Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Bill Sloman Newsgroups: sci.electronics.design Subject: Re: CCFL transformer Date: Sun, 28 Apr 2024 16:51:09 +1000 Organization: A noiseless patient Spider Lines: 112 Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Sun, 28 Apr 2024 08:51:10 +0200 (CEST) Injection-Info: dont-email.me; posting-host="01858230f45e989aa1d995adc277cfad"; logging-data="972169"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+6/rACy2xOmh1YwhiI5gAlpqy6HUh1QMk=" User-Agent: Mozilla Thunderbird Cancel-Lock: sha1:8OZkJjO/TM8eZOV2ZK3in7loWxI= Content-Language: en-US In-Reply-To: Bytes: 5766 On 28/04/2024 2:49 am, Klaus Vestergaard Kragelund wrote: > On 27-04-2024 08:18, Bill Sloman wrote: >> On 27/04/2024 10:17 am, Klaus Vestergaard Kragelund wrote: >>> On 25-04-2024 09:02, Bill Sloman wrote: >>>> On 24/04/2024 3:10 pm, Bill Sloman wrote: >>>>> On 24/04/2024 12:25 pm, John Larkin wrote: >>>>>> On Wed, 24 Apr 2024 01:57:36 +0200, Klaus Vestergaard Kragelund >>>>>> wrote: >>>>>> >>>>>>> Hi >>>>>>> >>>>>>> I need a low distributed capacitance winding transformer, for a HV >>>>>>> step-up function (3.5kV) >>>>>>> >>>>>>> I am zeroing in on similar concept as CCFL transformers with >>>>>>> sectionalized bobbin. >>>>>>> >>>>>>> For example: >>>>>>> >>>>>>> https://www.coilcraft.com/en-us/products/transformers/power-transformers/ccfl-transformers/fl/ >>>>>>> >>>>>>> Possibly using Triple Insulated Wire to create some distance >>>>>>> between the >>>>>>> individual turns. >>>>>>> >>>>>>> Not many sells CCFLs these days. >>>>>>> >>>>>>> Guess I will keep it alive.... >>>>>> >>>>>> Can you use a C-W multiplier? >>>>>> >>>>>> For low current, you can do resonant tricks too. >>>>> >>>>> It's easier to use a voltage doubler or tripler that it is to find >>>>> a multi-section former off-the shelf. The occasional high voltage >>>>> power supply that I've dismantled clearly used proprietary formers, >>>>> as do the Coilcraft parts >>>>> >>>>> I suppose one could use self-bonding wire to make a series of >>>>> self-supporting pancake windings, but I've never heard of anybody >>>>> doing it. >>>>> >>>>> The Baxandall configuration is definitely a resonant trick, and >>>>> copes with the interwinding capacitance by resonating it with the >>>>> winding inductance. >>>>> >>>>> There's nothing "low current" about it, but if you are working at >>>>> higher currents and powers you can justify even more elaborate >>>>> switching arrangements. >>>>> >>>>> http://sophia-elektronica.com/Baxandall1959JM.pdf >>>>> >>>>> Jim Williams talked about it a lot - application notes AN45, AN49, >>>>> AN51, AN55, AN61, AN65 - but described it as a "a current driven >>>>> Royer inverter" which is simply wrong. >>>>> >>>>> MOSFETs work better as switches than bipolar transistors, and don't >>>>> seem to "squeg". >>>> >>>> The Coilcraft data sheets don't say anything much about the resonant >>>> frequencies of their transformers - except "The FL Series of >>>> transformers is designed for use in cold cathode fluorescent lamp >>>> (CCFL) power supplies at operating frequencies up to 100 kHz" where >>>> the "up to 100kHz" gives them a lot of wriggle room. >>>> >>>> A primary inductance of around 50uH with a 100:1 step-up implies a >>>> 0.5H secondary inductance. 10pF parallel capacitance would give a >>>> 71kHz resonant frequency, which is less than 100kHz. >>>> >>>> Of course once you have one of the Coilcraft parts you can measure >>>> the resonant frequency. >>>> >>> >>> Measurement: >>> >>> https://www.electronicsdesign.dk/tmp/FL2015-4D_primaryL.png >>> >>> FL2015-4D, primary inductance is 43uH. Resonance is 332kHz, reflected >>> capacitance to primary is 5nF. Reflected to secondary 100mH is 2.3pF >> >> There's no "reflection" involved. The resonance reflects the >> oscillating flux in the core, and the parallel capacitances of the >> primary and secondary windings both get charged up and discharged >> during the cycle. >> >> The parallel capacitance of the secondary will be higher, and the >> voltages across it much higher, so it is dominant. >> >> The resonant current is flowing through the capacitances so may not >> heat the insides of the winding wires. >> >> Measuring the self-heating of a transformer being resonated might be >> an interesting exercise. > > I wrote "reflected", since the inductance on the primary was the > measurement. The resonance of the transformer is the same on all > windings, if the coupling is reasonable good. > > So like you wrote, the secondary is dominant, which is why the primary > resonance is due to reflection from the secondary. There's no "reflection" involved. Both the primary and the secondary winding take part in the same process, and make their own - more or less independent - contributions. It's a pedantic point, but getting a proper grip on what going on in transformer does seem to be difficult, and it does take a while. Getting close to precisely the right point of view probably helps. -- Bill Sloman, Sydney