POWER SUPPLIES
Reducing the size of your power supply
As expectations grow for ever smaller, more cost effective solutions how can engineers maintain the long-term reliability of these parts? Shane Callanan looks at some of the challenges
Even modular based AD/DC complex power supply designs with multiple outputs are not immune from the performance improvements that customers have come to take for granted. With each new generation of parts that are released, the expectation is set that that latest design will provide a smaller, more cost effective solution that the previous part. Here we focus on one of these aspects, namely the challenges associated with making things smaller. We not only need to understand how we can achieve this, but more importantly how we can continue to maintain the long-term reliability of these parts while simultaneously reducing the size of our power supplies. If we review the typical Watts per cubic inch over the last say 15 years or so we can quickly see the improvements in this from generation to generation of design.
graph above also shows the corresponding trend and what we can expect if the present rate of design continues.
In 1998 a typical part would deliver the equivalent of 4 Watts per cubic inch. This information is derived from on a typical 350 Watt multiple output design, that would have been available at that time. With the advancement of design control techniques, improvements, and even with the use of magnetic amplifiers we see a slight improvement in the power density. However, it is not until we see the use of synchronous buck controllers in 2006, that we see a significant step in the performance up to approx. 16 Watts
In the last few years, as the technology of power supplies has changed we have seen tremendous improvements in the power density. This is in direct response to customer demands of
power supply you need to ensure that component temperatures are kept low, or run the risk of reduced reliability.
Efficiency of a power supply If a power supply could convert all of the power entering into it into useable power output, it would be 100% efficient. However, a certain amount of energy is ‘lost’ during each conversation stage as you convert from one voltage level to another. In the process of these conversion stages a power supply will consume some energy. Typically this is expressed as a percentage.
= Pin / Pout Eqn (1) Where Pin = Input power Pout = Output power = Efficiency (%)
Any inefficiency in a power supply will be converted to heat, which will cause a temperature rise internally on components. This in turn will impact on the reliability of the power supply. The importance of these provisions cannot be
overstressed since the failure rate of some components will double for a 10°C increase in temperature.
How efficient? In order to maintain reliability, efficiency numbers intuitively must be high, but why is this the case and how high should this number be? From Eqn (1) (Figure 2) we can determine the power to be dissipated (Pdiss), since this is a function of efficiency
Pdiss = Pout ((1- / )) Eqn (2)
Where Pdiss = Power to be dissipated = Efficiency
Figure 1: Power Density of off the shelf AC/DC Multi output supplies in the 300 Watt to 1400 Watt range
The plot above shows some data from actual parts in the field between 1998 and 2010. The
smaller solutions, lower cost and increased reliability. However, as you reduce the size of your
12 CIE Power Supplement May 2013
Once you understand this, it will allow you start to consider if you need additional cooling in your design. If the answer is affirmative, you now have a number of options available.
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