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Switching power supply inductor selection

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Switching power supply has always been the main product of the power industry. However, as the global demand for energy-efficient products continues to increase, the market for linear power traditionally using cheaper but less energy-efficient converters will also be moving toward switching power supplies. During this transitional period, the power industry has worked tirelessly to increase the switching frequency to meet customers' demands for more power-hungry, smaller power supplies. This trend of development for the switching power supply has opened up new markets, and some design engineers face the market demand for switching power supply design. This article explains the basic points of using inductors for non-isolated smps. Examples are for ultrathin surface mount design applications like voltage regulation modules (vrm) and pol power supplies, but do not include systems based on larger backplanes. Figure 1 Typical buck topology Power Supply Figure 1 shows a buck topology power supply architecture, the framework is widely used in the output voltage is less than the input voltage of the system. In a typical buck topology circuit, when switch (q1) is closed, current begins to flow through the switch to the output and steadily increases at a rate that is dependent on the circuit inductance. According to Lenz's law, di = e * dt / l, the amount of change in the current through the inductor equals the voltage multiplied by the amount of time variation divided by the value of this inductance. Since the current flowing through the load resistor rl steadily increases, the output voltage increases in proportion. When a predetermined voltage or current limit is reached, the control integrated circuit turns the switch off, attenuating the magnetic field around the inductor and causing the bias diode d1 to conduct forward, thereby continuing to supply current to the output circuit until the switch is turned on again . This cycle is repeated, and the number of switching by the control IC to determine, and the output voltage regulation in the required voltage value. Figure 2 shows the voltage and current waveforms flowing through inductors and other buck topology circuit components over several switching cycles. Figure 2. Switching behavior of a switching power supply with buck topology. Inductance values are critical to maintaining the current flowing to the load during a switch-off event. Therefore, the minimum inductance necessary to maintain the buck converter output current must be calculated to ensure that sufficient current is still available to the load while the output voltage and input current are in their worst-case condition. To determine the minimum inductance, you need to know the following information: • Input voltage range • Output voltage and its specified range • Operating frequency (switching frequency) • Inductor ripple current • Operating mode; Continuous mode or discontinuous mode Table 1 Typical step-down power supply system specifications The following equations are used to calculate the required inductance for the buck converter: In continuous operation mode: di <(1-vo / (vin-von)) / (f * di) 1 / 2i In order to calculate the minimum inductance suitable for the entire operating conditions of the power supply, the value of the parameter must be chosen so that the selected inductor value will still hold the ripple current at the most unfavorable combination of parameters Within a specific range of values. The buck power supply, the most unfavorable combination of conditions: the input voltage and frequency are at their lowest value. In addition, the output voltage is also taken as its minimum rating to determine the minimum inductance required to maintain normal regulation. The designer can control these values in the way they are accustomed to in order to achieve the worst-case conditions. According to the data listed in Table 1, the minimum inductance is calculated as follows: l1 (min) = vo (min) (1-vo (min) / (vin (min) -von)) / ) l1 (min) = 4.95v (1-4.95v / (20v-0.7v)) / (693,000hz * 0.5a) l1 (min) = 10.6uh Therefore, in this particular application, the inductance of the inductor At least 10.6 μh, and its current rating should be above the lowest operating current of 20 amperes, and maintain a sufficient safety factor. Selecting an inductor with an inductance below this minimum will cause the buck converter to fail to maintain its output voltage within the specified range for the maximum current. After the inductor value is determined, the actual inductor design must meet the relevant electrical standards, system size and installation restrictions. Many magnetic component suppliers offer a wide range of standard products to meet most design standards. However, the use of off-the-shelf standard products in the design may result in inadequate performance and size of the inductor and may ultimately adversely affect product sales. Fortunately, some suppliers, including the Tyco Electronics coev magnetic components, can provide the necessary custom engineering support to meet the specific inductance, electrical performance and shape
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