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Figure 2: EMI/EMC input filter Diode Bridge RectifierĪs mentioned previously, the diode bridge rectifier is used to convert the ac input voltage to a dc voltage for use within the power supply. Thus, there may be the need for additional external components to enable your system to meet EMI/EMC regulatory requirements even if the supply already includes internal EMI/EMC components. Your application will probably not be operating in the same conditions as evaluated by the development team. When the EMI/EMC components are internal to the power supply the supply development team has selected the components based upon maximum rated output current into a resistive load. Protect the input power source and conductors if there is a failure of the power supply.Minimize the input surge current when voltage is first applied to the input of the power supply.Minimize the effects of voltage transients applied from the input voltage source.Minimize radiated and conducted noise on the input of the power supply.The EMI/EMC components may be required to serve the following functions: The EMI/EMC filter block can be either components placed internal to the power supply by the power supply designer or added as external components by the user of the power supply.
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Figure 1: Simplified block diagram of ac-dc switching power supply EMI/EMC Filter A rectifier circuit (diodes D1, D2, D3, D4) is required in ac-dc power supplies and is not needed for dc-dc power supplies, otherwise the power supply topologies can be identical. The difference in the block diagram between ac-dc versus dc-dc input supplies is the diode bridge rectifier. The block diagram in figure 1 is representative of many ac-dc or dc-dc power supplies. In this article we will present a power supply topology and discuss each of the internal functional blocks to provide a fundamental understanding regarding the major circuits internal to power supplies. While there is no need for you to be a power supply design expert, there are benefits to understanding the basic blocks internal to power supplies. Although you specify and use power supplies in your projects, they may be a ‘black box’ with unknown internals.