Tuesday, March 31, 2015

USB Ports and High Voltage Power Supplies

Customer Inquiry on a High Voltage Power Supply

VMI recently received an inquiry regarding a high voltage power supply powered via a USB connection.  We had to decline the design because it wasn't a good fit for unrelated reasons, but it got us thinking about USB ports.

USB Power Delivery

This particular application's output was about 10W, which, come to find out, is the maximum limit for the +5V power delivery via a USB connector, as defined in the USB Power Delivery Specification [1].  From a design point of view, it's not recommended practice to run at the maximum rated power.  I'm reasonably sure the PD specification has some built-in margin, but one never knows.  It might be that one instance where the maximum really is the maximum.  Maybe if the output power was 5W or 7W......

Information Credit

The USB PD specification was finalized in July 2012, and has since been revised.  The intent was to provide uniform charging of lap tops, tablets, cell phones, and other electronic devices, and was largely due to a collaboration between industry giants.

High Voltage Power Supply Idea

It's an interesting idea - to power a high voltage power supply via a USB connection.  It is probably totally do-able, given the right parameters.  Here's a potential block diagram.  It is definitely food for thought.

  1.  "USB 3.1 Specification". Retrieved 2014-11-11.
  2. http://commons.wikimedia.org/wiki/File:USB-Connector-Standard.jpg
  3. https://en.wiki2.org/wiki/RF_connector
  4. http://en.wikipedia.org/wiki/USB#PD

Thursday, March 26, 2015

VMI Logo - Manufacturer of High Voltage Devices
Voltage Multipliers Inc.

Florida Sales Representatives

VMI is happy to announce our association with QREP, our sales representatives in Florida.


QREP has a strong technical background in the South East's  aerospace, military, avionics, satellite, guidance, industrial, and medical, industries.


With six outside sales representatives and two customer service offices, QREP has the ability and resources to support our customers during the design, procurement, marketing, and sales process.

For more information, contact Mike Foresta at 321-427-1220

Area Coverage
Florida, USA

For the East Florida area (Melbourne, Titusville, Cape Canaveral, Palm Bay, Miami), contact Michael Foresta at (P) 321-427-1220.

For the Tampa and West Florida areas (Tampa, St. Petersburg, Clearwater, Palm Harbour, Fort Myers), contact Gary Pfeiffer (P) 813-416-4961.

For the Orlando and Florida Panhandle areas (Orlando, Apopka, Tallahassee, Fort Walton Beach, Lockheed Martin Orlando), contact Charlie Bevan at (P) 321-474-5671.

Tuesday, March 24, 2015

Z50FF3 Junction Capacitance Meets High Temperature

Diode data sheets can provide helpful information when doing circuit analysis or simulation.  

For instance, using the Z50FF3 as an example, the data sheet includes a graph indicating expected values of leakage current vs. temperature.  The graph can be used as an approximate impedance of the diode at high temperatures, which can be used to extrapolate information on junction capacitance at temperature.

In general, junction capacitance of the diode depends mainly on the applied voltage, the operating frequency of the diode, and the general wave shape the diode is exposed to during oscillation. Junction capacitance is also a function of the silicon material.  This is reflected in the "Typical Junction Capacitance vs. Reverse Voltage at 25 deg C" graph.   

Unfortunately, there is no graph on Cj vs. Temperature, but if there is a need for it, VMI may be able to arrange to do some testing.  Contact us with your specific request.


Wednesday, March 18, 2015

2 Factors in High Voltage Junction Capacitance

Voltage Multipliers Inc.
There are lots of different 'capacitances' in a.c. circuits.  There is capacitor capacitance, stray capacitance, transformer interwinding capacitance, and junction capacitance.

Stray Capacitance in a High Voltage Diode

In a diode, the biggest source of capacitance is that of junction capacitance.  This is the stray capacitance that occurs between the active junctions of the device.     

Cross-sectional Area

To minimize junction capacitance due to physical characteristics, there are two things to consider.  First is the cross-sectional area of the diode junction.  This translates to the area of two plates that define parallel plate capacitance (Eq.1).  Diodes with smaller junctions will have less area to act as a capacitor plate, and thus, will have lower stray capacitance.

(Eq. 1)       

C =  capacitance in Farads 
e = permittivity
A = area of the plates  
d = is the distance of separation between plates

The second thing to keep in mind is the number of silicon p-n junctions.

