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Frequently Asked Questions about Diodes

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Question: Are the overmolded diodes - SMF and SXF families of high voltage didoes - hermetically sealed?

Answer: The SMF and SXF families start with the basic, glass-body, hermetically sealed diode, and over mold it in a rigid epoxy. Following the encapsulation process, the leads are trimmed and formed. The diode itself is hermetically sealed. The epoxy is extremely moisture resistant, but not water-proof.


Question: What are the maximum storage and operating temperatures for silicon diodes?

Answer: VMI's diodes can easily withstand 175C and 200C non-operating (storage) temperatures.

During operation, junction temperature become very important. As a general rule of thumb, junction temperature should not exceed 125C.

Junction temperature is relative to the operating parameters in the application. If reverse recovery losses are not a factor, care should be taken to ensure the diode does not go into thermal runaway due to reverse current (Ir) losses.

If reverse recovery losses are an issue, the junction temperature should be kept below the point at which reverse recovery losses initial thermal runaway.

Reverse recovery losses can be attributed to reverse recovery time (Trr) of the diode, junction temperature, operating frequency, forward switching current in the circuit, diode series impedance in the circuit, and available thermal paths to get the heat away from the junctions.

Due to the uniqueness of each application, reverse recovery loss analysis must be performed during circuit design and testing.



Question: When diodes are used in a bridge or string configuration is it necessary to place equalizing resistors across the diodes? Also, is it necessary to place capacitors in parallel with the equalizing resistors to reduce noise and high voltage spikes?

Answer: Balancing resistors and capacitors are generally not required when using VMI diodes. A resistors connected in parallel with a rectifier is intended to balance the reverse bias voltage across the series rectifiers. As such, the resistor value needs to be selected such that the current through the resistor is fairly large compared to the reverse leakage of the diode, thereby making a stiff divider circuit. If there are relatively high leakage currents expected in the diode, this can result in a significant power loss in the balancing resistors.

Capacitors connected in parallel to resistors/diodes can be used to balance voltages across diodes during transient conditions, or ringing. Care must be taken to select the capacitors small enough so as not to impact the rectified waveform at the output of the diode.

VMI's diodes are well-balanced for Ir and reverse recovery time (Trr).

Many of VMI's diodes have up to twenty junctions in series. The series junctions operate under many kinds of applications, and all sorts of conditions without compensating resistors or capacitors with no problems.



Question: Is the dimensional information on the diode spec sheets nominal numbers, or maximums? If they are nominal, what are the maximum and minimum ranges for the packages VMI manufacture space-level diodes?

Answer: Diode dimensions are typically nominal with a tolerance of +/- .005 inches (.127mm) unless otherwise specified. If you have questions about specific dimensions, please contact us.



Question: Do you have any info on the failure rate (FIT rate) @55C and @60%CL for the MD90FF18J based on in-house life test data?

Answer: VMI does not have any in-house life test data, but the calculated MTBF (Mean Time Between Failure)at 55C and operating 60% in a stressful environment (Missile Launch conditions) is 43 years.

If you have questions about MTBF on specific diodes, please contact us.



Question: Does VMI manufacture space-level diodes?

Answer: The highest level QPL diode VMI manufactures is JANTXV. We have on many occasions, "up-screened" JANTXV diodes to the space level by following testing and processing guidelines outlined by NASA. Please contact us if you are interested in space level diodes.



Question: What is the most effective way to mount the K25UF epoxy diode to a printed circuit board?

Answer: The most effective method of mounting the K25UF diode (and most all axial-leaded epoxy diodes) is to make a cut-out in the board just a tad larger than the dimensions of the diode body and drop the diode in so the leads lay flat across the solder pads. If possible, avoid lead-forming. If space does not permit it, is important to support the diode body while forming the leads. The leads are relatively short and thick, which enhances the current carrying capacity of the device. The epoxy body is easily damaged if the leads are not supported near the body during the lead forming process.


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Question: I am trying to determine the polarity of a high voltage diode. My Fluke meter shows an open circuit in both directions for the 10KV Z100UFG diode. Is that normal?

Answer: Yes, and no. When trying to identify the cathode and anode end of a high voltage diode, the meter has to have enough voltage to overcome the forward voltage drop, Vf, of the device under test. We use a test setup that includes a 30V current limited power supply. When testing for polarity, the current should be limited to mA going through the diode when it is connected in the forward direction.


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Question: Will plastic diodes work in high temperature down-hole applications?

Answer: Plastic diodes have very high leakage current at extended high temperature, and are not recommended for the HT options.


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Question: What is the I 2T rating and how is it used?

 Answer: The I2T (Amps2*Seconds) rating is defined as the single cycle surge current, Ifsm, multiplied by a standard pulse width of 8.3mS (sine wave is assumed).

