Wednesday, June 18, 2008

900V Transistors

Real high voltage transistors can be difficult to find. What I mean by "real" is that the transistor has characteristics similar to low voltage transistors, but just have higher operating voltage capability. For example, some HV transistors have very low beta, and very high leakage current, thereby making them unattractive in many applications, particularly low current applications. Thus, the application spectrum is narrowed somewhat.

Infineon's CoolMOS™ 900V power MOSFETs are the industry’s first 900 V superjunction MOSFETs specifically intended for high-efficiency applications. For example, quasi-resonant flyback designs for LCD TV power supplies can benefit from a higher flyback voltage, which provides a longer primary duty cycle with reduced peak current, true zero-voltage switching and significantly lower voltage stress on the secondary side. Other applications include crowbar circuits and high voltage switching.

Devices with ON resistance of 0.34ohms are available now, and soon to be released are parts with 0.12 ohm ON resistance.

As with many high voltage components, it is important to review the data sheets, since there are often limitations not found with their low voltage brethren. For example, in Infineon IPW90 family, drain-source breakdown voltage rating varies significantly with temperature. While it is 900V at 25C, it goes down to 850V at -20C. Also, drain-source leakage current is just 100nA, but it is specified with no D-S voltage applied. If you look at the breakdown spec, it specifies a maximum of 250 microamps at 900V, however.

Friday, June 13, 2008

Isolate, Convert, Boost, and Rescale Process Signals

Even when you are not working with high voltage, it is useful to be able to isolate signals by kV.

The model TEVL-HVDC accepts a DC voltage input and provides an optically isolated DC voltage or current output that is linearly related to the input. This module is unique because it is field rangeable for voltage inputs from 100 VDC to 2000 VDC. Typical applications include signal isolation, signal conversion, signal attenuation or a combination of the three.The optical isolation between input and output makes it useful for ground loop elimination, common mode signal rejection or noise pickup reduction. The power supply is isolated, resulting in full 3-way (input, output, power) isolation.

The input and output range settings are configured by the factory to customer requirements, but they can be reconfigured in the field via internal switches. Common range settings are on the module label. A user specified range is available that can be factory configured to meet your specific requirements

  • High Voltage Input Ranges from 100 VDC to 2000 VDC

  • High Input Impedance

  • 2000 V Full Input/Output/Power Isolation

For more information see

Tuesday, June 3, 2008

High Voltage Power Supply Selection Issues - Part 2

A couple more high voltage issues to consider:

Some power supplies require a minimum load to regulate correctly. Particularly at low output current and low output voltage, this is a concern. Thus, after the minimum desired operating conditions are determined, it is worthwhile checking with the manufacturer to determine if the selected power supply can operate at those conditions.

It is also worthwhile investigating power supply operation when the output current reaches the limit. Some power supplies exhibit foldback, while others will limit the current to the max value. When the limiting is precise, then it is termed current regulation. Some power supplies can operate in current limit mode indefinitely, while others cannot, and they can overheat or fail.