# How to calculate power loss for a diode

voltage measurement

For example, the DSB20I15PA Schottky diode datasheet mentions a threshold voltage of 0.23V and a slope resistance of 7.2mΩ.

I need to calculate the power loss at different voltages, as the battery drains. e.g. 14V down to 10V.

Is it as simple as subtracting 0.23V from your source voltage, or is it more complicated than that? Do you need to factor in the slope resistance too? Does current play a factor? How about diode temperature?

I'm using a 12V battery and a multimeter to measure the drop (by measuring pin to pin on the diode). In my test circuit, the drop appears to be 0.325V when there is a load, but only 0.168V when there is no load. The load is a DC motor. The two parameters given are a linear approximation of a portion of the V-I curve of the Schottky diode at a certain junction temperature. Vf0 is a theoretical intercept, not something you can measure directly, whereas rF is the average slope of the curve (in the example curve, between the points of about 65A and 210A, so you need to be able to produce those currents to measure it).

Vf = Vf0 + If * rF

The power loss is the product of the instantaneous forward current and voltage.

Here is information on a different device, illustrating the approximation: The maximum is going to be higher than the typical, as shown in the curve.

The maximum power loss from forward current will be higher at lower junction temperatures since the forward voltage will be higher, but usually that's not a concern.