SC2544 datasheet(14/24 Pages) SEMTECH

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 2005 Semtech Corp.

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SC2544

POWER MANAGEMENT

TTTTTop Switc

op Switc

op Switch

The RMS value of the top switch current is calculated

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, R

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

. To clarify these, we sketch the typical MOSFET

switching characteristics under clamped inductive

mode in Figure 9.

Figure 9. MOSFET switching characteristics

)

(

rms

)(

(

In Figure 9, Q

gs1 is the gate charge needed to bring

the gate-to-source voltage V

gs to the threshold voltage

gs_th .

gs2 is the additional gate charge required for the

switch current to reach its full-scale value I

ds,

and Q

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

capacitance when V

ds is falling.

Switching losses occur during the time interval [t

3]. Defining tr = 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 =

Vg s

M iller p lateau

Id s

Vd s

Vg s th

Qg s1 Qg s2

Qg d

G ate ch arg e

V o lts

Vg s

M iller p lateau

Id s

Vd s

Vg s th

Qg s1 Qg s2

Qg d

G ate ch arg e

V o lts

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 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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