High Voltage

Simple High Voltage Power Supply Circuit

2008-07-09T11:43:00.000-04:00

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

2008-07-01T11:16:00.004-04:00

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

2008-06-18T13:08:00.003-04:00

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

2008-06-13T17:58:00.001-04:00

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

High Voltage Power Supply Selection Issues - Part 2

2008-06-03T18:18:00.002-04:00

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.

High Voltage Power Supply Selection Issues

2008-05-29T14:37:00.004-04:00

When comparing high voltage power supplies from different manufacturers, here are a few issues to look at:

Is ripple specified in peak-to-peak or rms? For many applications, it is the peak-to-peak that matters. If the ripple is specified as rms, it is not as simple as multiplying the rms by roughly 3 to get peak-to-peak. Due to switching regulator spikes or high voltage diode issues, the ratio between peak and rms may be much higher.

Are the monitors derived from separate sources as the feedback control signals? If so, then these independent signals can be used to determine when the power supply is not working correctly. On the contrary, when the same signal is used for both voltage monitor and voltage feedback, it is possible that the power supply output is much lower than the set point, and the monitor will not know.

What is the output connector? As high voltage connectors are not standardized, the output connector might even be the deciding factor. Issues include shielded vs. unshielded, the bend radius of the wire, size and cost. Lead time of the mating connector and cable can be substantial, and in some cases it can take longer to get a cable than the power supply.

Plasma Sterilization

2008-05-02T16:35:00.003-04:00

In still another use for high voltage, researchers have developed a plasma that operates at room temperature. The plasma is generated between a high voltage probe and the surrounding room air. One use envisioned is to sterilize medical instruments. A second use is to kill bacteria, specifically dental.

Generally, plasmas operate at high temperature, and thus are impractical to use on humans. However, these develpments appear promising.

The research paper is in Applied Physics Letters.

A Few Low Cost High Voltage Power Supply Circuits

2008-04-24T11:18:00.000-04:00

Here's a circuit that generates 100V from 25V. Output current is 25mA. You can find it at http://www.edn.com/index.asp?layout=article&articleid=CA46343

Another high voltage power supply circuit is at http://www.instructables.com/id/Cheap-Semi-High-Voltage-Power-Supply/ It is actually just a high voltage multiplier circuit, and is open loop. In other words, you put in an ac signal, and you get the high voltage output.

This is a low cost 50kV power supply. Like the one above, it is open loop, however in this case it includes a way to vary the output voltage. It uses parts that are available at low cost. See http://www.instructables.com/id/Build-a-Variable-High-Voltage-Power-Supply/

Alternating Current – Minimum Approach Distance

2008-04-19T16:19:00.004-04:00

The following table is from Occupational Safety & Health Standards Board of the California Cal/OSHA program website. http://www.dir.ca.gov/oshsb/rubberglove.html Please see this website for more information.

It is related to conductors and equipment energized over 7,500 volts

Alternating Current – Minimum Approach Distance

Normal Voltage Range (Phase to Phase)Kilovolt	Approach Distance Phase to Ground Exposure
Above 0.6 to 15	2 ft 1 in.
Above 15 to 36	2 ft. 4 in.
Above 36 to 46	2 ft 7 in.
Above 46 to 72.5	3 ft 0 in.
Above 72.5 to 121	3 ft 4 in.
Above 121 to 145	3 ft 7 in.
Above 145 to 169	4 ft 0 in.
Above 169 to 242	5 ft. 3 in.
Above 242 to 362	8 ft 6 in.
Above 362 to 552	11 ft. 3in.
Above 552 to 765	15 ft 0 in.

Human Body Resistance and Capacitance

2008-04-09T14:19:00.000-04:00

Here is an interesting piece written by Paul E. Schoen of P S Technology, Inc.

