Collapse effe5ct of field-effect transistors in power applications 5

But even if this abnormal peak voltage did not trigger the component to collapse, we still need to evaluate that the junction temperature of this field-effect transistor must be lower than the rated maximum temperature to ensure its reliability and reliability. Under steady-state conditions, the temperature of the joint surface can be derived from the following equation:

Tj=PDxRθjc+TC----------(1)

Where Tj: Joint surface temperature TC: Shell temperature PD: Overall power loss R θ jc: Thermal resistance conducted from the joint surface to the shell under steady-state conditions

In most applications, switch mode power supply lines use field-effect transistor as switches. Therefore, when a series of pulse switching field-effect transistors are used, their power consumption and changes in junction temperature depend on the peak power and pulse width. The instantaneous thermal resistance at this time can be derived based on the change in time as follows:

Zθjc(t)=r(t)xRθjc----------(2)

Where r (t) represents a parameter of heat dissipation capability. When the pulse width is very short, r (t) is also relatively small. But when the pulse width is very long, r (t) will approach 1, and the instantaneous thermal resistance will also be as high as in steady state. Figure 6 shows the instantaneous thermal resistance parameter curve provided by Fairchild Semiconductor. From this figure, the temperature of the joint surface under instantaneous conditions can be derived as follows:transistor

Tj=PDxZθjc(t)+TC----------(3)

For example, when a 2KW power pulse is applied to FQA11N90. html "target=" -blank "title=" FQA11N90 ">FQA11N90 with a pulse time of 1uS, we can calculate the temperature rise generated by this pulse power on the mating surface based on the curve in Figure 6

T=PDxZθjc(1uS)=2000x1.49x10-3»3℃