2017.03.09 Si Power Device
In the previous session, we explained MOSFET parasitic capacitances. This time, we will discuss the switching characteristics of MOSFETs.
MOSFET Switching Characteristics
In power conversion, a MOSFET is essentially used as a switch. As MOSFET switching characteristics, the turn-on delay time Td(on）, rise time tr, turn-off delay time td(off）, and fall time tf are generally indicated. The following data is excerpted from the datasheet for the Nch 600V 4A MOSFET F6004KNX, which features a low turn-on resistance and fast switching. The names and symbols of these parameters may differ somewhat among manufacturers. For example, the turn-on/off delay time may be called the on/off delay time, and the rise/fall time may be the rising/falling time, and so on.
These parameters related to switching are greatly affected by the signal source impedance and drain load resistance RL of the measurement circuit. Hence, reference measurement conditions are stipulated and a measurement circuit is suggested. In general, as in the example above, the conditions for VDD, VGS, ID, RL, and RG are indicated on the datasheet. The suggested measurement circuit adopts the stipulated conditions.
In addition, there are time-related parameters as well; these parameters may stipulate the time from one event to another.
>Turn-on delay time: Time from 10% of the rise of VGS until 10% of the rise of VDS
>Rise time: Time from 10% to 90% of the rise of VDS
>Turn-off delay time: Time from 90% of the fall of VGS until 90% of the fall of VDS
>Fall time: Time from 90% to 10% of the fall of VDS
Here, the expressions "rise/fall" of VDS may seem reversed when thinking of a waveform, but the output is inverted, and so these expressions are used. For example, the turn-on time may be interpreted as "the time from when VGS has risen to 10%, until the MOSFET is 10% turned on".
Temperature Characteristic of the Switching Characteristic
These switching times tend to increase slightly as the temperature rises, but since a temperature increase of 100°C results in a switching time increase of about 10%, the temperature dependence can be thought of as almost nil. Actual measurement examples are shown below.
Next, an explanation of ID-VGS characteristics is scheduled.
・MOSFET switching characteristics that are generally provided include the turn-on delay time, rise time, turn-off delay time, and fall time.
・Switching characteristics are greatly affected by the measurement conditions and measurement circuit, and so the suggested conditions should be confirmed.
・Switching characteristics are affected hardly at all by changes in temperature.