This inrush current can be an issue as it may be 100’s of times greater than the normal input operating current. This difference in voltage between the applied voltage and the bulk capacitor voltage can cause a large input surge current while the bulk capacitor is charging up to the input voltage. When the input voltage is first applied to a power supply the voltage across the input bulk capacitor will be 0 V. The input bulk capacitor filters the dc voltage from the rectifier diodes in ac-dc power supplies and acts as an input filter in dc-dc power supplies. Figure 3: Diode bridge rectifier Input Bulk Capacitor If a dc voltage is applied with a diode bridge present on the input of the supply, the dc voltage can be connected in either polarity and will pass through the diodes and appear on the input bulk capacitor. However, many power supplies designed for an ac input voltage have also been characterized to be powered by a dc input voltage. The rectifier circuit is not present in a supply designed for only a dc input voltage since the dc voltage is already present. 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. 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.
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