Filter Layout: Preventing Input

Input filters for DC/DC converters limit the ripple current drawn from the supply, attenuate conducted differential-mode interference and must do so without destabilizing the converter control loop. This article explains how to size the LC input filter, how to check the Middlebrook stability criterion and how to damp the resulting resonance, with a worked example for design engineers and component engineers who select the filter inductor and capacitors. The core method is adapted, with permission, from the Würth Elektronik DC/DC Converter Handbook [1]. The focus is on differential-mode LC input filters for non-isolated and isolated DC/DC converters, with the buck converter as the main example, because its pulsed input current makes it particularly demanding. Common-mode suppression, layout for radiated EMI and surge protection are covered briefly and linked to dedicated articles.

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Key Takeaways

A second-order LC input filter rolls off at 40 dB per decade above its cut-off frequency; placing the cut-off at one tenth of the switching frequency gives about 40 dB of ideal attenuation at the switching frequency, and parasitics reduce this in practice.
The filter capacitor sits on the supply side and the filter inductor faces the converter, whose input capacitor acts as a low-impedance noise source.
A switching converter with a regulated output draws constant power and therefore has a negative input resistance of about −V_in²/P_in; the filter output impedance must stay well below this value over the control-loop bandwidth.
The critical resonance is between the filter inductor and the converter input capacitor, not the filter capacitor.
A damping branch of a capacitor C_damp of at least three to four times C_in in series with a resistance R_damp of about 0.5 to 0.8 times √(L/C_in) limits the impedance peak; the electrolytic capacitor ESR can provide this resistance.
Calculations are a starting point: verify the effective MLCC capacitance under DC bias, the source impedance, the conducted emissions and the stability margin by measurement.

Why DC/DC Converters Need Input Filters

A switching converter draws its input current in pulses at the switching frequency. The input capacitor supplies most of the pulsed current, but the remaining ripple flows back into the supply and appears as conducted interference. As described in selection of capacitors for DC/DC converters, a well-designed input capacitor alone is often not enough, and an additional input filter is required in many applications [1]. The buck converter, probably the most widely used topology, is particularly affected because its input current is discontinuous. Input filters at the converter input have several roles:

limiting the differential-mode ripple current and voltage on the supply line,
helping the design meet conducted EMI limits,
protecting the upstream source and other loads on a shared bus from the converter’s switching noise,
preserving converter control-loop stability, which the filter can compromise if it is not damped.

Attenuation values of up to about 30 dB at the switching frequency are typically required in practice, and they are readily achieved with a classic second-order LC low-pass filter [1]. The underlying low-pass behavior is explained in basic filter circuits explained. The applicable emission standard sets the limits and the frequency range. Converters in multimedia and ICT equipment are assessed under CISPR 32, while automotive components are measured under CISPR 25, whose conducted voltage method covers 150 kHz to 108 MHz [2]. Differential-mode filters as described here address the low-frequency part of this range; common-mode interference and the upper megahertz range require additional measures.

LC Input Filter Configuration

The order of the filter elements follows from the impedances on both sides of the filter. Placing them the wrong way round removes most of the attenuation. The LC input filter must be configured as in Figure 1. The input capacitor C_in of the converter represents a low impedance, so the filter inductor, with its high impedance, has to face the converter. Together with the filter capacitor on the supply side, this forms an LC low-pass filter from the viewpoint of the converter, whose ripple voltage across C_in acts as the noise source [1].

Schematic of an LC input filter: filter capacitor on the supply side, filter inductor toward the low-impedance input capacitor of the DC/DC converter — Figure 1. Correct LC input filter configuration: the high-impedance filter inductor faces the low-impedance converter input capacitor C_in.

