- Copper loss
- DC copper loss caused by the DC Resistance (DCR) of the windings. It increases proportionally to the square of the current.
- AC copper loss caused by the AC Resistance (ACR). The AC resistance depends on the frequency and is based on
- skin effect: The higher the frequency the lower the skin depth. As the frequency becomes higher, there is a tendency for the current flow to become concentrated in the area near the conductor surface and for the effective resistance value to increase.
- proximity effect: The proximity effect increases the AC resistance as well. A current driven wire creates a magnetic field around the wire, which also creates a magnetic field around the wire, which also creates eddy currents.
- Core loss
- Hysteresis loss
- Eddy Current loss. Eddy current loss is proportional to the square of the frequency.
An ideal inductor would have zero capacitance and zero resistance.
The Figure below shows a graph of inductive reactance versus frequency . Inductive reactance increases linearly with frequency.
Figure: Inductive reactance vs frequency (ideal inductor)
A real inductor can be modeled by the following elements:
- a series inductor
- a series resistor or
- a parallel capacitor or : It is the distributed capacitance between the turns of the wire and is derived from the Self Resonant Frequency ().
- a parallel resistor : It represents the magnetic core loss of the inductor core.
The figure below shows a real-life impedance vs frequency graph.
Figure: Inductive reactance vs frequency (real inductor)
Self-Resonant Frequency (SRF) or in Hz: This is the frequency at which the inductance of the inductor resonates with the inductor’s distributed capacitance . Increasing or lowers . Decreasing or raises .
- the inductor will act as a pure resistor,
- the input impedance is at its peak,
- the Quality factor of the inductor is zero,
- the reactance of the inductor is zero,
- the capacitance is given by
At frequencies below the reactance is inductive and increases as the frequency increases.
At frequencies above the reactance is capacitive and decreases as the frequency increases.
According to Faraday’s law:
The term in front of in the equation we call it self inductance of the solenoid:
- Inductance L (tested at a certain test conditions for example 100 KHz, 0.1 , 0 A DC)
- Inductive tolerance: It is the allowed amount of variation from the nominal value specified by the manufacturer (e.g. ±20%).
- Rated operating voltage (across inductor)
- DC Resistance (DCR): The resistance of the inductor winding measured using DC current. The resistance in a component due to the length and diameter of the winding wire used.
The DC resistance has a constant value. The bigger the cross section, the lower the DC Resistance (DCR), the lower the copper losses. The smaller the cross section, the higher the DC resistance (DCR), the higher the copper losses.
- AC Resistance (ACR)
- Maximum DC current : Maximum DC current is the DC current at which the inductance falls to 90% of its nominal value or until its temperature rise reaches 30 °C.
Figure: Inductance vs DC Bias Load (or DC Bias Characteristic)
DC Bias current relates to a constant current element that is added to the AC signal.
- Incremental Current Rating: The DC bias current that causes an inductance drop of 5% from the initial zero DC bias inductance value.
- or RMS current:
- for a 20°C rise above 25°C ambient temperature
- for a 40°C rise above 25°C ambient temperature
- Saturation current : The DC bias current that causes the inductor to drop by a specified percentage (e.g. 10% or 20%) from its value without current. See Figure Inductance vs DC Bias Load (or DC Bias Characteristic)
- Q factor or Quality factor: The measure of the relative losses in the inductor.
Quality factor is defined as the ratio of the inductive reactance to the effective resistance . Both and are functions of frequency. The test frequency must be given when specifying .
- Self-Resonant Frequency (SRF) or in Hz
- Curie temperature (in degrees Celsius): It is the temperature at which the core material start to lose its magnetic properties.
- Inductance temperature coefficient: The change in inductance per unit temperature change. Measured under zero bias conditions and expressed in parts per million (ppm).
- Resistance temperature coefficient: The change in DC wire resistance per unit temperature change. Measured at low DC Bias (<1 ) and expressed in parts per million (ppm).
- Magnetic saturation flux density : At this value of flux density, all magnetic domains within the core are magnetized and aligned.
- with shield
- without shield
- Electromagnetic interference (EMI): It refers to the magnetic field radiated away from the inductor into space. The magnetic field may cause interference with other magnetically sensitive components.
- Core material
- Ferrite cores
- Iron powder cores
- Storage temperature range
- Operating temperature range
- Ambient temperature range not including self-temperature rise
- Product temperature range including self-temperature rise. The operating temperature is equal to the ambient temperature plus component’s self-heating . The maximum allowable temperature for an inductor is the maximum ambient temperature plus the maximum temperature rise.
- Moisture Sensitivity Level (MSL)
If we assume that a constant DC current has been flowing through the inductor for some time, then is zero and thus is zero.
Under DC conditions the inductor acts like a short circuit.
Inductive reactance is measured in Ohms and it tells as “how bad” the inductor passes the current.
Inductive susceptance is measured in Siemens and it tells as “how well” the inductor passes the current.
Practical inductor, RL in series: