Thursday, March 20, 2014

How to Kill a High Voltage Diode

So.  You want to research and characterize high voltage diodes, and you really, really, really want to kill a VMI diode.  There are ways it can be done, but it’s not easy.  Read on.

Multi-junction Diodes

VMI’s diodes are silicon based.  A diode with a Vrwm (reverse voltage) rating greater than 1kV has more than one active junction.  It is a “multi-junction” diode.  A rule of thumb is for every 1kV Vrwm, there is one active junction, so a 5kV diode has at least five active junctions.  A 10kV diode has at least 10 active junctions.  You get the idea.  Read on to find out why it's important.

Three Easy Ways to Kill a Diode


1.  Exceed its Maximum Vrwm Rating

It’s a well-known fact that silicon diodes are susceptible to over-voltage conditions.  There are special types of diodes to deal with reverse power surges, and ones that can recover from avalanche conditions such as TVS (Transient Voltage Suppressors), or zener diodes.  We’re not talking about those.   

Soft Failure

How fast a diode fails due to exceeding Vrwm depends on how much the maximum voltage rating is exceeded.  If the voltage surge is less than 1000V – or less than the blocking capacity of one junction – the diode may lose a junction.  If the remaining junctions are undamaged and no further voltage spikes occur, the diode may continue to block. 

The bad news is, one less junction reduces the overall blocking capacity of the diode, and makes it susceptible to additional damage.  The good news is, as long as the combined Vrwm is not exceeded, it will continue to operate.

Hard Failure

A hard failure is when the diode is mostly shorted in the reverse direction.  All or most of the active junctions have been damaged beyond repair.  This requires a large magnitude voltage spike or a series of smaller ones that take out one junction at a time.  Hard failures typically have large, black, arc paths across the junctions visible under the microscope after the glass is removed. 

2.       Exceed the maximum Io or I(surge) rating


If a diode operates in the forward mode and the average current rating is exceeded, the diode will eventually fail if there is no method to get the heat out.  When a diode gets hot, it can move into a thermal runaway condition where the forward voltage drop, Vf, goes down with temperature, which means the resistance goes down, which means the device draws more current, which means it gets hotter which means the Vf goes down…and so on.   

There are other factors involved, and multiple causes of thermal runaway,  but running the diode too hot is a sure fire way to jump start the thermal runaway process.  

Sometimes diodes see a huge current surge in the forward direction.  If the current is high enough to generate a lot of heat, the glass surrounding the diode may crack.  These cracks can be visible under magnification. 

Sometimes the diode will break in half in the plane perpendicular to the diode body.  In cases like that, the silicon has gotten super hot, super fast, and created a huge thermal mismatch between silicon and glass.  So much so that the glass actually breaks because the silicon is expanding so fast. 

Breaks like these are usually accompanied with large, black carbon marks.  The diode looks like it exploded from the inside.      

3.      Crank up the operating frequency so that it goes into thermal runaway


When a diode gets hot, not only does the Vf decrease, but the time it takes to recover increases too.  The longer it takes to recover – to transition from reverse voltage blocking to forward conducting – the more power is dissipated in the reverse direction, and the hotter the diode gets.  Which means it heats up even more, and the reverse recovery time increases, and …. 

Add forward conducting power losses in, and it’s easy to see why a diode might go into thermal runaway if it isn’t kept reasonably cool.   

Lest you get the idea that VMI diodes are not robust, banish the thought.  VMI’s reliability is second to none.  Ditto for quality.  Our guarantees are nothing to sneeze at either. 

Knowing the limits of any device in a specific application can provide priceless peace of mind.  We encourage you to characterize our diodes in your application, safely.  Please contact us if you have any questions.  We’re here to help. 


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