Inductor selection tips in switching power supply circuit design

The first part: Let me talk about what is the inductance

An inductor is an element that can store electrical energy into magnetic energy and store it. The structure of the inductor is similar to a transformer, but with only one winding. The inductor has a certain inductance that only blocks the change in current. If the inductor is in a state where no current is flowing, it will attempt to block current flow through it when the circuit is turned on; if the inductor is in a state where current is flowing, it will attempt to maintain the current when the circuit is turned off. Inductors are also known as chokes, reactors, and dynamic reactors.

Simply put: DC, blocking communication.

The second part of the actual inductance characteristics

Inductance is a commonly used component in switching power supplies. Because of its different current and voltage phases, the theoretical loss is zero. Inductors are often energy storage components and are often used with capacitors on input filtering and output filtering circuits to smooth current. Inductors, also known as chokes, are characterized by "very large inertia" of the current flowing through them. In other words, due to the continuous nature of the flux, the current on the inductor must be continuous, otherwise large voltage spikes will result.

The inductance is a magnetic component and naturally has the problem of magnetic saturation. Some applications allow inductor saturation, and some applications allow the inductor to start saturation from a certain current value, and some applications do not allow the inductor to saturate, which requires differentiation in specific lines. In most cases, the inductor operates in a "linear region" where the inductor value is a constant and does not vary with terminal voltage and current. However, switching power supplies have a problem that cannot be ignored, that is, the winding of the inductor will lead to two distributed parameters (or parasitic parameters), one is the unavoidable winding resistance, and the other is distributed related to the winding process and materials. Stray capacitance. Stray capacitance has little effect at low frequencies, but gradually increases with increasing frequency. When the frequency is above a certain value, the inductance may become a capacitive characteristic. If the stray capacitance is "concentrated" into a capacitor, the capacitance characteristics exhibited after a certain frequency can be seen from the equivalent circuit of the inductor.

Main characteristic parameters of the inductor
1, inductance L
The inductance L represents the inherent characteristics of the coil itself, regardless of the current magnitude. In addition to the special inductor (color code inductor), the inductance is generally not marked on the coil, but marked with a specific name. The unit has Henry (H), milli Henry (mH), and micro Henry (uH), 1H = 10^3mH = 10^6uH.
2, the anti-XL
The size of the inductive coil that blocks the AC current is called XL, and the unit is ohm. Its relationship with inductance L and AC frequency f is XL=2Ï€fL
3. Quality factor Q
The quality factor Q is a physical quantity indicating the quality of the coil, and Q is the ratio of the inductive reactance XL to its equivalent resistance, that is, Q = XL / R. The higher the Q value of the coil, the smaller the loss of the loop. The Q value of the coil is related to the DC resistance of the wire, the dielectric loss of the skeleton, the loss caused by the shield or the iron core, and the influence of the high frequency skin effect. The Q value of the coil is usually from several tens to several hundreds. With the magnetic core coil, multiple thick coils can increase the Q value of the coil.
4. Distributed Capacitance Also known as the inherent capacitance or parasitic capacitance, the capacitance between the turns and turns of the coil, between the coil and the shield, and between the coil and the bottom plate is called the distributed capacitance. The presence of the distributed capacitance reduces the Q value of the coil and the stability is deteriorated, so the smaller the distributed capacitance of the coil, the better. The segmented winding method reduces the distributed capacitance.

Cu in the figure is distributed capacitance
5. Allowable error: The percentage difference between the actual value of the inductance and the nominal value divided by the nominal value.
6. Rated current: refers to the maximum current allowed by the coil. It is usually represented by the letters A, B, C, D, and E. The nominal current values ​​are 50mA, 150mA, 300mA, 700mA, and 1600mA.

There are two types of rated current for power inductors:

In a DC-DC converter, the inductor is the core component next to the IC. Higher conversion efficiencies can be achieved by choosing the right inductor. The main parameters used in the selection of inductors are inductance value, rated current, AC resistance, DC resistance, etc. Among these parameters are the concepts specific to power inductors. For example, there are two types of rated currents for power inductors. What is the difference between them?

In order to answer such questions, we will describe the rated current of the power inductor here.

There are two reasons for the rated current

The rated current of the power inductor has two determination methods: "rated current based on self-temperature rise" and "rated current based on change rate of inductance value", which are of great significance. "Rated current based on self-temperature rise" is the rated current specification based on the calorific value of the component. When it is used outside the range, it may cause component damage and component failure.

