Monday, June 30, 2014

An Insider’s Look at High Voltage Fat Pack Rectifiers and Single Phase Bridges

In a previous post, we discussed VMI’s Slim Pack and Slim Pack Junior rectifier assemblies.  If you recall, these are axial-leaded assemblies containing one or more diodes in a string.  The diodes have been soldered or welded together, and then over potted. 

The advantages of high voltage sticks over discrete diodes are:

1) You can pack a lot of high voltage in one assembly

2) They can be configured in different ways such as building your own  single or three phrase bridge. 

3) Many of them can run in air without additional conformal coating or over encapsulation.

4) Reduced system component count.

5) Reduced system assembly time.  It’s faster to solder two leads than 22 leads.
Slim Packs (SP) are rated at 500mA and up to 20kV.

Slim Pack Juniors (SPJ) are rated at 50mA and up to 45kV.  Slim Pack Juniors are smaller and require conformal coating or encapsulation when operating at greater than 5kV.

Fat Pack High Voltage Stacks 

Okay, enough of the review already!  Continuing on in the line of rectifier assemblies, today’s topics include include VMI’s Fat Pack rectifier assemblies (FP) and our single phase bridges (SPB).  By the way, “Fat Packs” were called that by our manufacturing crew as a kind of verbal shorthand to distinguish them from “Slim Packs”.  The name stuck.  (To those Fat Packs who may take offense at being called a Fat Pack, no offense intended.  Perhaps you could slow down, look inward, and learn to love-thy-self.  You are perfect and beautiful just the way you are, and don’t let anyone tell you otherwise.  Plus, you can handle more than four times the current that a Slim Pack can).   

Fat Packs range from 2500V up to 25kV.  They can handle up to 2.2A of forward current, and are available in 100ns and 150ns reverse recovery times.  They measure 0.70” height, 0.400” width, and up to 7.9” in length.   They’re pretty heavy duty in comparison to most high voltage stacks.

Fat Pack Data Sheets
FP25F – 150ns

FP100F – 150ns

FP175UF – 100ns FP175F – 150ns
FP50F – 150ns

FP125F – 150ns

FP200UF – 100ns FP200F – 150ns
FP75F – 150ns
FP150F – 150ns
FP250U – 100ns

High Voltage Single Phase Bridges

Fat Packs, Slim Packs, and Slim Pack Juniors can all be configured to make a single-phase bridge.  It takes four of them to do so.  Or, you could go with a SPB assembly – VMI’s single phase bridges. 

These bridges come in a rectangular package ranging from 1.5” to 2.5” square and 0.6” tall (excluding terminations).  Terminal options include pins or four 4-40 inserts for mounting to a heat sink.

Each leg is made up of one or more diodes connected in series that have been tested for thermal response.  Thermal response testing verifies the internal integrity of the diode and makes sure the diode will dissipate internal heat in an efficient manner.  

Electrical properties range from 5kV to 20kV per leg.  All the bridges listed below are 70ns, 2A devices.,
5kV to 45kV High Voltage 1P Bridge with Inserts
"B" - 1P Bridge - Insert Terminals
High Voltage 1P Bridge with Pin Terminals - 5kV to 45kV per leg
"A" - 1P Bridge - Pin Terminals

The “A” suffix indicates pin terminals, as in SPB50UFA.
The “B” suffix indicates inserts are used, as in SPB50UFB. 

Data Sheets - Single Phase Bridges - 5kV to 20kV per leg 

So… summarize, VMI offers the following in high voltage stacks and single phase bridges - 

Type          Family     Voltage       Current        Trr
HV Stack   SP        2.5kV–25kV     500mA       70ns - 150ns
HV Stack   SPJ       10kV–45kV      50mA         70ns - 3000ns
HV Stack   FP        2.5kV–25kV      2.2A         100ns - 150ns
1P Bridge  SPB       5kV – 20kV       2.0A         70ns

If none of the above fits your needs, contact us.  Custom rectifier designs are welcome.

Thursday, June 26, 2014

The Insider's Guilde to Slim Pack and Slim Pack Jr. High Voltage Rectifiers

Rectifiers are usually thought of as a discrete diode that has been changed in some way, either by over molding it, or connecting more than one together to form an assembly.  

Examples of rectifier assemblies include single phase bridges which use more than one diode per leg to form a bridge, or Slim Pack (SP) and Slim Pack Junior (SPJ) high voltage rectifier stacks.  

The schematic representation of the SP and SPJ is identical to that of a discrete diode.  

SP, SPJ, discrete High Voltage Diode Schematic Symbol
Schematic of diode, SP, and SPJ

Slim Pack Rectifiers

Slim Pack rectifiers (SP) are long, skinny assemblies containing a string of high voltage diodes that have been soldered or welded together and encapsulated in a rigid epoxy. 

SP High Voltage Rectifier
Slim Pack Package
A few of the more obvious benefits include the ability to achieve higher reverse voltages over that of discrete diodes, longer creep paths between leads, and less assembly work and component count for the customer.  

Less obvious is the potential for running them in air, thus eliminating the need to over mold or conformal coat them.  Longer creep paths means it's possible to run them in air.  In the case of the 25kV version (SP25), the voltage stress between leads would be approx. 4.5V/mil at an operating voltage of 25kV.  This is less than the recommended 10V/mil isolation in air.  (Disclaimer:  Other factors may influence the decision to run in air, including humidity, temperature, relative humidity, to name but a few). 

 SP rectifiers are popular products, as are their smaller counterparts – the Slim Pack Junior rectifiers.

Slim Pack Ratings and Data Sheets

 Depending on the number of diodes in the string and the voltage rating of the device, package length ranges from 1.125” to 4.25” in length, .5” in height.  Slim Packs come in reverse recovery times from 70ns to 3000ns. 

