Tuesday, May 27, 2014

More “Three More Design Tips for Using High Voltage Diodes”



        In the last post, we discussed junction size tradeoffs and how mechanical characteristics can affect electrical ratings.  To summarize, the larger the cross-sectional area of the diode, the larger the current rating (Io), and the lower the forward voltage drop (Vf).

Diode junction size effects other electrical ratings too, including repetitive surge current (Ifrs), junction capacitance (Cj), and single cycle surge current rating (Ifsm), I2T.

      Below is a table containing the diode ratings for the discussion below.  Read on for three more diode design tips. 


Table I – 5kV Diode Ratings for discussion










Part
Number

If
Vf
Body Dia.
Cj @ 50V,
1Khz
Single Surge Current, Ifsm
Rep. Surge Current, Irsm
50mA
9V @ 25mA
0.095” (2.4mm)
1pf
4A
0.75A
1N6525 
150mA
7V @ 150mA
0.135” (3.4mm)
3pf
10A
2.0A
1N6517 
1000mA
8V @ 1000mA
0.185” (2.4mm)
16pf
100A
15A



 

Repetitive Surge Current

           Repetitive Surge Current is one of those vague ratings that are important, but imprecise.  VMI performs surge screening as part of our QPL processing, or when customers request it.  Surge testing is done in accordance with Mil-Std-750, Method 4066.
 

Using the JANTXV1N6519 diode, the QPL version of the commercial 1N6519, as an example, test conditions include:

  • Six surges at a test pulse duration of 8.3ms, (half 60Hz sine wave) at ambient temperature. 

  • Surge amplitude = 27.5A.

  • Electrical tests, Vf and Ir, are performed afterwards.



Tip:  Surge testing can help pinpoint problems related to interconnections.  If internal contacts are not super good, the surge testing may accelerate performance degradation that is detectable by Vf testing.  Vf will increase as internal connects are compromised.

Surge testing is considered a screening test and is usually done on a sample basis.  It is occasionally done on a 100% basis, for an additional charge. 

Junction Capacitance  


           

The nature of the silicon p-n junction means there is an inherent junction capacitance according to the following equation - 

    C = kA/r   

     where k = 1.04 x 10-10 F/m, the permittivity of silicon 
     A = area of the diode junction, usually A=(pi)r2
     Where r = radius of the diode junction


Diode junction capacitance follows the same rules as other capacitors.  Adding more diode junctions effectively reduces overall capacitance.  It is similar to connecting multiple capacitors in series.  A diode with a smaller junction (less capacitance plate area) will have lower junction capacitance than a larger diode with the same number of junctions. 



Tip:  If your application is extremely sensitive to loading, stray or junction capacitance, you can use diodes with smaller cross-sectional areas, or ones that have many junctions to keep junction capacitance to a minimum.  


Single Surge Current


Single surge current ratings are based on the I2T calculation, which is based on fuse technology, as best this writer can determine.  The thought is that the value of the rated I2T calculation is the point at which the diode acts like a fuse, and burns up.  However, this is speculation, and surely older and wiser experts may know more.  The instructions were, many years ago, when doing design calculations, to calculate I2T with the proposed operating conditions and compare the result to the I2T rating in the catalog using the rated Ifsm and 8.3mA for T.  The calculated value should be “much, much, MUCH less than the rated value”. 

For instance, if operating conditions were 1000A for 100ns, then the 1N6517 diode comparison would look like this - 


     I2T(rated) = (100A) 2 x (8.3ms) = 83A2sec
     I2T(oper) = (2A) 2   x (100uA) =  0.0004A2sec



Baring other operating conditions, it’s probably a safe bet that the 1N6519 would survive a single cycle surge current pulse of 2A for 100uA since 0.0004A2sec is much, much, MUCH less than 83A2sec.


Thermally Shocked Glass - Photo Credit
Keep in mind that power increases with the square of the current.  The typical surge current failure mode is localized heating that cause cracks.  You’ll see this when the diode fractures parallel to the junctions.  It can be quite catastrophic.


Tip:  Even rare spikes or surges can cause long-term damage due to thermal shock.  It’s like pouring boiling water into a cold glass.  Part of the glass expands so quickly, the surrounding area can’t catch up, and things start to crack.  To minimize the risk, eliminate surges where possible, or de-rate when total elimination isn’t possible.



Thursday, May 22, 2014

3 More Design Tips for Using Voltage Diodes



     Physical characteristics and electrical ratings are often inextricably linked in a high voltage diode. For instance, the size of the die affects current ratings and body diameter. 


A large diode junction area means the diode can handle higher currents without overheating. In smaller junction diodes, current density and power dissipation will increase due to the smaller cross-sectional area and generated heat. The larger junction devices have higher current ratings.

