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The Forward Biased Safe Operating Area curves define the maximum simultaneous drain±to±source voltage and drain current that a transistor can handle safely when it is forward biased. Curves are based upon maximum peak junction temperature and a case temperature (TC) of 25°C. Peak repetitive pulsed power limits are determined by using the thermal response data in conjunction with the procedures discussed in AN569, ªTransient Thermal Resistance ± General Data and Its Use.º

Switching between the off±state and the on±state may traverse any load line provided neither rated peak current (IDM) nor rated voltage (VDSS) is exceeded, and that the transition time (tr, tf) does not exceed 10 μs. In addition the total power averaged over a complete switching cycle must not exceed

(TJ(MAX) ± TC)/(RθJC).

A power MOSFET designated E±FET can be safely used in switching circuits with unclamped inductive loads. For reli-

able operation, the stored energy from circuit inductance dissipated in the transistor while in avalanche must be less than the rated limit and must be adjusted for operating conditions differing from those specified. Although industry practice is to rate in terms of energy, avalanche energy capability is not a constant. The energy rating decreases non±linearly with an increase of peak current in avalanche and peak junction temperature.

Although many E±FETs can withstand the stress of drain± to±source avalanche at currents up to rated pulsed current (IDM), the energy rating is specified at rated continuous current (ID), in accordance with industry custom. The energy rating must be derated for temperature as shown in the accompanying graph (Figure 13). Maximum energy at currents below rated continuous ID can safely be assumed to equal the values indicated.

I D , DRAIN CURRENT (AMPS)

VDS, DRAIN±TO±SOURCE VOLTAGE (VOLTS)

Figure 12. Maximum Rated Forward Biased

Safe Operating Area

350

ID = 6 A

300

250

200

150

100

50

0

TJ, STARTING JUNCTION TEMPERATURE (°C)

Figure 13. Maximum Avalanche Energy versus Starting Junction Temperature

Surface mount board layout is a critical portion of the total design. The footprint for the semiconductor packages must be the correct size to ensure proper solder connection interface

between the board and the package. With the correct pad geometry, the packages will self±align when subjected to a solder reflow process.

The power dissipation of the SO±8 is a function of the input pad size. This can vary from the minimum pad size for soldering to the pad size given for maximum power dissipation. Power dissipation for a surface mount device is

determined by TJ(max), the maximum rated junction temperature of the die, RθJA, the thermal resistance from the

device junction to ambient; and the operating temperature, TA. Using the values provided on the data sheet for the SO±8 package, PD can be calculated as follows:

The values for the equation are found in the maximum ratings table on the data sheet. Substituting these values into

the equation for an ambient temperature TA of 25°C, one can calculate the power dissipation of the device which in this case is 2.0 Watts.

PD = 150°C ± 25°C = 2.0 Watts

62.5°C/W

The 62.5°C/W for the SO±8 package assumes the recommended footprint on a glass epoxy printed circuit board to achieve a power dissipation of 2.0 Watts using the footprint shown. Another alternative would be to use a ceramic substrate or an aluminum core board such as Thermal Clad . Using board material such as Thermal Clad, the power dissipation can be doubled using the same footprint.

The melting temperature of solder is higher than the rated temperature of the device. When the entire device is heated to a high temperature, failure to complete soldering within a short time could result in device failure. Therefore, the following items should always be observed in order to minimize the thermal stress to which the devices are subjected.

•Always preheat the device.

•The delta temperature between the preheat and soldering should be 100°C or less.*

•When preheating and soldering, the temperature of the leads and the case must not exceed the maximum temperature ratings as shown on the data sheet. When

using infrared heating with the reflow soldering method, the difference shall be a maximum of 10°C.

•The soldering temperature and time shall not exceed 260°C for more than 10 seconds.

•When shifting from preheating to soldering, the maximum temperature gradient shall be 5°C or less.

•After soldering has been completed, the device should be allowed to cool naturally for at least three minutes. Gradual cooling should be used as the use of forced cooling will increase the temperature gradient and result in latent failure due to mechanical stress.

•Mechanical stress or shock should not be applied during cooling.

* Soldering a device without preheating can cause excessive thermal shock and stress which can result in damage to the device.

For any given circuit board, there will be a group of control settings that will give the desired heat pattern. The operator must set temperatures for several heating zones and a figure for belt speed. Taken together, these control settings make up a heating ªprofileº for that particular circuit board. On machines controlled by a computer, the computer remembers these profiles from one operating session to the next. Figure 16 shows a typical heating profile for use when soldering a surface mount device to a printed circuit board. This profile will vary among soldering systems, but it is a good starting point. Factors that can affect the profile include the type of soldering system in use, density and types of components on the board, type of solder used, and the type of board or substrate material being used. This profile shows temperature versus time. The

line on the graph shows the actual temperature that might be experienced on the surface of a test board at or near a central solder joint. The two profiles are based on a high density and a low density board. The Vitronics SMD310 convection/infrared reflow soldering system was used to generate this profile. The type of solder used was 62/36/2 Tin Lead Silver with a melting point between 177±189°C. When this type of furnace is used for solder reflow work, the circuit boards and solder joints tend to heat first. The components on the board are then heated by conduction. The circuit board, because it has a large surface area, absorbs the thermal energy more efficiently, then distributes this energy to the components. Because of this effect, the main body of a component may be up to 30 degrees cooler than the adjacent solder joints.

CASE 751±05

SO±8

ISSUE P

NOTES:

1.DIMENSIONS A AND B ARE DATUMS AND T IS A DATUM SURFACE.

2.DIMENSIONING AND TOLERANCING PER ANSI Y14.5M, 1982.

3.DIMENSIONS ARE IN MILLIMETER.

4.DIMENSION A AND B DO NOT INCLUDE MOLD PROTRUSION.

5.MAXIMUM MOLD PROTRUSION 0.15 PER SIDE.

6.DIMENSION D DOES NOT INCLUDE MOLD PROTRUSION. ALLOWABLE DAMBAR PROTRUSION SHALL BE 0.127 TOTAL IN EXCESS OF THE D DIMENSION AT MAXIMUM MATERIAL CONDITION.

STYLE 11:

PIN 1. SOURCE 1

2.GATE 1

3.SOURCE 2

4.GATE 2

5.DRAIN 2

6.DRAIN 2

7.DRAIN 1

8.DRAIN 1