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Considerations on Switching Frequencies

2015.12.06

Points of this article

・In most cases, efficiency, size, and noise are in a tradeoff relationship, requiring optimization.

・In the proper use of a switching regulator, a critical requirement is how to deal with switching noise (radiation/conduction).

・In the noise processing related to switching frequencies, the use of an IC capable of adjusting the switching frequency may offer significant advantages.

The switching frequency of a switching regulator IC ranges from several tens of kHz to several mega Hz, and some newer regulators claim to achieve even higher frequencies. The specific frequency to be chosen depends on several conditions under which the regulator operates.

One issue to be addressed is whether efficiency or size is the primary concern. High switching frequencies permit the use of small external inductor and capacitor values, which necessarily favors size reductions. This reduces the form factor which measures the mounting area times the height, contributing to space savings. The high-speed switching, on the other hand, increases switching loss, penalizing efficiency to an extent amounting to a few percentage points. For compact portable devices, these efficiency and size parameters are those for which tradeoffs must be avoided. At any rate, well-balanced optimization would be required.

Figure 62. Tradeoff between efficiency and size

Figure 63 shows the relationship between general evaluation items and the switching frequency. In items other than parts size and efficiency, the high switching frequency offers advantages. Although this is generally a valid point, in some applications there may be frequencies that are to be avoided, as witnessed by an AM radio frequency band, which ranges from 400 kHz to 1.8 MHz. In simple terms, the use of a power supply in this switching frequency range causes degradation of sensitivity and S/N. To avoid such an issue, an effective approach would be to use a switching frequency outside this frequency range.

Figure 63. Evaluation items vs. switching frequency

Thus, tradeoffs are always needed. One must undertake a design process with a thorough review of application requirements and the operating environment. Although a number of ICs exist that offer modifiable switching frequencies, it should be borne in mind that a modification not only involves the frequency, but it also requires a careful review of external part constants including inductor and capacitor.

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The basics of step-down switching regulators, including their operation and functions, are explained. Comparison with linear regulators, synchronous rectification and diode rectification, control method, auxiliary functions, etc. are also explained.

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