Eliminate the Guesswork in Controlling EMI

Jan 1, 2008 12:00 PM
By Majid Dadafshar and Jinho Choi, Field Application Engineers, Fairchild Semiconductor, San Jose, C

Power supplies are natural generators of narrow-band noise, present at the fundamental of the switching frequency and its associated harmonic frequencies. To contain the noise, it is critical that designers identify its source and the paths it takes in becoming radiated or conducted emissions. Direct radiation of noise can be stopped with simple measures such as enclosing the supply in a metal box or spray coating the case. Designers also need to pay attention to the internal layout of the supply and the wiring that goes in and out of the supply.

However, interference conducted through the input or output terminals is more difficult to suppress. But with proper transformer design, connection of the heatsink, and filter design, conducted interference also can be reduced, such that the supply can achieve EMI regulatory agency approvals without incurring excessive filter cost. Fig. 1 shows the typical conducted emissions of an ac power supply depicting the limits on EMI set by the European standard EN55022.

In a typical power-supply design, a high dc voltage is changed to a chopped or a pulsed waveform. This process generates discrete, unwanted narrow-band signals spaced at the repetition frequency that can be either conducted or transmitted through a system with harmonics over a wide range of frequencies.

Most of the conducted interference on the power line is the result of the main switching transistor or output rectifiers. It is very desirable for a power-supply designer to predict signal levels resulting from these types of waveforms to determine the degree of filtering or shielding needed to meet the EMI limits before actually building any power supply.

Let's examine a power supply with a 110-Vac input. The square waveform at the drain of the main switching transistor has three fundamental characteristics in the time domain that have a direct influence on calculating the predicted emission in the frequency domain. These characteristics, which are depicted in Fig. 2 and Fig. 3, are amplitude, rise time and duty cycle.

The fourier envelope of the spectral energy for Fig. 2 can be estimated by Fig. 3, with slope changes indicated by corner frequencies at F1 and F2. The location of each frequency (F1 and F2), which corresponds to a change in the amount of attenuation, can be tailored based on adjusting the duty cycle (D) and rise time (t_RISE) of any given waveform.

Assuming 90% of the spectral energy within a signal is contained in the first 10 harmonics, then by placing the location of F2 at a much lower frequency than the tenth harmonic, we can obtain an attenuation of 40 dB/decade with the proper rise-time setting. Harmonic energy content is a function of the waveform's rise time and can be estimated using Table 1.

A pulse train such as that of a switching waveform generates discrete, narrow-bandwidth signals spaced at the harmonics of the switching frequency. As Table 1 suggests, a pulse train with a faster rise time spreads these discrete, narrow-bandwidth noise signals over a wider frequency range, such that there are more, higher-order harmonics. In contrast, a pulse train with a slower rise time exhibits a fundamental with a broader bandwidth, while spreading the discrete, narrow-bandwidth noise signals over a narrower frequency range.

Keep in mind that tailoring the rise time involves a tradeoff of the amount of power dissipation through the main switching transistor versus the level of generated emissions. Fast rise and fall times limit power dissipation in the switching transistor, but produce greater emissions.

By calculating the harmonic content of a pulsed waveform with a given rise, duty cycle and amplitude, we can predict an EMI spectrum over a conducted range (such as 150 kHz to 30 MHz) for any given rise time:

where HRMC is the harmonic content of the pulse and T is the period of the switching waveform.

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