Hi Sean: Thank you for the comments. The project application is proprietary but I can discuss the power supply. The transformer is designed for 220 VAC @ 100 KHz primary and the secondary delivers 3000 VAC 100KHz @ 1 Amp. It is not a laminated core transformer or a "Metglass" core but a ferrite core. It is a torroid. I want to drive it with a sine curve because it is more efficient. I had the transformer designed for 100 KHz operation, not 60 Hz, because the package must be kept to a small footprint and mass, so I thought to reduce the magnetics. My application does not require a ferroresonant design, and that is not in my power supply design specification. As for primary impedance, the 16 ohms suggested by Russell is perhaps unlikely. At 50 Hz or 60 Hz you can use Ohms law to approximate the primary or secondary impedance by Z=Vp/Ip = 220/13.5 = 16+ ohms. But at 100 KHz the skin effect modifies the winding resistance R, and the impedance is more closely approximated by the square root of [R + (2 pi f)sqLsq] or Zp= sqr root of Rsq + Xsq. It is easier to just measure the impedance. I apologize for the equations. Since the volt amps of the secondary equal the volt amps of the primary, the primary current will depend on the current that the secondary draws. VIp=VIs. Given that Is = 1 Amp and Vs= 3KV, solving for Ip I get about 13.6 amps. So I think I am stuck with the primary drive that I have described, unless I am not seeing this correctly. About creating a 50 ohm tank, that is not a hard spec. My thinking proceeds as follows. Your critique is welcome. If I drive the transformer with a square wave it will work, not as efficiently, but it will work. However, I will also get all of the even harmonics. That is not good, even though they could be filtered. The output impedance of, say bipolars, will be a function of the junction characteristic and the output circuitry and will probably be lower than the transformer primary impedance. Normally, driving with a lower impedance is desirable but in this (what I consider higher power application) I am expecting some aberration of the sine wave that is driving the transformer. The matched impedance should mitigate the aberration of the curve. I have not designed anything with IGBTs. I can design a circuit with bipolars to produce a fairly clean sine wave. But the problem with bipolars is the low beta. That means the base drive is going to be high. I can use power Darlington, but I will have to accept that they will run a bit hotter due to the higher VCEsat for darlingtons. HexFets might work because the multiple FETS in parallel reduce the on resistance. In a switching application I could expect ringing from the internal contact wire but this is a linear application and using a HexFet might not be a good choice. I thought of using a complementary symmetry N-Channel and P-Channel power FET, if I could find a pair without resorting to parallel operation, which could work I am in the early stages of the design so I can be flexible. The driving factor is the application, which has defined the transformer. So the circuit needs to service the transformer. I will not commit to a prototype circuit until everything is worked out on paper. I will appreciate any caveats that are pointed out because I cannot say that I have never done anything stupid. In fact I better not embarrass my self by saying how many stupid mistakes I have made, The only redeeming factor is that I am careful to make them once. Thank you for your input, Sean. I look forward to your response. ----- Original Message ----- From: "Sean Breheny" To: "Microcontroller discussion list - Public." Sent: Sunday, August 10, 2008 2:52 AM Subject: Re: EE > Hi Rich, > > Is there a reason why you are using such a high frequency to transfer > power? What does the secondary voltage/current look like? What kind of > core material does your transformer use? > > When the coupling between primary and secondary is close enough to 1 > and the impedance of the load is significantly smaller than the > inductive reactance of the windings, then a transformer looks "ideal" > in that the input impedance looks like the square of the turns ratio > times the load impedance. > > I do not think that you want to create a 50 ohm load - you have > already specified the input voltage and current so your input > impedance is determined (do you really have a setup which will draw a > constant 14A at 230V?). > > I also doubt that you want to create an LC circuit. That is usually > done when filtering is desired and the coupling between pri and sec is > much less than 1 (so that the Q of the LC circuit isn't too spoiled by > the load resistance). In "wireless" power transmission it is also > sometimes done to get around some of the effects of having poor > coupling coefficient (due to distance between the pri and sec). > > IGBTs or any other switching element for that matter will NOT of > itself create a sine wave. If you go that route, you will either be > driving the transformer with a square wave or you will have to do PWM > which is of a significantly higher freq than the sine wave you want to > generate. You could also go back to making an LC circuit and create a > class C amplifier, but that will likely make things bulky and somewhat > inefficient. > > Could you say a bit more about your application? The focus on > transmitting kilowatts at 100KHz seems odd. > > Sean > > > > On Sun, Aug 10, 2008 at 1:23 AM, Rich wrote: >> I wonder if I am mistaken here. The transformer primary is an inductive >> element. True, there is distributed coil capacity, and a frequency >> component that makes the system complex. So there will be some "natural" >> resonance that may not be at 100 KHz and may have a very low Q, >> regardless >> of the fact that the transformer was designed to operate at 100 KHz. But >> by >> adding the properly selected components, could one not design starting >> from >> the LCR characteristic of the primary winding to create a tuned circuit >> at >> 100 KHz and 50 ohms Z? Also, would it be true that if the Q is high the >> stability of the oscillator must be high in order to stay within the >> resonant bandwidth? So, a high Q may not be desireable and the R >> component >> would be a factor? All comments, criticisms, "Oh what stupidity" >> comments >> are welcome. >> >> >> ----- Original Message ----- >> From: "Vasile Surducan" >> To: "Microcontroller discussion list - Public." >> Sent: Saturday, August 09, 2008 9:07 AM >> Subject: Re: EE >> >> >>> Nice way of computing... A square signal bumped with a positive glich >>> at the end of the rising edge will increase the current with 10-20% at >>> the same secondary load. >>> Does the primary impedance seen by the driver will be different ? >>> >>> Vasile >>> >>> On 8/9/08, Apptech wrote: >>>> > Where did you get 16 ohms from? >>>> >>>> R = V/I. >>>> The driver sees the load reflected via the transformer. If >>>> 230 VAC causes 14 amps to flow then the AC is seeing 230/14 >>>> ~= 16. >>>> >>>> In the absence of load the actual transformer impedance will >>>> be seen but it will very usually b swamped when loaded. >>>> >>>> >>>> Russell >>>> >>>> >>>> I have to drive a transformer primary at 230 VAC @ 14 >>>> >>>> Amps @ 100KHz. I have not yet measured the primary >>>> >>>> impedance, but I will. >>>> >> >>>> >> The impedance you see will be about 16 ohms - ie the >>>> >> load, >>>> >> more or less regardless of the transformer's unloaded >>>> >> impedance. >>>> >> >>>> >> >>>> >> Russell >>>> >>>> -- >>>> http://www.piclist.com PIC/SX FAQ & list archive >>>> View/change your membership options at >>>> http://mailman.mit.edu/mailman/listinfo/piclist >>>> >>> -- >>> http://www.piclist.com PIC/SX FAQ & list archive >>> View/change your membership options at >>> http://mailman.mit.edu/mailman/listinfo/piclist >> >> -- >> http://www.piclist.com PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist