Monday, December 30, 2013

VMI Honors 30-year Employees

Every year VMI hosts an employee appreciation luncheon in the month of December.  This year, several people celebrated their 30-year anniversary.  Pictured from left to right, are -
 

Ignacio F., Karen S., Thelma M., Cora P., Linda S., Jeannie B., Kathy L., Nancy C., and VMI's President, Dennis Kemp.

VMI employs over 200 people, and has been in business since 1980.  Since 1990, the technical staff has more than quadrupled.  We've come a long ways in diode manufacturing, and the future is bright.  2013 was a great year, and 2014 looks even brighter!   

Thursday, December 26, 2013

It’s cute as a bug and useful too! The OC025 - 2500V Optocoupler



Manufactured by VMI, the OC025 2500V optocoupler is both small and useful.  

2500V High Voltage Optocoupler from VMI - OC025
OC025 High Voltage Optocoupler

What Makes an Optocoupler?

An optocoupler is made up of a light source, usually one or more LEDs, and at least one photodiode.   
The OC025 uses one photodiode and two LEDs.  The device is encapsulated in an optically clear material, and comes in an amber colored Ultem shell.  It is moisture and solvent resistant. 


The OC025 uses a special, high voltage photodiode as the light-sensitive component.  Designed to be ultra sensitive to light, the photodiode generates leakage current proportional to the light exposure from the LEDs when the photodiode is reversed bias.  


Leakage current from the photodiode is often used as a feedback and control mechanism in instrumentation systems.  Leakage current is also referred to as “reverse current”, and is represented by “Ir”.  “Reverse current” and “leakage current” are often used interchangeably.

High Voltage Optocoupler Operation

The highest levels of leakage current are generated when two conditions are met. 

First, the photodiode must be reversed biased.  Reverse biased, in this case, means the anode voltage is more negative than the cathode voltage, and the device is ‘blocking’.  No forward current is flowing through the photodiode. 


Second, the photodiode junctions must be exposed to light.  The more intense the light, the more leakage current is created.  The greater the range of leakage current, the greater the degree of sensitivity and resolution available to the external control system circuitry.     

Optocoupler Concept of Operation Illustration
High Voltage Optocoupler Concept of Operation
Photodiode leakage current can be adjusted, monitored, and controlled by varying the amount of current through the light-generating LEDs.  Photodiode leakage current can serve as a feedback mechanism for controlling the forward current through the LEDs, which, in turn, determines
the amount of leakage current in the photodiode.
 

Physical Dimensions

Physical dimensions, excluding axial leads, are 0.460” square x 0.325”H (11.7mm x 11.7mm x 8.3mm height).  

Electrical Specifications 

The OC025 is rated at 2500V reverse bias voltage.  In addition,
  •  The DC transfer ratio is 0.76% minimum.
  • Typical T(on) and T(off) are 2us max.
  • The photodiode can dissipate up to 125mW.

For More Info

Contact VMI for more information. 

For the OC025 data sheet or information on other high voltage optocouplers, visit our optocoupler catalog.


***

Thursday, December 19, 2013

From Optodiode to Optocoupler in Three Easy Steps

High Voltage (10kV and 15kV) Optodiodes
10kV OZ100SG and 15kV OZ150SG Optodiode

VMI's 10kV and 15kV optodiodes make it possible to build your own high voltage optocoupler using your choice of LEDs as a light source in three easy steps.
  • Reverse bias the photo-diode
  • Run forward current through one or more LEDs, thus exposing the photo-diode junctions to light
  • Monitor the leakage current, Ir, of the photo-diode.  The level of Ir serves as feedback for the level of forward current through the LED while providing system isolation.  Varying forward current in the LED varies leakage current in the photo-diode.

Applications


Just a few applications include -

  • High Voltage Switches and Remote Control Circuitry
  • HV Linear Regulators
  • Spectroscopy
  • Pockels Cells

Any application that needs high voltage isolation between subsystems, or ground-loop elimination can benefit from the use of optodiodes or optocouplers.

OZ100SG and OZ150SG diodes are 100% tested for Ir, and forward voltage drop, Vf. The overall package is very small - .45x length x .180x x .180x [11.43 mm length x 4.57 mm width x 4.57mm height].

