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.