SC2545TSTRT datasheet(14/24 Pages) SEMTECH

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SC2545

POWER MANAGEMENT

TTTTTop Switc

op Switc

op Switch

The RMS value of the top switch current is calculated as

The conduction losses are then

tc = IQ1,rms

ds(on).

ds(on) varies with temperature and gate-source voltage.

Curves showing R

ds(on) variations can be found in

manufacturers’ data sheet. From the Si4860 datasheet,

ds(on) is less than 8m

Ω when Vgs is greater than 10V.

However R

ds(on) increases by 50% as the junction

temperature increases from 25oC to 110oC.

The switching losses can be estimated using the simple

formula

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 9. MOSFET switching characteristics

)

)(

(

In Figure 9, Q

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

ds,

and Q

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 :

gt = Rgi+Rge+Rg.

gsp is the Miller plateau voltage shown in Figure 9.

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

tg =

Vgs

Miller plateau

Ids

Vds

Vgs th

Qgs1 Qgs2

Qgd

Gate charge

Volts

Vgs

Miller plateau

Ids

Vds

Vgs th

Qgs1 Qgs2

Qgd

Gate charge

Volts

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 propor-

tional to the input voltage. Switching loss however

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.

Applications Information (Cont.)

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