Junction size also determines lead diameter, and is a large contributing factor in Leakage Current, Ir, during reverse mode operation.

Thermal qualities are impacted by size. Larger leads provide for faster heat transfer via a larger cross-sectional thermal path, aiding the designer who needs cool-running components.


Junction Size 

 

High Voltage, Low - High Current, Diodes

Standard VMI glass-body diodes come in three different junction sizes. You’ll notice, for instance, keeping Vrwm and Trr the same, there are three different current ratings. A general rule of thumb is, the smaller the die size, the lower the current rating, the higher the Vf (due partly to an increase in current density), and the smaller the body diameter.

For example, three 5kV, 70ns diodes that illustrate the above are: 



Part Number
Current Rating (Io)
Forward Voltage (Vf)
Nominal Body Diameter 
50mA
9V @ 25mA
0.095” (2.4mm)
150mA
7V @ 150mA
0.135” (3.4mm)
1000mA
8V @ 1000mA
0.185” (2.4mm)

        The 1N6533 has a body diameter of .095” (2.4mm), while the 1N6517 has a body diameter of .185” (2.4mm). 


     The 1N6517 has a junction diameter just under four times the 1N6533, and can handle 20 times the current. 

Thicker Leads


While all three diode sizes are made the same way, other mechanical differences include thicker diameter leads. Larger leads provide extra mechanical strength, and a larger cross-sectional thermal path. This is especially important in higher current rated diodes since the extra thermal path is an advantage.

Leakage Current and Die Size


Leakage current also depends on die size. Given the same conditions, large diameter diodes will typically have higher leakage current. Fortunately, since Ir is often in the nano-amp range, it is insignificant in most applications.

As in life, with higher current ratings, there are trade-offs. Higher current diodes are larger (think 1N6517), and generally cost more. However, the increased current carrying capacity can often save on overall component count.

 

Knowing your optimum current level and selecting a high voltage diode based on current-carrying requirement will help keep unit costs down and product reliability up. 



Not sure which diode to specify?  Give us a call.

Tuesday, May 20, 2014

3 Design Tips for Using High Voltage Diodes




     High voltage design is a little different than low voltage design.  Some things that are not a concern at low voltage take on more significance as you move up the voltage scale. 

Read on for a few high voltage design tips.

 Isolation between leads


                High voltage diodes will usually be reversed bias at some point during operation.  Reversed bias means the diode is operating in the blocking (i.e. reverse) mode. 

In forward mode, the cathode voltage is more positive than the anode voltage.  In reverse mode, the cathode voltage is more negative than the anode voltage, and blocks voltage.  Ideally, no current flows.   

Realistically, a minute amount of reverse current is generated during blocking mode, and is called “Leakage Current”, Ir.  It is many magnitudes less than current conducted in the forward direction – nanoamps compared to milliamps, or more.    

If your diode is rated at greater than 5kV, and it is operating at that level, you might consider adding extra isolation between the terminals.  In air, you might experience arcing from cathode to anode at reverse voltages higher than 5kV.

 There are several methods to increase isolation voltage.  Adding a conformal coating, or encapsulating the diode to increase distance between the leads are common approaches.  Running the circuit in a dielectric fluid or gas is another method.  Lastly, if other approaches don’t work, you might consider using a rectifier assembly that uses a bigger body size to provide more distance between leads.    

Thermal Management – Getting the Heat Out

 

Simply stated, the cooler you can keep your diode, the better.  Most of the heat will travel through the leads since silver is a much better thermal conductor than glass.  Maximize heat transfer, while keeping isolation between leads in mind, by soldering it to a PCB with lots of copper, or running it in circulating dielectric fluid, or placing it in close proximity to water cooled base plates. 
              
If that doesn’t work, start adding diodes to share the voltage or current load.  This effectively reduces the load each individual diode may see.     

Vrwm – Specifying Vrwm

 

                Vrwm is defined as the reverse working maximum voltage.  Don’t exceed it.  If you do, you run the risk of killing the diode. 

The rate of change of voltage w.r.t. time, dV/dT, is also a factor in selecting a diode that can handle the voltage swings, and the wave form.  Square waves are harder on diodes than sine waves because of faster dV/dTs.  Square waves also can have more switching voltage spikes. 

Use the magnitude of the maximum voltage spike when specifying a diode, and be sure to include a little safety margin.

Every application is different, and there are often many unforeseen events that can occur.  Give yourself a safety margin when selecting components, and then test, test, TEST your circuit. 

Give VMI a call if you have any questions about our diodes.  We can make the selection process much easier.  We’re here to help.