DC/DC Converters Research Guide - Serial 2
Time：2022.12.30
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DC/DC Converter Research Guide
3. Basic working principle of DC/DC converter
We take the most basic basic type to explain the DC/DC converter circuit boost and step-down working principle. Other forms of circuitry using coils can be seen in combinations of booster and step-down circuits or application circuits.
Figure 3 and Figure 4 illustrate the operation of the booster circuit. FIG. 3 shows the current path when the FET is ON. Although the dashed line is a tiny leakage current, it will make the efficiency of light load worse. The current energy is accumulated in L at the time of FET ON. Figure 4 shows the current path when the FET is OFF. Even if the FET is OFF, L is still working to maintain the current value before OFF. The left end of the coil is forced to be fixed to VIN for voltage boost to provide enough power to connect the voltage to V OUT. Therefore, the longer the ON time L of the FET, the more current energy accumulated, the more power can be obtained. However, if the ON time of the FET is too long, the power supply time to the output side will be very short. When the FET is ON, the loss will increase and the conversion efficiency will become worse. Therefore, the maximum duty cycle is usually limited so as not to exceed the appropriate ON/OFF time ratio (duty cycle). The step-up work is repeated as shown in FIG. 3 and FIG. 4.
Figure 5 and Figure 6 illustrate the operation of the step-down circuit. Figure 5 shows the current path when the FET is ON. Although the dashed line is a tiny leakage current, it will make the efficiency of light load worse. When the FET is ON, it accumulates current energy in L and supplies power to the output. FIG. 6 shows the current path when the FET is OFF. Even if FET is OFF, L is working and the current value before OFF should be maintained, so that SBD is ON. At this point, the voltage of the VOUT drops as the left end of the coil is forced to drop below 0V. Therefore, the longer the ON time L of the FET, the more current energy accumulated, the more high-power power can be obtained. When the FET is ON, it can also supply power to the output, so there is no need to limit the maximum duty ratio. Therefore, when the input voltage is lower than the output voltage, the FET is always ON and cannot be boosted. Therefore, the output voltage is also reduced to below the input voltage. The depressurization work is repeated as shown in Figure 5 and Figure 6.
4. Design points of DC/DC converter circuit
The required specifications of DC/DC converter circuit should pay attention to the following items:
(1) stable operation (= will not be damaged by abnormal vibration and other misoperation, burning, overvoltage)
(2) High efficiency
(3) The output ripple is small
(4) Good load transient response
These can be improved to some extent by changing the IC and peripheral components of the DC/DC converter. The weighting of these 4 items varies according to the specific application. The following illustrates how to improve these 4 items from the point of view of selecting each component.
The above content is from Tris Semiconductor Co., LTD. In the series...