At the same time, the "rated current based on the rate of change of the inductance value" is defined as the rated current of the degree of decrease in the inductance value. When the range is exceeded, the IC control may be unstable due to an increase in ripple current. Further, depending on the magnetic circuit configuration of the inductor, the tendency of magnetic saturation (i.e., the tendency of the inductance value to decrease) is different. Fig. 1 is a schematic view showing changes in inductance values ​​caused by different magnetic circuit configurations. With regard to the open circuit type, as the DC current increases, a relatively flat inductance value appears until the predetermined current value, but the value of the boundary current decreases sharply with the predetermined current value. On the contrary, as the DC current type increases, the value of the permeability decreases gradually, so the inductance value slowly decreases.

The rated current parameter in the power inductor specification only indicates the saturation current Isat value of the medium.

Little common sense: the difference between Isat and rms

Isat and Irms are technical terms that our engineers often encounter, but because of some customer problems, they often confuse the two, causing engineering errors. What is Isat and Irms, and what does Chinese mean? How are Isat and Irms defined, and what are they related to? What are we defining in inductor design?
Isat: refers to the saturation current of the magnetic medium. In the BH curve below, it refers to the amount of DC current required for the magnetic medium to reach the Hm corresponding to Bm. For the inductor, that is, the magnitude of the current after the inductance drops to a certain ratio, such as SRI1207- For 4R7M products, the current with a 20% drop in inductance is 8.4A, then Isat=8.4A. Isat calculates the formula as follows:
Let the cross-sectional area be S, the length is l, the magnetic permeability is μ on the iron ring, and the tight coil N匝 is wound, and the current passing through the coil is I. According to the magnetic circuit law:
Hl/0.4Ï€=NI=0.7958Hl
For the same material and the core H1 of the crucible changes according to the BH curve, but under the same slope, Hl is constant, therefore:
N1*I1=Hl/0.4Ï€=N2*I2
which is:
N1/N2=I2/I1

Irms: refers to the rated current of the application of the inductor product, also known as the temperature rise current, that is, the DC current corresponding to the surface when the product reaches a certain temperature.

The following is a comparison of the current rated current Irat, saturation current Isat, and temperature rise current Irms in the industry by taking the 4.7uH stacked power inductor in the 2520 series as an example.
Stacked power inductor (ferrite high current inductor) parameter comparison table


The status quo will mislead engineers to select types and create hidden dangers;

At present, there are quite a few manufacturers of laminated power inductors whose current rating is defined by the rated current rating of the laminated inductor used in traditional signal filtering. The rated operating current is defined according to the temperature rise current of the inductor. In this case, the product design engineer will often measure the rated operating current in the actual circuit according to the traditional power inductor selection experience and the rated current value defined in the supplier's inductance specification, which is likely to cause saturation due to the inductor. The current is lower than the actual operating current of the circuit, and there are hidden dangers as follows:
A). When the inductor is actually working, it will be saturated due to excessive current, causing the inductance to drop too much, causing the current ripple to exceed the maximum allowable specification range of the latter circuit, causing circuit interference, which may not work properly or even be damaged;
B). The actual operating current in the circuit exceeds the saturation current of the inductor, which may cause mechanical or electronic noise due to the decrease in inductance saturation inductance;
C). The actual operating current in the circuit exceeds the saturation current of the inductor, which will cause the inductor to be saturated. The inductance decreases, causing the output voltage & current to be unstable when the power supply is loaded, causing unstable abnormalities such as other unit circuit systems.
D). Inductor rated current (including saturation and temperature rise current) selection of insufficient margin will lead to excessive surface temperature during operation, reduced efficiency of the whole machine, accelerated inductance itself or aging of the whole machine to shorten the life

Part III Output Inductor Selection of Switching Power Supply

The parameters of the inductor we need to focus on:

1. Equivalent resistance: affecting efficiency

2. Inductance value: affecting ripple current

Calculating the correct inductor value is important for choosing the right inductor and output capacitor for the smallest output voltage ripple.

As can be seen from the figure below, the current flowing through the switching power supply inductor consists of two components, AC and DC. Because the AC component has a higher frequency, it will flow into the ground through the output capacitor to generate the corresponding output ripple voltage dv. =di×RESR. This ripple voltage should be as low as possible to avoid affecting the normal operation of the power system. Generally, the peak-to-peak value is 10mV~500mV.

