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.

Table 5: Standard L values and rated current values for switching frequencies
Item Condition Recommended Values
L value Switching frequency When light-load time weighted Standard value When heavy-load time weighted
30kHz, 50kHz330µH220µH100µH
100kHz220µH100µH47µH
180kHz100µH47µH22µH
300kHz47µH22µH10µH
500kHz33µH15µH6.8µH
600kHz22µH10µH4.7µH
900kHz10µH4.7µH3.3µH
1.2MHz6.8µH3.3µH2.2µH
2MHz3.3µH2.2µH1.5µH
3MHz2.2µH1.5µH1.0µH
Rated currentStep-up circuitApprox. 2 to 3 times of Max. input current
Step-down circuitApprox. 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)

L:VLF10045T(L:VLF10045T(22µH、33µH、47µH)

Figure 15: Relationship between L value and ripple (step-down: XC9220A093)

L:VLF10045T(22µH、33µH、47µH)
CL:22µF、Tr:2SJ616

Figure 16: Figures 14 and 15 Circuit used for measurements shown in XC9220A093 (PWM=CE=VIN)

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