5 Things You Should Know About AC-DC Power Conversion

April 4, 2018
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ac dc power systemAC-DC power conversion presents a range of challenges for power system designers. Here are five things you really should consider when developing a product powered from AC.

 

The Need for Power Factor Correction

The power factor of an AC system is defined as the real power flowing to the load divided by the apparent power in the circuit. In AC systems there are two main causes of increased power factor. In linear loads, reactive elements cause power to be delivered to the load and then returned to the network, increasing the apparent power in the load. In non-linear loads, for example those using switching power supplies, the current in the system is distorted and is no longer a sinusoid, meaning the current contains harmonics of the supply frequency.

Low power factors increase costs for the distribution network. Restrictions are placed on the power factor of loads to ensure stability and reliability of the electrical grid, meaning that low power factor must be corrected for many systems before they can be sold.

Power factor in linear systems can be corrected using passive components. For example, a motor is a highly inductive load, resulting in a low power factor. By adding capacitance, the power factor can be brought closer to 1. Non-linear loads are more difficult to correct. Filtering can help, but typically more complex active power factor correction circuits are required to achieve compliance. Some products, such as the Vicor PFM, build-in power factor correction.

 

Earthing or Double Insulation for Safety

Safety regulations aim to ensure that a single failure cannot result in danger to the user. For example, earthing a metal case ensures that if a wire disconnects inside the equipment and touches the case, the fuse will blow and there will be no risk of shock.

Today many AC products don’t use earthing, providing double insulation where the case is insulated, and the internal wiring is within a second insulating enclosure. Double-insulated devices don’t require a good ground connection for safety and therefore applications that could have poor earthing, such as power tools that may be used on construction sites, should use this approach.

 

Selecting Fuses or Circuit Breakers

Fuses and circuit breakers are used to protect AC-DC systems in the case of overcurrent by breaking the connection between the AC power and the converter. Fuses are very simple, low-cost protection devices that are slow to respond to over-current, have limited accuracy and require manual replacement before the system can be restarted. Although the performance of fuses is limited, their convenience and low cost means they are still widely used for protection.

Circuit breakers are similar to fuses in terms of performance, but can be reset more easily. Circuit breakers, however, are much more expensive than fuses and can be susceptible to internal corrosion in harsh environments. Circuit breakers still require manual resetting.

Polymeric positive temperature coefficient devices (PTCs) dramatically increase resistance when an overcurrent condition causes them to heat up. As they cool, they return to a low-resistance state and so can be considered self-resetting fuses. As with all thermal devices, they are slow and are relatively inaccurate, and their on-resistance can reduce efficiency.

There are a number of active current shutdown methods to detect overcurrent. These can be made to have precise trip points and very fast response, but are significantly more expensive than other solutions.

 

A Wide Range of EMI Standards

There are a vast range of EMI standards for AC-DC systems, and there are different regulations that apply locally in countries and regions around the world. In the USA, the FCC defines standards, while in Europe the EN standards are defined by CEN, CENELEC and ETSI. Global standards are also defined for EMI in AC systems by CISPR, part of the IEC.

 

Universal Voltage

Voltages and frequencies differ around the world, and universal AC-DC power systems must be able to handle the full range. In the USA the supply is 120V at 60Hz, in Europe is it 230V at 50Hz, Japan uses 100V with a frequency of 50Hz or 60Hz depending upon whether you are in East or West Japan. A full list of worldwide voltages is available on Wikipedia. Converters are considered to support universal voltage if they can accept from 85VRMS to 264VRMS.

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