Advantages

Advantages of the OZ100SG and OZ150SG include providing high voltage isolation, and flexibility to your control system in the smallest package available.

Use it as a building block to create your own optocoupler.  

Description of Operation


 The optodiodes represent a breakthrough in opto-diode technology. Capable of operating at up to 10,000V or 15,000V reverse bias (10kV or 15kV Vrwm) the optodiodes are comprised of a minimum of twelve active, stacked, silicon junctions.

The key to operation is light sensitive junctions. Molded in optically clear epoxy, the OZ100SG and OZ150SG are light-sensitive devices. When exposed to light, leakage current is the result. Leakage current is directly proportional to light intensity, so by varying the source's light intensity (such as by varying forward current through multiple LEDs), the leakage current in the photo diode will vary accordingly.  The typical range for leakage current (Ir) is from 1uA to 1nA.

Controlling the exposure to light makes it possible to control leakage current in the photo-diode, thus providing a closed feedback loop. This makes for a useful way to remotely monitor electronically sensitive systems.

Other light sources can be used to generate leakage current in the photo diode.

Factors Affecting Ir in a Photodiode 


Photo-diode Reverse Leakage (Ir) is linear within stated operating parameters.

Leakage current is directly proportional to

  • Level of reverse bias, Vrwm
  • Distance of the light source from the junctions
  • Wavelength of the light source

High Voltage Gain 


When creating an optocoupler using VMI's optodiodes, the High Voltage Gain is defined as
  • Optodiode current divided by LED (light source) forward current x 100%

The optimum light wavelength has been experimentally determined to be
  • Between 890nm and 940nm

Electrical Specifications

 

Electrical specifications for the OZ100SG and OZ150SG include -
  • Vrwm = 10kV (OZ150SG), 15kV (OZ150SG)
  • Io = 50mA
  • Trr = 3000ns

Technical Data Sheets

Dimensions

 

Physical dimensions for both devices are 0.450x length x 0.180x x 0.180xl [11.43 mm length x 4.57 mm width x 4.57mm height]


Pricing 

 

Contact VMI Sales for pricing and delivery.

Monday, December 9, 2013

Not Sure Your High Voltage Diode is Blocking? How to Test It for a Catastrophic Failure.



In manufacturing testing, dedicated high voltage testing equipment is used to check all the important performance characteristics of a diode.  These include Vf, Ir, and Trr.  If you don't have HV test equipment, and suspect your diode is no longer blocking, you can get a ballpark picture of what's going on by monitoring the leakage current when the diode is reversed bias.

Precautions


But first, a few precautions -
  • Consider using a TVS diode to protect your meter
  • Use a current limited HV power supply (approx. 500ua) with current meter
  • Make sure the power is turned off before connecting anything
  • Practice good electrical safety.  Always. 

Okay, now that that's out of the way, the basic process is to apply a reverse voltage to the diode and monitor the leakage current through it.  When a diode loses it's ability to block, leakage current normally increases at a much greater rate as reverse voltage is applied.  The diode is operating in the far (-x, -y) region of the graph below, only the reverse voltage needed to get past the knee of the curve is shifted way to the right, closer to the origin.  Very little reverse voltage generates a large leakage current, relatively speaking. 

Typical Curve Tracer signal for HV Diodes

Testing Steps


The basic testing steps are as follows:

1.  Clean the diode using IPA or a similar solvent (debris can conduct when a reverse voltage is applied and may yield a false-failed reading).

2.  Submerge the diode in clean dielectric fluid such as Fluorinert, or in an inert gas such as SF6.  This is to provide isolation between the leads since isolation through air is about 10kV per inch.  In a diode that is .5", an applied voltage of more than 5kV may cause an arc between leads.  Better safe than sorry.

3.  Using a current limited HV Power Supply (to approx. 500uA) equipped with a current meter,  connect the diode cathode lead to the output of the power supply.  Connect the anode lead to ground. 

Most diodes have a band or dots at one end that indicating the cathode.  If it's unmarked, test it one way, and then flip it around. 

4.  Turn on the power supply and gradually, slowly increase the voltage across the diode while monitoring the leakage current through it.  In a blocking diode, the measured leakage current should be much, much less than 1uA  The nA range is typical.  

