AN1703 - APPLICATION NOTE

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required to calculate the junction to PCB thermal resistance (°C/W).

The procedure has two main steps. First is the characterization of the body diode. Second is the

temperature rise measurements and calculation of the thermal resistance.

As an inherent part of the MOSFET structure, the body diode makes the ideal sensor to measure the

For this the diode's temperature coefficient is needed to get an accurate representation of the junction

temperature. The forward voltage is measured with a low level current flowing through it to ensure there

is no self heating, which would make the junction temperature measurement inaccurate.

The MOSFET being characterized is soldered on to the thermal test PCB and has the gate shorted to the

source, to insure the MOSFET cannot turn on. The copper mounting pad reaches the remote connection

points through fine traces that do not contribute significant thermal dissipation but serve the purpose of

electrical connections.

connected with two power supplies.

Figure 1: Schematic of temperature measurements

The first power supply forces current through the body diode to heat the junction with a fixed power level,

when the switch is in the "heating" position. The second power supply provides the sensing current for

position.

the devices and the relevant arising junction temperature:

This computation is made from pulsed operation to steady state in order to achieve the whole thermal

magnitude and time duration, so it is an input data.

HEATING

CURRENT

Vf

HEATING

SENSING

Pmos

D.U.T.

Sense

Current

HEATING

CURRENT

Vf

HEATING

SENSING

Pmos

D.U.T.

Sense

Current

D

C

J

D

PCB

THJ

P

T

T

P

T

R

−

=

∆

=

−