and as such can be considered a high failure risk. The primary piece parts of an optocoupler are a photo-detector IC and an infrared emi5ng LED (typically Gallium Arsenide). Experimental analysis has shown that the LED is the only portion of the optocoupler that has a significant impact on life, with light output degradation leading to a decrease in CTR.

Furthermore, it is the actual current through the LED which is by far the most dominant factor. For longest possible service life therefore, it is desirable to allow at least 50% margin for a reduction in CTR over 'me and to drive the LED at as low a current as possible for the required CTR.

Next _me we will look at Spike & Surge—the primary natural cause of catastrophic psu failure in the field.

Advance Product Services Ltd

www.advanceproductservices.co.uk

Paul Horner is Managing Director at Advance Product Services Ltd

.

Design for life - part 3

Last month we looked at film capacitors and saw how selecting the wrong material for the application can have very serious consequences. This week we will look at Power MOSFETS and uncover some surprising factors which have a big impact on long term reliability.

Power MOSFETs

Generally speaking, power semiconductors are among the group of components least prone to ageing effects. Assuming they are used within their maximum ratings and are well thermally managed, they are very reliable. However they account for more than half of all service return failures.

Typically this is because their maximum ratings have been exceeded through knock-on effects of other component failures, poor circuit design, and environmental influences such as spike or surge, over-temperature or mechanical stress.

In terms of the circuit design however, there are subtleties that can contribute to a surprisingly large proportion of failures which tend to be far less well appreciated:

Problems can occur in MOSFETS where a high rate of rise of drain to source voltage (dVds/dt) causes capacitive charging of the FET gate. This can switch the FET back on while it is turning off—usually a destructive event!

This is especially problematic where the “off” drive connects the gate to a voltage slightly above zero, rather than to a negative potential. A negative drive holds the gate well below the threshold voltage as the drain-source cap charges and generally provides a much more robust solution. It should be noted that the gate threshold voltage typically reduces to less than 70% of its 25°C value at maximum junction temperature.

A high dVds/dt can also cause the parasitic transistor (present in the construction of all FET devices) to turn on, especially at high temperature where more thermally generated minority carriers exist within it. If the body diode of the FET is used to clamp the drain to source voltage (as in a zero voltage switching ‘ZVS’ resonant converter), its reverse recovery Time can be very long. This is due to the FET body diode only being moderately fast and the fact that the reverse voltage is only the “on” voltage the FET, typically around 1V.

As the body diode is in fact the collector-base junction of the parasitic transistor, the unrecovered charge carriers cause the parasitic transistor to turn on when Vds rises rapidly, allowing large currents to flow in the device. To make matters worse, the diode recovery time is even longer at higher temperatures.

There is a final scenario which sounds like it has come straight from science fiction! It is known as Single Event Burnout (SEB). SEB research carried out as long ago as 1996 showed that a high voltage MOSFET, biased off, supporting a voltage near to its maximum rating can suffer an avalanche failure caused by a single sub atomic particle colliding with a silicon nucleus.

Subsequent research has shown that even at ground level, neutrons from cosmic ray collisions in the upper atmosphere can cause random failures in high voltage. MOSFETs over and above the rate predicted by MTBF data from manufacturers life tests. Reducing the maximum Vds by even 6% has been shown to decrease SEB failures by an order of magnitude.

Advance Product Services Ltd

www.advanceproductservices.co.uk

Paul Horner is Managing Director at Advance Product Services Ltd.

Design for life - part 2

Last month we looked at electrolytic capacitors, their limitations and why you need to pay careful attention to ripple current rating.

This week we will look briefly at film capacitors and see how selecting the wrong material for the application can have very serious consequences.

Part 2—Film capacitors

It is not often appreciated that the ac rms voltage rating of film capacitors must be greatly de-rated for frequencies above approximately 1kHz. A popular cap, rated at 400Vdc & 250Vac is specified at just 1Vac maximum at 100kHz, so it can be easy to exceed the high frequency ac voltage rating in a power circuit. The image below is the result of exceeding the HF ac voltage rating of a 470nF 250Vac cap which occurred suddenly after 38 months in the field.

Film caps are also vulnerable to failure as a result of exceeding the repetitive  rate of change of voltage (dV/dt).

Metallised polyester snubber caps across switching semiconductors have been found to fail due to excessive dV/ dt, where the use of polypropylene, ceramic or foil film would have been preferable.

Surface Mount Multlayer Ceramic Capacitors (SMD MLC caps)

The larger sizes (1812, 2220) of SMD multilayer caps are prone to failure when mounted on fibre glass or composite PCBs due to the different coefficients of thermal expansion of the cap and the substrate. These components can fail short circuit with devastating consequences if they are connected across a power rail.

