Noise Management in DC/DC Converters

The application of high-density switching DC/DC converters has forced people to fundamentally redefine the standards for size, performance and reliability. These power components, often called "bricks", provide power system designers with unprecedented design and development flexibility.

However, because these converters exhibit fast voltage and current switching transients, they generate conducted and radiated noise. Therefore, it is very important to understand the relationship between converter topology and generated noise.

Converter topology

There are several different topologies of DC/DC converters in use today, but no topology is superior in all aspects. However, among the major topologies, quasi-resonant zero current switching (ZCS) generates much less conducted noise than traditional pulse width modulation (PWM) converters mounted on circuit boards.

a) DC/DC converter modules using zero current switching topology have fixed pulse width and variable repetition rate; (b) For modules using pulse width modulation, the situation is just the opposite.


For applications that require extremely low conducted noise, are very sensitive to noise, or are limited by space and cannot add filtering circuits, the quasi-resonant zero current switching topology with inherent low noise characteristics should be considered.

The low noise characteristics of the ZCS converter are largely determined by the topology of the power line. The sinusoidal current waveform of the zero switching converter produces less harmonic noise. Moreover, the more continuous the waveform, the less excitation it produces to parasitic elements, and the parasitic noise it produces is much smaller.

In modules using quasi-resonant topology, the pulse width is fixed and the repetition rate is variable; in modules using pulse width modulation, the situation is just the opposite - the repetition rate is fixed and the pulse width is variable (see figure). Each topology will produce a characteristic noise spectrum.

However, since the ZCS structure is basically a half-wave rectified sine wave, It has much lower high-frequency components associated with the rising and falling edges of the current waveform. The spectral content of the ZCS waveform is smaller in amplitude and contained in a narrower frequency band. In addition to the topology, other components - such as the transformer and the module packaging - also generate noise.

Parasitic capacitance

When the input and output terminals are designed close to each other to ensure effective magnetic coupling, the transformer will induce electrical coupling through parasitic capacitance. In addition, DC/DC converters with higher isolation - compared to converters with lower isolation or no isolation between the primary and secondary - have lower input-output parasitic capacitance and therefore generate less noise.

The high correlation of di/dt with PWM topology will lead to the stimulation of construction-method parasitics. Various components (such as diodes and MOSFETs) are mounted on an isolated substrate because they generate heat, and then the substrate is mounted to the aluminum base plate of the module.

For heat dissipation considerations, a narrow gap is left between the diode or FET and the base plate, thus forming a capacitor. Although this construction is conducive to heat dissipation, it also increases parasitic noise, thereby increasing noise coupling. By carefully controlling the geometry, both parasitic capacitance and noise coupling can be minimized.