According to precedence, MOSFET datasheets show output capacitance at a single measured voltage. While these values were good enough for relative comparison between products in the past, it is misleading to use these values for modern devices. A better representation of product capacitance is needed.
The output capacitance of a MOSFET is voltage dependent; therefore a single point measurement does not accurately represent the capacitance characteristic of the device. Curve fitting can be used to find an output capacitance equation from this single point. Equation 1 below is an example based on capacitance at 25 V.
This formula’s integral could then be used in place of a single value capacitance in applicable equations. As shown in figure 1 and figure 2, equation 1 worked fairly well for planar devices however, more complex structures, like super-junction, are poorly represented leading to excessive error in any calculations.
Rather than creating a different equation to better fit the capacitance characteristic of each new device architecture, effective capacitance measurements can be used. Effective capacitance values represent the capacitance that results in the same charge time or charge energy up to a given voltage. These values take the change of capacitance into account without the need for complex formulas or integration like would be required when using equation 1.
Figure 1. Planar Output Capacitance Calculated per Equation 1
Figure 2. Super-Junction Output Capacitance Calculated per Equation 1
Using Effective Capacitance
Effective capacitance can be used in modeling energy loss and designing resonant topologies. When modeling energy loss for hard switching topologies, the energy stored in the output capacitance is lost as heat every switching cycle. As the switching frequency increases the switching losses approach conduction losses, dramatically affecting efficiency. The relationship of output capacitance to power loss is shown in Equation 2 , . This is only one component of switching loss, but it