Cut-off Frequency and Attenuation

It is useful to base the filter design primarily on the converter’s switching frequency f_sw. If the fundamental is attenuated by, for example, 30 dB, its harmonics are attenuated even more, because they lie further above the cut-off frequency [1]. The characteristic frequency f_LC of the filter, formed by the filter inductance L_filter and the filter capacitance C_filter, follows from Thomson’s equation, Eq. (1):

f_{LC} = \frac{1}{2\pi\sqrt{L_{filter} \cdot C_{filter}}} \tag{1}

f_LC is the resonant frequency of the LC network. It equals the −3 dB point only for a damped filter with Q ≈ 0.7; an undamped LC filter shows a resonance peak at this frequency. Above f_LC, the ideal filter attenuates at 40 dB per decade, which can be approximated for f ≫ f_LC by Eq. (2) (Figure 2):

A(f) \approx 40 \cdot \log_{10}\left(\frac{f}{f_{LC}}\right)\ \mathrm{dB} \tag{2}

Bode magnitude plot of an ideal LC low-pass filter falling at 40 dB per decade above its cut-off frequency — Figure 2. Frequency response of an ideal LC filter with an attenuation of 40 dB per decade.

If f_LC is set to one tenth of the switching frequency, for example 100 kHz for a 1 MHz converter, the ideal filter provides 40 dB of attenuation at the switching frequency according to Eq. (2). In practice, the equivalent series inductance (ESL) of the filter capacitor and the PCB inductance flatten the curve at higher frequencies, so the full 40 dB per decade is not reached. With a careful choice of components, 30 dB at the switching frequency is still readily achievable [1].

Estimating the Required Attenuation

A first estimate of the required attenuation follows from the measured peak-to-peak input ripple voltage V_ripple,pp at the switching frequency and the permitted noise level V_max (in dBµV) of the applicable standard at that frequency, Eq. (3) [3]:

A_{req} = 20 \cdot \log_{10}\left(\frac{V_{ripple,pp}}{1\,\mu\mathrm{V}}\right) – V_{max} \tag{3}

For example, 50 mV peak-to-peak ripple corresponds to about 94 dBµV. Against an assumed limit of 54 dBµV at the switching frequency, the filter would need 40 dB plus the design margin. The measurement should be made at the highest input current, which is the worst case for the ripple.

Input Filter Topologies and Component Choices

The basic LC filter covers most low- and medium-power converters. Higher attenuation requirements or noisy input environments lead to more elaborate networks and change the requirements on the components.

LC, π and Multi-Stage Filters

The simplest and most widely used input filter is the single LC low-pass filter in Figure 1. Where conducted EMI limits are more demanding, a π filter (C-L-C) or a multi-stage filter is used instead of, or in addition to, the basic LC filter. Additional capacitors and filter stages improve high-frequency attenuation, but they also change the damping and the inrush behavior, so the complete network must still satisfy the stability criterion described below.

The progression from a single LC stage to multi-stage networks follows power level and noise environment. The Passive Components for Power Converters dossier maps simple single-stage filters to adapters and low-power converters and multi-stage filters with additional choke stages to systems in the kilowatt range; a filter designed for a 65 W adapter does not scale directly to a 10 kW system.

The LC and π networks discussed here provide differential-mode attenuation between the positive and negative input lines. In isolated converters, a common mode choke and Y capacitors to chassis or protective earth are often added at the input to suppress common-mode noise. These measures complement the differential-mode filter and are covered in EMC filters explained, from component to design.

Capacitor Technologies

The choice of capacitor technology strongly influences filter performance. Multilayer ceramic capacitors (MLCCs) offer very low ESR and ESL and therefore excellent high-frequency attenuation, but their low losses increase the quality factor of resonances and may require additional damping. Class 2 MLCCs also lose capacitance with DC bias and temperature, so the effective capacitance at the operating voltage must be used in the calculation, for example from manufacturer models that include these dependencies [4]. The mechanism is explained in DC bias characteristics of ceramic capacitors.

