IRF7807VPBF_15 datasheet(3/8 Pages) IRF

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IRF7807VPbF

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Control FET

Special attention has been given to the power losses

in the switching elements of the circuit - Q1 and Q2.

Power losses in the high side switch Q1, also called

the Control FET, are impacted by the R

ds(on) of the

MOSFET, but these conduction losses are only about

one half of the total losses.

Power losses in the control switch Q1 are given

by;

loss = Pconduction+ Pswitching+ Pdrive+ Poutput

This can be expanded and approximated by;

loss = Irms

2 × R

ds(on )

()

+ I ×

× V

in × f

⎛

⎝

⎜

⎞

⎠

⎟ + I ×

gs 2

× V

in × f

⎛

⎝

⎜

⎞

⎠

⎟

+ Q

g × Vg × f

()

oss

×V

in × f

⎛

⎝

⎞

⎠

This simplified loss equation includes the terms Q

gs2

and Q

oss which are new to Power MOSFET data sheets.

gs2 is a sub element of traditional gate-source

charge that is included in all MOSFET data sheets.

The importance of splitting this gate-source charge

into two sub elements, Q

gs1 and Qgs2, can be seen from

Fig 1.

gs2 indicates the charge that must be supplied by

the gate driver between the time that the threshold

voltage has been reached (t1) and the time the drain

current rises to I

dmax (t2) at which time the drain volt-

age begins to change. Minimizing Q

gs2 is a critical fac-

tor in reducing switching losses in Q1.

oss is the charge that must be supplied to the out-

put capacitance of the MOSFET during every switch-

ing cycle. Figure 2 shows how Q

oss is formed by the

parallel combination of the voltage dependant (non-

linear) capacitance’s C

ds and Cdg when multiplied by

the power supply input buss voltage.

Figure 1: Typical MOSFET switching waveform

Synchronous FET

The power loss equation for Q2 is approximated

by;

loss = Pconduction + Pdrive + Poutput

loss = Irms

2 × R

ds(on)

()

+ Q

g × Vg × f

()

+ Qoss

× V

in × f

⎛

⎝

⎜

⎞

⎠

+ Q

rr × Vin × f

(

)

*dissipated primarily in Q1.

Power MOSFET Selection for DC/DC

Converters

Drain Current

Gate Voltage

Drain Voltage

VGTH

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