If the diode is not blocking, or blocking at a much lower level, the leakage current will be greater than 1uA.  In a multi-junction diode, excess leakage current can increase rapidly around voltage levels greater than 2kV.  (This assumes at least one junction is still operational).  


What's Next

 

If the diode appears to be a dead short, try flipping it around.  If it doesn't block voltage in the other direction, you have a shorted diode.  Throw it away.


That's when you give VMI a call.   
      

Wednesday, December 4, 2013

Design Tips for Getting the Best Performance from Your High Voltage Diodes


VMI's definition of "high voltage" is anything over 1000V.  (Forgive me if I repeat myself). 

The selection process can be intimidating.  There are many diodes to choose from, making it difficult to know where to start.   


Simplify the selection process by keeping the following three things in mind and prevent headaches down the road. 


Keep in Mind the Isolation Between Anode and Cathode

Isolation Between Anode and Cathode Terminations

 

When operating in air, anything over 5kV should probably have added isolation between the anode and cathode leads.  

At low voltages, isolation between the anode and cathode is not usually an issue.  However, if the diode body is .5" (1.27cm) in length, and it's running at 10kV, it is likely to arc in air.  Isolation through air is approx. 10kV/inch (394V/mm), depending on humidity, temperature, and other operating conditions.  At 10kV, the voltage stress on a .5" (1.27cm) long diode is about double what air can support.

Methods to Increase Isolation Voltage Between Terminations

 

There are several ways to increase isolation between the leads.  Over-encapsulation is one method.  If a diode is part of a power supply, many times the entire power supply will be encapsulated.  As long as the encapsulation material adheres to the surface of the diode, it will increase the isolation voltage. 

Conformal coating is a second method.  Parylene conformal coating is common, and is usually done at the pcb or system level.  

Running the part in a vapor, such as SF6, works in some applications.  This requires a sealed environment and is used in specialized applications.  

If air operation is necessary, try connecting several lower voltage diodes in series.  This approach increases the component count and takes up extra space, but it will work in air.  

A second approach to high voltage air operation is to look at a high voltage stick.  A stick uses multi-junction diodes connected in series, which are then encapsulated.  The advantages are greater lead-to-lead isolation and fewer components compared to single junction diodes.   

Design Consideration - Use Hermetically Sealed Diodes in Dielectric Fluid
 

 Running in Oil - Glass or Epoxy?

 

When running in an oil or liquid environment, a glass body diode is recommended. Glass body diodes are hermetically sealed and operate quite well in an isolating fluid such as dielectric oil. 
While epoxy diodes like the K-bodies can handle higher currents, they are not hermetically sealed.  Over time, any fluid may penetrate the epoxy body or wick up through the leads, ultimately causing the diode to fail.

K-bodies will work in a gas, such as SF6, but are not recommended for fluid operation.

Design Consideration - Keeping Your Diodes Cool for Ultimate Performance


How Hot is HOT? -Thermal Considerations 

      Note:  Thermal considerations are more relevant in higher current, high voltage applications but should be given some thought even in lower current apps.


High voltage can mean high reverse power dissipation.  In contrast to low voltage applications, leakage currents can be higher too.  Since dissipated reverse power - in the form of heat - is a function of reverse voltage, reverse current, and reverse recovery time, it makes sense that reverse power losses can no longer be ignored. 
 
Two things happen when a silicon diode heats up.  First, the reverse recovery time (Trr) slows down.  That means the diode sees a longer interval of reverse voltage and reverse current before switching to forward conducting mode.  The product of reverse voltage and reverse current is 'reverse power'.  


As reverse power goes up, so does the temperature of the diode, which means it slows down, which means the dissipation interval gets longer which means the diode heats up which means.....this cycle will continue to escalate until eventually the diode fails due to thermal runaway.  The diode is unable to block.  It will conduct in the forward and reverse directions.    

Secondly, for a given forward current (Io) the forward voltage (Vf) will decrease.  As silicon heats up it becomes less resistant until eventually it will act as a short.  A very hot diode with a lower resistance means it will draw more current.  More current means it heats up more, which lowers Vf which means it draws more current.....As you can see, dissipated power - both forward and reverse - become significant.  