Film capacitors provide stable capacitance over temperature and bias, moderate ESR and good ripple-current capability, at the expense of larger size. Aluminum electrolytic capacitors bring high capacitance and a comparatively high ESR, which is useful for damping and energy buffering at lower frequencies, but they are less effective at high frequencies and require attention to ripple current and lifetime. Their losses are characterized by the ESR and dissipation factor described in capacitor losses: ESR, impedance, DF and Q.

Filter Inductors

The same trade-offs apply to inductors. Shielded inductors are preferred when the component is placed close to the converter or to sensitive circuitry, because the closed magnetic path reduces stray-field coupling into nearby traces. Core material and saturation current rating must keep the inductor in its linear range over the full load and transient current profile of the converter, including the inrush current at start-up; the effect of exceeding it is described in inductor saturation current explained.

The inductance of the filter inductor should be selected as low as possible. A low inductance gives a small DC resistance for a given component size and keeps the characteristic impedance of the filter low, which helps to meet the stability criterion.

Component type	Strengths in input filters	Limitations and notes
MLCC	Very low ESR and ESL, excellent high-frequency attenuation, compact size	High-Q resonances, capacitance loss with DC bias (Class 2), limited bulk energy storage
Film capacitor	Stable capacitance, good ripple-current capability, moderate ESR helpful for damping	Larger volume, typically higher cost per µF
Aluminum electrolytic capacitor	High capacitance, ESR useful for damping, low-frequency energy storage	Limited high-frequency performance, finite lifetime, ripple-current and temperature checks required
Shielded inductor	Reduced stray field, less coupling into nearby traces, compact	Slightly higher cost and loss than unshielded types
Unshielded inductor	Lower cost and often lower DC resistance	Stronger stray field, risk of noise coupling near sensitive nodes

Table 1. Typical input filter component options.

Calculating the Filter Inductance

It is generally practical to choose the filter capacitor first and then determine the inductance that places f_LC at one tenth of the switching frequency. Rearranging Eq. (1) gives Eq. (4) [1]:

L_{filter} = \frac{1}{4\pi^{2} \cdot C_{filter} \cdot \left(0.1 \cdot f_{sw}\right)^{2}} \tag{4}

A good compromise between component size, cost and parasitic behavior for many DC/DC converters lies at filter inductances of 0.47 µH to 22 µH and filter capacitances of 1 µF to 100 µF [1]. For a 1 MHz converter with C_filter = 4.7 µF, Eq. (4) gives 0.54 µH, so a standard 0.56 µH inductor places f_LC at about 98 kHz.

Input Stability Criteria

An input filter changes the impedance seen by the converter. If this impedance becomes too high near the filter resonance, the filter and the converter control loop interact.

Placing a filter in front of a DC/DC converter can, under certain conditions, lead to undesirable effects:

a deeper output voltage dip and a longer output voltage overshoot after a load step,
a pronounced oscillation of the voltage at the converter input.

Negative Input Resistance and the Middlebrook Criterion

The cause lies in the input characteristic of a regulated converter. Because the control loop holds the output voltage, and therefore the output power, constant, the input current falls when the input voltage rises. Within the control-loop bandwidth, the converter behaves as a negative incremental input resistance, Eq. (5) [5]:

Z_{N} \approx -\frac{V_{in}^{2}}{P_{in}} \tag{5}

where V_in is the input voltage and P_in the input power. For a buck converter this equals −R/D², with the load resistance R and the duty cycle D [6]. The smallest magnitude, and therefore the worst case, occurs at the minimum input voltage and the maximum input power.

A converter with an input filter remains stable if the output impedance Z_o of the filter, seen from the converter input, is significantly lower than the magnitude of the converter input impedance over the frequency range in which the control loop is active [5], [6], Eq. (6):

\left|Z_{o}(f)\right| \ll \left|Z_{N}(f)\right| \tag{6}

Above the control-loop crossover frequency, the input impedance of the converter becomes positive again and is set by the power stage and the input capacitor [5]. Figure 3 shows an example of the dynamic input impedance of a buck converter together with the output impedance of an LC input filter.

