The present invention relates to switching mode power supplies. More particularly, the invention provides methods and apparatuses for reducing electromagnetic interference (EMI) of switching mode power supplies.
Many electronic components require clean DC power sources that may obtained from other DC or AC power sources using switching mode power regulation systems. Generally, transformer may be used to convert a DC power source to the desired DC power. In order to obtain a small size of transformers, the switching frequency has to be high relative to the alternating current (AC) power line. However, the relative high switching frequency can be coupled back to the AC power line and interfere with the operation of other radio frequency operating equipment such as radio or television receivers. Traditionally, EMI filters must be added to the inputs of the DC source to prevent EMI from leaking out of switching mode power supply back to the DC source. The EMI filter conventionally uses inductors and capacitors to form passive band-stop filters having a notch bandwidth matching the EMI frequencies. The analog EMI filter approach not only is cumbersome because it requires numerous trials of different inductors and filters (i.e., on a trial-and-error basis), but also is expensive and requires large system area for mounting the passive components. Furthermore, passive filters consumes additional power.
EMI is a critical issue in the design of a switching mode power supply. With regards to conventional pulse width modulation (PWM) power converters, the energy of the electromagnetic radiation has its maximum value at the fundamental switching frequency, the radiation energy decreases with higher harmonics. The major portion of the electromagnetic radiation energy resides in the fundamental switching frequency and its lower harmonics. In order to reduce EMI, different frequency jittering techniques can be used. For example, switching frequencies may be varied in order to spread out the electromagnetic radiation energy across a relatively large frequency range.
Many publications have proposed the reduction of EMI using frequency jittering techniques. For example, “Frequency jittering control for varying the switching frequency of a power supply” by Balu Balakrishnan, et al., U.S. Pat. No. 6,249,876, Jun. 19, 2001 (hereinafter “the '876 patent”) proposed digital and analog frequency jittering circuits. However, the prior art circuits generate undesired ripples at the power supply.
A digital frequency jittering circuit shown in FIG. 1 of the '876 patent uses a seven-bit binary counter that is clocked by a primary oscillator. The counter outputs are provided to a series of frequency jittering current sources whose outputs are added to the primary oscillator to vary its frequency. This approach has many drawbacks. For example, the frequency jitter is discontinuous due to the digital nature of the binary counter. In this example, the output will toggle with every 8-clock periods of the primary oscillator. This non-continuous frequency change causes high magnitude spikes in a power source. Another drawback is the relative large circuitry of the binary counter that consumes a large silicon area; the silicon area can be quite large if the frequency variation needs to be continuous, i.e., more counter stages and therefore more complexity are required.
An analog frequency jittering circuit, shown in FIG. 3 of the '876 patent, uses a primary oscillator whose frequency is controlled by a primary current source. A second analog oscillator produces a low frequency triangular waveform that is used to control a current mirror. A mirrored current is then added to the primary current source to vary the primary oscillator in a narrow range to reduce the EMI noise. The continuous characteristic of the triangular waveform allows the jitter of the switching frequency to vary continuously with time, hence avoiding the discrete changes of the primary oscillator that cause spikes that radiate EMI emission in the power supply. This jitter circuit generates a secondary current in a second oscillator, but still adds the secondary current to the primary current of the oscillator to vary the oscillator frequency.
From the above, it is seen that an improved technique for reducing EMI of switching mode power supplies is desired.