How to Select a Switching Power Supply Inductor

Switching power supplies have long been a staple of the power supply industry. However, as the global demand for energy-efficient products continues to increase, the market, which has traditionally used cheaper but less efficient linear power supplies, will also turn to switching power supplies. During this transition period, the power supply industry has been working tirelessly to increase switching frequencies to meet customer requirements for more powerful power supplies that take up less space. This development trend has opened up new markets for switching power supplies and has exposed some design engineers to the market's demand for switching power supply designs.

This article will explain the basics of inductor selection for non-isolated switch mode power supplies (SMPS). The examples given are for applications in ultra-thin surface mount designs, such as voltage regulator modules (VRMs) and point-of-load (POL) type power supplies, but not for larger chassis based systems.

Figure 1 Typical buck topology power supply

Figure 1 shows the architecture of a buck topology power supply, which is widely used in systems where the output voltage is less than the input voltage. In a typical buck topology circuit, when the switch (Q1) is closed, the current begins to flow through the switch to the output and increases steadily at a rate that depends on the circuit inductance. According to Lenz's law, di=E*dt/L, the change in current flowing through the inductor is equal to the voltage multiplied by the change in time, divided by the inductance value. As the current flowing through the load resistor RL increases steadily, the output voltage increases proportionally.

When the predetermined voltage or current limit is reached, the control integrated circuit turns off the switch, thereby 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 switches is determined by the control integrated circuit, and the output voltage is regulated at the required voltage value. Figure 2 shows the voltage and current waveforms flowing through the inductor and other buck topology circuit components during several switching cycles.

Figure 2 Switching action waveform of a switching power supply using a buck topology

The inductor value is critical to maintaining current to the load during the switch off period. Therefore, the minimum inductor value required to maintain the output current of the buck converter must be calculated to ensure that sufficient current can be supplied to the load under the worst-case output voltage and input current conditions. To determine the minimum inductor value, the following information is required:
· Input voltage range
· Output voltage and its specified range
· Operating frequency (switching frequency)
· Inductor ripple current
· Operation mode; continuous operation mode or discontinuous operation mode

Table 1 Typical buck power system specifications

The following formula is used to calculate the required inductor value for the buck converter:

L1 = Vo(1-Vo/(Vin-Von))/(f*dI) In continuous operation mode: dI < 1/2I

In order to calculate the minimum inductance value applicable to the entire operating conditions of the power supply, the parameter values ​​must be selected to ensure that the selected inductance value can still keep the ripple current within a specific range under the most unfavorable combination of various parameters. For step-down power supplies, the most unfavorable combination condition is when the input voltage and frequency are at their respective minimum values. In addition, the output voltage must also be taken to its minimum specified value to determine the minimum inductance value required to maintain normal regulation. Designers can control these values ​​in their own way to achieve the state where the worst condition is established.

According to the data listed in Table 1, the minimum inductance value is calculated as follows:

L1(min) = Vo(min)(1-Vo(min)/(Vin(min)-Von))/(f(min)*dI)

L1(min) = 4.95V(1-4.95V/(20V-0.7V))/(693,000Hz * 0.5A)

L1(min) = 10.6uh

Therefore, in this specific application, the inductor should have an inductance of at least 10.6 μh and a current rating above the minimum operating current of 20 amps with a sufficient safety margin. Choosing an inductor with an inductance below this minimum will result in the buck converter being unable to maintain its output voltage within specification at maximum current.

After the inductance value is determined, the actual inductor design must comply with relevant electrical standards, system size, installation method and other restrictions. Many magnetic component suppliers provide various types of standard products that can meet most design standard requirements. However, the use of standard products available off the shelf in the design may result in deficiencies in the performance and size of the inductor, and may ultimately have an adverse effect on product sales. Fortunately, some suppliers, including Tyco Electronics CoEv Magnetic Components Division, can provide the necessary customized engineering design support to meet the specific inductance value, electrical performance and appearance restriction requirements combined in a fully mature product to promote design optimization. Make full use of the industry's professional technology, thereby minimizing the design and testing time and accelerating product launch.