SC2542 datasheet(19/27 Pages) SEMTECH

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SC2542

POWER MANAGEMENT

Top Switch

The RMS value of the top switch current is calculated as:

The conduction losses are then:

)

(

rms

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

Curves showing Rds(on) variations can be found in manu-

facturers’ data sheet. From the Si4860 datasheet, Rds

(on) is less than 8m

Ω when Vgs is greater than 10V.

However Rds(on) increases by 50% as the junction tem-

perature increases from 25°C to 110°C.

The switching losses can be estimated using the simple

formula:

()(1

)

O IN

I V f

where tr is the rise time and tf is the fall time of the

switching process. Different manufactures have differ-

ent definitions and test conditions for tr and tf. To clarify

these, we sketch the typical MOSFET switching charac-

teristics under clamped inductive mode in Figure 17

Figure 17. MOSFET switching characteristics

In Figure 17, Qgs1 is the gate charge needed to bring the

gate-to-source voltage Vgs to the threshold voltage

Vgs_th. Qgs2 is the additional gate charge required for

the switch current to reach its full-scale value Ids,.and

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

capacitance when Vds is falling.

Switching losses occur during the time interval [t1, t3].

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

()

gdgt

gsp

−

where Rgt is the total resistance from the driver supply

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

internal impedance Rgi, external resistance Rge and the

gate resistance Rg within the MOSFET:

Vgsp is the Miller plateau voltage shown in Figure17.

Similarly an approximate expression for tf is:

()

gsp

Only a portion of the total losses Pg = QgVccfs is dissi-

pated in the MOSFET package. Here Qg is the total gate

charge specified in the datasheet. The power dissipated

within the MOSFET package is:

The total bottom switch losses are then:

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.

Bottom Switch

The RMS current in bottom switch is given by:

)(1

)

Qrms

=−

The conduction losses are then:

Applications Information (Cont.)

)

Qrm s

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