SP Data Sheets


SP25UF- 70ns 
SP25F- 150ns
SP25S- 3000ns 
SP100S- 3000ns  
SP100F- 150ns
SP100UF- 70ns
SP200UF- 70ns 
SP200F- 150ns 
SP200S- 3000ns




SP50UF- 70ns
SP50F- 150ns 
SP50S - 3000ns 

SP125UF- 70ns
SP125F- 150ns 
SP125S- 3000ns

SP250UF- 70ns
SP250F - 150ns
SP250S- 3000ns


SP75UF- 70ns
SP75F- 150ns
SP150UF- 70ns 
SP150F-  150ns 
SP150S- 3000ns

Add a “TX” suffix to the part number (i.e. SP50UFTX), and you can specify slim packs made with JANTX diodes.

Slim Pack Juniors (SPJ)

Slim Pack Juniors (SPJ) offer an extended voltage range – up to 45kV – in smaller packages than the SP.  SPJ dimensions are 0.250” width x 0.250” height in lengths ranging from 1.0” to 2.5”, excluding leads.   They too are axial-leaded, encapsulated diodes.

Dimensions of Slim Pack Junior (SPJ) Rectifiers
Slim Pack Jr. - Smaller than Slim Pack

Note:  SPJ rectifier assemblies are not designed to run at full rated voltage in air.  VMI recommends conformal coating or over potting them when operating at voltages over  5kV in air. 

  SPJ Data Sheets
SPJ100F – 70ns
SPJ100S  – 3000ns
SPJ300F – 70ns
SPJ300S – 3000ns
SPJ150F- 70ns  
SPJ150S- 3000ns
SPJ350F – 70ns
 SPJ350S – 3000ns
SPJ200F – 70ns
SPJ200S  – 3000nsSPJ400F – 70ns
SPJ400S – 3000ns
SPJ450S – 3000ns
SPJ250F – 25kV, 70ns
SPJ250S – 25kV, 3000ns
SPJ450F – 70ns  
SPJ450S – 3000ns

So whether you're in the market for a longer high voltage stack, or a miniature one, VMI has several options to choose from.

The SP and SPJ stacks are popular and available.  Call or contact VMI for pricing and delivery.   

Wednesday, June 25, 2014

4 Challenges in High Voltage Power Supply Design

High Voltage Power Supply

High Voltage Power Supplies

For some, anything over 50V is ‘high voltage’. At VMI, we specialize in custom power supply designs in the 5kV range and higher.

There are a few similarities between a low voltage and high voltage power supply (HVPS). Most power supplies have an input filter, a switcher/oscillator, control circuitry, built-in self-diagnostics, and an automatic self-test sequence.

On top of all that, HVPS usually include a high voltage transformer, or HV rectifier & multiplier, a HV output filter, HV control sensors, and specially designed HV connectors.

Portions of the HVPS may be encapsulated – usually the multiplier/rectifier section, and sometimes the transformer. A low voltage power supply may be partially or completely exposed, especially in high power designs that generate heat.

Challenges in High Voltage Design


Corona and partial discharges can be more of a problem in HVPS. High voltage stresses tend to polarize certain materials – like air voids trapped in potting material – creating phenomena similar to ‘plasma etching’. Over time, this plasma wears down insulation, can enlarge voids, and creates weaknesses in materials. It also causes EMI and contributes to the formation of carbon deposits, which can ultimately lead to arcing and circuit failure. 


To prolong the life of a power supply, removing contaminants and practicing good housekeeping throughout the assembly process should be standard practice. The encapsulation process can be vulnerable to contamination. Contaminants can interfere with adhesion to surfaces, which can be critical to isolation between high voltage components. Encapsulation materials can be susceptible to the formation of air pockets, or voids. In extreme cases, depending partly on the material in use, encapsulation can be done under a vacuum to keep voids to a minimum, and the process conducted in a clean room with positive pressure or under flow hoods.

Voltage Multipliers Inc.‘s processes include rigorous cleanliness standards and encapsulation processes. Our processes have been optimized for reliable, long-term operation of the HVPS under the harshest of conditions. Our operators are well trained and highly skilled in potting techniques for many different materials and catalysts.

High Voltage Connectors

Another characteristic of HVPS are the size of the output connectors. They are typically much larger than those used in low voltage applications, and made of different materials. Silicone is common, and many of the plastics. Larger connectors are needed to provide longer creep paths between conductors, and for dealing with increased electric field stresses.

In the case of leaded connectors, potting adhesion to silicone wire insulation can be problematic.  There are steps that can be taken to increase adhesion, but it's best to check the compatibility early on in the design.


For commercial devices, corona testing may be done during the design stage, or on a sample basis for pre-production runs. Typical candidates for corona testing include transformers and capacitors, potted sub-assemblies such as voltage multipliers, and sensor units.

Other tests include dielectric strength, which is a measure of isolation voltage between terminals, or terminals-base plate, arc resistance, and short-circuit tests. The exact test parameters and methods are determined by the power supply ratings.  At higher output voltages and current, it may be necessary to build test equipment that can generate sufficient inputs and measure the outputs reliably.  

Environmental testing such as temperature cycling and burn-in can be helpful in determining limits of operation of particular designs.

For military grade power supplies, the above tests are common. Additional testing is often required.

VMI tests our power supplies 100%. Standard testing includes temperature cycling, and burn-in. The conditions are determined by power supply characteristic.

Next time you’re in the market for a high voltage power supply design, give us a call. You’ll be amazed at our level of personalized customer service.

High voltage power supply design is what we do.