What factors should be considered in switching frequency?

Typically, the switching frequency will vary around a typical value in the middle of the specified range. According to its datasheet specifications, the oscillators built into switching regulator ICs are generally available over a very wide frequency range. For example: the monolithic ADP2386 buck converter IC ensures that its switching frequency is within ±10% of the set value. Other commonly used switching regulator ICs are specified within a range of ±20% or higher of the set value. Since the ADP2386 switching frequency has a ±10% variation range, in extreme cases, the ADP2386 uses RT to set the switching frequency to 600 kHz, which allows switching at 540 kHz and 660 kHz frequencies.

Figure 1. ADP2386 buck converter with switching frequency set by resistor RT

When designing the circuit, it must be considered that the switching frequency may vary by 20%. Since the peak current flowing through the inductor will vary with the actual switching frequency, the inductor current ripple will directly affect the output voltage ripple.

Figure 2. Peak-to-peak coil current ripple affected by switching frequency changes

Figure 2 shows the effect of switching frequency on inductor current ripple. In the figure, the nominal switching frequency of 600 kHz is shown in blue. The minimum (540 kHz) switching frequency is shown in purple and the maximum (660 kHz) switching frequency is shown in green. At a nominal set frequency of 600 kHz, a peak-to-peak ripple current of 1.27 A is seen when the regulator switches at 540 kHz. However, at the same frequency setting of 600 kHz, the switching regulator is also capable of switching at 660 kHz, with a corresponding ripple current of 1.05 A. In this example, the 220 mA difference in coil current ripple may be due to changes in switching frequency of different components in the circuit. This exceeds the entire allowable temperature range.

The current limit setting of the switching regulator must take this factor into consideration. The peak current must be low enough to ensure that any existing overcurrent protection is not activated during normal operation.

Note that this example does not take into account all other possible variations, such as inductor and capacitor value changes.

Figure 3 shows the corresponding output voltage ripple values ​​for different current ripple changes. The circuit is designed to produce a ripple voltage of 4.41 mV at a switching frequency of 600 kHz. At a switching frequency of 540 kHz, the ripple voltage is 5.45 mV; at a switching frequency of 660 kHz, the ripple voltage is 3.66 mV.

Figure 3. Changes in output voltage ripple caused by changes in switching frequency in switching mode regulator ICs

In this case, the only variable considered is the change in switching frequency over the allowed temperature range. In real applications, there may be many other variables, such as changes in the actual values ​​of inductors and capacitors. These are also affected by operating temperature. However, we can also assume that in most cases the actual change in switching frequency will not reach the ±10% limit. Typically, the switching frequency will vary around a typical value in the middle of the specified range. In order to systematically consider all dynamic variables in the power supply, we can find the answer through Monte Carlo analysis. In which changes in the parameters of different components and variables are weighted according to their probability of occurrence and correlated with each other.