There is some more information [regarding the resistance of a human body] at http://van.physics.uiuc.edu/qa/listing.php?id=6793 , where it states that the external human body resistance is about 1k to 100k Ohms, and the internal resistance is 300 to 1000 ohms. Only a thin layer of dry skin separates the internal resistance from an external object.

The human body capacitance to a far ground is 100-200 pF, which is really a minimum value. This correlates to an impedance of about 13 megohms at 60 Hz, which corresponds to a minimum of 9 uA at 120 VAC to ground. This is enough to be sensed and used for capacitively operated light dimmers.

Here is a way to measure your body capacitance: http://web.mit.edu/Edgerton/www/Capacitance.html

The inside of your body can be considered a conductor, and thus if you place your hand flat on a metal plate, you will form a capacitor with an area of perhaps 15 square inches, with a thin (maybe 0.005”) insulating layer of dry skin, which will form a capacitor much higher in value than the 200 pF stated above. According to a formula in http://www.sayedsaad.com/fundmental/11_Capacitance.htm , this would be C = 0.2249 * k * A / d = 1350 pF, (assuming k for skin is 2, about like dry paper). This will be an impedance of about 2 megohms , and current of 60 uA. This is still below the normal threshold of sensation, and still far below the usual safe current levels of 1 to 5 mA.

The actual thickness of the epidermis (per http://dermatology.about.com/cs/skinanatomy/a/anatomy.htm ) varies from 0.05 mm (0.002”) for eyelids to 1.5 mm (0.06”) for palms and soles, but the actual outer layer of the epidermis that is a good insulator is composed of flat, dead cells, which is much thinner. So the capacitance could be much higher than the quick estimate above.

Probably the main reason for electrical current to reach levels high enough for electrocution to occur (6 to 200 mA for 3 seconds, according to http://www.codecheck.com/ecution.htm ), is when skin becomes sweaty or otherwise loses its dry protective layer, which quickly exposes the underlying 1000 ohms or less, which will conduct 120 mA at 120 VAC.

There are safe ways to measure the body’s resistance and capacitance using realistic higher voltages, skin conditions, and contact surfaces, but I’m not going to suggest anyone try it. Suffice it to say that ohmmeter readings are misleading, and any carelessness around any kind of voltage source can be dangerous.

For very high voltages, there are standard minimum distances that must be maintained between a worker and an energized line: http://www.dir.ca.gov/oshsb/rubberglove.html . I found this on a search for rubber glove testing.

The field intensity near high voltage lines is so great that it might be fatal to touch them even if you were suspended in free air. You may notice that birds can sit on lower voltage transmission lines which are 5kV to 50 kV or so, but not on 200kV+ lines.

Extremely low cost high voltage power supplies

2008-04-02T08:59:00.006-04:00

If you are looking for an extremely low cost high voltage power supply, a good place to look is consumer products that incorporate high voltage. For example there are electronic fly swatters that claim to produce 4kV! Of course the current is very low, and performance in terms of ripple, regulation and stability is not all that good. The electronic fly swatter may not even put out nearly the voltage claimed. However, if these performance related issues are not important in your application, or just need to generate a spark, you're in luck. Besides, you can find them for under $10, and sometimes even under $5. Other products to consider as a source of low cost high voltage are air purifiers, mosquito zappers, LCD display backlights, piezoelectric spark generators, and vehicle electronic ignitions.

Welcome Package for Opto Isolators

2008-03-27T16:38:00.001-04:00

New high voltage opto isolators are always a welcome addition to a designers. Optek has a new configuration that you may find handy.

They have axial fiber optic isolators offered in isolation distances from 18mm to 80mm. Providing power electronics engineers with a means to isolate high voltage input/output circuits over varying distances, TT electronics OPTEK Technology has developed a series of axial optical isolators using plastic optical fiber. Designated the OPI1270 Series axial fiber optic isolators, the devices are constructed using a visible red LED emitter and a phototransistor detector in separate opaque molded plastic modules, linked by a shrouded optical cable that shields the optical signal from dust and other contaminations.

High Voltage Comments

2008-03-21T16:37:00.002-04:00

You may noticed a recent change in our high voltage blog. You now have an opportunity to leave a comment on any of the posts. Feel free to indicate what you like/dislike, what you agree/disagree with, or you may submit ideas for future posts. Questions are always welcomed. Of course we expect all comments to be in keeping with the professional nature of the blog.

We look forward to your comments!

High Voltage in the News

2008-03-14T11:43:00.002-04:00

Parts were recently stolen from towers carrying 115kV wires. The thieves took hundreds of steel struts, nuts and bolts from three 115kV high voltage towers in Thailand. The 130 foot high towers leaned much more than normal during a windy evening, causing an investigation. The parts were probably taken in order to sell them as scrap metal.

Imagine what would have happened if the towers fell down!

High Voltage Chip Resistors

2008-03-03T11:30:00.000-05:00

It's often tough to find good resistors with high enough voltage and resistance ratings. Stackpole has expanded its HVC product line in both areas. Their surface mount resistors are now available up to 10Gohms, and with voltage ratings up to 25kV.

Stackpole claims that their manufacturing process yields resistors with lower noise and better stability.

Other specifications include:

Temperature coefficient ratings are as low as 25 ppm per degree C.

Tolerance as low as 0.5%

Voltage ratings from 5kV to 25kV for chip sizes 0603 to 2512, respectively.

Find Corona

2008-02-11T08:24:00.000-05:00

If you suspect that corona is an issue in your system, here's one simple way to detect corona. The CoronaFinder, manufactured by Syntronics, is a real-time viewing system that looks for the ultraviolet glow of corona. If you look at their website at http://www.syntronics.net/CoronaFinder.htm, you'll see a couple of dramatic photos. One shows wires using normal light, and the second shows the same view as seen through the CoronaFinder.

Comparison: Ceramic and Film High Voltage Capacitors

2008-02-06T09:45:00.000-05:00

Both ceramic and film capacitors are widely used in high voltage applications. Ceramic high voltage capacitors have the following advantages, as compared to film:

Lower equivalent series inductance (ESL) and equivalent series resistance (ESR)
Smaller size for the same value (capacitance & voltage)
Better temperature coefficient available, as well as a choice of temperature coefficient characteristics

Disadvantages of ceramic capacitors include:

Higher voltage coefficient, resulting in substantially lower capacitance as the applied voltage is increased. (This characteristic is often overlooked, and its effects are often more severe than you would think.)
Lower maximum available capacitance rating
Lower maximum available voltage rating
Susceptibility to piezoelectric effects
Typically smaller lead-to-lead distance

High Voltage Op Amp

2008-01-19T11:27:00.000-05:00

National Semiconductor has just introduced two high voltage op amps, types LME49811 and LME49830. Operating with supply voltage up to 200V rail-to-rail, this supports a typical output voltage up to 192V, peak-to-peak. With a properly designed output stage, an output power up to 500W can be generated. While the op amp is aimed at audio applications, it can also be used for high voltage power supply and high voltage amplifier applications. These op amps might also serve as post-regulators or active ripple reducer circuits on a high voltage power supply.

High Voltage Regulator

2008-01-02T18:19:00.000-05:00

For a high voltage shunt regulator, Victoreen Corotron was often used. However, they have been discontinued for some time. The good news is that there is a replacement: AnaLog Services has two families of what they call a Codatron. These shunt regulators are available in voltages up to 3750 volts.

10kV High Voltage Transistor

2007-12-15T11:54:00.000-05:00

Panasonic reported that they have developed a gallium nitride power transistor with an extremely high breakdown voltage of more than 10kV.

This GaN transistor was described at IEEE International Electron Devices Meeting (IEDM 2007) in Washington DC.

The data taken on this high voltage transistor included a breakdown voltage of 10.4kV and on-state resistance of 186Ωcm².

