Chokes
A choke is an inductor used mainly to impede an unwanted alternating or rapidly changing current while passing wanted DC or lower-frequency current. The name comes from the idea of “choking off” the unwanted component of the signal.
Power-supply chokes
Older linear power supplies often placed a large iron-core choke between filter capacitors. The choke resisted ripple current while allowing DC load current to flow. Choke-input and capacitor-input filters have different voltage, current, and regulation behavior.
Modern switch-mode supplies use smaller ferrite or powdered-core inductors at much higher switching frequencies, but the filtering principle is related.
RF chokes
An RF choke provides high impedance to a radio-frequency signal while passing DC bias. It may feed DC to an amplifier transistor without allowing much of the RF signal to escape into the power supply.
At the wanted blocking frequency, the choke should have enough impedance to separate the RF circuit from the DC supply. Real chokes also have winding capacitance, so impedance does not continue increasing forever with frequency.
Self-resonance matters
Every real inductor has parasitic capacitance. That capacitance resonates with the inductance at the self-resonant frequency (SRF). Near SRF, impedance may be very high, which can be useful for a choke. Above the first resonance, however, the part may behave more like a capacitor than an inductor.
Common-mode chokes
A common-mode choke contains two or more windings on one magnetic core. Wanted load current flows through the windings in opposite magnetic directions, so their magnetic flux largely cancels. Common-mode noise flows in a way that makes the flux add, creating high impedance to the unwanted noise.
Common-mode chokes are widely used on:
- AC mains inputs.
- USB and other data cables.
- Automotive wiring.
- Ethernet and communication interfaces.
- Power-converter input and output lines.
Differential-mode chokes
Differential-mode noise is current flowing out on one conductor and back on the other, just like the wanted load current. A common-mode choke does not necessarily provide enough inductance to suppress this component, so separate series inductors or specially designed combined filters may be used.
Ferrite beads
A ferrite bead is closely related to a choke but is intentionally lossy over a frequency range. Instead of merely storing and returning unwanted high-frequency energy, the ferrite converts part of that energy into heat. Beads are useful for suppressing RF noise on power leads and signal lines.
Choosing a choke
- Frequency range to be attenuated.
- Required impedance across that range.
- DC or RMS load current.
- DCR and resulting voltage drop.
- Saturation or bias behavior.
- Self-resonant frequency.
- Voltage and insulation requirements.
- Common-mode versus differential-mode noise.