High voltage and high current protection devices in RF Design

Hi ,

I am new in RF design , as i compare RF with Digital design . i see here in RF there is no current protection fuse , voltage protection Mov or Diodes in Power supply .why dont we need protection devices in RF power .

pls correct me if am wrong


Thanks

If u see any active RF component data sheet, U can find the MAX voltage and input power ratings, beyond which component will damage. as for the passive circuits the design will malfunction if the dielectric of the design substrate is broke down. As per the protection fuses and all, Probably u can find them in High Power EM design but my knowledge is less in that area.

jack_pcb said:

Hi ,

I am new in RF design , as i compare RF with Digital design . i see here in RF there is no current protection fuse , voltage protection Mov or Diodes in Power supply .why dont we need protection devices in RF power .

pls correct me if am wrong


Thanks

Click to expand...

To make a fast enough current protection circuit you need to use the same or faster RF transistor that that to be protected. During RF amplifier development, often a number of expensive devices is lost.
RF overvoltage is a bit rare case as the typical line impedance is 50 Ohms, so even under quite a high power the peak voltages are not exceedingly high. The high-power short-wave and VHF/UHF transmitters are now built together with an output reflectometer that adjusts the power device current according to the load match. If mismatched, the reflected RF power can damage the RF power device if this protection circuit is not used. At frequencies > 300 MHz, isolators are used to separate load mismatch from the RF power device.

well, if u look at a system that hooks up to an outdoors antenna, u will likely find lightning protection circuits. Luckily microwave frequencies pass thru very small capacitances, so the first line of defense is a small series capacitor (maybe 5 pF) and a really good low ohm return path to earth ground from the coaxial cable shield.

After that, u can add MOV devices (IF they are very low capacitance) or spark gap gas tubes across the RF center connector to ground. If you have a narrowband application, u can use a short circuited stub one-half wavelength long in parallel with the input connector.

Sometimes u need to add passive protection from high RF power blowing stuff up, so they make PIN "limiter" diodes that automatically turn into momentary short circuits if 1 watt or higher RF power hits them.

Other than that....for most modern low voltage RF equipment, there is no protection. Fuses blow...but then someone has to drive to the mountain top to replace them....so they are not as useful as one might think!!!!

No protection in RF amps at all, other then colouring output connectors red, so the inputs are disconnected first. The actual aerial systems are heavily protected so that lightning strikes/ water leaks will do as little damage as possible. High power transmitters have VSWR that switch of the transmitters if the reflections exceed 2%.
Frank

Inside some RF IC, there is some ESD parts. May be you can find them in some IC's layout.