Impedance versus frequency plot: buck converter input impedance near 12 ohms and LC filter output impedance rising to a resonance peak just below 100 kHz — Figure 3. Example frequency response of the dynamic input impedance of a buck DC/DC converter (black) and the output impedance of an LC input filter (red).

In Figure 3 the red curve comes close to the black curve in a narrow frequency range. Here the filter inductance resonates with the input capacitor of the converter, not with the filter capacitor, as is often wrongly described [1]. Because the filter capacitor on the supply side is a low impedance at this frequency, the converter sees L_filter in parallel with C_in. Both are low-loss components, so the impedance rises sharply at the resonant frequency f₀ and reaches about the characteristic impedance R₀ multiplied by the quality factor of the resonance, Eq. (7):

R_{0} = \sqrt{\frac{L_{filter}}{C_{in}}}, \qquad f_{0} = \frac{1}{2\pi\sqrt{L_{filter} \cdot C_{in}}} \tag{7}

Eq. (7) also shows why a low filter inductance helps: it lowers R₀ and therefore the height of the impedance peak for a given input capacitance.

Damping the Resonance

To reduce the quality factor Q, and with it the impedance peak, a damping capacitor C_damp with a series resistance R_damp is connected in parallel to the input capacitor, as shown in Figure 4 [1]. The resistance can be the ESR of the capacitor, for example an aluminum electrolytic capacitor, or a low-inductance SMT resistor in series with it. C_damp blocks the DC voltage, so that R_damp does not dissipate power from the DC input [7].

Schematic of an LC input filter with a damping branch of Cdamp in series with Rdamp connected in parallel to the converter input capacitor Cin — Figure 4. Reducing the filter quality factor Q with an additional lossy branch (C_damp, R_damp) in parallel to the input capacitor C_in.

C_damp should be at least three to four times larger than C_in [1], [3], [8]. The reason is that the reactance of C_damp must be lower than R_damp at the resonant frequency, so that the resistance, and not the capacitor, sets the impedance of the branch [7]. Because only a purely ohmic loss can damp the resonance, the resistance must be present in every case; a low-ESR capacitor alone shifts the resonance without damping it.

With C_damp large compared with C_in, the damping branch acts at resonance as a resistance in parallel to L_filter and C_in. The quality factor of this parallel resonant circuit and the required damping resistance are given by Eq. (8):

Q \approx R_{damp} \cdot \sqrt{\frac{C_{in}}{L_{filter}}} = \frac{R_{damp}}{R_{0}}, \qquad R_{damp} \approx Q \cdot \sqrt{\frac{L_{filter}}{C_{in}}} \tag{8}

A quality factor in the range of 0.5 to 0.8 gives a response that is neither over-damped nor under-damped and avoids the impedance rise that could violate the stability criterion [1]. With Eq. (8), this corresponds to R_damp ≈ 0.5 R₀ to 0.8 R₀. A larger resistance leaves the L_filter–C_in resonance under-damped, and a smaller one effectively connects C_damp directly in parallel to C_in and creates a new, lower-frequency resonance that is also poorly damped.

For the ratio n = C_damp/C_in, the optimum damping resistance and the resulting peak output impedance of the filter are given by Eq. (9) and Eq. (10) [7], [6]:

Q_{opt} = \frac{R_{damp}}{R_{0}} = \sqrt{\frac{(2 + n)(4 + 3n)}{2n^{2}(4 + n)}} \tag{9}

\left|Z_{o}\right|_{max} = R_{0} \cdot \frac{\sqrt{2(2 + n)}}{n} \tag{10}

For n = 4, the optimum is Q_opt ≈ 0.61 and the peak output impedance is about 0.87 R₀; for n = 3 the values are 0.72 and 1.05 R₀. A larger damping capacitor lowers the impedance peak further, at the cost of size. This confirms the practical rule of a damping capacitor of three to four times C_in with Q between 0.5 and 0.8. The damping resistor must be rated for the ripple current that flows through the damping branch, and the electrolytic capacitor for its ripple current and lifetime.