But not to worry! 


What to Do About Hot Diodes 

 

If it turns out that your diodes dissipate high reverse power in your app, there are a couple of things you can do.  First, pick a diode that has a faster recovery time.  The faster Trr limits the amount of dissipated reverse power by reducing the length of time the diode takes to recover and transition to a forward conduction state.

Secondly, de-rate, de-rate, de-rate.  If the diode is only idling as opposed to running full blast, it will stay cooler.  A cooler diode is a happy diode.  


If your app requires 50mA, but the operating temp is 100C, you might consider using a diode rated for 500mA.   

Third,  add heat sinks or heat spreaders.  This can be done by soldering heat sinks to the diode leads, or near the diode body.  If the heat sink is conductive, like copper or aluminum, care must be taken to prevent arcing through the material from the diode leads to the heatsink.

Fourth, forced air, water cooled baseplates, or circulating liquid are excellent techniques for getting the heat out of a diode. 


Lower Power Applications



When selecting a high voltage diode, it's important to keep isolation through air, hermeticity, and thermal dissipation in mind.  Thermal considerations are more relevant to higher power applications, but can pop up in lower voltage apps.     

Selecting the right diode can save you time, money, and effort.  If you're not sure which one to choose, give us a call.  We can help you make the best selection.   

Tuesday, November 26, 2013

Three Surprising Things about VMI's 5kV Diodes

 

 Did you know that....


1)  VMI offers over 45 different 5kV diodes? 


Neither did I!  I got to thinking about it because our 5kV diodes are popular devices.  The question came up because a customer called and wanted a recommendation.  I knew we offered quite a few, but I was surprised to learn there are more than 45 of them.  I know because I counted them.

45+ different 5kV diodes sounds intimidating.  Fortunately there are a couple of pretty quick questions that can help narrow down the search.

Three main diode characteristics include Vrwm, Io, and Trr (reverse voltage, forward current, and reverse recovery time, respectively).  There are more, but these define the device.  Different applications have different requirements.  As is true with many things in life, the answer to, "What's the best 5kV diode you make?", is, "It depends".

Higher operating frequencies require faster Trr (lower Trr).

Sometimes high operating temperature means you want to add a lot of derating, so even though the application might only use 2kV, if the temperature is high or there are a lot of voltage spikes, using a diode with a higher Vrwm rating can be helpful.

  • Current ratings of 5kV diodes range from 40mA up to 2.2A.
  • Trr ranges from 30ns to 3000ns.

And then there are the 5kV diodes that have reverse power capabilities, like the ZR50SG.  It's a 5kV, standard recovery diode that can handle up to 10mJ of reverse power.  The reverse power capability means under arcing conditions or voltage spikes of less than 10mJ, the diode will recover.

High reliability tested and QPL diodes are available too.  The options include axial-leaded glass body diodes tested to different levels of processing.  Military grade diodes use the JANTX, or JANTXV prefix such as JANTXV1N6525.

2)  Termination and Package Styles Abound?


The second surprising thing about VMI's 5kV diodes is the variety of termination and package styles.

Termination styles include surface mount, through-hole, and formed-lead.

Package styles include glass, epoxy, and molded bodies.

There are various combinations too, like a surface mount, molded diode (SMF6533 for example), and a formed-lead glass body diode, like the 1N6517LL.

3)  VMI's Diodes are "Multi-junction"?

There are many surprising things about high voltage diodes, not the least of which is that any diode over approximately 1kV is going to have more than one junction.  Essentially a 5kV has multiple die 'stacked' together in series.  The die come from the same wafer so differences are minimized.  Stacking silicon die is about the only way to achieve high voltage blocking capabilities. 

Get Your List of 5kV Diodes 


It's easy to get your list of VMI's 5kV diodes.  Next time you visit VMI's website, click on the "Diode Search" button in the upper right corner.  When the search page appears, select 5kV in the Vrwm (V) pull-down menu, and press the 'Search' button.

The results page includes a summary of all the 5kV diodes in pdf format, and links to their data sheets.  You can save the results page to your desktop, or print it out.

And if you're not sure what you need, give us a call.  We'll help you make the right selection for your application.
 

