RF power amplifier / oscillator for dielectric heating - Page 1
Forum section: electronics / rf-microwave

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: July 25, 2018, 09:07:09 am
content: I want to check out Dielectric heating. To start off I want to build something in the range of 100W to 400W. I searched around web and found some designs of Class C power amplifiers. I'm attaching those files here.Bascilly I can start with something like 100W and in the frequency range of 10Mhz.I was thinking of using a 10Mhz oscillator, are there any suggestions in building an 100W / 10Mhz power oscillator, something like a mazilli zvs / royer configuration. After building the oscillator how do I connect my load to it?, I mean the material to be heated would be placed in between to parallel plate electrodes so how do I connect it to the oscillator?

author: CopperCone
author_rank: Super Contributor
timestamp: July 25, 2018, 12:35:25 pm
content: don't you just use a H-bridge with mosfets for that frequency if you just wanna heat?it sounds kind of like a commercial 13.56MHz plasma power supply. They are on ebay cheap.It would also be good to use 13.56MHz for heating / industrial use since the band is approved legally for such purposes. 13.56 and its harmonics are in the ISM band so you won't jam anything important with a nasty square wavehttps://www.microsemi.com/document-portal/doc_view/124244-drf1400-13-56-mhz-class-d-half-bridge-rf-generatorhttps://ixysrf.com/wp-content/uploads/2017/09/3ap_3_5kw13_56mhz_gen.pdfthe schematic shows a typical impedance matching network.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: July 25, 2018, 03:09:44 pm
content: Thanks for the info. The plasma power supplies you are reffering to is a supply for Plasma TV? because when I find loads of them on ebay when I put in that search term.Currently I don't need high voltage. 100V or so would do and frequency above 1Mhz.

author: CopperCone
author_rank: Super Contributor
timestamp: July 25, 2018, 07:06:36 pm
content: no they are for making the phase of matter plasma typically under moderate vacuum for things like cleaning, plasma etching and other industrial processes that involve the generation of plasma.search forplasma generatornot plasma supply on ebay. plasma supply will give you TV crap.

author: capt bullshot
author_rank: Super Contributor
timestamp: July 25, 2018, 07:36:14 pm
content: Quote from: CopperCone on July 25, 2018, 07:06:36 pmLike one of these. It does 13.56MHz, adjustable 0 ... 300W into 50 Ohms and is most probably idiot proof in terms of impedance mismatch, open and shorted load.Didn't cost anything but a few words to the janitor ... errr facility manager

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: July 26, 2018, 05:27:45 am
content: Quote from: CopperCone on July 25, 2018, 12:35:25 pmI thought RF amplifiers operate in the linear region and not in switching mode, so square waves are not used? So how do these H bridge type mosfets operate in the linear region?Quote from: CopperCone on July 25, 2018, 12:35:25 pmThe schematic you are reffering to is in the first link or second?

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: July 26, 2018, 05:31:07 am
content: Quote from: capt bullshot on July 25, 2018, 07:36:14 pmSo these do high voltage and high frequency? Is the high voltage adjustatble. I mean at present I don't want to start of with high volatge. Something around 100 - 200V would do, but high frequency is a must, something above 1Mhz.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: July 26, 2018, 01:32:35 pm
content: RF amplifiers can be run as switching amplifiers., especially if you don't care too much about distortion (efficiency is far higher in switching modes).

author: capt bullshot
author_rank: Super Contributor
timestamp: July 26, 2018, 01:52:41 pm
content: These generators are specified for output power into a specific impedance, not voltage. This is usual for most kind of RF stuff. And this kind of generator is fixed to 13.56MHz.300W into 50Ohms equals 122Vrms, and the output power is adjustable, so the output voltage is adjustable as a result. But there's a caveat, as with most RF stuff, you'll have to load the generator within the specified range. Technically speaking, this means, you'll have to load it with a 50Ohms resistor to use it, otherwise it may limit its output, reversibly shutdown or irreversibly break.If you need other voltages, you can build a suitable RF transformer, but this has to be loaded with the correct impedance also.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: July 26, 2018, 03:48:35 pm
content: Thanks for all your valued inputs. Highly appreciated!So if a RF generated is generally matched to a 50ohm load:1. How do you determine that your load matches this 50ohm. In my case I have 2 plates in between which there will be some material for heating.2. If I am making my own oscillator how do we connect these plates to it, are there any known steps to follow.

author: capt bullshot
author_rank: Super Contributor
timestamp: July 26, 2018, 04:05:38 pm
content: In general, the load is matched to the generator by using a matching network. This is often a Pi-network (C-L-C) with adjustable capacitors and inductors. Amateur radio operators often use such matching networks for their antennas, but they're also applicable for your kind of load. Companies that build such generators also sell matching networks, manually operated and also automatic. Can't help you with the design of your own network, but this would be the answer for your second question (building a matching network or a transformer), too.You'll want a means to measure forward and reverse power at the output of your generator / oscillator, to find the best load match (max. fwd power and min. reverse power).

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: July 26, 2018, 05:21:28 pm
content: Quote from: capt bullshot on July 26, 2018, 04:05:38 pmThis is some pretty amazing info to get me started. I have a better understanding now than when I first posted this question.Thanks a million!