SC2453_05 datasheet(15/22 Pages) SEMTECH

15 / 22 page

 2005 Semtech Corp.

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SC2453

POWER MANAGEMENT

)(

(

where t

r is the rise time and tf is the fall time of the

switching process. Different manufactures have different

definitions and test conditions for t

and t

. To clarify

these, we sketch the typical MOSFET switching

characteristics under clamped inductive mode in Figure

Figure 6. MOSFET switching characteristics

Where,

gs1 is the gate charge needed to bring the gate-to-source

voltage V

gs to the threshold voltage Vgs_th,

gs2 is the additional gate charge required for the switch

current to reach its full-scale value I

and

gd is the charge needed to charge gate-to-drain (Miller)

capacitance when V

ds is falling.

Switching losses occur during the time interval [t

1, t3].

Defining t

r = t3-t1 and tr can be approximated as:

gsp

)

(

−

where R

gt is the total resistance from the driver supply

rail to the gate of the MOSFET. It includes the gate driver

internal impedance R

gi, external resistance Rge and the

gate resistance R

g within the MOSFET i.e.:

gt = Rgi+Rge+Rg

gsp is the Miller plateau voltage shown in Figure 11.

Similarly an approximate expression for t

f is:

gsp

)

(

Only a portion of the total losses P

g = QgVccfs is dissipated

in the MOSFET package. Here Q

g is the total gate charge

specified in the datasheet. The power dissipated within

the MOSFET package is:

The total power loss of the top switch is then:

t = Ptc+Pts+Ptg

If the input supply of the power converter varies

over a wide range, then it will be necessary to weigh

the relative importance of conduction and switching

losses. This is because conduction losses are inversely

proportional to the input voltage. Switching loss how-

ever increases with the input voltage. The total power

loss of MOSFET should be calculated and compared for

high-line and low-line cases. The worst case is then

used for thermal design.

Bottom Switch:

The RMS current in bottom switch can be shown to be:

)

)(

(

rms

−

The conduction losses are then:

bc=IQ2,rms

ds(on)

where R

ds(on) is the channel resistance of bottom MOSFET.

If the input voltage to output voltage ratio is high (e.g.

in=12V, Vo=1.5V), the duty ratio D will be small. Since

the bottom switch conducts with duty ratio (1-D), the

corresponding conduction losses can be quite high.

Due to non-overlapping conduction between the top and

the bottom MOSFET’s, the internal body diode or the

external Schottky diode across the drain and source

terminals always conducts prior to the turn on of the

bottom MOSFET. The bottom MOSFET switches on with

only a diode voltage between its drain and source

terminals. The switching loss is:

)(

(

is negligible due to near zero-voltage switching.

The gate losses are estimated as :

The total bottom switch losses are then:

b=Pbc+Pbs+Pbg

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