SC2441AEVB datasheet(30/37 Pages) SEMTECH

30 / 37 page

 2006 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2441A

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 t

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, R

ds(on) is less than 4.5mOhm 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. To clarify these, we sketch the typical

MOSFET switching characteristics under clamped

inductive mode in Figure 21.

Figure 21. MOSFET switching characteristics

Applications Information

If D

1>0.5 and D2 > 0.5, then

(

)

)(

(

Cin

Power MOSFET Selection and Gate Drive

Main considerations in selecting the MOSFET’s are power

dissipation, cost and packaging. Switching losses and

conduction losses of the MOSFET’s are directly related

to the total gate charge (C

g) and channel on-resistance

ds(on)). In order to judge the performance of MOSFET’s,

the product of the total gate charge and on-resistance is

used as a figure of merit (FOM). Transistors with the same

FOM follow the same curve in Figure 20.

FOM:100*10^{-12}

FOM:200*10^{-12}

FOM:500*10^{-12}

On-resistance (mOhm)

Cg 100 Rds

()

Cg 200 Rds

()

Cg 500 Rds

()

Rds

Figure 20. Figure of merit curves.

The closer the curve is to the origin, the lower is the FOM.

This means lower switching loss or lower conduction loss

or both. It is difficult to find MOSFET’s with both low C

and low R

ds(on). Usually a trade-off between Rds(on) and Cg

has to be made.

MOSFET selection also depends on applications. In many

applications, either switching loss or conduction loss

dominates for a particular MOSFET. For synchronous buck

converters with high input to output voltage ratios, the

top MOSFET is hard switched but conducts with very low

duty cycle. The bottom switch conducts at high duty cycle

but switches at near zero voltage. For such applications,

MOSFET’s with low C

g are used for the top switch and

MOSFET’s with low R

ds(on) are used for the bottom switch.

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