Friday, November 22, 2013

High Voltage Turkeys and Holiday Catalogs

Maybe the same thing happens to you.  Since the beginning of November, my mail-box has runneth-over with mail-order catalogs!  Every day two, three, four, or more catalogs are delivered.  

Holiday Catalogs

There are basically two categories of catalog mailings.  Some are from companies I have actually placed orders with before, in which case case they seem to amp up the rate of their delivery.  Sometimes it's as frequent as one every two or three days.  The second category is that of companies I have never ordered from, but are similar to companies I have ordered stuff from.

One year I collected all the covers of all the catalogs I received between November 1st and November 30th just to see how many I'd get.  I put up a big sheet of 36" wide butcher paper and pasted all the catalog covers on it.  When I filled that strip up, I added a second one, and then a third.  By the time my "art piece", entitled "Chri$tma$, was completed, it required three strips of floor-to-ceiling butcher paper and covered an area of 108 sq. feet.  I keep that in mind when the catalogs start showing up in the mail.

It's a major mystery to me how they got my mailing address!  Okay, not really, but I AM surprised at how SOON they got it.  I have not been at my current address very long, and when I moved I made a 'clean break' of it, meaning I discontinued some subscriptions and culled the mail-order catalogs (this was during my return-to-brick-and-mortar purchasing habits phases when it seemed local stores were threatened by the rapid rise in Internet purchases).

Oranges?  

Of course since then things have mellowed out again, but there is something to be said about buying local. For instance, the other day I noticed the oranges I'd purchased were from Australia.  I have nothing against Australia whatsoever, except that I happen to live in the biggest citrus belt in the WORLD!!! "Why", I asked myself, "am I buying oranges from half-way around the world, when it's orange-season right now?!"  The answer is, "Because that is what was available in the grocery store that I happened to be shopping in, and I needed oranges".  Sheesh!

High Voltage Turkeys and High Voltage Diodes

In the U.S., Thanksgiving is less than a week away.  Next Tuesday afternoon VMI will be distributing holiday turkeys to each and every employee just like they have for at least the last 23 years I've been here.  It's their way of making the holidays happier for everyone, including temporary and part-time employees.

And it is greatly appreciated!  Turkey, high voltage turkey, is part of Voltage Multipliers, Inc.'s holiday tradition.

I don't know what exactly high voltage diodes have to do with turkey, except that happy employees make for quality products, and quality products make for happy customers, and happy customers make for happy employees.  

Here's hoping your Thanksgiving holiday is a happy one!  

Monday, November 18, 2013

2kV and 3kV High Voltage, Higher Current Diodes


You may recall the post from November 6, 2013 on the differences between the 5kV, 70ns, 1N6517 axial-leaded and the equivalent 1N6517LL formed lead diode.

Not every application needs a 5kV diode.  Why buy a 5kV diode when a 2kV or 3kV diode will work, especially if it costs more?  Enter the 2kV 1N6513LL and 3kV 1N6515LL diodes.   


Cost Drivers and Advantages


Generally speaking, the higher the reverse voltage rating, the higher the unit cost.  Likewise, as the current rating (Io) goes up, or the reverse recovery time (Trr) goes down, the unit cost goes up.  There are other factors involved, but that generally holds true. 

The same advantages of decreased thermal impedance, higher forward current ratings, and pre-formed leads, holds true for the 1N6513LL and 1N6515LL diodes.  


2kV, 3kV, and 5kV high voltage diodes
High Voltage Formed Lead Diode
Axial-leaded high voltage diode style
High Voltage Axial-leaded Diode

 

Differences in Current Ratings and Thermal Impedance


Formed lead diodes have a higher current rating than the axial-leaded equivalent.  The larger diameter 99.99% silver leads are excellent thermal conductors.  The larger lead mass serves as a built-in heat sink. 

 

Diode Ratings and Comparisons
  

The 1N6513 is a 2kV, 70ns diode, as is the 1N6515LL formed lead diode.  Below is a comparison of the rated current and thermal impedance of the two devices.     

