High Voltage, Low Noise DC/DC Converter Circuit Designs

Have you seen the Linear Technology application note on high voltage, low noise DC/DC converters? Ap notes don't get any better than this one. It includes circuits for high voltage power supplies rated from 200V to 1000V. It also addresses noise issues, and the effects of various voltage feedback techniques on transient response.

The ap note describes resonant Royer topologies (which produces power by utilizing sinusoidal waveforms instead of switched waveforms) and also flyback converters. It also describes methods of measuring output noise.

If you are planning to design a high voltage power supply for photomultipliers (PMT's) or other applications requiring low noise, low power in the range of hundreds of volts, you might want to peruse the ap note. It is Linear Technology Application Note 118.

Isolated RS-232 Transceiver

When there are high voltage circuits, such as in a high voltage power supply, and you need to control them via an RS-232 interface, it is usually a good idea to isolate the computer from the analog electronics. Here is information about a new isolated RS-232 transceiver that may be of interest.

Fully Isolated RS-232 Transceiver Readies For Harsh Environments
Claiming title as the industry’s first fully isolated single-package surface mount RS-232 transceiver with integrated dc/dc converter to supply isolated power, the ADM3251E transceiver integrates a line driver, line receiver, oscillator, rectifier, regulator, voltage doubler, voltage inverter, and transformers into a single chip that isolates both the data and power lines on chip. The transceiver incorporates the company’s iCoupler and isoPower isolation technologies to enable a 2.5 kV isolation rating and it complies with industry-standard isolation regulations including UL1577 and DIN VDE 0884-10. Additionally, the ADM3251E comes in a lead-free, 13 mm × 10.56 mm 20-lead WSOIC surface-mount package. Price is $2.99 each/1,000. Analog Devices, Inc., Norwood, MA. (800) 426-2564.

New High Voltage Laboratory in India

This week, the M P Varshney High Voltage Laboratory was dedicated at the Zakir Husain College of Engineering and Technology, Aligarh Muslim University.

Professor (Dr.) M P Varshney, an alumnus of this University during 1942-46 has graciously made a large donation for the extension of the High Voltage Laboratory, named after him.

It's been said, "The electrical department of this college boasts of the best high voltage laboratory in the entire north India.

This is but one of several excellent high voltage laboratories in India.

Simple High Voltage Power Supply Circuit

There's a Design Idea in EDN magazine that uses just one IC in a simple, high voltage power supply. See the June 26, 2008 issue. It's a fixed output supply with limited adjustment capability. If you need a small, inexpensive power supply, this is a good possibility.


The output voltage can be increased by adding more secondaries, and stacking them up, as in the design as shown.


The regulation is not great, but if that is all you need, then that is ok. However, if you prefer better regulation, and if you don't need a floating output, then the circuit would work better if the voltage feedback was taken from the output (with a voltage divider) rather than from a separate secondary winding.


You can find the article at http://www.edn.com/article/CA6571003.html

Beware of Conterfeit High Voltage Transistors

Apparently, counterfeit high voltage transistors are around. Here's what Geoff found:

Our design had worked for 20 years in the field, then suddenly last year we had failures. My gut reaction was counterfeiting, even though I had never experienced it before, and this was without even laying a CRO probe on the circuit. A month ago, our RF engineer and I looked into it, wondering what was wrong with the design. After a week of bench tests and simulations we went back to basics. We measured the VCEO and found the problem! When I etched the metal from some samples, we found a tiny die on the faulty parts. This occurred with ST BU505's and some ON Semiconductor MJE8502's, though these are obsolete.

It is likely this won’t affect many, but we have good evidence of counterfeit high voltage transistors from some suppliers. The types so far affected are BU505 and MJE8502. These are 1500V VCES and 700V VCEO. We noted failures in our products and traced to a very low VCEO which I checked with a current limited voltage source. We found ST brand BU505’s failing at slightly more than 500V VCEO and the same for ON Semiconductor branded MJE8502. I etched a couple of BU505’s with nitric acid to remove all metal. One was a good tested one and one the other a failed one. There was a completely different die size between the two. The good one had a 3.3mm die and the bad one a 1.8mm die. The good one had the markings etched and the bad one had markings printed. These devices are often used for switchmode supplies and horizontal drive in CRT’s, where circuit designs may not ever allow the base to be open circuit, i.e., driven from a low impedance source, so I guess that many users many not see the failures.

I presented the data to St Microelectronics and they confirmed the likelihood of counterfeit devices. Now we will have to do incoming VCEO checks of all batches until the situation improves. We try to get these normally from mainstream suppliers, but sometimes supply problems prevent this. It looks like we will have to get smarter with purchasing though. This is the first time I have personally come across this in 20+ years of engineering.

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.

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 http://www.ferret.com.au/common/clickthrough2.asp?intType=2&strTarget=http%3A%2F%2Fwww%2Eveederline%2Ecom%2Eau&intOrigin=3&strID=239085