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Common Mode and Differential Mode Filter Design for DC-DC Converters

Published Date
Author Cadence System Analysis

Key Takeaways

  • In differential mode, noise sources appear across power supply lines, and the differential mode noise current takes the same direction as the converter input current.

  • Conducted and radiated emissions from common mode and differential mode noises cause the majority of DC-DC converter electromagnetic interference issues.

  •  LC filters are used as common mode filters as well as differential mode filters. 

DC-DC

Filters are incorporated in DC-DC converters to mitigate common mode and differential mode noise effects

Power supplies are the lifeline of any electronic system. We use gadgets and equipment with advanced features, and these functionalities can be harnessed only when the device is supplied by the input voltage. Electrical and electronic devices are powered either by plugging into utility sockets or through battery power. If the power supply is DC, DC-DC converters are involved in the internal circuit.

When using DC-DC converters, common mode and differential mode noise can cause issues. Filters are incorporated in DC-DC converter designs to mitigate the effects of this noise. Effective common mode and differential mode filter designs are crucial in DC-DC converters, as they reduce harmonics and prevent voltage spikes from reaching the main converter circuit. 

Common Mode and Differential Mode Noises in DC-DC Converters 

Differential Mode Noise

Consider a boost converter with DC (+) and DC (-) terminals. The current from the input DC supply flows into the boost converter circuit through the DC (+) terminal and out from the converter circuit to the negative terminal of the DC supply, through the DC (-) terminal. The noises present in the current flowing through the DC (+) and DC (-) terminals can be collectively referred to as differential mode noise. 

In differential mode noise, noise sources appear across the power supply lines, and the differential mode noise current takes the same direction as the boost converter input current. The differential mode currents in DC (+) and DC (-) terminals are opposite each other. This type of noise can also be called normal mode noise. 

Common Mode Noise

The boost converter enclosed inside a housing can be placed over a chassis that is connected to a reference ground. There is stray capacitance between the boost converter PCB and the housing, and between the housing and the reference ground. The noise currents can flow from the DC (+) terminals and DC (-) terminals to the reference ground via the stray capacitance. These types of noises are called common mode noises. The switching of the power electronic devices in the boost converter resulting in high dv/dt and di/dt is one of the reasons for the generation of common mode noises. In common mode noises, the noise currents flowing through the DC (+) and DC (-) terminals are in the same direction.

Common mode and differential mode noises in DC-DC converters can be conducted or radiated emissions. These emissions cause the majority of DC-DC converter electromagnetic interference issues. It is necessary to eliminate or minimize this noise so that DC-DC converters function smoothly and reliably. 

Common Mode and Differential Mode Filter Designs

For common mode and differential mode filter designs, LC filters are often used. The connection of the inductor (L) and capacitor (C) varies with the type of noise it is intended to filter. 

Differential Mode LC Filters

The L and C values are designed according to the frequency of noise. The L allows DC power to pass through it, whereas C bypasses high-frequency components. In the differential mode, the noise present in the DC (+) and DC (-) lines needs to be eliminated. To achieve this, an inductor of half the designed value is placed in the DC (+) terminal line connecting to the DC-DC converter. Similarly, the same value inductor is placed in the DC (-) terminal line and the capacitor is connected across the DC (+) and DC (-) terminals. 

Common Mode LC Filters

In common mode LC filters, common mode chokes are used as inductors. These chokes are connected across the input DC supply on one side and the other side connects across the DC-DC converter input terminals or to the differential-mode LC filter. At the junction of the common mode choke and DC-DC converter positive input terminal, a capacitor is connected to the reference ground. Similarly, from the junction of the common mode choke and DC-DC converter negative input terminal, a capacitor connects to the reference ground. The common mode choke (along with two capacitors) forms the common mode LC filter.

Common mode and differential mode filter designs are essential to prevent noise from affecting a DC-DC converter’s operation. Cadence provides a suite of design tools that help when designing DC-DC  converters with filters.

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