Axial-leaded 1N6513 - 2kV 70ns 

             
Current Rating:  2A @ 55°C
Thermal Impedance
      Lead Length
           @ 0.0":  3°C/W
           @ 0.125”:  6°C/W
           @ 0.250”:  12°C/W


Formed Lead 1N6513LL - 2kV 70ns 


Current Rating:  3.5A @ 55°C
Thermal Impedance: 
      Lead Length
             @ 0.0":  5°C/W
             @ 0.125”:  5°C/W
             @ 0.250”:  5°C/W


The 1N6515 is a 3kV, 70ns diode, as is the 1N6515LL formed lead diode.  Below is a comparison of the rated current and thermal impedance of the two devices.     

Axial-leaded 1N6515 - 3kV 70ns 

          
Current Rating:  1.5A @ 55°C
Thermal Impedance
      Lead Length
           @ 0.0":  3°C/W
           @ 0.125”:  6°C/W
           @ 0.250”:  12°C/W


Formed Lead 1N6515LL - 3kV 70ns 


Current Rating:  2.5A @ 55°C
Thermal Impedance: 
      Lead Length
             @ 0.0":  5°C/W
             @ 0.125”:  5°C/W
             @ 0.250”:  5°C/W

 

Quick Summary 

The same device in a formed lead format will have a higher forward current rating and lower thermal resistance over the axial-leaded device.  The 1N6515LL will have higher current ratings than the 1N6515.

Between devices, the 1N6515 family has higher Vrwm ratings and lower forward current ratings (Io) compared to the 1N6513 family.  this is true for the formed lead diodes too.  The 1N6515LL has a higher Vrwm and lower Io rating than the 1N6513LL.  Sound confusing?  Don't worry - here's a link to the data sheet.


Cost

An added feature of the 1N6513 family is one of the lowest forward voltage drops of any diodes VMI makes.  This is also a cost driver.  Unless you need a 70ns, high current, low Vf device, go with the 1N6515 family device if your application is cost sensitive.

The formed lead devices are typically less costly than the copper tabbed devices (i.e. 1N6515U, or JANTX1N6515US).


More Choices

Other choices are available.  Chose from different reverse recovery times, package styles, and current or voltage ratings.  VMI has one of the most comprehensive lines of high voltage diodes.  You won't want to miss out!


Wednesday, November 13, 2013

VMI’s Power Supply Design Team – Meet the Engineers


VMI’s high-voltage-power-supply design team is stellar.  The three main design engineers are Randy Bethel, Derek Onstott, and Alex Jiang.  Between them, they have over 40 years of design experience.  Bonus – they’ll be at the VMI Sales Seminar in 2014, so plan to come!



HeNe Laser High Voltage Power Supply
High Voltage Power Supply 
VMI excels at solving tough, high voltage problems.  Our most successful ‘success stories”, defined in terms of customer delight and continued production, almost always originates with a custom seeking help for a difficult design problem no other company is willing to tackle.  The reasons are obvious – other companies just didn’t have the staying power or resources to make it through the necessary multiple iterations that custom solutions often require.  Experience has taught us that dedication, focusing on providing the customer with what they want and need, and the patience to deal with moving targets, are all necessary ingredients to a successful outcome.  Design specifications change, or are unknown in the beginning, or even untestable.  It’s not uncommon to design tests and test procedures right alongside the actual high voltage power supply.

A lot goes into the design of a high voltage power supply, much of it ‘behind the scenes’.  In the beginning, the design engineers are highly visible and play a central role.  Nevertheless, they cannot and should not do it all.  They have the unflagging support of the Purchasing, Sales, and Production departments, plus testing technicians, manufacturing engineers, drafters, the machine shops, and plastic shops.

Electrical specifications are usually specific to the customer and their application.  They range in output voltage, output current and average power.  Specialized voltage and current outputs are also possible.    

In addition to electrical specifications, other customizable features include -

·         Custom housings – metal and non-metal

·         Customized encapsulation

·         Single, dual, multiple voltage and current outputs

·         Signal monitoring and control circuits

·         Feedback networks

·         Test points

·         Custom testing hardware & software

·         Specialized testing fixtures for burn-in and temperature cycling

Our expertise includes power supplies up to 125kV and 250W. 

Our designs are found in mass spectrometers, portable & hand-held x-ray applications, commercial paint sprayers, medical defibrillators, CRT displays, powering sensors & detectors, and other industrial and commercial applications.

