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across the load element is recommended, see

Figure 3.

The energy not consumed in switching losses must be

absorbed by the over-voltage protection element. Most

protective devices are designed to withstand certain

peak power, in the case of a Transient Voltage

Suppressor (TVS); or maximum avalanche energy, in

the case of a MOSFET. Understanding the switching

losses and load dynamics is absolutely essential.

One simple way to reduce the amount of stored

inductive energy is to increase the energy dissipated

in the switch. This can be accomplished by adding a

larger capacitor in parallel with the gate-drain

connection of the MOSFET, however care must be

taken so that the rise time and peak current do not

exceed the Safe Operating Area (SOA) rating of the

transistor.

The consequence of increasing the gate-drain

effective capacitance is reduced dV/dt tolerance.

When used in a circuit with an inductive load,

precautions must be taken to prevent damage to the

circuit from inductively generated voltage spikes. The

circuit shown in Figure 3 includes such protection

across the inductive load.

5.1 Other Protection Techniques

Switching loads with higher inductance characteristics

requires consideration of other circuit protection

techniques, device ratings, or protector types. Of

paramount importance is that the designer know the

characteristics of the load being switched.

Figure 3. CPC1580 Over-Voltage Protection for Inductive Loads

6. Application Switching Losses

During the transition intervals, the application and load

components change energy states and, in the

process, incur switching losses. The switching losses

are manifested as heat in the application circuit and

must be addressed by the designer to ensure that no

one component exceeds its power rating. The

designer must understand the details of the load

behavior in order to adequately size and protect the

application circuit. There are three general cases to

observe: (1) purely resistive loads,

(2) inductive/resistive loads, and (3) loads with

significant capacitance. Inductors and capacitors are

energy storage elements that require special

consideration for switching.

During the switching periods, energy is conserved.

Inductors turning off transfer their stored energy to

MOSFET switching losses, to the capacitance of the

load and application circuit, and to the protector.

During the turn-on interval, the inductor energy is zero,

and so the capacitive energy in the load and parasitic

elements of the switching application must be

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