Ensuring a package’s isolation characteristics are sufficient for environmentally harsh conditions.
Electrical and electronic applications must be designed to ensure human safety, as well as systems reliability from high-voltage conditions. Thus, the power semiconductors used must also fulfill these safety and distance standards set by organizations such as Underwriter Laboratories (UL) or the International Electrotechnical Commission (IEC).
Even though the TO247 package has been widely used in power electronics, its isolation characteristics are sometimes not sufficient for certain environmentally harsh conditions. Take the example of an air conditioning (AC) unit. In this system, the power conversion circuit is installed in the outdoor unit and its ambient environment is likely to be very vulnerable to contamination and condensation. When the power conversion circuit is exposed to such environmentally harsh conditions, the electric isolation characteristics of the power semiconductor devices will be highly deteriorated and may eventually result in system failure.
Figure 1 shows such an example in the outdoor unit of an air conditioning system. In fact, this device failure occurs due to the tracking discharge that progresses gradually over a long period of time after the system is installed. Prescreening at the manufacturing stage is impossible.
Fig. 1: Device failure in an outdoor AC unit.
The standard TO247-3 package has 2.6 mm and 2.8 mm minimum pin to pin clearance and creepage distances respectively because of flat package body shape and wider pin stoppers. By comparison, the new TO247-3-HCC package has 3.4 mm and 4.8 mm of enhanced pin to pin clearance and creepage distances respectively. This is achieved through the groove share of the package body and the thinner pin stoppers. Because of this, the permissible isolation voltage is increased, thus enhancing the reliability of the system as well. What’s more, the new TO247-3-HCC package is fully compatible with the PCB design for existing the TO247-3 package due to the similar dimensions of the body and pin pitches between both packages.
Fig. 2: Clearance comparison between the TO-247-3 and the new TO-247-3-HCC.
As shown in figure 3, the DC or RMS voltage acceptance of the TO247-3-HCC package drastically improved from 400 V to 676 V, where pollution degree II, and the material group II (400 ≤ CTI < 600) are applied. Meanwhile, when it comes to the condition with pollution degree II, inhomogeneous field, and the altitude below 2000 m, the standard TO247-3 PKG with clearance of 2.6mm, and the new TO247-3-HCC PKG with clearance of 3.4mm can ensure the transient voltage of 3.6 kV and 4.4 kV respectively.
In other words, both are sufficient for functional isolation of most high voltage devices. However, applications exposed to environmentally harsh conditions such as welding power sources and photovoltaic solar inverters should be designed for pollution degree 3 or higher. For these cases, it is still required to cover the pins by coating, potting, or molding, for example, in order to fulfill insulation standards.
Fig. 3: Coordination results of acceptable voltage for the standard TO247-3 and new TO247-3-HCC packages in accordance with [1].
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Great article, thank you!
Could you comment on creepage requirements between TO-247 device fastening means (clamp, screw through the device, etc.) and the heatsink? I am trying to figure out if and how I can use metallic screw to mount the device onto the heatsink and still meet creepage requirements.