If you have a tough, high voltage application, give us a call.  We’ll talk.  Moreover, there’s no obligation on your part.

 VMI is ISO9001:2008 certified.  We’ve earned DSCC’s award of Lab Suitability, and we’re an IPC member.

Wednesday, November 6, 2013

High Voltage Diodes - Formed Lead vs. Axial-leaded Terminations



So.  What’s the difference between a formed lead diode and an axial-leaded one?  Well, the most obvious differences are in the leads themselves.  

Axial Lead and Formed Lead

An axial lead is typically smaller in diameter and longer in length.  The leads are straight and ready to be trimmed and formed into a variety of shapes and lengths.  The axial-leaded devices are designed for through-hole connections.  
 

In contrast, a formed lead device is shipped with the leads already bent and trimmed in a surface mount configuration.   
 

Differences in Current Ratings and Thermal Impedance

Each has it’s strong point when it comes to electrical specifications.  The formed lead diodes have a higher current rating, largely due to a lower thermal impedance.   The formed leads are larger in diameter than the axial-leads.  In addition, the leads are 99.99% silver and excellent thermal conductors so the bigger lead serves as a kind of built-in heat sink.
 
For example, the axial-leaded 1N6517 is a 5kV, 70ns diode, as is the formed lead 1N6517LL.  Below is a comparison of the rated current and thermal impedance.



Axial-leaded High Voltage Diode - 1N6517
1N6517 5kV 70ns Axial-leaded   
1N6517 (Axial-lead) 
             
Current Rating:  1A @ 55°C
Thermal Impedance 
      Lead Length
           @ 0.0":  3°C/W
           @ 0.125”:  6°C/W
           @ 0.250”:  12°C/W




Formed Lead High Voltage Diode - 1N6517LL 5kV 70ns
1N6517LL 5kV 70ns Formed Lead

1N6517LL (Formed Lead)


Current Rating:  1.5A @ 55°C
Thermal Impedance: 

      Lead Length
             @ 0.0":  5°C/W
             @ 0.125”:  5°C/W

             @ 0.250”:  5°C/W

 

 

 

 

Pad Layout


The pad layout is different for each device.  They are not drop-in replacements, but they’re close.  Depending on your board layout, the changes could be minor, or a little more than minor.  

Cost

In terms of cost, the formed lead diodes are in the middle of axial-leaded devices and tabbed surface mount devices (1N6517U, for example).  They are considerably less expensive than the tabbed QPL devices such as the JANTXV1N6517US. 

The formed lead diodes (1N6517LL, for example), are a cost effective  alternative to the commerical and military versions of the tabbed devices when low thermal impedance and higher current ratings are needed.

Tuesday, October 29, 2013

Yes, Virginia, CRT power supplies are still made in the USA!

 

CRT Displays 

Back in the day, the only kind of monitor available was a CRT type, be it computer, T.V., cockpit displays, oscilloscopes, etc. They were big, bulky and weighed a ton! And they used a lot of power.

If I left my small, color T.V. on in the morning, I’d come home from school later that day to find the temperature in my room about five degrees warmer than the rest of the house (I kept the door closed because my mom hated looking at the mess in my room. She could have made me clean it up, but she picked her battles wisely, and that wasn’t one of them. A story for a different day perhaps….). 

CRT Displays and CRT Power Supplies 

 

Fifteen or twenty years ago, VMI used to sell a lot of high voltage multipliers to a well-known maker of oscilloscopes. The ‘scopes were analog back then. The company had their own version of CRT power supplies, and we supplied the high voltage section. 
 
CRS Series - High Voltage CRT Power Supplies

From the outside, the high voltage sections looked like a big, black brick with an anode connector on one end, and flying leads on the other. They weighed as much as a brick too. 

Most CRTs and CRT power supplies have a couple of things in common. In a high voltage module, there can be various voltage or current taps, feedback networks, and such, but they all have an anode, a cathode, and one or more electrodes. 

Anodes are used to accelerate electrons, cathodes are used as the source of electrons, and electrodes are used to focus the electrons. The stream of electrons form an electron beam, which is directed to a luminescent screen. The beam is directed either by electric or magnetic deflection, depending on the application and size of the monitor. 

