CPC1580

R00G

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9

5. 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

dissipated by the MOSFET, in order for the load to

change state.

To calculate the stored inductive energy in Joules:

5.1 Resistive Load Losses: The Ideal Case

For purely resistive loads, the energy dissipated by

changing states occurs primarily in the MOSFET.

The equation describing MOSFET energy dissipation

during rise time, in Joules, is:

The average power of the MOSFET for any load type

in Watts is:

time, in Joules.

5.2 Inductive/Resistive Loads

If the load is resistive and inductive, and the

inductance doesn't saturate, the load current during

in Volts is:

The instantaneous power in the MOSFET will be the

product of the two equations and the energy will be the

integral of the power over time.

5.3 Capacitive Loads

The energy absorbed by the MOSFET for loads that

are more capacitive in nature occurs during the

MOSFET turn-on as opposed to the turn-off. The

energy absorbed by the MOSFET will be a function of

the load, the TVS (or other protector), and the

MOSFET drain capacitance. The MOSFET energy,

data sheet. As mentioned earlier, the MOSFET

switching losses occur at different times, either rising

or falling, so loads with a combination of inductance

and capacitance can also be calculated by the energy

equations described above.

5.4 dV/dt Characteristics

The application circuit shown in Figure 1 dissipates

significant energy caused by large dV/dt events. Fault

voltages across the MOSFET will turn it on for the

same reason the part turns off slowly. For dV/dt events

on-resistance and current/power capability, so higher

load designs are more sensitive.

The CPC1580 provides an internal clamp to protect

the gate of the MOSFET from damage in such an

event. The part can withstand 100mA for short

periods, like dV/dt transients.

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