DC Filters

Capacitors

In a DC input filter, electrolytic capacitors can help prevent ringing and dampen oscillations due to their ESR characteristics. With capacitance values in the range of several hundred µF, they can also act as buffer elements.

Overvoltage Protection

Multilayer Varistors are capable of diverting high currents, but with a rather high clamping voltage. For semiconductor-based overvoltage protection, e.g., TVS diodes, the clamping voltage lies close to the operating voltage, but the power dissipation is lower. Therefore, a two-stage protection concept is often useful.

PCB Ferrites

SMD ferrites capable of handling high inrush currents are suitable for applications requiring effective EMI suppression.

MLCC & Aluminium Polymer Capacitor

Keep the effects of DC bias and SRF in mind when selecting MLCCs for filtering. Film capacitors don’t have a DC bias but they require significantly more space in the design.

DC & Data Line Chokes

Differential mode noise <30 MHz is best attenuated by ferrite chokes. 

Short Introduction to DC Filter

A DC input filter acts as a barrier against electrical noise, protecting both the device and its power source. It blocks unwanted noise from entering the device (boosting immunity) and stops the device from polluting the supply line (reducing emissions). To tackle both common-mode and differential-mode noise, a common-mode choke is paired with line filters and capacitors. If overvoltage protection is required, it belongs right at the filter’s input.

Learn More About DC Filter

Schematic representation of the signal range.

Characteristic curve for a unidirectional overvoltage protection device: During normal operation, the signal should remain within the voltage range depicted in gray. If the voltage continues to rise, excess currents are initially diverted and the voltage is clamped, before the red area is reached at even higher overloads, causing damage to the overvoltage protection device.

You can see the characteristic curve for a bidirectional overvoltage protection element

Characteristic curve for a bidirectional overvoltage protection element: Under normal operation, the signal should remain within one of the gray shaded voltage areas. If the voltage continues to increase (in either a positive or negative direction), excess currents are initially diverted and the voltage is clamped, before reaching the red area at even higher overloads, where the overvoltage protection element may become damaged.

The graphic shows the dependence of the attenuation on the frequency.

Since manganese-zinc cores operate at lower frequencies than nickel-zinc cores, NiZn cores are mainly used for data lines and MnZn cores for DC lines.

The picture shows a Bifilar and Sectional Winding

For DC filters, common mode chokes with bifilar (see left) or sectional (see right) winding are available. Due to the additional differential mode attenuation, the sectional winding is recommended for DC lines.

The graphic shows the dependence of the Impedanz on the frequency.

With the online simulation platform RedExpert, you can calculate the appropriate impedances of SMT ferrites for your application based on frequency, DC bias, system impedance, and required attenuation.

The graphic shows the dependence of the max. pulse currenton the pulse length.

For high peak currents, the SMT ferrites from the WE-MPSB family are particularly suitable, as they can handle a very low DC resistance Rdc and a rated current of up to 10.5 A.

The graphic shows the dependence of the Impedance on the frequency.

SMT ferrites can be characterized according to four application areas: 1) high currents, 2) high frequencies, 3) wide bandwidth, and 4) high speed. The diagram shows the impedance characteristics as a function of frequency for the four application areas.

The graph shows the impedance over frequency for capacitors with different capacitance values.

The frequency/impedance diagram shows the self-resonant frequencies of the MLCC capacitor types with 100 nF, 10 nF, and 1 nF. The self-resonant frequencies increase as the  capacitances decrease.

The image shows how capacitance changes with increasing DC-Bias voltage.

The capacitance of a capacitor is typically also dependent on an applied direct current voltage (DC Bias) and can become significantly lower with increasing voltage. From the DC Bias/capacitance diagram, it is clearly visible that with increasing miniaturization of the component, the nominal value of 470 nF decreases (red curve).

On the Picture you can see the Board Flex.

For applications with high mechanical stress, it is advisable to use soft termination capacitors. These have a termination with Ag-polymer/Ni/Si and minimize the risk of cracks occurring.

The graph illustrates the change in capacity for the different capacitor models.

Capacitors should maintain their capacitance as consistently as possible across temperature variations. Three of the materials used, NP0, X7R, and X5R, fulfill this requirement significantly better than Y5V, as can be clearly seen in the diagram. NP0 shows virtually no changes in capacitance over a wide operating temperature range.

The lifespan of a capacitor depends on the maximum allowable operating temperature, the ambient temperature, the ripple current, and the applied voltage. The simplified formulas disregard the influence of current and voltage. In RedExpert, the lifespan can be calculated in detail for your application.

Illustration of a real simulation model.

For the simulation of capacitors for power supplies, real models based on the shown equivalent circuit with equivalent series inductance (ESL) and equivalent series resistance, as well as parallel resistance for leakage currents, are available.

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