ST AN2385 APPLICATION NOTE

AN2385

Application note

Power dissipation and its linear derating factor, silicon Limited Drain Current and pulsed drain current in MOSFETs

Introduction

Datasheets of the modern power MOSFET devices, either of low voltage or of high voltage, show in the section entitled "Absolute Maximum Rating" the values of some important parameters that regard the SOA (safe operating area). As it is well kwown in literature, SOA is the area that includes all the ID-VDS operating points where the device works in safety conditions.

These important parameters are studied in this technical article. In particular, attention will be focused on the Power Dissipation and its Linear Derating Factor, Silicon Limited Drain Current and Pulsed Drain Current. This technical article will explain what these parameters are and how they can be calculated. It will recall some basic and simple technical concepts and can be a useful tool for customers to understand and facilitate reading of a power MOSFET datasheet.

June 2006

Rev 1

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Contents	AN2385

Contents

1	Determination of the SOA limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 5
2	Silicon limited drain current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
3	Determination of the power dissipation and its derating factor . . . . . .	9
4	Pulsed drain current (IDM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
5	Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12
6	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13

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AN2385	List of figures

List of figures

Figure 1. Typical FBSOA of a Power MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 2. RDSON limit for MOSFET's FBSOA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 3. Typical RBSOA of a Power MOSFET. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 4. Silicon limited drain current vs case temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 5. Power dissipation vs T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 6. IDM determination considering the output characteristic . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 7. Junction to case maximum effective transient thermal impedance for TO-220 package.. . 10 Figure 8. Determination of the maximum rectangular pulse duration . . . . . . . . . . . . . . . . . . . . . . . . 11

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List of tables	AN2385

List of tables

Table 1. Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

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AN2385	Determination of the SOA limits

1 Determination of the SOA limits

SOA is the acronym of Safe Operating Area. It includes all the ID-VDS operating points inside where the device works in safety conditions. There are two kinds of SOA. The first one is named Forward Biased Safe Operating Area (FBSOA), while the second one is named Reverse Biased Safe Operating Area (RBSOA). FBSOA is the SOA during the device on state, while RBSOA is the SOA when the MOSFET switches off. Supposing that the ID and VDS axis are in log scale, a typical FBSOA can be depicted as in Figure 1.

Figure 1. Typical FBSOA of a Power MOSFET

IDmax is the maximum drain current limit of the MOSFET. It is usually fixed by the wires that connect the drain and source pads to the package pins respectively. BVdss is the maximum

drain-source voltage that the device can sustain (breakdown voltage). Pmax(t) is the maximum power that the device can dissipate. It depends on the junction temperature and

power pulse interval time and on the package used. In fact, if the junction temperature overcomes typically 150'C or 175'C, as defined in the automotive devices, the MOSFET could fail or, however, the device works out of the guaranteed temperature spec.

Furthermore, increasing the case temperature, the Pmax(t) value decreases due to a lower energy, which is necessary to bring the junction temperature to the maximum guaranteed

value. Pmax is also a function of the power pulse interval time because, fixing the power pulse, the energy dissipated in the MOSFET, as well as the junction temperature rises, increasing the power interval time. In particular, when the power pulse interval time increases, Pmax(t) decreases and the SOA area decreases too.

Another limit for FBSOA is established by RDSON of the device. In fact, when the device is in on state without loads connected to the component, all the voltage feed is applied on the drain-source terminals and, thus, the maximum drain current that can flow in the transistor is fixed by VDD and RDSON (Figure 2.).

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