Worked Example: 12 V Buck Converter Input Filter

The example applies the equations to the operating point used for the REDEXPERT example later in this article: 12 V input voltage, 1 A input current and 500 kHz switching frequency, with 40 dB of attenuation required at the switching frequency. The converter input capacitance is assumed to be an effective 10 µF of MLCCs after DC-bias derating.

Cut-off frequency: f_LC = 0.1 · 500 kHz = 50 kHz.
Filter components: with C_filter = 22 µF, Eq. (4) gives 0.46 µH. A standard 0.47 µH inductor places f_LC at 49.5 kHz, and Eq. (2) predicts about 40 dB of ideal attenuation at 500 kHz.
Resonance with C_in: Eq. (7) gives R₀ = √(0.47 µH / 10 µF) ≈ 0.22 Ω and f₀ ≈ 73 kHz.
Damping: C_damp = 47 µF gives n = 4.7. Eq. (9) gives Q_opt ≈ 0.56, so R_damp ≈ 0.12 Ω, which can be the ESR of a suitable electrolytic or hybrid polymer capacitor or an added resistor. Eq. (10) gives a peak output impedance of about 0.17 Ω.
Stability check: at a minimum input voltage of 10.8 V and 12 W input power, Eq. (5) gives |Z_N| ≈ 9.7 Ω. The damped filter peak of 0.17 Ω lies about 35 dB below this value, which satisfies Eq. (6) with a large margin.

Without damping, the peak impedance of the L_filter–C_in resonance would be limited only by the small ESR of the MLCCs and the DC resistance of the inductor, and could approach or exceed the converter input impedance. The calculation must be repeated with the effective capacitance at the worst-case DC bias and temperature, because a lower C_in raises both R₀ and f₀.

Interaction with the Source and System Requirements

The input filter does not operate in isolation. Its behavior depends on the source that feeds it and on the protection components around it.

Source Impedance

A laboratory power supply with fast current limiting, a long cable harness, an upstream DC bus or a battery pack each presents a different source impedance, which shifts the resonant frequencies and changes the damping of the input network. When the source impedance is high or poorly defined, the LC filter can interact with it and produce deeper voltage dips or prolonged ringing unless sufficient damping is provided at the converter input. Where several converters share one upstream filter, their combined input impedance must be used in the stability check, and the complete system should be verified with all converters operating.

Inrush Current and Start-up

Large input and damping capacitors also determine the inrush current. At power-on or hot-plug, charging these capacitors can draw a high current from the source, which may trigger protection or cause voltage drops on shared buses, and the filter inductor must not saturate during this event. Inrush-limiting measures such as NTC thermistors, dedicated inrush controllers or controlled start-up sequences may be required in addition to the filter; the thermistor approach is described in how to use NTC thermistors for inrush current limiting.

Transient and Surge Protection

In automotive and industrial applications, the input filter is often combined with transient and surge protection. TVS diodes, fuses and surge absorbers are typically placed at the very input to handle load dump, voltage spikes and ISO 7637-2 pulses, with the LC input filter following them to reduce steady-state ripple and conducted EMI. The voltage ratings and energy handling of these elements must be coordinated with the filter capacitor and inductor values to achieve both EMC compliance and robustness over the full operating profile.

Practical Implementation and Verification

Layout and measurement decide whether the calculated attenuation and stability margin are achieved on the board.

PCB Layout

The loop formed by the input capacitor, the filter inductor and the converter input pins carries high di/dt and should be as small and compact as possible, with short, wide traces or planes to minimize stray inductance. The filter inductor should be placed close to the converter input, and the return current path should follow a tight, direct route, so that the noisy switching node does not couple into the input filter or sensitive signal traces. The filter input and output should be kept apart, because coupling between them bypasses the filter at high frequencies.

