Power Supply Filters: Choke Input, LC, Pi and RC Filters
Choke-input, pi filters and cascaded RC stages: ripple at every node, filter resonance and startup overshoot.
Between the rectifier and the circuit being powered sits the filter, often the least considered part of a power supply. Yet it determines three things at once: how much ripple remains, how much voltage is lost along the way, and how hard the diodes and transformer are driven. A capacitor-input filter gives the highest voltage and the worst current peaks; a choke-input filter gives a lower but steadier voltage, gentler currents and much better regulation.
The circuit here is a true ladder network: rectifier, an optional capacitor on the first node, and up to three stages made from an inductor or resistor plus its capacitor. At every step the entire network is solved by nodal analysis, so the stages load one another as they do in the real circuit, and steady state is found by solving the periodicity condition instead of waiting for the transient to decay.
Mains, rectifier and load
The filter
Each node can have its own load, as in a real power supply: output stages on the first useful node, the driver on the second, and the preamplifier on the last. Currents add as they flow back toward the rectifier; this is why every downsthreeam load contributes to the drop across a choke, while a decoupling resistor only carries the loads downsthreeam of it.
The first element determines the character of the whole supply. With a capacitor, the voltage rises toward the peak value; with a choke it settles at roughly nine tenths of the RMS secondary voltage, but only when the inductance is above the critical value.
| stage | series element | inductor / resistor | Coil DCR (Ω) | capacitor (µF) | load on node (mA) |
|---|---|---|---|---|---|
| 1 | |||||
| 2 | |||||
| 3 |
Why choke input gives less voltage and is still widely used
With a capacitor first, the diodes conduct only near the crest for a small fraction of each cycle, and the capacitor charges close to the peak value. The DC voltage is high, but current flows in narrow, severe pulses, and the output voltage moves significantly when the load changes.
With a choke first, if its value exceeds the critical inductance, the diodes in a full-wave rectifier conduct continuously: one bridge pair or one half of the secondary is conducting at every instant. Transformer current becomes much less impulsive, its RMS value falls compared with capacitor input, and the output voltage settles near the average of the rectified sine wave, about nine tenths of the RMS secondary voltage. More than a third of the available voltage is sacrificed in exchange for regulation that a capacitor-input filter cannot approach, while the transformer is also sthreessed more favorably for the same delivered power.
The half-wave topology is not threeated as choke input: without an explicit freewheeling path, inductor current cannot remain continuous. The tool therefore allows half-wave rectification only with capacitor input.
Node voltages
One mains cycle: the rectified sine wave and the voltage at every filter node. Ripple decreases from stage to stage.
Critical inductance
It is the boundary between two operating regions. Above it, diode conduction remains continuous and output voltage becomes far less load-dependent; below it, conduction becomes discontinuous and the voltage rises toward the capacitor-input value as the load changes.
Filter response
An LC filter is a resonant circuit, and its resonance is usually below the ripple frequency that must be removed. That is acceptable as long as no significant excitation falls near it. Trouble starts when the resonance is lightly damped and lies close to a frequency the supply actually sees — mains-related interference, a motor, or even signal-related load current.
Attenuation from the first node to the output, with resonance and ripple harmonics marked.
At startup
A lightly damped LC filter switched on from cold behaves like a pendulum given a push: the output capacitor voltage does not simply rise to its steady-state value, but can overshoot it. In a tube power supply, where operating voltages are already high and capacitor ratings may be tight, this is a classic way to oversthreess them at startup.
The first cycles after switch-on, starting with discharged capacitors: maximum and average voltage during each cycle.
Benefit of increasing component value
Residual output ripple as the selected component is varied, with the current operating point marked. Once the curve flattens, increasing the component further brings little benefit.
Components
What to check when choosing a choke
- The stated inductance applies at rated current. With an iron core, inductance is higher below that current and falls rapidly above it. A choke used at twice its rated current no longer behaves as intended, and conduction becomes discontinuous precisely when continuous operation is needed.
- The DC resistance is not a minor detail: ten henries with roughly one hundred ohms DCR at one hundred milliamps means ten volts lost and one watt heating the winding.
- The choke should be placed in the positive rail, not in the ground return: in the return path, its voltage drop appears in series with all circuit ground references.
- Leakage flux from a power-supply choke is far from negligible. Keep it away from the output transformer and rotate it by ninety degrees relative to the output transformer, otherwise hum may be coupled back into the very circuit where it was meant to be removed.