DC/DC Converters Research Guide - Serial 4
Time：2023.03.23
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7. Coil selection
The optimum L value varies for different switching frequencies because the coil current is proportional to the ON time of the FET and inversely proportional to the L value. The loss caused by the coil is the sum of the coil winding resistance RDC and the loss caused by the ferrite core. However, for a switching frequency of about 2MHz, it can be assumed that most of the losses of the coil are caused by the RDC, and the coil with small RDC should be selected first. However, if a coil with a small L value is selected to reduce the RDC, the current value is too large during the time when the FET is ON, and the heat loss generated by the FET, SBD, and coil becomes large, and the efficiency decreases. Moreover, the ripple increases due to the increase of current.
On the contrary, if the L value is too large, the RDC becomes large, not only the efficiency at heavy load becomes poor, but also the ferrite core becomes magnetically saturated, and the L value decreases rapidly, so that the performance of the coil cannot be realized, and it falls into the dangerous state of overheating caused by excessive current. Therefore, in order to flow large currents in coils with large L-values, the shape must be of a certain size to avoid magnetic saturation.
In summary, the appropriate L-value has been limited in terms of both form factor and efficiency relative to the switching frequency. Table 5 shows the standard L-values for each switching frequency value.
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Figure 12 and Figure 13 show examples of how the efficiency and ripple of the XC9104D093 (boost) circuit shown in Figure 14 change only the L value.
The above content is from Tris Semiconductor Co., LTD. In the series...Figure 12 and Figure 13 show examples of how the efficiency and ripple of the XC9104D093 (boost) circuit shown in Figure 14 change only the L value.
Similarly, Figures 15 and 16 show examples of the efficiency and ripple of the XC9220A093 (buck) circuit shown in Figure 17.
In both examples, the maximum output current value decreases by increasing the L value, and the efficiency increases and ripple decreases at light loads when the coil structure is the same. This shows that it is important to choose the L value that is appropriate for the output current.
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8. Selection of SBD
8. Selection of SBD
The SBD loss is the sum of the forward heat loss VF×IF and the heat loss caused by the reverse leakage current IR for the same reason as the FET. Therefore, it is ideal to choose a product with small VF and IR. However, VF is inversely proportional to IR, and generally depends on the load current, VF is large at heavy loads, and considering that IR is independent of the load for a certain value, it is better to choose products with small IR at light loads to improve efficiency, and to choose products with small VF at heavy loads. The above contents are summarized in Table 6 below.
Figure 18 shows the efficiency change in the XC9220A093 circuit shown in Figure 19 with only the SBD change shown in Table 7. It can be seen that compared to XBS203V17, XBS204S17 has good efficiency at light load and poor efficiency at heavy load due to large VF and small IR.
Figure 18 shows the efficiency change in the XC9220A093 circuit shown in Figure 19 with only the SBD change shown in Table 7. It can be seen that compared to XBS203V17, XBS204S17 has good efficiency at light load and poor efficiency at heavy load due to large VF and small IR.