Parasitic inductances and capacitances limit the real-world attenuation, especially at higher frequencies. The ESL of the filter capacitor and the inductance of traces and vias reduce the effectiveness of the capacitor above its self-resonant frequency, as explained in effects of ESL on capacitor performance. Several capacitors in parallel, with different values and case sizes, placed close to the inductor and the converter pins, extend the useful attenuation range; their anti-resonances should be checked.

Measurement

Filter performance and stability should be verified in the laboratory rather than relying on calculation alone. A line impedance stabilization network (LISN) with a spectrum analyzer or EMI receiver measures conducted emissions against the relevant CISPR limits [3]. Probes at the converter pins show the input ripple voltage and current, while impedance or frequency-response measurements confirm that the filter output impedance remains below the converter input impedance over the control-loop bandwidth. A load-step test at minimum input voltage and maximum load is a simple practical check of the stability margin.

REDEXPERT EMI Filter Designer

Online tools that use measured component data shorten the iteration between calculation and component selection. Würth Elektronik’s REDEXPERT platform includes an EMI Filter Designer for conducted differential-mode noise, for example from a DC/DC converter, which evaluates the filter response with real components [9]. Figure 5 shows its input page with the 12 V, 1 A, 500 kHz example.

The noise source impedance is determined by the input capacitor of the DC/DC converter, so a reference value of 0.1 Ω can be assumed. For a purely differential-mode design, 100 Ω is entered as the LISN impedance, corresponding to the two 50 Ω LISN measurement impedances in series [1]. Based on these impedances and the required attenuation, the tool suggests a topology. Instead of the attenuation, a cut-off frequency, usually one tenth of the switching frequency, can be entered as the selection criterion.

Suggested CL filter with C1 56 µF, 63 V, 3.90 milliohm and L1 470 nH, 1.40 A, 100 milliohm — Figure 6. Filter suggestion from the EMI Filter Designer for an attenuation of about 40 dB at 500 kHz switching frequency.

The components in Figure 6 are selected from the current and voltage specifications of the input page and can be changed at any time. The suggested 470 nH and 56 µF give f_LC ≈ 31 kHz according to Eq. (1), which is lower than the one-tenth rule and leaves margin for the parasitics that reduce the ideal attenuation. Selecting the double-arrow symbol opens a list of suitable alternative parts, or specific parts can be added. The insertion loss and the input and output impedances are plotted immediately, and the summary tab compiles the bill of materials and plots into one document, as shown in Figure 7.

REDEXPERT summary page with the bill of materials, insertion loss and impedance plots of the selected input filter — Figure 7. Resulting bill of materials and data from the EMI Filter Designer for the selected filter components.

The tool and its use in a complete design are presented in filter calculation and selection with the REDEXPERT EMI Filter Designer. The output impedance plot of the tool can be used directly for the stability check against the converter input impedance.

Input Filter Design and Selection Checklist

The checklist summarizes the points to confirm before an input filter design is released.

Requirement: required attenuation at the switching frequency, derived from the measured ripple and the applicable limit, plus margin.
Topology: filter capacitor on the supply side, inductor toward the converter; π or multi-stage filter only where a single LC stage is insufficient.
Filter capacitor: effective capacitance at the operating DC bias and temperature, voltage rating, ripple current, ESL and case size.
Filter inductor: inductance as low as the attenuation allows, DC resistance, saturation current including inrush and transients, shielded construction near sensitive circuits.
Stability: |Z_N| = V_in²/P_in at minimum input voltage and maximum power; filter output impedance peak well below it.
Damping: C_damp of at least three to four times C_in, R_damp ≈ 0.5 R₀ to 0.8 R₀ from ESR or an added resistor, ripple-current and lifetime ratings checked.
System: source impedance, shared filters, inrush limiting and coordination with TVS diodes and fuses.
Catalog versus custom: standard inductors and capacitors from tool-supported libraries are usually sufficient; custom magnetics are mainly needed at high power.
Qualification: automotive or industrial qualification of all filter components where required, and verification by conducted-emission and load-step measurements.

