Understand how to choose the right inductor for switching power supply-EDN electronic technology design

What is inductance? Inductor is an element that can convert electrical energy into magnetic energy and store it. The structure of an inductor is similar to a transformer, but it has only one winding. The inductor has a certain inductance, it only hinders the change of current. If the inductor is in a state where no current is flowing, it will try to block the current from flowing through it when the circuit is on; if the inductor is in a state where current is flowing, it will try to maintain the current when the circuit is off. Inductors are also called chokes, reactors, and dynamic reactors.

Inductance is a commonly used element in switching power supplies. Because of its different current and voltage phases, theoretically the loss is zero. Inductance is often an energy storage element, and is often used in input filtering and output filtering circuits together with capacitors to smooth current. Inductance is also called a choke, which is characterized by "very large inertia" in the current flowing through it. In other words, due to the continuity of magnetic flux, the current on the inductor must be continuous, otherwise a large voltage spike will be generated.

Inductance is a magnetic component, and naturally there is a problem of magnetic saturation. Some applications allow the inductor to saturate, some applications allow the inductor to enter saturation from a certain current value, and some applications do not allow the inductor to saturate, which requires distinction in specific circuits. In most cases, the inductor works in the "linear region", at this time the inductance value is a constant, and does not change with the terminal voltage and current. However, there is a problem that cannot be ignored in the switching power supply, that is, the winding of the inductance will cause two distributed parameters (or parasitic parameters), one is the inevitable winding resistance, and the other is the distribution related to the winding process and materials. Stray capacitance. Stray capacitance has little effect at low frequencies, but it gradually becomes apparent as the frequency increases. When the frequency is higher than a certain value, the inductance may become a capacitive characteristic. If the stray capacitance is "concentrated" into a capacitor, then the equivalent circuit of the inductance can be seen from the capacitance characteristics that appear after a certain frequency.

The inductance L represents the inherent characteristics of the coil itself and has nothing to do with the current. Except for the special inductor coil (color code inductor), the inductance is generally not specifically marked on the coil, but marked with a specific name. The units are Henry (H), millihenry (mH), micro-henry (uH), 1H=10^3mH=10^6uH.

The hindering effect of the inductance coil on the alternating current is called the inductive reactance XL, and the unit is ohm. Its relationship with inductance L and AC frequency f is XL=2πfL

The quality factor Q is a physical quantity that represents the quality of the coil, and Q is the ratio of the inductive reactance XL to its equivalent resistance, namely: 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 tens to hundreds. Using magnetic core coils, multiple thick coils can improve the Q value of the coil.

Also known as inherent capacitance or parasitic capacitance, the capacitance existing between the turns of the coil, between the coil and the shield, and between the coil and the base plate is called the distributed capacitance. The existence of distributed capacitance reduces the Q value of the coil and the stability becomes worse. Therefore, the smaller the distributed capacitance of the coil, the better. The segmented winding method can reduce the distributed capacitance.

The percentage of the difference between the actual value of the inductance and the nominal value divided by the nominal value.

Refers to the maximum current allowed by the coil, usually represented by the letters A, B, C, D, and E respectively. The nominal current values ​​are 50mA, 150mA, 300mA, 700mA, 1600mA.

In the DC-DC converter, the inductor is the core component second only to the IC. By choosing an appropriate inductor, a higher conversion efficiency can be obtained. The main parameters used in the selection of inductors include inductance, rated current, AC resistance, DC resistance, etc. These parameters also include the unique concept of 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 are here to explain the rated current of the power inductor.

There are two methods for determining the rated current of power inductors: "Rated current based on self-temperature rise" and "Rated current based on the rate of change of inductance value", which are of great significance. "Rated current based on self-temperature rise" refers to the rated current regulation based on the calorific value of the component. Using it outside of this range may cause component damage and component failure.

At the same time, the "rated current based on the rate of change of inductance value" is a rated current that uses the decrease of the inductance value as an index. When used outside this range, the IC control may become unstable due to the increase in ripple current. In addition, depending on the magnetic circuit structure of the inductor, the tendency of magnetic saturation (that is, the tendency of decreasing inductance value) is different. Fig. 1 is a schematic diagram showing changes in inductance caused by different magnetic circuit structures. For the open magnetic circuit type, as the DC current increases, the inductance value is relatively flat until the specified current value, but the inductance value drops sharply at the specified current value. On the contrary, the closed magnetic circuit type gradually decreases with the increase of the DC current, so the inductance value slowly decreases.

The power inductor specifications only indicate the saturation current Isat value of the medium for the rated current parameter.

Isat and Irms are technical terms that our engineers often encounter, but due to some customer problems, they often confuse the two, resulting in engineering errors. What do Isat and Irms mean, and what does Chinese mean? How are Isat and Irms defined, and what factors are they related to? How do we define inductance design?

Refers to the saturation current of the magnetic medium. In the BH curve in the figure below, it refers to the amount of DC current required for the magnetic medium to reach Hm corresponding to Bm. For inductance, that is, the current after the inductance drops to a certain percentage, such as SRI1207-4R7M , The current when the inductance drops 20% is 8.4A, so Isat=8.4A. Isat calculation formula is as follows:

Suppose the cross-sectional area is S, the length is l, and the magnetic permeability is μ, the tight coil is wound with N turns, and the current passing through the coil is I. According to the law of magnetic circuit:

Hl/0.4π=NI=0.7958Hl

For the iron core of the same material and size, Hl changes according to the BH curve, but under the same slope, Hl is unchanged, so:

N1*I1=Hl/0.4π=N2*I2

which is:

N1/N2=I2/I1

Refers to the application rated current of inductive products, also known as temperature rise current, which is the DC current corresponding to the surface temperature when the product is applied.

The following uses the 4.7uH laminated power inductor in the 2520 series as an example to compare and illustrate the current industry’s identification of the inductor’s rated current Irat, saturation current Isat and temperature rise current Irms.

Multilayer power inductor (ferrite high current inductor) parameter comparison table

The current situation will mislead engineers to select models and cause hidden dangers;

At present, a considerable number of laminated power inductor manufacturers follow the traditional signal filtering processing with laminated inductor rated current standards to define their product rated current specifications, which define their rated operating current according to the temperature rise current value of the inductor. In this case, product design engineers often use the traditional power inductor selection experience and the rated current value defined in the supplier’s inductor specification to measure the rated operating current in the actual circuit, which is likely to result in inductance saturation. If the current is lower than the actual working current of the circuit, there will be the following hidden dangers:

A). When the inductor is actually working, it is 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 subsequent circuit, causing circuit interference, and thus failing to work properly or even damage;

B). The actual working current in the circuit exceeds the saturation current of the inductor, which may cause mechanical or electronic noise due to the decrease of the inductor saturation and inductance;

C). The actual working current in the circuit exceeds the saturation current of the inductor, which will cause the inductor to saturate, and its inductance will decrease, causing the output voltage & current to be unstable when the power supply is loaded, causing other unit circuit systems to crash and other unstable abnormal situations;

D). Insufficient selection margin of the inductor rated current (including saturation and temperature rise current) will cause the surface temperature to be too high during operation, reduce the efficiency of the whole machine, accelerate the inductance itself or shorten the life of the whole machine

We need to consider the inductance parameters:

1. Equivalent resistance: affecting efficiency

2. Inductance value: affects ripple current

Calculating the correct inductance value is very important for selecting the appropriate inductor and output capacitor to obtain the smallest output voltage ripple.

As can be seen from the figure below, the current flowing through the inductor of the switching power supply is composed of AC and DC components. Because the AC component has a higher frequency, it will flow into the ground through the output capacitor and generate the corresponding output ripple voltage dv. =di×RESR. This ripple voltage should be as low as possible so as not to affect the normal operation of the power system. Generally, the peak-to-peak value is 10mV~500mV.

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

During the switching process of the switch tube, the change of the current on the inductor.

In the process of switching the switch tube, the Ohm's law of the inductance is applied, and the calculation:

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 greater the inductance of the inductor, the smaller the output ripple current. But the problem is that the dynamic response (response time) slows down. If the inductance value is small, if the output voltage ripple is also small, the switching frequency needs to be increased, so that the switching loss on the MOS tube increases and the circuit efficiency decreases.

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

Alternative power controller model: MP4420A (A means: CCM mode, H means: light load frequency reduction mode)

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

Manufacturer: MPS

Power output: 3.3V

Power range requirement: 5%

Power ripple requirement: 2% 0.066V

Switching frequency: 410kHz (320~500kHz)

Duty cycle: 12V to 3V3: 27.5%

After we select the 10uH inductor, the ripple current is determined:

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

The ESR of our selected ceramic capacitor:

The meaning is the ripple current/voltage value that the capacitor can withstand. They have a close relationship with ESR and can be expressed by the following formula: Urms = Irms × R In the formula, Urms represents the ripple voltage Irms represents the ripple current R represents the ESR of the capacitor.

It can be seen from the above that when the ripple current increases, even when the ESR remains the same, the ripple voltage will increase exponentially. In other words, when 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, the equivalent series resistance (ESR) inside the capacitor causes heat, which affects the service life of the capacitor. Generally, the ripple current is proportional to the frequency, so the ripple current is relatively low at low frequencies.

Therefore, for the output capacitor, the withstand voltage requirements and capacity can be appropriately reduced a bit. The requirement of ESR is higher, because what must be guaranteed here is enough current throughput. But it should be noted here that ESR is not as low as possible. Low ESR capacitors will cause the switching circuit to oscillate. The complexity of the anti-vibration circuit will also increase the cost. In the board design, there is generally a reference value here, which is used as a component selection parameter to avoid the increase in cost caused by the vibration elimination circuit.

We set ESR to 1 ohm:

We set ESR to 10mΩ:

Significantly reduced

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.

According to the datasheet of ceramic capacitor

Around 410kHz, its ESR is about 2mΩ

So the ripple voltage=0.75A*2mΩ=1.5mV

Much less than the ripple requirement of 66mV.

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

The hot spot temperature rating of the inductive component is related to the insulation performance of the coil wire group, working current, transient initial current and 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, and we need to select an inductor with a rated current of 2.5A~3A as the output. Isat and Irms choose the smaller one as the rated current.

We choose 10uH inductors with Irms and Isat greater than 2.5A and a relatively small DCR, and finally consider cost and volume.