What are MOSFETs? – MOSFET Switching Characteristics and Temperature Characteristics

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 T_d(on）, rise time t_r, turn-off delay time t_d(off）, and fall time t_f 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 R_L 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 V_DD, V_GS, I_D, R_L, and R_G 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 V_GS until 10% of the rise of V_DS
  >Rise time: Time from 10% to 90% of the rise of V_DS

  >Turn-off delay time: Time from 90% of the fall of V_GS until 90% of the fall of V_DS
  >Fall time: Time from 90% to 10% of the fall of V_DS

Here, the expressions “rise/fall” of V_DS 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 V_GS 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