All in all, it’s pretty complicated, but well-established. The whole process happens extremely fast, and repeats continuously.
 
Image Credit:  Mediahex

LCD and CRT Displays 


These days, LCD screens are more common. They are less expensive, less bulky, and lightweight. They are not without trade-offs though. They do not have the color fidelity that CRT type displays have, and they are not as responsive. 

In comparison to CRT types, they are limited in the number of colors available. CRTs are great for high-end applications needing a wide range of colors, deep blacks, accurate color rendition, speed, and fidelity. For most CRTs, you need a CRT power supply. 

CRS Series of High Voltage CRT Power Supplies 

Anyway…..when it comes to CRT type power supplies, VMI makes the best. Our CRS series is adjustable up to 18kV (anode voltage), and ranges from zero to 550uA (anode output current). 

The CRS series features very low ripple (no noise or fuzziness in your display), excellent regulation (no droopy signals), and is arc & short circuit protected. 

Two RoHS compliant models are available, and, as always, guaranteed. The standard power supplies feature an insulated high voltage leads.

If you need a custom lead, send us your spec. We may be able to customize your power supply. VMI is ISO9001:2008 certified. 

Sources

Thursday, October 24, 2013

Single Phase Bridge Thermal Paths


Single-phase bridges are hot!  Especially high voltage ones.  Bordering on power modules, and highly application specific, high voltage (1kV and higher) 1P bridges are the thermal divas of the rectifier world.  Why is that?  Well, partly due to the magnitude of high reverse voltages, high leakage currents, and forward conduction losses.

During the recovery phase of a diode, it is blocking high voltage.  In a less-than-ideal world, the high reverse voltage generates a ‘leakage’ current in the diode.  This happens in low voltage systems too, but because the reverse voltage is much lower, it is generally ignored.  Ignore it in high voltage systems at your own risk!  Here’s why….

Peak Reverse Power

Peak reverse power is defined as the product of peak reverse voltage and leakage current, and is expressed as

 

P(reverse) = Vrwm * Ir


At Vrwm = 10kV, Ir = 500nA, P(reverse) = 5mW.  No a big deal, right? 

 

Forward Dissipated Power

Okay, so now let’s add in the forward power expressed as 

 

P(forward) = Vf * If


Where Vf = Forward Voltage drop, and If = Forward Current.  If Vf = 8V, and If = 1A, then P(forward) = 8W.

 

Total Dissipated Power

Total power, Ptot, is the sum of P(reverse) and P(forward) and is expressed as  


P(tot) = P(reverse) + P(forward)


In this example, Ptot is mostly Vf * If, and is slightly more than 8W (8.005W to be exact).  That’s at room temp (25°C).  But what if the base plate temperature is 50°C, and the thermal impedance of the device is 3°C/Watt?  That means the diode junction temperature is approximately


T(diode junction) = T(base plate) + T(Thermal Imp) °C/W * W



Or, T(diode junction) = 50°C + 3C/Watt * 8 Watt = 74°C.

 

The Impact of Reverse Recovery Time and Thermal Runaway


Did you know that for every 25-degree rise in temperature, leakage current triples?  It’s partly explained by temperature-sensitive reverse recovery times.  At temperatures go up, reverse recovery time gets longer which means the time to dissipate reverse power increases too.   At a diode junction temp of 74°C, the leakage current will be approximately six times the leakage measured at room temp.  In our example, at T(base plate) = 50°C, leakage current of the diode will be approx. 6 * 500nA = 3uA.  Now reverse power becomes 30mW, compared to 5mW at 25°C. 

As the temperature goes up, so does reverse power, which means the diode junction gets hotter, which means the reverse recovery time gets longer resulting in even more reverse power dissipation.  It’s easy to see how and why thermal runaway happens. 

The good news is thermal runaway can be prevented using one or more techniques.  Keep operating temperatures as low as possible, add heat-sinking capabilities, and derate components as much as as you can.  

Of course, your numbers will vary from this example.  There are other mitigating factors such as frequency and duty cycle, input signal, and the absence or presence of heat sinks.

Don't hesitate to call us if you have any questions about our single phase bridges or diodes.  We're here to help.