Circuit Design Guide for DC/DC Converters（3/7）
Selecting the Coil
An optimal L value varies with switching frequency as the coil current is in proportion to the duration of activation of the FET and is in reverse proportion to the L value.
Loss by coil appears as a sum of the coil's wire-wound resistance RDC and the loss generated in the ferrite core. In switching frequencies of up to 2MHz, it is considered that the RDC of the coil is mainly responsible for the coil losses. Therefore, firstly select a coil with a small RDC value. However, if minimizing RDC results in selection of too small a L value, the current value while the FET is activated becomes too large, increasing heat losses from the FET, SBD and coil, and reducing the efficiency. Also, the ripple becomes larger due to this increased current.
On the contrary, if the L value is too large, the RDC value becomes larger, degrading the efficiency at the heavy-load time, and magnetic saturation occurs in the ferrite core, rapidly reducing the L value. In this state, the coil cannot properly function, and heat generated by over-current becomes dangerous. Therefore, to allow large current flow in the coil with a large L value, the dimensions of the coil need to be increased to some extent to avoid magnetic saturation.
From the above mentioned, an appropriate L value for an individual switching frequency is determined by considering both dimensions and efficiencies.
Table 5 shows the standard L values for individual switching frequencies. This is reference data for a typical DC/DC converter circuit where the input and output voltages (VIN & VOUT) are 6.0V or less. For higher Voltage circuits the recommended values will be slightly different.
|L value||Switching frequency||When light-load time weighted||Standard value||When heavy-load time weighted|
|Rated current||Step-up circuit||Approx. 2 to 3 times of Max. input current|
|Step-down circuit||Approx. 1.5 to 2 times of Max. output current|
Figure 12 shows example of variations of efficiency respectively when only the L value is varied in the XC9104D093 (step-up) circuit shown in Figure 13.
Figure 14 and Figure 15 show the examples of efficiency and ripple in the XC9220A093 (step-down) circuit shown in Figure 16.
In both examples, if the coil structure is identical, increasing the L value decreases the maximum output current, increases the efficiency at the light-load condition, and reduces the ripple. This result shows that selecting the L value optimal for output current is very important.
Figure 12: Relationship between L value and efficiency (step-up: XC9104D093 )
Figure 13: Test circuit for XC9104D093 shown in Figures 12.
Figure 14: XC9220A093 Relationship between L value and efficiency (step-down)
Figure 15: Relationship between L value and ripple (step-down: XC9220A093)
Figure 16: Figures 14 and 15 Circuit used for measurements shown in XC9220A093 (PWM=CE=VIN)