Number of P-N Junctions

To make high voltage diodes (diodes capable of blocking reverse voltages of more than 1kV), the p-n junctions are metallurgically bonded together.  This effectively stacks the junctions in series.  The total junction capacitance is a function of the number of p-n junctions, as expressed by (Eq. 2).  The more junctions there are, the smaller the total junction capacitance.  

(Eq. 2)

Minimizing Junction Capacitance

To minimize junction capacitance, look for high voltage diodes with the smallest body diameter and the greatest number of p-n junctions.  Forward current ratings are indications of junction diameter.

If you compare two 5kV diodes, the one with the higher current rated diode will have the larger body size.

In the screen shot below, comparing the 1N6517 and the 1N6525, they are both 5kV, 70ns diodes, but the current ratings, body diameter, and junction capacitance are different.  

From the data sheets,


Body diameter = 0.185" (4.7mm) nominal
If = 1A
Cj= 16pf


Body diameter = 0.135" (3.4mm) nominal   
If = 150mA
Cj = 3pf

To summarize - the larger cross sectional area of the diode, the higher the Cj.  The more junctions in the diode, the less Cj.

So, if you are looking for our devices with the absolute lowest capacitance, you will probably want to consider our smallest cross-sectional area highest voltage device: the M160UFG diode.  It's a 10kV, 10mA, 70ns, 0.5pf Cj diode.

Electrical Factors Effecting Junction Capacitance

Other factors can effect junction capacitance too, like operating frequency and reverse voltage, but those are topics for another day.

Thursday, March 12, 2015

Qrr Charge Carriers in High Voltage Diodes

Every so often a customer request information on charge carriers for VMI diodes.

The amount of charge carriers, Qrr, is a measure of the charge carriers that flow in the reverse direction when the diode is transitioning from a forward conduction condition to reverse voltage blocking.  current 

As a result of a recent request, we did a little research, and found the following article on calculation Qrr of a diode, based on it's reverse recovery  

VMI diodes are characterized as ‘soft recovery’.  The equation below uses diode ratings commonly found on diode data sheets.  

As an example, our 1N6519 (10kV, 500mA, 70ns) has a the following reverse recovery characteristics:

  • TRR = 70ns MAX
  • IF = 0.5A
  • IR = 1.0A
  • IRR = 0.25A


Qrr is calculated to be 8.75 nano-Coulombs.
This is assuming the diode actually has a reverse recovery of 70ns which is the rated maximum for the part.   A more typical reverse recovery time is 60ns for this diode, which would yield a Qrr of 7.5nC.

Tuesday, March 3, 2015

Z50FF3LLHT - 5kV, 200C, 30ns, High Voltage Diode for Harsh Environments

Harsh environments can be tough to operate in, especially in downhole oil and gas industries.  Diodes are often used in the well logging side of things, in the control circuitry.  Even in that application (as opposed to being on the drill head), managing key material characteristics such as leakage current, and maximizing thermal transfer characteristics in packaging, can be challenging.  Active cooling is not always possible or practical.

At a geothermal gradient of about 25°C/km, things get heated pretty quick.

Enter VMI's 5kV, 200C, 30ns Z50FF3LLHT  diode.  It has been optimized to get the heat out, and because the diode body is ever-so-slightly elevated, it won't sit on a substrate or printed circuit board and provide a leakage path.

Z50FF3LLHT High Voltage, High Temp, 5kV, Hyper-fast, Diode

Advanced Process Technology

Specially selected and targeted wafers are used to minimize voltage stresses in the diode. The selected wafers perform well under higher operating temperature extremes. 

Electrical Specifications

Operating Temperatures:  -65°C to +175°C
Storage Temperatures:  -65°C to +200°C

Vrwm = 5kV
Io = 800mA
Trr = 30ns

More Features

The Z50FF3LLHT still features formed lead diodes for higher current-carrying capacity.

Part Number Breakdown - 

  • Z50 - 5kV diode
  • FF3 - Freaking fast, 30ns, Trr
  • LL - Formed lead
  • HT - High Temp 

The Z50FF3LLHT 30ns Trr means it has a super fast reverse recovery time - less time to dissipate power during the transition from forward conduction mode to reverse voltage blocking mode.  Because the Trr is super-duper fast, that means it will operate faster than a standard recovery diode at higher temperatures, reducing the reverse power dissipation even more.    

The Z50FF3LLHT is hermetically sealed, so you don't have to worry about moisture, ESD sensitivity, or substrate leakage paths.

All in all, the Z50FF3LLHT diode is robust and sturdy. Exactly the characteristics you need for downhole or military grade applications.

Free Samples

Contact VMI for free samples, or to discuss your application.

Z50FF3LLHT Data Sheet