A boost converter normally requires two main passive components for storing energy during power conversion: an inductor and an output capacitor. The inductor affects the steady state efficiency (including the ripple and efficiency), transient behavior, and loop stability, which makes the inductor the most critical component in application.

When selecting the inductor and the inductance, the other important parameters are:

• The maximum current rating (RMS and peak current should be considered)
• The series resistance
• Operating temperature

The TPS61378-Q1 has built-in slope compensation to avoid subharmonic oscillation associated with current mode control. If the inductor value is too low and makes the inductor peak-to-peak ripple higher than 2 A, the slope compensation may not be adequate, and the loop can be unstable. Therefore, it is recommended to make the peak-to-peak current ripple between 800 mA to 2 A when selecting the inductor.

The inductance can be calculated by Equation 4, Equation 5, and Equation 6:

Equation 4.

Equation 5.

Equation 6.

where

• Δ_IL is the peak-peak inductor current ripple
• V_IN is the input voltage
• D is the duty cycle
• L is the inductor
• ƒ_SW is the switching frequency
• Ripple % is the ripple ration versus the DC current
• V_OUT is the output voltage
• I_OUT is the output current
• η is the efficiency

The current flowing through the inductor is the inductor ripple current plus the average input current. During power up, load faults, or transient load conditions, the inductor current can increase above the peak inductor current calculated.

Inductor values can have ±20%, or even ±30%, tolerance with no current bias. When the inductor current approaches the saturation level, the inductance can decrease 20% to 35% from the value at 0-A bias current, depending on how the inductor vendor defines saturation. When selecting an inductor, make sure the rated current, especially the saturation current, is larger than its peak current during the operation.

The inductor peak current varies as a function of the load, switching frequency, and input and output voltages. The peak current can be calculated with Equation 7 and Equation 8.

Equation 7.

where

• I_PEAK is the peak current of the inductor
• I_IN is the input average current
• Δ_IL is the ripple current of the inductor

The input DC current is determined by the output voltage. The output current can be calculated by:

Equation 8.

where

• I_IN is the input current of the inductor
• V_OUT is the output voltage
• V_IN is the input voltage
• η is the efficiency

While the inductor ripple current depends on the inductance, the frequency, the input voltage, and duty cycle are calculated by Equation 4. Replace Equation 4 and Equation 8 into Equation 7 and get the inductor peak current:

Equation 9.

where

• I_PEAK is the peak current of the inductor
• I_OUT is the output current
• D is the duty cycle
• η is the efficiency
• V_IN is the input voltage
• L is the inductor
• ƒ_SW is the switching frequency

The heat rating current (RMS) is can be calculated with Equation 10:

Equation 10.

where

• I_{L_RMS} is the RMS current of the inductor
• I_IN is the input current of the inductor
• Δ_IL is the ripple current of the inductor

It is important that the peak current does not exceed the inductor saturation current and the RMS current is not over the temperature-related rating current of the inductors.

For a given physical inductor size, increasing inductance usually results in an inductor with lower saturation current. The total losses of the coil consists of the DC resistance (DCR) loss and the following frequency-dependent loss:

• The losses in the core material (magnetic hysteresis loss, especially at high switching frequencies)
• Additional losses in the conductor from the skin effect (current displacement at high frequencies)
• Magnetic field losses of the neighboring windings (proximity effect)

For a certain inductor, the larger current ripple (smaller inductor) generates the higher DC and also the frequency-dependent loss. An inductor with lower DCR is basically recommended for higher efficiency. However, it is usually a tradeoff between the loss and foot print. Table 9-2 lists some recommended inductors.