Hello,
I am fairly new to electronics and need some advice regarding impedance matching.
I wish to impendance match a half bridge to a parallel tuned circuit (LC) Via series inductor ie LCLR netwowk. Here is a link to what I am trying to achieve.
Questions :- How does one find the impendance of the Half-Bridge? How do I calculate the L-match inductance / capacitance required to match the inverter (h-bridge) to the load?
Regards SRG
It's more or less equal to the on resistance of the FETs... plus some inductance thrown in for good measure. BUT: He's not trying to match the impedance of the half-bridge to that of his coil; he's considering the half-bridge to be (approximately) an ideal voltage source, and therefore trying to drop the impedance of the coil so that he can get decent amounts of power into it while using reasonable voltages (hundreds of volts rather than thousands of volts).
You have to know what his "work resistance," R, is. I suspect he determined this empirically -- it's going to be a little different depending on exactly what you insert. I'd imagine he first ran the circuit without the matching network and simply measured the power consumption at resonance to ascertain R (he probably chose the total capacitance -- some of which eventually becomes part of the matching network -- to resonate his coil at, say... 150kHz). He probably then did a few "back of the envelope" calculations to see whether or not building a matching network to 10 ohms was reasonable and discovered that, yes, it was. (The main point here is: A lot of his design appears to have been done empirically.)
---Joel
achieve.http://www.richieburnett.co.uk/indheat.html
Joel already answered most of your questions. To put the "impedance of the half-bridge" into perspective, consider that if you put a load on the power line at a standard wall socket (120V 60Hz AC in the US, for example), you do NOT want to match impedances to the line! You would blow a circuit breaker or fuse (or start a fire). Instead, you want to put a load on it that accomplishes whatever it is you want to do: the power to run a motor, or light a light. Same with your induction heater: you want to deliver enough power to the object you put in the coil to heat it to some temperature, typically. The half- bridge just puts out a reasonably constant voltage at some frequency, and you want to present a load to that which will put the appropriate amount of power into the object to be heated.
For impedance matching, I can recommend that you get a copy of the free program, RFSim99. A google search will let you know where to get it. You can play with L's C's and R's to your heart's content, and you can even let the program determine a match for you, if you know the impedance you want to present as a load. Just set the RFSim99 generator to that impedance and ask it to do a match for you.
Cheers, Tom
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