Current, power, and temperature rise

Inductors are typically not rated based on power, but current and resistance data sheet specifications can be used to estimate an approximate value of the power handling capability of hollow or ceramic chip inductors. Example: 1 μ H, the Irms rating of the chip inductor is 480 mA, and the maximum DCR rating is 1.2 ohms. The Irms rating is equivalent to a 15 ° C increase in ambient temperature. The maximum allowable ambient temperature is 125 ° C, therefore the maximum component temperature allowed for a temperature rise of 15 ° C is~(125+15)=140 ° C. To estimate power capability, calculate Irms2 × DCR. If we assume that the nominal DCR is 80% of the specified maximum DCR, the calculation result is: (0.48 A) 2 × (0.8 × 1.2 Ohms)=0.221 W=221 mW. Therefore, a power of approximately 221 mW will cause the temperature of the inductor to rise by about 15 ° C. At RF frequency, ESR is much higher than DCR. Therefore, the amount of current that causes the same temperature rise is significantly reduced. For example, if the RF signal is 100 MHz, the ESR of the inductor is 8.14 ohms (almost seven times the DC resistance), so the Irms AC current corresponding to the same power (and temperature rise) is only about 161 mA, which is opposite to the rated value of 480 mA at DC, as shown below. If there are current related losses or other loss mechanisms at higher frequencies in the inductor (not part of low current ESR measurements), the estimation may be inaccurate. 14 ohms (almost seven times the DC resistance), therefore the Irms AC current corresponding to the same power (and temperature rise) is only about 161 mA, while the rated current at DC is 480 mA. If there are current related losses or other loss mechanisms at higher frequencies in the inductor (not part of low current ESR measurements), the estimation may be inaccurate. 14 ohms (almost seven times the DC resistance), therefore the Irms AC current corresponding to the same power (and temperature rise) is only about 161 mA, while the rated current at DC is 480 mA. If there are current related losses or other loss mechanisms at higher frequencies in the inductor (not part of low current ESR measurements), the estimation may be inaccurate.

Power consumption of inductors

As shown in the figure below, the purpose of the inductor in the bias T-junction is to provide DC bias for the amplifier while preventing high-frequency RF signals from entering the DC source. Ideally, any RF signal applied to the bias line would be filtered out by a series inductor. For this discussion, we assume a lossless (e.g. hollow or ceramic core) inductor with only copper losses (AC and DC) - no core losses.

The total DC power consumed by the inductor is Pdc=Idc2 × DCR. The total AC power consumed by the inductor is Pac=Irms2 × ESR, where Idc is the DC current passing through the inductor. Irms is the amplitude of the alternating current (RF signal) passing through an inductor (which may be lower if the inductor is close to the ideal state). DCR is the direct current resistance of an inductor. ESR is the effective series resistance of an inductor at RF signal frequency (assuming only a single RF frequency). The total AC power consumed by a DC inductor is: Ptotal=Pdc+PacorPtotal=Idc2 × DCR+Irms2 × ESRAs shown in the figure. In the simplest case of a single frequency AC signal on the RF line, in order to determine the AC power consumed by the inductor, the ESR of the inductor at the RF frequency, and the Irms value of the RF current passing through the inductor must be known. For more complex multi frequency noise signals to be filtered, the total AC power consumed by the inductor is the sum of all Irms2 × ESR contributions, where ESR varies depending on the frequency.

Broadband RF choke coil

The broadband performance of broadband RF chokes is the result of using high permeability magnetic core materials such as iron powder or ferrite. When the RF signal passes through an inductor, the magnetic core losses related to frequency and current will add additional heat to the total heat generated by the inductor. Simple ESR measurements (usually conducted at very low currents) will not capture these losses. Therefore, the above estimation method is not applicable and may incorrectly predict a lower temperature rise than the actual result. Inductors will be hotter than expected. The same applies to any inductor with a high magnetic permeability (ferrite, iron powder, composite material) core. For high penetration core products, we recommend measuring the temperature rise of the inductor under all frequency and current conditions, which may result in determining the worst-case temperature rise for your application.