All sizes, but more especially the larger ones, are prone to failure due to mechanical stress. An example encountered recently used several SMD MLC caps under a dc power output screw terminal block which was subject to flex whenever the terminal was pressed down by tightening or loosening the screws. The subsequent fracture of the cap burnt a hole right through the pcb, as the ceramic cap body remains mostly intact even when red hot.

Avoid large SMD MLC caps on circuit boards involved with intense thermal cycling unless substrates are matched, and never place in areas of mechanical flex or stress.

Next time we will look at MOSFETS and uncover some very surprising factors which have a big impact on long term reliability.

Advance Product Services Ltd www.advanceproductservices.co.uk

Paul Horner is Managing Director at 
Advance Product Services Ltd.

Design for life - Part 1

Design for life

It's interesting. Being involved with switch mode power supply design for over 40 years, you learn what works and what doesn't. What once seemed like quite complex theory becomes second nature and you instinctively have a feel for what is required to make a design work.

However, working in the lab as a prototype is one thing, working faultlessly for the next 20 years is quite another.

It's feedback that the design engineer seldom has the opportunity to benefit from. You rarely see the end application, let alone the condition of the components after years of operation in an industrial environment.

This is probably of little concern to the glut of far eastern manufacturers with products at throw-away prices, but there are sill plenty of applications where long term reliability and build quality are paramount, and this remains a stronghold of UK design and manufacturing. Lets face it, if you are manufacturing in the UK and you are not focusing on quality - you are dead in the water.

Of course, there are a host of general parameters that effect long term reliability of a power supply. Fundamental circuit design, component selection, mechanical construction, assembly process, storage and handling all play a big role. However these tend to be well appreciated at the design and manufacturing stages.

Looking at things from the service return side gives the engineer an entirely new perspective. It allows a unique appreciation of what, in practice causes power supplies to fail in the field. And it's not always obvious.

Part 1 — Electrolytic capacitors

The drying out of wet electrolytic capacitors is perhaps one the most widely recognised causes of age related failure, and it is certainly prevalent. Modern demands for ever decreasing sizes can result in thinner dielectric materials and less volume of electrolyte. Although the loss of electrolyte is by some means the natural wear out mechanism, it can be slowed considerably by reducing the core operating temperature of the capacitor. Locating caps away from other high dissipation components is one obvious example, but the core temperature is also very much influenced by the ripple current flowing through the ESR (equivalent series resistance), namely the electrolyte.
A typical 105°C rated capacitor has a ripple current typical in a 105°C ambient, giving a core temperature of approx 115°C. The specified load life under these conditions can be as low as 1,000 hours (42days), although in practice most caps will continue to operate for longer than this, albeit with reduced capacitance and or higher ESR.

Most practical applications do not subject passive components to more than 50°C, so it can be tempting to increase the ripple current above the rated maximum. This is not recommended because the temperature rise is proportional to the square of the ripple current multiplied by the ESR. Because ESR increases with time, end of life failure will occur sooner, faster than for a cap operating at 105°C and maximum rated ripple current.

Vented capacitors at end of life failure

Output capacitors on small ‘flyback' power supplies, operating in the discontinuous current mode are especially vulnerable to early failure due to the large ripple currents inherent in this topology, so they need specifying carefully. By comparison, continuous current flyback and ‘forward' converters have typically a 20% peak to peak current ripple compared to the 100% of the discontinuous mode flyback converter.
Conversely, small (approximately 6 x 12mm) electrolytic caps commonly used in power supply control circuitry can cause problems in high local ambient temperatures, even when run at very small ripple currents. These capacitors are often used in conjunction with a high resistance connected to a HT rail to provide a start up supply to the control circuit. Due to the very small amount of electrolyte they contain, they can dry out before any other component fails and prevent the power supply starting at turn on due to high impedance or current leakage. Often this can go completely unnoticed until the first mains blackout and subsequent restart attempt.

Careful electrolytic capacitor selection is becoming increasingly important as more and more far-eastern manufactured components enter the market and it is important to pay a good deal of attention to the detailed specification of such components. Cutting costs by using inferior capacitors is rarely money well saved when it results in a dramatic reduction in service life, potentially high warranty costs and a blemished reputation.

Better cooling, larger capacitors or solid electrolyte capacitors are alternative solutions. Niobium solid electrolyte caps are a cap alternative to tantalum caps, which are becoming more expensive as tantalum reserves diminish.