Thank you, Sean. I have not taken the opportunity to work with IGBTs so I really do not know much about them. I work alone in this lab so I don't have anyone to bounce ideas of. This list is invaluable for people like me who are not in a corporate setting. Fortunately, I have some good test equipment. It seems that both you and Richard agree that the square wave is the better approach. Certainly it is easier to design than than the sine wave generator. The only good news about the Sine wave generator is that the frequency is fixed and therefore the oscillator can be rock bound. I was not sure whether or not the square wave would create unacceptable losses in heat and core losses. The secondary output of 3KVA is very tight. I think I will proceed to design with the square wave approach and test the transformer under full load (pulling 1 Amp off the secondary). I have a spectrum analyzer that covers the 100 KHz easily and a 100 MHz digital scope so I can see pretty much what is going on. I had great success with a 5 KV@ 50 MA 25 KHz transformer by driving it with a sine wave. It was a variable output device. But I overdesigned everything. The transformer primary is 48 VAC on that. The transformer runs nice and cool. But it was a less demanding application. This is an interesting project, to say the least. Thank you for your valuable input. ----- Original Message ----- From: "Sean Breheny" To: "Microcontroller discussion list - Public." Sent: Sunday, August 10, 2008 8:27 PM Subject: Re: EE > Hi Rich, > > Ok, that sheds quite a bit more light on the issue. This is not all > that much different from a switching power supply. I think that you > will be able to get away with squarewave drive. I'm not sure about the > resonance issue (i.e., whether or not you want to allow extra leakage > inductance in the transformer and then tune it in order to get more of > a sine wave). > > I had only suggested IGBTs because they can handle much higher > voltages than MOSFETs. However, after seeing Rich P's post, I looked > on Digikey and there are indeed a few power MOSFETs with high enough > voltage ratings to do what you want. MOSFETs are preferable to IGBTs > if you can make them work in your circuit (which I now believe you > can) because they are much faster. Typical max switching frequencies > with IGBTs are around 20kHz due to saturated switching transit and > delay times in the BJT part of the device. Typical max for low Rds_on > power MOSFETs is around 1MHz (although switching losses - loss due to > the time spend turning on and turning off - will be rather high up in > this region). > > You definitely do NOT want to try to make a sine wave by using the > FETs or IGBTs or other active devices as linear amplifiers. That will > cause horrible inefficiency (much worse than the harmonics in the > wire/ferrite will). > > Sean > > > On Sun, Aug 10, 2008 at 2:18 PM, Rich wrote: >> 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 >> > -- > 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