Vishay Siliconix

AN822

Document Number 71681

03-Mar-06

www.vishay.com

3

THERMAL PERFORMANCE

Introduction

A basic measure of a device’s thermal performance is

the junction-to-case thermal resistance, R

θjc, or the

junction to- foot thermal resistance, R

θjf. This parameter

is measured for the device mounted to an infinite heat

sink and is therefore a characterization of the device

only, in other words, independent of the properties of the

object to which the device is mounted. Table 1 shows a

comparison of the PowerPAK 1212-8, PowerPAK SO-8,

standard TSSOP-8 and SO-8 equivalent steady state

performance.

By minimizing the junction-to-foot thermal resistance, the

MOSFET die temperature is very close to the tempera-

ture of the PC board. Consider four devices mounted on

a PC board with a board temperature of 45 °C (Figure 4).

Suppose each device is dissipating 2 W. Using the junc-

tion-to-foot thermal resistance characteristics of the

PowerPAK 1212-8 and the other SMT packages, die

temperatures are determined to be 49.8 °C for the Pow-

erPAK 1212-8, 85 °C for the standard SO-8, 149 °C for

standard TSSOP-8, and 125 °C for TSOP-6. This is a

4.8 °C rise above the board temperature for the Power-

PAK 1212-8, and over 40 °C for other SMT packages. A

as 20 %.

Spreading Copper

Designers add additional copper, spreading copper, to

the drain pad to aid in conducting heat from a device. It

is helpful to have some information about the thermal

performance for a given area of spreading copper.

Figure 5 and Figure 6 show the thermal resistance of a

PowerPAK 1212-8 single and dual devices mounted on

a 2-in. x 2-in., four-layer FR-4 PC boards. The two inter-

nal layers and the backside layer are solid copper. The

internal layers were chosen as solid copper to model the

large power and ground planes common in many appli-

cations. The top layer was cut back to a smaller area and

at each step junction-to-ambient thermal resistance

measurements were taken. The results indicate that an

area above 0.2 to 0.3 square inches of spreading copper

gives no additional thermal performance improvement.

A subsequent experiment was run where the copper on

the back-side was reduced, first to 50 % in stripes to

mimic circuit traces, and then totally removed. No signif-

icant effect was observed.

TABLE 1: EQIVALENT STEADY STATE PERFORMANCE

Package

SO-8

TSSOP-8

TSOP-8

PPAK 1212

PPAK SO-8

Configuration

Single

Dual

Single

Dual

Single

Dual

Single

Dual

Single

Dual

20

40

52

83

40

90

2.4

5.5

1.8

5.5

Figure 4. Temperature of Devices on a PC Board

2.4 °C/W

49.8 °C

PowerPAK 1212

20 °C/W

85 °C

Standard SO-8

PC Board at 45 °C

52 °C/W

149 °C

Standard TSSOP-8

40 °C/W

125 °C

TSOP-6