The most basic and important decision to be made in the design of a power system for a piece of electronic equipment is the selection of the power architecture to meet its application needs.
In our private and work lives, we are surrounded by an increasing array of electronic products that help entertain us, enable us to communicate effectively and generally make our lives easier and more enjoyable. Virtually every piece of electronics equipment requires some sort of power conversion. Whether converting ac mains power to dc or, conditioning dc to suit load requirements, the power supply and conversion element of a product is a key part of its design. In its simplest form the power system of any electronic product is comprised of the power source, the power converter and the load.
Most electronics components in a design require specific levels of dc voltage in order to function. With the bandwidth and high levels of functionality present in modern electronics equipment it is very often the case that more than one dc voltage is needed to support the various loads. These could include devices such as processors, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) and memory as well as fans and disk drives etc.
The most basic and important decision to be made in the design of a power system for a piece of electronic equipment is the selection of the power architecture to meet its application needs. This analysis typically involves the trade-off of the often conflicting requirements of cost, efficiency, size and flexibility.
Types of conversion
There are two general product groups we can consider when discussing power conversion. The stage that takes the ac input voltage and produces a dc output voltage is known as the ‘power supply'. A front end power supply provides a single regulated dc bus voltage typically used in a distributed bus architecture. While no single industry-wide standard format for ac-dc power supplies exists, there are some common configurations and dimensions; these include CompactPCI, cable TV (CATV) and dimensions such as 3" x 5" or 1U rack unit height. Ac-dc power supplies can be supplied in either open frame or closed box formats depending on the need of the application. (See figures 1, 2a and 2b).
Modules that take the dc voltage feed from a source such as a battery, front end power supply or another dc-dc converter and convert it to a dc level required to power the load are known as dc-dc converters. Many pieces of electronic equipment have multiple loads with different power requirements so several dc-dc converters may be required in a single design. Both isolated and non-isolated DC/DC converters are available in a variety of through-hole and surface-mount packaging formats. Isolated converters provide electrical separation of the input supply from the output load.
The transformer used in an isolated dc-dc converter provides a safety barrier as the input and output grounds are isolated. This enhances human and electrical safety, satisfies certification requirements and is required in some applications for proper functioning. Non-isolated dc-dc converters can take their input from an isolated dc-dc bus converter and are used at the point-of-load to limit voltage distribution losses in distributed power and intermediate bus architectures. Their output voltage is typically programmable by an external resistor or voltage source.
Power conversion specifications
For those seeking to familiarize themselves with the basics of power conversion, understanding some of the key specifications and terminology can be useful.
Input voltage range - These are the maximum and minimum limits that the power supply or converter can accept at its input. A wide input voltage range offers design flexibility as it helps to reduce the restrictions placed on the engineers tasked with designing the product. A typical input voltage range might be 36 to 75Vdc with the nominal being 48Vdc.
Output voltage - is the nominal or typical dc voltage at the output terminals of a power converter. Devices may have single or multiple outputs. Designers may seek power converters with a high degree of output voltage stability to ensure the load being supplied does not have its operation, life expectancy or reliability adversely impacted by fluctuations in supply.
Power loss - Expressed in Watts (W), this is the difference between the input power and output power. Losses can comprise heat, noise, light or other forms of energy expended in the execution of the conversion. Minimizing losses is especially important as equipment designers become increasingly conscious of making their designs as energy efficient as possible in response to environmental, legislative, and end user demands.
Efficiency - Similar to power loss but expressed as a percentage, this is the ratio of total output power to input power. Designers and manufacturers such as Murata Power Solutions can now achieve efficiency levels of up to 95%. Organizations and programs such as the Climate Savers Computing Initiative and 80 Plus foster the design and adoption of high efficiency products and make it easier for end users to recognize products with the best energy usage credentials.
Line regulation - This specification states as a percentage, the variation of an output voltage due to a change in the input when all other factors remain constant. Load regulation meanwhile is the variation of the output voltage due to a change in the output (load) with all other factors held constant and is also expressed as a percentage.
Voltage adjustment or trimming - is a feature where the output voltage can be adjusted within a given range. This is often implemented to anticipate loss of voltage across output load lines as well as to optimize the output voltage for specific applications. Trimming can compensate for static losses when the load current is constant.
Remote Sense - allows voltage regulation to be maintained at the load itself rather than at the output pins. The remote sense lines monitor the voltage at the load and adjust the power converter output, thus compensating for the loss of voltage across the output load lines. Remote sense is typically required when output voltages are low and currents are high. Where output voltages are higher (12V or more) and currents are lower, remote sense is less commonly required.
Cooling and thermal management - All electronic devices and modules generate some amount of heat when in normal operation; heat is effectively an energy loss.
Protection features
In general, the greater the efficiency of a power converter the lower the amount losses it generates. In less demanding scenarios, natural convection cooling is often sufficient to take heat away from the module and keep it operating within its temperature specification. In some cases forced convection cooling using fans or conduction cooling may be required. Conduction cooling is achieved by attaching the module to a metal chassis or heatsink to draw more heat away. Keeping any electronic device or module within its specified operating temperature range through effective thermal management is vital to ensure its performance, efficiency and reliability.
Protection features - Power converters may include a range of protection features to prevent damage to, or in the event of converter failure, the end equipment or indeed the equipment user. Common protection features include over-current and over-temperature shutdown, often with automatic restart once conditions have normalized.
Standards and certification - the amount of legislation impacting the electronics industry as a whole is growing, mainly in response to environmental safety and sustainability demands. Such legislation includes the continually evolving RoHS standards, REACH, EuP and the Batteries Directive. In addition to this there exist a number of power standards that are designed to ensure the design of safe products that avoid the likelihood of injury to users and damage to other equipment.
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