Switching power supplies obtain energy storage from output capacitors
On the voltage charge relationship diagram, the capacitor is in a diagonal line, and the energy storage in the capacitor is the area contained below that line. Although the output capacitance of Power MOSFET is nonlinear and varies according to the drain source voltage, the energy storage in the output capacitance is still the area under the nonlinear capacitance line. Therefore, if we can find a straight line that gives the same area as the variable output capacitance curve shown in Figure 1, then the slope of the line is exactly the equivalent output capacitance that produces the same energy storage.
For some old-fashioned planar technology MOSFETs, designers may use curve fitting to find the equivalent output capacitance, which is based on the output capacitance values in the data table at the typically specified 25V drain source voltage.
(3) Therefore, energy storage can be obtained by a simple integration formula.
(4) Finally, the effective output capacitance is
(5) The measured values of the output capacitance and the fitting curve obtained from formula (3) are displayed. Compared to the old-fashioned technology MOSFET in Figure 2 (a), its performance is good. However, for MOSFETs that use new technologies such as super junction technology and have more nonlinear output capacitors, simple exponential curve fitting is sometimes not good enough. Figure 2 (b) shows the measured output capacitance values of the new technology MOSFET and the fitting curve obtained using formula (3). For the equivalent output capacitance value, the gap between the two in the high voltage region can lead to a huge difference, as the voltage is multiplied by the capacitance in the integration formula. The estimation in Figure 2 (b) will result in a much larger equivalent capacitance, which may mislead the initial design of the converter.
Output capacitance estimation, (a) old MOSFETs, (b) new MOSFETs
If the output capacitance value changes based on the leakage source voltage, the energy storage in the output capacitance can be calculated using formula (4). Although the capacitance curve is displayed in the data sheet, it is not easy to read the capacitance value from the chart * *. Therefore, according to the drain source voltage, the energy stored in the output capacitor is given by the chart in the * new Power MOSFET data table. By using the curve shown in Figure 3 and formula (5), the equivalent output capacitance at the desired DC bus voltage can be obtained
