SC2441ITSTRT datasheet(18/35 Pages) SEMTECH

18 / 35 page

 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2441

The losses in power MOSFET’s consist of

a) conduction loss due to the channel resistance R

ds(on),

b) switching loss due to the switch rise time t

r and fall time

f and

c) the gate loss due to the gate resistance R

Top Switch:

The RMS value of the top switch current is

)

(

rms

Its conduction loss is 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 Si7882DP datasheet,

ds(on) is less than 4.5mΩ when Vgs is greater than 5V.

However R

ds(on) increases by nearly 40% as the junction

temperature increases from 25°C to 125°C.

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 11.

Figure 11. MOSFET switching characteristics

In Figure 11,

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

voltage V

gs to the threshold 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. 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

tg =

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 loss is 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.

Bottom Switch:

The RMS current in bottom switch can be shown to be

)

)(

(

rms

−

Applications Information (Cont.)

charge

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