It can be seen from the impedance frequency curve that when the operating frequency is lower than the resonant frequency, the inductive device exhibits inductance, and the impedance increases with the increase of the frequency: when the operating frequency is higher than the resonant frequency, the inductive device exhibits capacitive, and the impedance decreases with increasing frequency.
In the application, the inductor whose resonant frequency point is higher than the operating frequency should be selected as the power filter, and the following points should be paid attention to.
①When inductance and capacitance form a low-pass filter, the inductance value is a very critical parameter. The nominal inductance value of the inductance device data is the value at which the operating frequency is lower than the resonant frequency. If the operating frequency is higher than the resonant frequency, the inductance value will decrease sharply with the increase of the operating frequency, and gradually become capacitive.②When the inductor is used for power supply filtering, the voltage drop caused by its DC resistance needs to be considered.
③ When used for power supply filtering, the working current of the inductor must be less than the rated current. If the operating current is greater than the rated current, the inductance will not necessarily be damaged, but the inductance value may be lower than the nominal value.
Several main related parameters①Inductance value range: 1-470uH ②DC
resistance: There are various DC resistances to choose from. The larger the inductance value, the larger the corresponding DC resistance. Generally, the DC resistance of the signal inductor is larger than that of the high frequency signal inductor and the power supply inductor. The minimum DC resistance is generally several milliohms, and the larger one is several ohms.
③Self-resonant frequency: tens of megahertz to hundreds of megahertz. The larger the inductance value, the smaller the corresponding self-resonant frequency.
④ Rated current: a few milliamps to tens of milliamps. The larger the inductance value, the smaller the corresponding rated current.
When the operating frequency is lower than the resonant frequency, the inductance value is basically stable; but after the operating frequency exceeds the resonant frequency, the inductance value will increase first, and will decrease rapidly after reaching a certain frequency.
If the ear can hear howling
(squeak), it is certain that there is a switching current of about 20HZ-20KHZ (human ear range) at both ends of the inductor.
For example, the inductance whistle of DC-DC circuit, because the load current is too large, there is a current-limiting protection circuit inside the DC. When the load exceeds the switch (MOS) current inside the IC, the current-limiting detection circuit judges that the load current is too large and will immediately Adjust the duty ratio of the internal switch of the DAC, or immediately stop the switching operation until the detected load current is within the standard range, and then restart the normal working switch. The time period from the stop switch to the restart switch is just a few KHZ frequency, and it is precisely because of the switching frequency of this period that howling occurs.
Improvement measures: reduce the load current, replace the DC-DC with a slightly larger power, and change the output capacitor.
Noise problems caused by inductance due to excessive load current or voltage: Inductance - induced voltage due to current changes causes transmission line effects, abrupt changes, crosstalk, switching noise, track collapse, ground bounce and most sources of electromagnetic interference (EMI).
For example:digital circuits are noisy, saturated logic (such as TTL and CMOS) will briefly draw large currents from the power supply during switching, so the noise caused on the digital ground will be large, but due to the noise immunity of the logic level up to Hundreds of millivolts or more (due to inductance - induced voltage caused by current changes)
The main purpose of adding an inductor to the magnetic core is to increase the inductance (or mutual inductance) of the inductor coil.
Back EMF:① Back EMF refers to the tendency to resist current changes and generate electromotive force, which is essentially an induced electromotive force.
②The origin of the back electromotive force: the change of the current causes the change of the magnetic field. According to Maxwell, an electric field will be generated around a changing magnetic field, and the electric field will have an electric field force on the charges in it. The electric field force is a non-electrostatic force and generates an electromotive force.
③ When the current increases from small to large, the direction of the generated back electromotive force is the same as that of the original voltage. When the current goes from large to small, the direction of the generated back electromotive force is opposite to that of the original voltage.