Wednesday, August 14, 2013

New High Voltage Catalog is Here!

VMI's new catalog of high voltage diodes, bridges, power suppliers, optocouplers, and more.  Includes App Notes and technical data sheets.

Voltage Multipliers Inc. offers high voltage solutions designed for high reliability, standard, and custom applications.  Through VMI’s manufacturing technologies, it provides superior performing diodes, diode assemblies, optocouplers, power supplies, and more.
Inside you'll find technical data sheets on our products, application notes, design guidelines, step-by-step thermal calculations, and more.    
VMI’s quick response on custom designs means faster prototype-to-manufacturing time for our customers.
Contact us now to get your copy of our new catalog.  Quantities are limited.  Do it now!      

Friday, August 9, 2013

High Voltage Rectifier Stacks – Also Known as High Voltage ‘Sticks’.

I don’t know when or how the phrase, “high voltage stick” morphed into “high voltage stack”, or really, which came first, but here at VMI, we use the term ‘stack’.

High Voltage Rectifier Assembly - HV Stack from Voltage Multipliers Inc.
High Voltage Stack - up to 40kV
High Voltage Stack - SP, FP, SPJ Families” implies a rectifier assembly containing a string of high voltage diodes in series.  For applications requiring higher output current, multiple strings of high voltage diodes are connected in parallel, then encapsulated (FP family).  Aside from discrete diodes, it is the simplest and most straightforward of rectifier assemblies.   

High Voltage Stack Schematic - FP, SP, and SPJ Product Lines
HV Stack Schematic
Very high blocking voltages can be achieved using multiple diodes in series.    

High voltage stacks are also useful for creating custom circuits where a modular approach is needed.  For instance, four stacks can be used to create a single-phase bridge of up to 40kV 2.2A per leg using VMI’s standard HV stacks.  Three phase bridges can be created using HV stacks too.  A third application involves using a HV stack to rectifier signals tapped from the secondary winding of a HV transformer.    

A second advantage is that because stacks are longer and wider than discrete diodes, isolation voltages are higher, and surface creepage distances are increased.  This means your device is less likely to arc between leads.  Depending on the device rating and your application, many times HV stacks do not have to be over-encapsulated or operated in a dielectric medium to prevent arc-over. 

Package sizes range from 2.5 inches for a 5kV device to 6.50 inches for a 20kV package.  These are the FP or SP devices.  Shorter lengths are available in the SPJ packages.  The SPJ family features high reverse voltages at 50mA in a compact package.

The two terminations are 16AWG tin-plated copper wire solder-dipped in a RoHS compliant Sn96 solder for most HV stacks.   

HV stacks from VMI are available in three different current ratings – 50mA, 500mA, and 2.2A.  Vrwm ratings are available up to 40kV.  Trr options include 3000ns and 70ns.

If none of the above work for your application, send us your specification.  We welcome Custom Rectifier Design Request.

Quotes are free. 

Thursday, August 1, 2013

High Voltage Full-Wave Bridge Rectifiers

High Voltage Single Phase Bridge Schematic
I’ve always been a little uncertain of the definition of “rectifier”.  It’s a term that’s used a lot in connection with high voltage diodes and assemblies that use high voltage diodes. 
Single-phase, three-phase, poly-phase bridges use one or more diodes per leg, as do doublers, and positive or negative center taps.  They are ‘rectifiers’ too, since they take an AC signal and rectify it to a DC signal.      

Rectifier Assemblies

Rectifiers are much more than diodes.  Take bridge rectifiers, for example.  Most electrical engineers are familiar with single-phase and three-phase bridges.  By definition, a full-wave, single-phase bridge has four inputs – “+”, “-“, and two AC signals.  One leg is, for example, the diode appearing between the “-“ terminal, and the AC terminal).AC Input Signal and Rectified Output Signal for High Voltage Single Phase Bridge

Single Phase Bridges

Did you know that single-phase bridges are available in high voltages?  VMI’s SPB family of Single Phase Bridges share 2A, 70ns ratings.  The devices range from 5kV per leg to 20kV per leg, Vrwm.  When looking at the data sheet of VMI devices, the ratings are per leg.   
Select from pin terminations or insert terminals for interconnect options.
Lots of fancy things can be included in a custom bridge assembly, such as the addition of a smoothing capacitor, or a resistor network.  Quik-Connect (TM) terminals, flying leads, and other termination schemes are possibilities too. 

Select the Right Single Phase Bridge for Your Application

5kV, 10kV, 15kV, or 20kV Single Phase Bridge with Pin TerminationsWhen considering a bridge for your application, there are a few things to keep in mind.  These include, but are not limited to the following.   

1)       Voltage isolation
2)      Thermal management
3)      Input signal
4)      Operating frequency  

Voltage Isolation

5kV, 10kV, 15kV, and 20kV Single-phase Bridge with Insert Terminations
As is true for most high voltage applications, voltage isolation can be a concern.  Voltage isolation, loosely defined is the amount of isolation through a given medium (air, oil, epoxy, etc.) between ground and a non-ground potential.  It isn’t usually an issue in low voltage apps.  Given that isolation through air is approximately 10V/mil, if you’re working with 5V, you only need .5 mils (5V/(10V/mil)) – or half a mil between terminals.  If you’re working with 5000V, you’ll need 500 mils (5000V/(10V/mil)), or half an inch between terminals.  

Thermal Management

The cooler you can keep the bridge, the more reliable the device will be.  It’s kind of like the difference between driving your car hard and fast for 100,000 miles before it dies, or driving it slow and easy and having it last for 250,000 miles.  Your choice.
Cooling methods include using heat-sinking schemes such as mounting the assembly on a water-cooled base plate, and running the part in oil or some other medium with high thermal transfer characteristics.  De-rating is another common strategy.  For instance, if you really need 500mA average Io per leg, selecting a device with 2A Io will result in less stress in the device.

Input Signal & Operating Frequency

The operating frequency can make or break your selection.  At high voltage, Trr losses can become an issue because the peak voltage is typically higher.  Power dissipated in the reverse direction becomes a factor, and is dependent on such factors as slew rate (dV/dT), operating frequency, and operating temperature. 
In general, a 70ns device can operate in the range of 100kHz sine wave, or 50kHz square wave.  This is dependent on temperature and other environmental conditions. 
When the power losses become too great, a condition known as ‘thermal runaway’ can occur.  As a device heats up, the Trr of the internal components increases.  As a general rule of thumb, for every 25C rise in temperature, Trr triples.  So at room temp – 25C – Trr might be 70ns.  At 100C, Trr may increase to 210ns.  As Trr increases, so do reverse power losses.  More power loss means more heat, which results in an ever-increasing Trr, which means more heat….so on and so forth.  Eventually the thermal runaway will result in a catastrophic failure.    

Need Help Selecting the Best Single Phase Bridge?

Single-phase bridges are a great way to rectify signals from a high voltage transformer, or other AC signal generating device.
Performance is often influenced by less-than-perfect operating conditions.  Humidity, moisture, contaminants, voltage spikes, current spikes, and all sorts of other phenomena can effect performance.   
Keeping in mind a few critical parameters, selecting the right device for your application is a breeze.
What’s in your specification?  Not sure what the critical parameters are?  Give us a call.  We’ll help you sort it out.