Switching Power Supply inductor value to determine – HC net electrical industry
Switching power supply is one of the main power supply products, but with the worldwide demand for energy-saving products on the increase, those who have traditionally used the lower prices are less efficient linear regulator power supply application, it will turn to switching power supply (switchmode powersupplies, SMPS). This change will promote the improvement of the industry constantly switching frequency to meet customer demand for a smaller space, more power requirements. This SMPS manufacturers opened new markets, and the corresponding in turn have a number of areas SMPS design engineers how to meet the design requirements facing this problem.
Figure 1 Structure of a typical buck power
Fig.2 Buck-type switching power supply switching waveforms structure
This article will discuss how to select a non-isolated SMPS inductor problem. The examples given apply to thin surface mount design, such as voltage regulator module (VRM), point of load power (POL)) and a larger rack system.
Figure 1 shows the use of Buck (buck) topology power supply, it is commonly used in the output voltage is lower than the input voltage of the occasion. In a typical buck circuit, when the switch (Q1) closed, the current began to flow through the output, and a steady increase in its rate depends on the inductor value. According to Lenz's law, di = VL × dt / L, which flows through the inductor is equal to voltage multiplied by current changes in the changes of time and then divided by the inductance value. With the flow of current through the load resistance RL steady increase in the output voltage directly proportional to increase.
When the voltage or current to a predetermined limit, the control IC to switch off, the inductor in the magnetic field would gradually disappear, leaving the forward biased diode D1 to feed current to the output circuit until the switch closes again. This cycle repeats go, its closure and the number of times off the decision by the control IC, so that the output voltage can be adjusted to the desired value.
Figure 2 shows the number of cycle flows through the inductor and other components of the voltage step-down circuit and current waveform. Value for the inductor when the switch off to maintain the load current is critical. To be able to in the most extreme conditions the output voltage and input current to guarantee the minimum inductance value is still supported buck converter output current, output current to the load, the minimum inductance value is needed to determine a value. To calculate the minimum inductance value, and need the following information: input voltage range, output voltage and its rated range, operating frequency (the switching frequency), the inductor ripple current and mode (continuous or discontinuous).
The following formula is used to determine the required inductance of a buck converter value:
The formula for the continuous mode is, dI <I.
Identify all possible working conditions are applicable within the minimum inductance value, we must try to ensure the necessary inductance to the load current, and also ensure that all parameters are the worst conditions of ripple current still within the targets set. Buck for buck power supply for the worst conditions in the input voltage and frequency are the minimum value of the case. Let the output voltage is equal to its minimum allowable voltage regulator can be obtained to maintain the required minimum inductance. Designers based on the worst possible conditions for the capacity to select the inductance value.
Given value using Table 1, by the following formula to calculate the minimum inductance.
L1 (Min) = 10.6mh Thus, for this particular application, the minimum inductance value can 10.6mH, the current rating of 20A at the maximum operating current above still has a reassuring safety margin. Use a lower inductance inductor buck converter may result in the required maximum current, unable to maintain its output voltage within the range specified.
Conclusion Once the inductance value, we can according to electrical standards, size, and system installation limitations to design the actual inductance. Many magnet suppliers to meet most of the design specifications of standard products, however, the use of existing products, may need to compromise performance or size. In many cases, the use of magnet manufacturers have technical expertise the ability to form inductance, electrical performance and physical constraints of the optimal design are satisfactory, you can improve performance and reduce time to market.
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