Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – Insulating material
Reexamination Certificate
1999-01-15
2001-04-03
Lee, Eddie C. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
Insulating material
C257S341000, C257S712000, C257S720000
Reexamination Certificate
active
06211567
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to IGBTs which have high thermal capacitance and more specifically relates to IGBTs which are particularly useful for motor controller application. A very thick heatsink is in contact with the junction or top side of an IGBT die surface to act as a thermal capacitance for the die.
BACKGROUND OF THE INVENTION
Insulated gate bipolar transistors (IGBTs) are commonly used in the inverter circuit section of motor drive circuits. Inverter circuits of this kind are shown, for example, in U.S. Pat. No. 5,825,641.
When the motor of a motor drive circuit turns very slowly, the power output of the inverter is at very low frequency, for example, 3 Hz. The relationship between IGBT transient thermal impedance and its peak junction temperature rise is a function of the output frequency of the motor drive inverter. The same output current is generally required at all output frequencies. The rated output current of an IGBT die for a particular circuit is, therefore, limited to that obtainable at the lowest output motor current frequency, typically 3 Hz, since the junction temperature is at its highest at this frequency. More specifically, while the IGBT device will switch at a frequency between 1 KHz to 20 KHz, the meter current which it carries may have a frequency as low as 3 Hz at slow spaced rotation.
If the transient thermal impedance can be reduced, the ratio of peak to average junction temperature will be reduced, and increased output current can be obtained at 3 Hz for a given IGBT die.
More specifically, the ratio between peak and average junction temperature at 3 Hz can be approximated as:
T
JPK
T
JAV
=
1
+
2.2
R
th
75
ms
R
th
DC
,
where R
th75ms
is the transient thermal resistance for a 75 ms current pulse and R
thDC
is the DC thermal resistance of the IGBT. T
JPK
is the peak junction temperature and T
JAV
is the average junction temperature. For IGBT devices presently made by the International Rectifier Corporation of El Segundo, Calif., R
th75ms
/R
thDC
is about 0.8, which gives T
JPK
/T
JAV
=2.8. It would be desirable to reduce the transient thermal resistance since that would allow the use of a smaller die for a given motor controller application.
BRIEF DESCRIPTION OF THE INVENTION
Transient thermal resistance is reduced, in accordance with the invention, by placing a thermal mass on the top of the IGBT die within its package. The system of die and mass and package then has a thermal inertia, or thermal capacitance. This is preferably achieved by soldering or otherwise thermally connecting a slug of copper, or other material, to the top of the die. The die may have a solderable top metal as shown, for example, in U.S. Pat. No. 5,047,833, entitled Solderable Front Metal Contact for MOS Device, in the name of Gould.
It can be estimated that a 0.2 inch×0.2 inch area, 0.4 inch high copper slug soldered to the top of a size 5 IGBT die (sold by the International Rectifier Corporation of El Segundo, Calif.) would give a reduction in T
JPK
/T
JAV
from the present value of 2.76 to 1.22. This would yield a corresponding increase in inverter output current capability of about 80% for the same die size.
Less dramatic, but still significant increases in output current would be achieved with a less thick slug. For example, a 0.1 inch slug would give a reduction in T
JPK
/T
JAV
of 1.88/2.76=0.68, and a corresponding increase in output current of about 20%.
REFERENCES:
patent: 4965710 (1990-10-01), Pelly et al.
patent: 5786230 (1998-07-01), Anderson et al.
patent: 5966291 (1999-10-01), Baumel et al.
patent: 6031723 (2000-02-01), Wieloch
Kwok K. Ng, Complete Guide to Semiconductor Devices 1995, AT&T Bell Laboratories NJ, 356.
Cruz Lourdes
International Rectifier Corp.
Lee Eddie C.
Ostrolenk Faber Gerb & Soffen, LLP
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