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Design Techniques I Power Supplies

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 electrolytic caps 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 very quickly 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 vital and cutting costs by using inferior capacitors is rarely money well saved when it results in a 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 cheaper alternative to tantalum caps, which are becoming more expensive as tantalum reserves diminish.

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 on the right is the result of exceeding the HF ac voltage rating of a 470nF 250Vac cap. 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 multilayer ceramic capacitors

The larger sizes (1812, 2220) of SMD multilayer ceramic 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. All sizes are prone to failure due to mechanical stress. An example recently encountered 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 due to the fact that the ceramic cap body remains mostly intact even when red hot. So avoid intense SMD MLC caps on circuit boards involved with large thermal cycling unless the substrates are matched, and never place in areas of mechanical flex or stress.

Power MOSFETs Power semiconductors are among the group of components least prone to ageing effects. However they account for more than half of all service return failures. This is because their maximum ratings have been exceeded through knock-on effects of other component failures, poor circuit design, environmental influences, over- temperature or mechanical stress. 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. 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. There is another scenario known as Single Event

Burnout (SEB). SEB research 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 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 manufacturer’s life tests. Reducing the maximum Vds by even 6% has been shown to decrease SEB failures by an order of magnitude.

Optocoupler ageing 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 time. A degraded opto can and often 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 emitting 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 time and to drive the LED at as low a current as possible for the required CTR.

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 after spending less than £100K on improved immunity. A relatively small proportion of energy fluctuations on the grid originate from lightning strikes, and 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. 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).

It is not widely appreciated that 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 often overlooked or poorly optimised. 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 or the main input fuse to blow. 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, ultimately leading to thermal runaway of that particular device. Therefore 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 which leads to current hogging and premature failure of the device. Hence 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, 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 (Transorbs or 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. This makes diodes ideal for suppressing sub-nanosecond spikes which do their damage, and are gone before MOV’s even notice. Diodes also 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.

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. Gas discharge tubes also have a finite life and can only handle a few very large transients. By far the most effective suppression networks utilise a combination of components to give high energy, high current capability with a very fast response time. 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.

Advance Product Services Ltd |

Paul Horner is Managing Director at Advance Product Services

Components in Electronics July/August 2011 29

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