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.

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