Switching regulators convert the input voltage to a higher or lower output voltage, for which an inductor is used to temporarily store electrical energy. The size of the inductor depends on the switching frequency of the switching regulator and the expected current flowing through the circuit, so how do I choose the correct inductor value? The inductor value can be determined using a common formula that incorporates the inductor current ripple.
In most datasheets for switching regulators, as well as in most application notes and other explanatory texts, the inductor current ripple is recommended to be at 30% of nominal load operation. This means that the inductor current peaks and inductor current valleys are 15% higher and 15% lower, respectively, than the average current at the nominal load current. Why choosing 30% inductor current ripple or current ripple ratio (CR) can be considered a good compromise?
For buck converters, such as the one shown in Figure 1, Equation 1 applies:
Figure 1. corresponding inductor current ripple when using a buck converter.
This formula calculates the required inductance for a buck converter based on the current ripple ratio CR, L. This ratio is typically specified as 0.3, or 30% peak-to-peak ripple. In this formula, D represents the duty cycle and T represents the cycle time, depending on the respective switching frequency.
What happens when you use a different inductor current ripple?
In Figure 2, the red line represents the circuit's inductor current ripple (current ripple ratio (CR) of 30% with an output current of 3 A. This is a common compromise choice in switching regulator circuit design. The blue waveform corresponds to an inductor current ripple of 133% and the green waveform corresponds to an inductor current ripple of 7%.
Fig. 2. Inductor current ripple (red), small inductor current ripple (blue), and large inductor current ripple (green) for a ripple current ratio of 30% at nominal load.
Figure 3 shows the same circuit running with a partial nominal load as the output current (e.g. 1A). At high inductor current ripple, as shown by the blue waveform in Fig. 3, the inductor will completely discharge at each cycle. This mode is called discontinuous conduction mode (DCM). In this mode, the stability of the control loop changes and may produce higher output voltage ripple.
Fig. 3. Inductor current ripple (red), small inductor current ripple (blue) and large inductor current ripple (green) with a ripple current ratio of 30% at partial load.
So a certain ripple current ratio needs to be used to avoid DCM. a good compromise is obtained at a ripple current ratio of 30%. If the ripple current ratio is low, the system will operate in continuous current conduction mode most of the time, even at partial loads. So, by optimising the circuit, it is possible to operate in this mode.
What happens if I select a ripple current ratio that is too high?
Ripple current ratios above 30% result in smaller inductor sizes and lower costs. However, the peak currents are significantly higher, generating a large amount of electromagnetic interference (EMI), much higher than typical circuits can accept. In addition, to use continuous conduction mode (CCM), the load current has to be even higher. This is not yet a problem, but the operating characteristics change in this mode and this must be taken into account when designing the circuit.
In addition, a higher output voltage results from a lower inductor current ripple compared to a lower inductor current ripple.
What happens if I choose a ripple current ratio that is too low?
Ripple current ratios below 30% result in larger inductor sizes and higher costs. Because of the large size of the energy storage device, the load transient response will be somewhat lower. For example, when quickly disconnecting a high load current, the power stored in the inductor must be transferred somewhere. This causes the voltage across the output capacitor (COUT) to rise. The more electrical energy in the inductor, the higher the output voltage. Overvoltage may damage the supply circuitry.
After weighing the advantages and disadvantages of different inductor current ripple ratios, we have found that for most applications, a current ripple ratio of about 30% or so is more suitable. However, in some cases it is possible to deviate from this, as long as the results are acceptable.
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