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Power Handling

 

The voltage of Nicola Tesla's man-made lightning can be calculated from altitude and gap

When we talk about power handling, we mean power levels beyond which damage occurs (the magic blue smoke is released), usually well beyond the dynamic range of a system. Typical power handling issues are receiver protection during transmit for a system that the transmitter and receiver are duplexed to the same antenna. But any lab course should consider this subject, because exceeding power handling limitations of expensive test equipment can cost a lot of money!

Two limitations generally exist.

  • Average power kills hardware through heating effects
  • Peak power kills hardware through breakdown effects (like arcing)

By the way, this seemingly simple subject of power handling quite often requires very complex analyses, you can make a career out of it!

Average power

In this case we need to consider the combined heat due to average RF and DC power forms, and heat transfer to a heat sink. The better the heat sink, the more power you can apply.

The analysis of heat transport is a complex subject, and we are not going to get into it in any detail now...

Peak power

In this case we need to consider the combined maximum voltage due to RF and DC power forms, while voltage limits must be applied to peak waveforms.

Ionization breakdown in air occurs when a critical field strength is exceeded. Field strength is specified in volts/meter or volts per centimeter. In air, at 1 atmosphere, the critical field is often quoted at about 3,000,000 volts/meter, or 30,000 volts/cm, or about 76 volts per mil. Generally, this breakdown voltage is NOT a function of frequency, but it is a strong function of altitude and gap size.

One way to greatly improve power handling is to add a dielectric in place of air. Therefore, coax cables with PTFE fill have far greater power handling than air coax. The same same goes for stripline. For high-voltage applications that must work at high altitude (such as microwave tube power supplies for airborne radar) the entire part is often "potted" with a dielectric filler, this often makes repair impossible.

One thing to look out for is that when you mate connectors, even if they are filled with dielectric, the interface can have just a slight air gap. Guess where voltage breakdown is going to occur?

Paschen's Law

Back in the 19th century, Friedrich Paschen came up with Paschen's Law, an equation that describes the relationship between voltage breakdown, distance and gas pressure:

Vb=(apd)/(ln(pd)+b)

The constants a and b take on different values for different gases. Vb is the breakdown voltage in Volts, p is the pressure, d is the gap distance.

Power Handling in Switches

When we say "power handling" for switches, what we mean is the power level that will correspond to the onset of gain compression. Once you exceed the calculated power handling, the insertion loss of your switch starts to go up.  For more detail, check out our Switch Power Handling page.

Power Handling in Coax

Coax power handling is a complicated subject, but it can be broken down into two phenomena. High peak power can cause failures due to arcing, while high average power can cause failure due to heat.  Our Coax Power Handling page gives you equations, examples, and more.

Power Handling in Waveguide

Waveguide power handling is subject to both heat (average power) and voltage breakdown (peak power) limitations. Power handling in waveguide

Multipaction

Multipaction is the phenomenon that can cause breakdown due to high electric fields in a vacuum or near vacuum.

Multipaction

Semiconductor failure

Failure can occur in semiconductors, when breakdown occurs in the "channel" or "junction". Voltages must be kept below a critical threshold in semiconductor devices or failure results. The voltage is instantaneous voltage, the sum of DC and RF components.

 

 

Author : Unknown Editor

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