Panasonic indicated that they used GaN film on a highly resistive sapphire substrate with a new structure.

Great Circuit for Highly Isolated Floating Power Supply

2007-11-27T14:22:00.001-05:00

Every so often you have an application where you need to provide power to a circuit that is floating at high voltage. In general, isolated power supplies can be an expensive, complex endeavor. Sometimes undesirable effects, such as substantial ripple injected into the powered circuit can cause problems. In EDN Design Ideas, November 22, 2007 issue, there is a circuit for a floating power supply that can withstand a separation of 10cm between the ground section and the floating section. This flexibility can come in very handy. I might consider a changes with the bridge rectifier diodes, and I might consider some protective components if the output section is floating at a very high voltage.

For details on this excellent idea, please see EDN Design Ideas: Wireless “battery” energizes low-power devices

Isolated signal measurement

2007-11-20T10:24:00.000-05:00

It's often useful to measure voltages that are floating at a high voltage. One technique is to use analog to digital converters, feed the digital signals into fiber optic links, then digital to analog converters (or just use the digital signals) at ground level. This can also be done with voltage-to-frequency converters and frequency-to-voltage converters. Other techniques include capacitive pick up, transformers, etc. Linear optoisolators can be used in some applications. There are a few manufacturers that can make them, or you can assemble your own. There is an application note from Agilent that can provide additional information.

For less voltage isolation, other techniques can be used, such as isolation amplifiers.

There are a few commercially available devices provide isolated voltage measurement.

High Voltage Resistor Types

2007-11-08T08:21:00.000-05:00

Once the resistor voltage rating goes above a few hundred volts, there are very few manufacturers to choose from. The main types of high voltage resistors are:

Surge high voltage resistors, which are used in series with circuit elements that might experience arcs or intentional transients. Examples include resistors in series with an arc lamp or the resistor in an RC filter since they will need to carry current due to arcs.
Precision resistors (used for high voltage dividers). Typically these are thin film resistors.
General purpose high voltage resistors, generally thick film. Compared to precision high voltage resistors, these are not as stable, have worse voltage coefficients, worse temperature coefficients, and worse tolerances. However, they are lower cost and can be rated at higher power. Applications include high voltage dividers where precision is not important, load resistors and bleeder resistors.

More than with low voltage resistors, it is important to select a high quality manufacturer to obtain a part with good performance and reliability. Also, it is important in many applications to specify non-inductive types.

Built-in Protection for Voltage Regulator

2007-11-07T19:14:00.000-05:00

Whenever working with high voltage systems, there is a concern that components may fail during an arc or other transient event. Generally much care must be taken when selecting components, adding protective parts such as tranzorbs, and in defining the layout. Linear Technology has introduced an "overvoltage protection regulator" which might be useful. Since regulators seem to be vulnerable to the effects of high voltage transients, this is a welcome addition.

Maximum input voltage is 100V, and maximum output voltage is 80V. While this can come in handy, it is also good to know that for lower voltage operation, there is a wide safety margin.

The regulator has built-in protection for -30V or reverse input voltage, and it also has an inrush current limiter.

In applications where electronic systems must cope with high voltage surges of short duration, the LT4356 provides solid front-end protection for valuable, safety critical downstream components.

Water Forms Floating Bridge at High Voltage

2007-11-05T20:47:00.000-05:00

When exposed to a high-voltage electric field, water in two beakers climbs out of the beakers and crosses empty space to meet, forming the water bridge. The liquid bridge, hovering in space, appears to the human eye to defy gravity.

Upon investigating the phenomenon, the scientists found that water was being transported from one beaker to another, usually from the anode beaker to the cathode beaker. The cylindrical water bridge, with a diameter of 1-3 mm, could remain intact when the beakers were pulled apart at a distance of up to 25 mm.

Can you think of a practical use? If so, please let us know!

These excerpts are from an article at http://www.physorg.com/news110191847.html which contains additional details on this phenomenon.