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Explore internally-insulated packages for power converters

Posted: 10 Apr 2015 ?? ?Print Version ?Bookmark and Share

Keywords:TO-220? DO-220? insulation? heat-sink? Power converter?

Switched-mode power supplies are power converters that convert the mains AC power into a DC output in order to feed a load, such as a personal computer or a server. They are generally made of a high-voltage side (grid-side) and low-voltage side (load-side).

Common power-packages
Among the most popular packages for electronic power components, such as diodes used in switched-mode power supplies, are the TO-220 and DO-220 packages. Their low thermal resistance ensures a decent evacuation of the heat coming from the die out of the package. They are specifically designed to be mounted on a heat-sink to improve the thermal performance even further.

However, the drawback of those packages is that they are not isolated, meaning that the die is directly connected to the backside metal-frame of the package (figure 1). In the case of a diode, for example, it means the cathode (backside of the die) is accessible on the metal-frame.

Figure 1: Internal structure of a TO-220 package. The die is directly connected to the metal-frame that is usually directly connected to an external heat-sink.

For safety reasons, heat-sinks used in power supplies usually need to be connected to the ground to avoid anybody manipulating the board to be exposed to a high potential.

In the case of a Power-Factor Corrector (PFC) circuit, the cathode of the boost diode is connected to the DC-output of the PFC (usually set to 400 V). If the diode is housed in a DO-220 package and the use of a heat-sink is necessary, the package and the heat-sink must be isolated from one another.

Common isolation solution
The most common solution used to isolate a package from the heat-sink are implemented by inserting a isolation-foil in between both and using an isolating ring for the screw (figure 2).

Figure 2: Assembly of a DO-220 package on a heat-sink. The use of an isolating-foil is mandatory to isolate the package and the heat-sink.

Those foils have good dielectric properties, but they require some good dexterity to assemble as they can easily be cracked or torn, not guaranteeing a good isolation. They can also slip off the heat-sink or tend to rotate around the screw axis when the package is mounted.

Lots of power converter manufacturers complain about the loss of time and reliability on their assembly lines linked to those issues. It is estimated that the additional time required to mount a mica-foil is in the range of 10 to 20 seconds per package. Additionally, their intrinsic thermal-resistance has a non-negligible impact on the thermal performance of the system.

Another traditional solution consists of using power components housed in the fully-moulded TO-220FP full-pack. However, this package features a degraded thermal resistance that limits the dissipation and thus, the amount of current it can handle.

The solution: internally-insulated packages
In order to help manufacturers solve this problem and increase their UPH, some semiconductor vendors have developed internally-insulated packages, such as the insulated DO-220I or TO-220I.

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