DC/DC Converters Research Guide - Serial 5
Time：2023.05.09
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9. Selection of CL
The larger the CL is, the smaller the ripple will be, but if it is too large, the shape of the capacitor will be larger and the cost will be higher. First, aim for a ripple size of 10mV to 40mV, starting from the capacitance value in Table 8 for boost and Table 9 for buck. However, there is a risk of abnormal oscillation in DC/DCs that do not support low ESR capacitors, so if you want to use a low ESR capacitor in continuous mode, you should check the load transient response in advance to make sure that the output voltage can be stabilized in time (it converges within 2 oscillations).
Figure 20 shows the output ripple variation in the XC9104D093 with only the CL replaced as shown in Figure 21. The ripple is proportional to the ESR and increases inversely with the capacitance value. In the case of aluminum electrolytic capacitors, it is difficult to obtain the output current with too large ESR if there is no ceramic capacitor connected in parallel.
10. Choice of CIN
Although not as big as the influence of CL on the output stability, CIN is also the larger the capacitance value, the smaller the ESR, the better the output stability and the smaller the ripple. To some extent, the effect of reducing the output ripple will become smaller, in the sense of preventing electromagnetic interference (EMI) to the input side, the capacitance value should be explored from about half of CL. Figure 22 also shows how the input-side ripple size changes when CIN in Figure 23 is changed. The CIN does not oscillate at the output because the ESR is too small, so it is appropriate to use low ESR capacitors as much as possible.
11. RFB1, RFB2 selection
When using FB (feedback) products, RFB1, RFB2 used to determine the output voltage, the same output voltage can sometimes consider a variety of combinations. At this time, the choice of RFB1 + RFB2 = 150kΩ ~ 500kΩ is appropriate. Here becomes a problem is the efficiency of the light load and the output stability of the heavy load. Because the current flowing to RFB1 and RFB2 is not used as output power and is treated as loss of DC/DC converter, RFB1 and RFB2 should be set larger (RFB1+RFB2<1MΩ or so) to improve the efficiency at light load. And to improve the transient response at heavy load, be prepared for the poor efficiency at light load, set RFB1 and RFB2 1 digit smaller than the standard value, so that the voltage stability of the FB terminal can be improved.
12. CFB selection
CFB is the capacitor for ripple feedback adjustment, and the value also affects the load transient response. The CFB value in Table 10 is the best value according to the L value. The stability of operation is poor if the value is too small or too large.
The figure illustrates the effect of CFB using the XC9220C093 as an example. In the circuit of Figure 27, RFB1=82kΩ, CFB for fZFB=10kHz is about 390pF. (Fig. 24=39pF), (Fig. 25=390pF) and (Fig. 26=1000pF) are comparisons of the load transient response when changing the CFB. 39pF, the voltage drops sharply when the load becomes heavy and the time to return to a constant state is short, while 1000pF, the instantaneous voltage drop when the load becomes heavy is small, but the voltage takes a long time to return to a constant state.
On the other hand, in a special case, the ripple feedback from CFB to FB terminal is too large when the load is heavy and the output is unstable. When this happens, using the CFB without connection can sometimes make the operation stable. Although the necessary load current and transient response characteristics must eventually be considered, it is best to start by exploring the standard CFB.
The above content is from Tris Semiconductor Co., LTD. In the series...
The above content is from Tris Semiconductor Co., LTD. In the series...