It is used as a rule-of-thumb to gauge surge capability at different pulse widths. It works because at higher currents, Vf (forward voltage drop), is dependent on the resistive element of the diode. Vf becomes proportional to current in the diode expressed by Vf = Rdiode * If. The I2T calculation yields energy dissipated in the diode during the pulse duration. Power multiplied by pulse time gives the energy pulse. Energy dissipated during a surge current pulse is proportional to I2T and is usually the driving force behind a failure. The energy pulse causes localized heating which induces mechanical fractures or disruption of the silicon crystal structure. Calculating the maximum I2T can help determine if a diode will survive a current surge.

Example: A diode has an Ifsm rating 100A. Will it work at 150A surge for 1uS?

Solution:

1. Calculate I2T

I2T = (100A)2 * 8.3mS = 83A2S

2. Determine if I2T under the new conditions is much less than the original calculation. Is the I2t calculation at 150A, 1uS much less than 83A2S?

Is 83A2S >> (150A)2*1uS?

= .023 A2S

Yes, .023A2S is << 83A2S

The diode should be able to handle a surge of 150A for 1uS.

Comment: When approaching fast pulse times (i.e. ns range), I2T using Ifsm is used as an upper limit. Operating I2t should not exceed half of the upper limit.


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Question: Is it possible to connect diodes in series to increase reverse voltage capability? Is it possible to connect them in parallel to increase the forward current?

Answer: Yes. Diodes can be connected in series and/or parallel in order to increase the reverse voltage and/or forward current capacities.

There are practical considerations to keep in mind such as thermal management, voltage isolation, and component count, to name a few.

When connecting diodes in parallel it is usually a good idea to match Vf. Differences in Vf between diodes will get worse as they heat up. Diodes with lower Vf will initially conduct more current, increasing their power dissipation and further lowering their Vf. This in turn increases their current share, continuing to heat up the junction, and so on until a catastrophic failure occurs.


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Question: Do VMI diodes have avalanche capability?

Answer: Yes and no. 100% of our diodes are tested under avalanche conditions at 100uA. Yes, all of our diodes have some avalanche capability.

The real question becomes, “How much?” That varies from diode to diode and depends on several factors including number of junctions, junction area, resistivity of the wafers, the energy or power dissipated, and more.

In avalanche mode, a diode is reverse biased. Leakage current increases exponentially as the reverse voltage increases. Dissipated power increases as leakage current increases, which heats up the diode junction. Heating up the diode junction increases the leakage current, which increases the dissipated power, and so on until the diode fails catastrophically. If the diode is kept cool, it will continue to operate indefinitely.

VMI diodes are tested in such a way as to ensure reverse leakage current is low enough at the rated PIV to help prevent thermal runaway commonly associated with avalanche mode operation.

If the customer knows what the avalanche requirement is, VMI can determine if the diode in mind will meet it.


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Question: Do VMI diodes have a hard or soft recovery time (Trr)?

Answer: Soft recovery.


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Question: Can diodes be matched for Trr?

Answer: Occasionally in high frequency applications where voltage rise times are fast, matching Trr can be advantageous. This only works for single junction diodes (1KV or less). Individual junctions in high voltage multi-junction diodes (greater than 1KV) cannot be accessed.

Multi-junction diodes work very well in high frequency applications.


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Question: Can I operate an axial-leaded 10KV diode without it arcing from lead-to-lead?

Answer: A rule-of-thumb for determining whether additional encapsulation is required is the 10KV/inch isolation in air rule, which assumes relatively low humidity and clean air (no particulates on the diode). For example, using the 10kV/inch rule for the 1N6517 diode (5kV, 1A, 70ns), based on body size and distance between leads, it could operate up to 2900V in clean, dry air. Above 2900V, or in less clean conditions, the diode should be encapsulated or operated in oil or some other dielectric fluid.


Many factors can effect whether the 10KV/inch rule is effective in applications. Additional factors include humidity levels, thickness of the glass passivation, body length between leads, ambient temperatures, vacuum levels, and power levels.

Generally speaking, additional isolation should be provided for diodes operating a levels of 5KV or higher.

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Question: Can I operate the recommended MD90 F18 and MD90U25 replacements without over potting them?


Answer: The recommended repalcement for the MD90F18 is the SXF20FF3. The recommended replacement for the MD90U25 is the SXF6523. If the diodes are operated at full rated voltage, the voltage stressed between leads will exceed the general design guidleline of 10V/mil in air. It is recommended that the isoaltion voltage between leads be increased through the use of a conformal coating, the use of a dielectric liquid or vapor.

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Last Update: 07 April 2015