AN4501 datasheet(34/48 Pages) STMICROELECTRONICS

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Selection of the switching MOSFET

AN4501

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DocID026394 Rev 1

The Rdson is temperature dependant:

RON,M1 = RON,M1@T25°C x (1 + α (Tj - 25))

Where:

•

RON,M1@T25°C is the RDSON at 25°C

•

Tj is the junction temperature of M1

•α is the temperature coefficient, which is in general in the range of 6·10-3 K-1

In return the junction temperature depends on the power losses. Therefore, an iterative

calculation is necessary for an accurate estimation of the conduction loss. Figure 18

displays the flowchart for the estimation of the MOSFET power losses and RDSON.

8.3.1

Switching losses of M1

Some care must be taken for the calculation of the switching losses. Often, the turn-on and

the turn-off times which are specified in datasheets of the MOSFET are applicable for a

resistive load under very specific conditions (current and drain-source voltage).They are not

valid for the switching of inductive loads.

Switching losses in the M1 MOSFET occur when the drain-source voltage is high, while the

M1 current is not negligible. We can split the switching losses between the turn-on and the

turn-off transitions: PM1,SWON and PM1,SWOFF.

Switch-on phase

During t1, the gate-source voltage of M1 (vM1,GS) ramps up to the gate-source threshold

voltage, VM1,GS,TH. During this phase, there is no change in the drain-source voltage of M1

(VM1,DS) and M1 is not yet conducting. The inductor’s current (IL,VALLEY) still flows through

the diode and there is no switching loss in M1 (see Figure 16).

M 1 CO ND

ON M1

OU T

MAX

–

-------------------------





 2

MAX

∼

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