Key Terms

C_in: effective input capacitance of the DC/DC converter at its input pins.
C_damp: damping capacitor connected in parallel to C_in to reduce the resonance Q.
R_damp: loss element, the ESR or an external resistor, in series with C_damp that provides the damping.
R₀: characteristic impedance √(L_filter/C_in) of the resonance between the filter inductor and the input capacitor.
Q: quality factor of a resonant circuit; a high Q means low damping and a pronounced impedance peak.
f_LC: characteristic frequency of the LC filter, typically chosen at about one tenth of the switching frequency.
LISN: line impedance stabilization network, used in EMC testing to define the source impedance and measure conducted emissions.

Conclusion

An LC input filter for a DC/DC converter is designed in two steps that must both be satisfied. The attenuation step places the filter cut-off at about one tenth of the switching frequency, with the filter capacitor on the supply side and the inductor toward the converter. The stability step checks the resonance between the filter inductor and the converter input capacitor against the negative input resistance of the converter, and damps it with a capacitor of three to four times C_in and a resistance of about 0.5 to 0.8 times the characteristic impedance.

Low filter inductance, effective capacitance values under DC bias and a compact layout keep the impedance peak low and the attenuation close to the calculated value. Tools such as the REDEXPERT EMI Filter Designer speed up component selection, and conducted-emission and load-step measurements confirm the design.

FAQ

Why do DC/DC converters need input filters?

A switching converter draws pulsed input current at the switching frequency. The input capacitor supplies most of it, but the remaining ripple flows into the supply as conducted interference and can disturb other loads on the same bus. An LC input filter typically provides about 30 dB or more of attenuation at the switching frequency, so that the design meets conducted EMI limits and the source sees a smooth current.

How is the cut-off frequency of an input filter chosen?

A common starting point is one tenth of the switching frequency. An ideal second-order LC filter attenuates by 40 dB per decade above its characteristic frequency, so this choice gives about 40 dB at the switching frequency, for example a 100 kHz cut-off for a 1 MHz converter. Capacitor ESL and PCB inductance reduce the real attenuation, so tools often choose a somewhat lower cut-off frequency.

Why must the filter inductor face the converter?

The input capacitor of the converter is a low impedance and acts as the noise source. A series inductor toward this low impedance and a shunt capacitor toward the supply form an effective LC low-pass filter. Reversing the order places the capacitor in parallel with the converter input capacitor and the inductor toward the supply, which removes most of the attenuation the filter is meant to provide.

What is the Middlebrook stability criterion for input filters?

A regulated converter draws constant power, so within its control-loop bandwidth it behaves as a negative input resistance of about minus input voltage squared divided by input power. The output impedance of the input filter, seen from the converter, must stay well below this magnitude over the loop bandwidth. The worst case is the minimum input voltage at maximum power, where the converter input impedance is lowest.

Which capacitor does the filter inductor resonate with?

The critical resonance is between the filter inductor and the converter input capacitor, not the filter capacitor. At this frequency the filter capacitor on the supply side is almost a short circuit, so the converter sees the inductor in parallel with its input capacitor. Because both components have low losses, the output impedance of the filter can rise sharply and approach the converter input impedance.

How do you damp an input filter resonance?

A damping branch of a capacitor in series with a resistance is connected in parallel to the converter input capacitor. The damping capacitor should be at least three to four times the input capacitance, and the resistance about 0.5 to 0.8 times the square root of filter inductance divided by input capacitance. The ESR of an aluminum electrolytic capacitor or a small SMT resistor can provide this resistance.

Why should the filter inductance be as low as possible?

