Thank you for the information, Richard. You and Sean have already given me direction that will make the design go easier. I tried Digikey without success and I put power FET in the search bar with no real luck. So I think I will follow your lead on this. Again, TNX. ----- Original Message ----- From: "Richard Prosser" To: "Microcontroller discussion list - Public." Sent: Sunday, August 10, 2008 10:11 PM Subject: Re: EE > Rich, > I'm not directly involved in the design of our power converters, but > do work with the guys that are and have some exposure to their > problems. For reference, the equipment is "Telecom" AC-DC "rectifiers" > converting AC mains to DC output at nominally 48V. (actually 53V or > so). This is accomplished by rectifying the input voltage, boosting it > to about 420V with pfc and then converting the DC back down to 48V > using an "LLC" converter and a transformer. The boost stage operates > at about 70kHz and the DC- DC in the 150kHz - 400kHz range. Power > levels are typically 1-3kW and efficiency about 92%. > > For both the boost and DC-DC stage we use N channel MOSFETs, the DC-DC > ones having a DC rating of 600V. (The boost ones can be higher - > 800V?). These are prettty heavy-duty items, packaged in T0-247 style > packages. Rds_on is around the 100mOhm area. AFAIK we have never used > P channel MOSFETS in complementary style output arrangements but have > used parallel MOSFETS in the boost stage. Matching does not appear to > be too much of an issue due to the positive temperature coefficent of > the MOSFET resistance.. > > A good source of datasheets / part numbers (& application notes) > would be the IR, ST, etc. websites. These can also supply data on > suitable drivers for the MOSFETs, including high side capable drivers > which use a capacitively coupled bootstrap circuit to provide a gate > drive supply above the input voltage. > > Possibly something like the STW26NM60 could be a starting point ? > (Taken from one of our older units so their may be a more modern, > improved version available). > > > Richard P > > > 2008/8/11 Rich : >> Richard, Thank you for the excellent advice. I am so pleased that there >> are >> many persons on the list that are more knowledgeable than I. I did not >> design the transformer. Years ago I designed a lot of E - I (refers to >> shape) laminated core transformers, computing the window, core material >> wire >> size (round or square) and so on. But I did not design high frequency >> torroid and I would have to invest some significant time to study up, so >> I >> had a magnetics engineer design and build the transformer. From what you >> say, I need to focus more on power FETs. I can parallel them for the the >> current load. >> >> I have had experience designing oscillators and power amplifiers >> including >> high frequency power amplifiers. One caveat here (100 KHz) is >> controlling >> crossover distortion, which, of course, is more challenging at higher >> current loads and unstable heating. So some basic design principles are >> in >> order. These currents at this voltage is a bit beyond what I normally >> design. I have designed high current or high voltage but not both >> together, >> so I need to identify the semiconductors first. >> >> It seems that you recommend power FETS, and that sounds plausible. I >> will >> try to find power FETs that I can match for parallel operation. Unlike >> bipolars in parallel where base resistors are required, that is not the >> case >> with FETs; no gate resistors are needed because the gates do not draw >> current, so the design is easier. >> >> I have not been able to find a 300 Volt @ 16 Amp (minimum spec) device. >> I >> would prefer a 400 V @ 20 Amp N- channel and matching P-channel. >> Otherwise >> I will have to use 2 N-channel FETs and drive them with a phase inverter >> on >> the lower section. It would be so much easier if there is a COS pair at >> the >> 300 V@ 16 Amp minimum, spec. >> >> ----- Original Message ----- >> From: "Richard Prosser" >> To: "Microcontroller discussion list - Public." >> Sent: Sunday, August 10, 2008 4:04 PM >> Subject: Re: EE >> >> >>> Rich, >>> >>> 1. You need to include ferrite losses in your calculations. The loss >>> will be highly dependent on the ferrite grade used, along with the >>> drive level (how close you are to core saturation). At 100kHz you may >>> also want to look at Litz wire. (Since you're using a toroid, I'm >>> guessing the core is not gapped. Core saturation and material magnetic >>> tolerances need to be considered.) It's not a trivial design task. >>> >>> 2. The transformer _may_ be more efficient with a sinewave input but >>> the linear sinewave generating process will not be. For overall >>> efficiency a squarewave generation followed by a tuned winding should >>> be OK if you get things right. A tuned winding may give you soft >>> switching capability and lower switching losses. >>> >>> 3. 100kHz is starting to push things for IGBT switches. MOSFETS are >>> more common at this frequency. We use 600V types to switch a 420V bus >>> in a half or full bridge configuration at power levels to about 2kW. >>> Frequency 150kHz - ~400kHz output. Highside MOSFET driver chips for a >>> squarewave are very easy to locate. (PWM can be more difficult if you >>> want continuous 0 - 100% pwm). >>> >>> 4. Overall, a lower frequency is likely to be easier to design & get >>> working but will take more space.. >>> >>> Richard P >>> >>> >>> 2008/8/11 Rich : >>>> 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 >> > -- > 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