DC-DC Converter Input Filter Design

A switching regulator draws a pulsed current from its input, and everything upstream of it sees that pulse train as noise. The input filter is what keeps the noise out of the supply, the cable and the rest of the system, and its design is a compromise between attenuation, size and stability.

The Two Noise Paths

differential mode noise flows out on the positive input conductor and returns on the negative one. It is produced by the switching action itself, and its amplitude depends on the input capacitor and the loop area of the power stage.

common mode noise appears on both conductors together and returns through the earth or the chassis. It is produced by the fast voltage transitions on the switching node coupling to the environment through parasitic capacitance.

The two are measured differently, they are attenuated by different components, and a filter that only addresses one of them will fail an emission test on the other. That is why a practical filter has both a capacitor bank and a common mode choke.

Setting the Corner Frequency

The filter corner is placed below the switching frequency with enough margin that the fundamental is already attenuated. A converter switching at two hundred kilohertz with a filter corner at twenty kilohertz gives an attenuation of about forty decibels with a two pole filter.

The attenuation then rises with frequency at forty decibels per decade for a two pole design, which is usually enough to bring the harmonic content below the limit. A converter with a fast edge rate has more energy high up the spectrum, and a faster roll off is needed there.

Conducted emission limits start at a hundred and fifty kilohertz, so a converter switching below that frequency has its fundamental inside the measured band and the filter has to attenuate the fundamental itself rather than only its harmonics.

The filter damping Problem

A second order filter made of an inductor and a capacitor has a peak at its corner. If the converter impedance presented to the filter is higher than the damped filter impedance, the peak can cause the input voltage to oscillate, and the converter may become unstable.

filter damping is the answer, and the simplest form is a resistor in series with a capacitor across the filter output. The resistor value is chosen to be comparable with the characteristic impedance of the filter, and the capacitor is several times larger than the filter capacitor.

An electrolytic capacitor with a significant equivalent series resistance provides damping for free. It is often used alongside a ceramic capacitor, with the ceramic supplying the high frequency attenuation and the electrolytic damping the corner.

Component Choices

The inductor carries the full input current including its ripple, and it must not saturate at the peak. A saturating inductor stops behaving as an inductor exactly when the noise is largest, which is worse than having no inductor at all.

The self resonant frequency of the inductor sets the point above which it looks capacitive. Above that frequency the attenuation falls away, and the ceramic capacitors take over. Choosing an inductor with a resonance well above the frequencies of concern is part of the component selection.

The capacitors see the ripple current and heat accordingly. A ceramic type has a low equivalent series resistance and tolerates the ripple well, while an electrolytic type degrades with temperature. The ripple current rating is a specification to check rather than an afterthought.

Common Mode Choke Selection

A common mode choke passes the differential current without impedance because the flux from the two conductors cancels, and it presents a high impedance to the common mode current because the fluxes add. That selectivity is what makes it useful.

The leakage inductance of the choke also acts as a differential mode inductor, which helps the differential attenuation at the cost of a small series impedance in the power path. A high leakage inductance is a useful feature in a filtered input rather than a defect.

Saturation of the choke is a risk when the differential current is large, because a real choke has a finite core and the differential magnetisation is not zero. The rated current and the saturation current are different figures, and both have to be respected.

Layout and Loop Area

The input capacitor of the converter has to be placed with the shortest possible loop to the switch and the return. That capacitor carries the pulsed current, and every millimetre of trace adds inductance that raises the impedance at the switching frequency and increases the emitted field.

The filter itself should sit between the connector and the converter, with its own small loop for the capacitor that returns the high frequency current. A long trace between the filter output capacitor and the converter input capacitor lets the noise bypass the filter.

The ground of the filter should be a solid plane rather than a long thin trace. The return current from the filter capacitor flows at high frequency, and any impedance in the return path converts the differential current into a common mode voltage across the board.

Measuring the Result

Conducted emissions are measured with a line impedance stabilisation network between the supply and the product. A current probe on the input pair gives the differential mode, and a probe around both conductors gives the common mode, and taking both measurements localises the source of a failure.

A spectrum analyser with a peak detector shows where the energy is, and a near field probe on the board shows which part of the circuit radiates it. Working from the near field result back to the filter component is much faster than changing components at random.

Repeat the measurement at the extremes of input voltage and load, because the noise depends on the duty cycle and therefore on the operating point. A filter that passes at nominal conditions and fails at minimum load is a common experience. The wider set of measures for containing a switching circuit is described in our guide to EMI suppression design principles.

Verification and Stability

Verify the filter as part of the converter, not on its own. An impedance analyser can measure the filter impedance, and an injection transformer can measure the loop gain of the converter with the filter attached, which shows whether the damping is sufficient.

Check the start up behaviour with a full load applied. A filter with an undamped resonance produces a large overshoot on the input when the supply is connected, and the converter may see a voltage above its rating for a few microseconds.

The release checks that catch a wrongly placed capacitor or an unshielded filter loop are the same for any switching design, and they are collected in our PCB design release checklist. The assembly points that we inspect on a finished board are listed in judging PCB quality.

FAQ

Why does my converter oscillate with an input filter? The undamped resonance of the filter interacts with the converter input impedance. Add damping with a resistor and capacitor across the filter output.

Do I need a common mode choke? If the product has a cable or an earthed chassis, yes. Differential mode filtering alone will not keep the current on the pair balanced.

Where should the input capacitor be placed? As close as possible to the switch and its return, with the smallest loop area the layout allows.