A low inductance gives a small DC resistance and low losses for a given component size. It also lowers the characteristic impedance of the resonance between the filter inductor and the converter input capacitor, which reduces the impedance peak and makes the stability criterion easier to meet. The inductance still has to provide the required attenuation at the chosen cut-off frequency together with the filter capacitor.

Which capacitor technologies suit DC/DC input filters?

MLCCs give the best high-frequency attenuation in a small size, but their effective capacitance falls with DC bias and their low losses raise the resonance Q. Film capacitors are stable with moderate losses. Aluminum electrolytic and hybrid polymer capacitors provide bulk capacitance and an ESR that is useful for damping. Combinations of these technologies are common to cover the full frequency range and lifetime requirements.

How to Design an Input Filter for a DC/DC Converter

Step-by-step procedure for designing a damped LC input filter for a DC/DC converter, from the attenuation requirement to stability verification.

Determine the required attenuation
Measure the peak-to-peak input ripple at the switching frequency at maximum input current and compare it with the limit of the applicable standard to obtain the required attenuation plus margin.
Set the target cut-off frequency
Place the filter cut-off frequency at about one tenth of the switching frequency, for example 100 kHz for a 1 MHz converter, which gives about 40 dB of ideal attenuation at the switching frequency.
Select the filter capacitor
Choose a filter capacitor, typically between 1 µF and 100 µF, using its effective capacitance at the operating DC bias and temperature, and check voltage rating, ripple current and ESL.
Calculate and select the filter inductor
Calculate the inductance from the chosen capacitance and cut-off frequency, typically between 0.47 µH and 22 µH. Prefer the lowest suitable value and check DC resistance, saturation current and shielding.
Configure the LC filter correctly
Place the filter capacitor on the supply side and the filter inductor toward the low-impedance input capacitor of the converter, so that the network forms an LC low-pass filter from the viewpoint of the converter.
Check stability and add damping
Calculate the converter input impedance as input voltage squared divided by input power at minimum voltage and maximum power. Add a damping capacitor of at least three to four times the input capacitance with a resistance of about 0.5 to 0.8 times the characteristic impedance.
Verify with tools and measurements
Check the filter output impedance against the converter input impedance with a tool such as the REDEXPERT EMI Filter Designer or a simulation, then confirm conducted emissions with a LISN and stability with a load-step test.

References

DC/DC Converter Handbook – SMPS Topologies from an EMC Point of View, 1st edition (order code 9999669), Würth Elektronik eiSos, 2024.
Basic design considerations and layout recommendations for EMC robustness, Infineon Technologies, application note.
AN-2162 Simple Success with Conducted EMI from DC-DC Converters, SNVA489C, Texas Instruments, application note.
Dynamic Model of High Dielectric Constant Type Chip Monolithic Ceramic Capacitors and Power Inductors, Murata Manufacturing, technical note, v1.02, 2025.
R. D. Middlebrook, Input Filter Considerations in Design and Application of Switching Regulators, IEEE Industry Applications Society Annual Meeting, 1976.
R. W. Erickson, D. Maksimović, Fundamentals of Power Electronics, Chapter 10: Input Filter Design (lecture slides), University of Colorado Boulder.
R. W. Erickson, Optimal Single Resistor Damping of Input Filters, IEEE Applied Power Electronics Conference (APEC), 1999.
M. Sclocchi, Input Filter Design for Switching Power Supplies, SNVA538, National Semiconductor / Texas Instruments, application note.
REDEXPERT design platform – EMI Filter Designer, Würth Elektronik eiSos.
Würth Elektronik – EPCI premium supplier page, Passive Components Blog.

Article revised and technically reviewed in November 2026: corrected the damping-resistance equation and the description of the cut-off frequency, added the negative input resistance and Middlebrook criterion, optimum damping equations, required-attenuation estimate and a worked 12 V example, converted equation images to math, removed broken links and added referenced sources.

Source: DC/DC Converter Handbook