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All colleagues unite and focus their efforts to write a new chapter of developmentWhat is a filter inductor? What are its calculation formulas in switch-mode power supplies?
所属分类： Industry News
发布时间：2020-05-19
What is a filter inductor? Filter inductors typically use ferrite materials, which can be easily combined with feedthrough capacitors to form composite filters. In high-performance filters, wire-wound inductors are also employed. However, it should be noted that ferrite materials can experience magnetic saturation under high current, thereby reducing the filter’s effectiveness. So, how is a filter inductor calculated?
Switching power supply The output voltage Uo on the secondary coil is in a pulsed state.
To convert a pulsed square wave into DC suitable for circuit use, it must be smoothed. A commonly used smoothing circuit consists of a rectifier diode, a filter capacitor, and a filter inductor.
(1) Principle of Smoothing Processing
(Figure 2) In the circuit, inductor L both stores energy and presents high impedance to AC components, thereby blocking their passage. Capacitors C1–C4 exhibit low impedance to AC signals, allowing AC components to pass, while offering high impedance to DC, thus preventing its flow.
Inductive reactance: XL = 2πfL
An inductor presents high impedance to high-frequency components; the greater the inductive reactance, the higher the voltage drop across it for high-frequency signals, and the stronger its ability to attenuate high-frequency content.
Capacitive reactance: XC = 1 / (2πfC)
A capacitor presents high impedance to DC, thereby blocking its passage, while it exhibits low impedance to AC components; the smaller the capacitive reactance, the more readily AC can pass.
In Figure 2, the larger the product of L and C, the more effective the smoothing of high-frequency components. To determine the optimal inductance value, design calculations can be performed as described in the next section.
(ii). Ripple Noise of Switching Regulated Power Supply Output
Output waveform of the switching power supply after smoothing filtering
It is readily apparent that the DC output, after being smoothed by the circuit shown in Figure 2, still contains a certain amount of ripple noise.
It is divided into two parts:
Ripple: The waveform that is synchronized with both the primary input mains frequency and the switching frequency is referred to as the ripple.
Noise: The needle-like spikes on the ripple are noise.
The combination of the two types of waves is referred to as ripple noise.
(3) Design and Calculation of the Inductor L in the Output Smoothing Circuit
The calculation of inductance L involves the following formulas:
The ripple current ΔIl flowing through the inductor L is 2% to 5% of the output current Io.
That is:
△Il = (0.02~0.05) Io
△Il = Tonmax (Umin - Vf - Vo) / L
L=DTonmax（Umin-Vf-Vo）/△Il
Uomin=T(Vomax+Vf+VI)/D
D=Totalmanx/T
(4) Computational Example
Output voltage Vo = 5 V, 10% ±
Output current Io = 20 A
Switching frequency F = 200 kHz
Duty cycle D = 0.42
Switching period: T = 1/F = 1/200×10³ = 5 μs
Maximum on-time: Ton max = TD = 5 × 0.42 = 2.1 μs
Maximum DC output voltage: Vo max = 5 × 10% = 5.5 V
Minimum voltage across the secondary coil: Umin = 5 × (5.5 + 0.2 + 0.4) / 2.1 = 14.5 V
Under normal conditions, the ripple current ΔIl in the filter inductor is typically 2% to 5% of the load current Io; in this example, we take 5%.
According to Equation ②, L△Il = Ton max(Umin - Vf - Vomax)/△Il = 2.1(14.5 - 0.4 - 5.5)/1 = 18.06 μH.
The inductance L is 18 μH, and the current flowing through it is 20 A.
When Io is less than 1 A, magnetic beads made of amorphous alloy magnetic material can be used and slipped onto the output lead of the rectifier to form an inductor. When the current exceeds 1 A, a magnetic core or a wound toroidal core should be selected. To minimize the DC resistance of the inductor, reduce losses, and decrease inductor heating, the current density J for the enameled wire used to wind the filter inductor should be set at a relatively low value. In this example, J = 4 A/mm², which corresponds to a bare wire diameter d = 20 ÷ 4 = 5.0 mm².
In this example, the filter inductor uses relatively thick wire; a MOX magnetic core with a diameter of 50 mm is selected, yielding approximately 3 μH per turn. However, due to variations in core performance, the actual inductance should be determined based on measured test data during winding.
Note: The voltage drop across the filter inductor, Vl = 0.2 V, is specified by the designer. To minimize inductor losses, the voltage drop across the filter inductor should be less than 0.5 V; in this example, it is set to 0.2 V.