The magnitude of the ripple current also affects the size of the inductor and output capacitor. The ripple current is typically set to 10% to 30% of the maximum output current. Therefore, for a step-down power supply, the current peak through the inductor is higher than that of the power supply. The output current is 5%~15% larger.

The change in current on the inductor during the switching of the switch.

In the process of switching the switch, the Ohm's law of the inductor is applied and calculated:

The output current ripple is inversely proportional to the inductance value and inversely proportional to the switching frequency.

It can be seen from the above formula that the larger the inductance value of the inductor, the smaller the output ripple current. But the problem is that the response time is slower. If the inductance value is small, if the ripple of the output voltage is small, the switching frequency needs to be increased, so that the switching loss on the MOS tube increases and the circuit efficiency decreases.

Part IV Actual Circuit Design

BUCK type switching power supply specification requirements: 5V0~24V0→1V~5V0 Output current: 2A

Power controller alternative model: MP4420A (A means: CCM mode, H means: light load down mode)

PIN2PIN compatible: MPQ4420A-DJ (industrial grade), MPQ4420A-DJ-A (automobile grade)

Manufacturer: MPS

Power output: 3.3V

Power range requirement: 5%

Power supply ripple requirement: 2% 0.066V

Switching frequency: 410kHz (320~500kHz)

Duty cycle: 12V to 3V3: 27.5%

After we selected the 10uH inductor, we determined the ripple current:

Ripple current = (12V-3.3V)*0.275/(0.00001*320000)=0.75A

The ESR of our chosen ceramic capacitor:

The meaning is that the capacitor can withstand the ripple current / voltage value. They are closely related to the ESR and can be expressed by the following equation: Urms = Irms × R where Urms indicates that the ripple voltage Irms indicates that the ripple current R represents the ESR of the capacitor.

It can be seen from the above that when the ripple current increases, the chopping voltage is multiplied even when the ESR remains unchanged. In other words, as the ripple voltage increases, the ripple current also increases, which is why the capacitor is required to have a lower ESR value. After the ripple current is added to the stack, heat is generated due to the equivalent series resistance (ESR) inside the capacitor, which affects the life of the capacitor. In general, the ripple current is proportional to the frequency, so the ripple current is also low at low frequencies.

Therefore, for the output capacitor, the withstand voltage requirements and capacity can be appropriately reduced. The requirements for ESR are higher, because there is enough current throughput to be guaranteed. However, it should be noted here that the ESR is not as low as possible, and the low ESR capacitor will cause the switching circuit to oscillate. The complexity of the vibration absorbing circuit also leads to an increase in cost. In the design of the board, there is generally a reference value here. This is used as a component selection parameter to avoid the increase of the cost caused by the vibration damping circuit.

We set the ESR to 1 ohm:

We set the ESR to 10mΩ:

Significantly reduced amplitude

If we use two 1Ω, 100uF capacitors, we will find that the ripple voltage is further reduced. On the one hand, the impedance of the capacitor at the switching frequency point is further reduced by paralleling. On the other hand, ESR is actually equivalent to paralleling. The essence is that the ESR is connected in series with the capacitor in parallel, resulting in a significant reduction in the impedance of the output capacitor at the switching frequency point.

The series-parallel formula of ESR and capacitor is equivalent to the series-parallel formula of resistance.

Datasheet based on ceramic capacitor

Near 410kHz, its ESR is about 2mΩ

So the ripple voltage = 0.75A * 2mΩ = 1.5mV

Far less than 66mV ripple requirements.

So in fact, when we design, we take into account the accuracy range of the inductance value and temperature drift. Therefore, according to our cost and PCB space requirements, we can also reduce the size of our inductance. However, when reducing, it is also necessary to consider the case where the inductance value is the worst, and evaluate the ripple.

The fifth part of the inductance derating

The hot spot temperature rating of the inductive component is related to the insulation performance of the coil wire set, the operating current, the transient initial current, and the dielectric withstand voltage.

Note:

1) THS is the rated hot spot temperature.
2) Only applicable to chokes.

According to our design requirements, if our transient current is 2A, we need a rated current of 2A/0.9=2.22A. We need to select the inductor with rated current of 2.5A~3A as the output. Isat and Irms chose the smaller one as the rated current.

The sixth part of the inductor selection

We chose Irs and Isat to be larger than 2.5A, DCR relatively small 10uH inductor, and finally consider cost and volume.