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# Do you understand all this knowledge of inductance?

2023-03-18【Archive】

**The current of an inductive element cannot change abruptly**. (After removing voltage from inductor, current will not disappear immediately, so a reverse electromotive force will be generated)

Inductance is a type of energy storage element that is used in LC tank circuit, medium and low pass filter circuit, DC-DC power conversion, etc., and its application frequency range rarely exceeds 50MHz.

**Basic Options**

①Inductance range: 1-470uH

②DC resistance: There are a variety of DC resistances to choose from. The larger inductance value, larger corresponding DC resistance. Inductors for general purpose signals have more DC resistance than inductors for high frequency signals and power supplies. The minimum DC resistance is usually a few milliohms, and more is a few ohms.

③Natural resonant frequency: from tens of megahertz to hundreds of megahertz. The larger inductance value, lower corresponding natural resonant frequency.

④ Rated current: from a few milliamps to tens of milliamps. The larger inductance value, smaller corresponding rated current.

When operating frequency is lower than resonant frequency, inductance value remains basically stable, but when operating frequency exceeds resonant frequency, inductance value first increases, and after reaching a certain frequency, it rapidly decreases.

From impedance frequency graph, it can be seen that when operating frequency is lower than resonant frequency, inductive device exhibits inductance, and impedance increases with frequency: when operating frequency is higher than resonant frequency, inductance of device exhibits capacitance. The impedance decreases with increasing frequency. Therefore, when applying, you should choose a reactor with a resonant frequency higher than operating frequency. When choosing a reactor for power filtering, you need to pay attention to following points.

①When inductor and capacitor form a low pass filter, inductance value is key parameter. The nominal inductance value of inductance device data is value at point where operating frequency is lower than resonant frequency. If operating frequency is higher than resonant frequency, value of inductance will decrease sharply with increasing operating frequency, and gradually become capacitive.

②When an inductor is used for power filtering, voltage drop caused by its DC resistance must be taken into account.

③When used for power filtering, operating current of inductor should be less than rated current. If running current is greater than rated current, inductor cannot be damaged, but inductance value may be lower than rated value.

**Causes of inductive noise**

If ear hears a howl (beep), you can be sure that a switching current of about 20 Hz-20 kHz (human ear range) is present at both ends of inductor.

For example, DC circuit inductance whistles due to excessive load current.

There is a current limiting protection circuit inside DC. When load exceeds switch current (MOS) inside IC, current limit detection circuit determines that load current is too large and immediately regulates load. internal DAC switch cycle or stop switch immediately. until detected load current is within standard range, normal operating switch is restarted. The time period from stopping switch to reclosing switch is exactly a few kHz of frequency, because switching frequency of this cycle produces a howl

Countermeasures to improve: reduce load current or replace DC-DC with slightly more power, replace output capacitor, etc.

Load current or voltage is too high

**Problems with noise caused by inductance:**

Inductance - transmission line effects, sudden changes, crosstalk, switching noise, track collapse, ground bounce, and most electromagnetic interference (EMI) sources due to induced voltage due to current changes

Example:

Digital circuits are noisy, and saturated logic (such as TTL and CMOS) briefly draws large currents from power supply during switching, causing a lot of noise on digital ground, but due to logic level. Up to hundreds of millivolts or more (caused by inductance - an induced voltage generated by a change in current)

The main purpose of adding inductance to a magnetic core is to increase inductance (or mutual inductance) of an inductor.

**Back EMF:**

Reverse electromotive force refers to tendency to resist a change in current to create an electromotive force that is essentially an induced electromotive force.

The origin of back electromotive force: A change in current causes a change in magnetic field. According to Maxwell, an electric field will be generated around a changing magnetic field, and electric field will act on charges in it with force of electric field. The force of an electric field is a non-electrostatic force that generates an electromotive force.

When current increases from small to large, direction of generated reverse electromotive force is same as direction of original voltage. When current changes from large to small, direction of generated reverse electromotive force is opposite to direction of original voltage.

**Arc reason:**

Disconnect inductor circuit, inductance will generate high voltage to resist disconnected voltage. In physics, this is called **induced emf** or back emf. So there will be an arc through switch.

When switch is off, there is no current loop, so there is some energy stored in inductor?

---No? At moment breaker is turned off, due to characteristics of inductance, a large reverse potential will be generated to maintain current, while on breaker.

In terms of energy: when switch is turned off, energy of magnetic field disappears and is converted into energy of electric field. It seems that current in coil will become less, and voltage across switch will increase.

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