Film-Based Insulators Target Power Supplies

Sep 1, 2005 12:00 PM
By David Morrison, Editor, Power Electronics Technology

For the PDF version of this article, click here.

From Laird Technologies Thermal Products, the T-gard 3000 is a thermally conductive, low-cost, film-based electronic insulator. The insulator is engineered to solve heat problems experienced in switch-mode power supply (SMPS) designs, particularly those used in PCs and telecom equipment. Furthermore, the product is designed to resist cut-through in screw-mounting applications, while providing a more-consistent breakdown voltage versus other insulator types. According to the vendor, T-gard 3000 offers SMPS manufacturers greater dielectric strength and better consistency than fiberglass insulators, and is also less expensive.

T-gard 3000 insulators can be installed between noninsulated power component packages — such as TO-220s, TO-3s, TO-264s — and their heatsinks (see the figure). Although film-based insulators are not new, the high thermal conductivity exhibited by T-gard 3000 is said to be unique. The insulators exhibit a thermal resistance of 0.45°C-in²/W at 50-psi pressure and 0.35°C-in²/W at 400-psi pressure. The latter performance translates to a total thermal resistance of 2.2°C/W on a TO-220 device.

The insulators also feature a nonblocking surface such that pieces of the film won't stick to one another. Consequently, T-gard 3000 insulators do not require powder to reduce surface tack. The product also offers a dielectric breakdown voltage of greater than 6000 Vac, a volume resistivity of greater than 10¹² Ω-cm, a dielectric constant of 3.3 at 1 MHz, and an electrical RTI temperature rating of 150°C per UL 756B.

Whereas fiberglass-based insulators rely on the thickness of their material to achieve their breakdown voltage ratings, the film-based insulators achieve breakdown voltage rating from their film core. Another way to explain the difference between fiberglass- and film-based insulators is how they are manufactured. The fiberglass insulator takes a thermally conductive material and coats it to provide electrical insulation. The compound coated onto fiberglass has to deliver both thermal and dielectric properties. Any variation in raw materials and compounding can affect the dielectric breakdown voltages.

With the Laird Technologies' product, an electrically insulating, high-temperature film is conformally coated with a ceramic-filled silicone to provide its thermal conductivity. With this approach, the insulator's dielectric property is guaranteed by the film, so the compound coating only has to deliver thermal performance. In addition, the soft conformal coating is said to provide an excellent mating surface for low-pressure clip mounting applications.

Thinner and more flexible than fiberglass insulators, the film-based insulator, also promises superior cut-through resistance. This latter characteristic is significant since cut-through damage leads to electrical shorts.

T-gard 3000 is priced between 2 cents and 6 cents per square inch depending on volume of purchases. However, in the future, the company plans to introduce complementary product lines with even lower cost — perhaps 1 cent to 4 cents per square inch. More information about T-gard 3000 and the high-durability film-based insulator is available at www.lairdtech.com/pages/products/T-gard-3000.asp.

Total Thermal Resistance
Pressure (psi)	Units	10	50	100	200	400
Modified ASTM D5470	°C-in2/W	0.60	0.45	0.40	0.35	0.35
TO-220	°C/W	3.5	2.7	2.5	2.2	2.2

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