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Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
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PWM'ing any variable resistance electronic switch will reduce its overall current rating
Instead of just switching into "saturated mode" (where its resistance is the lowest) and staying there, you are switching back and forth through the "partially conductive" region several thousand times a second.
Partial conduction = heat generation. The longer you take to "transit" this partial conducting mode, the hotter your switch will get. This is why high power controllers are all about the "gate drive" of your mosfets. Since Mosfet gates are capacitive, forcing them to suddenly swing from one voltage (off) to another (on) very quickly takes a *lot* of "push" from the drive circuit to change the charge state of the gate capacitance.
Servo-Motor electronics are designed to PWM a tiny little motor that would probably be lucky if it managed to suck 1 amp of current. So the drive to the SSR's input is not likely to have a lot of oomph behind it, meaning fairly long switch times are likely, which will mean the SSR will get hot at a rating much lower than its "saturated on" rating. How much, depends on what the input capacitance of the SSR is.
Generally high-power Fets have higher gate capacitance (larger channel to effect with the field) and are thus harder to switch quickly (requiring more gate drive) than low power fets. Just like throwing a huge 50 amp switch takes more effort than a little 1amp toggle does. Paralleling multiple fets increases the gate capacitance just like parallel capacitors does, with the same slow-down effect.
So, thats the theory behind why PWM'ing a switch with a low-drive capable circuit will reduce its maximum current. How much so depends on how fast the drive can switch the load capacitance.
The only way to tell for sure how much without a boat load of data sheets and a calculator is to get out a *fast* oscilloscope and look at the "slope" on the edge of the square waves PWM'ing the input. Squarer, nice vertical edges is good, slopey gentle curves is bad. (long time in the transitional zone). _________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
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Thu Mar 29, 2007 2:14 am |
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