| Bosium |
Hi there
Quick question. I have breadboarded a smps I have designed by picking bits up here and there. It is unregulated, but I'm not too worried because it's for car use. I plan to build two of these smps's and use them to drive a leach amp each, which I have already built (I can't praise this amp enough).
The problem is, I get an acceptable waveform from the push-pull stages, and the FETs switch, but they get so hot I could fry an egg on them. I tried increasing dead-time by increasing the timing cap like it says in the app notes of the sg3524, but still nothing. Have I wound the transformer wrong?
My osc runs at 50KHz btw. The outputs overlap at ground (or near ground) potential, but there is a fair amount of deadtime between the high states of each output. I've used the core from the current choke on the +5v line in a PC smps as my transformer core.
I have wound 5+5 turns on the primary and 10+10 on the secondary in the opposite direction. I know this will give me the wrong output voltage for leach amps, but this is just testing atm, not the "real deal". Can anyone help?
Thanks in advance.
Gareth |
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| Claude Abraham |
Dear friends,
Once again an inductorless schematic is presented and the circuit doesn't work. There seems to be no end to these no-inductor circuits. An SMPS operates on the following fundamental principal. A power transistor (usually MOSFET) is turned on and current is delivered from the input supply to an inductor, storing energy in the inductor's magnetic field. Also, in forward topologies, the output cap is being charged and the output is being supplied while the FET is on. When the FET turns off, its energy is transferred to the output cap and load. As the input voltage varies, the PWM controller adjusts the duty cycle to keep the output voltage constant. At light loads, pulses are skipped to prevent overcharging the cap. Without an inductor it is hard to maintain constant output voltage over varying input voltage conditions as well as varying output current conditions. The inductor also filters the switching pulses very well. For some reason a lot of folks don't believe in using an inductor, and I just don't understand it. Best regards. |
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| li_gangyi |
| so he needs to add an inductor for the circuit to work?? |
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| Claude Abraham |
| In order to make it work with good tight regulation, lowest noise, and highest efficiency, yes, inductors are needed. Two are needed in this case since there are two outputs, plus and minus. To improve cross-regulation, the two inductors should be magnetically coupled. Texas Instruments has good application notes regarding this. Also, the IRF540 MOSFETs are 100 volt devices. These are much more than required in terms of voltage. The automotive system voltage (12-14 volts) limits are 9 to 16 volts. A push-pull circuit places twice the voltage on each MOSFET, or 32 volts max. A 40 volt rated part provides some safety margin. If one wants bigger margins a 55 volt part is more than enough. The 100 volt IRF540 has much higher on resistance than a 55 volt part, the IRF1010, for example. THe IRF1010 is pin compatible, and should drop right in. The IRF1010 should run much cooler. Heat sinks should be used. Best regards. |
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| Bosium |
Hi.
Thanks for the reply, but it was a bit off topic. I know there's no inductor. that is purely academic at this stage.
My problem is that my mosfets heat up terribly without even switching a square wave into my primary. I need to rectify this before i can start playing with the secondary side of the smps. For the record, I do intend to put inductors on the output rails, and I want to use IRFZ44's, they just didn't happen to be in my cad program.
The FETs switch on and of fine, but when the transformer primary is connected they fry. I'm not a complete imbecile.
This design is built on a breadboard, so this should emphasise that I'm just trying to get it to switch FULL STOP.\
Anyone else have any idea? I would really appreciate some feedback.
Gareth |
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| jackinnj |
| quote: | Originally posted by Bosium
I have breadboarded a smps
I tried increasing dead-time by increasing the timing cap like it says in the app notes of the sg3524, but still nothing. Have I wound the transformer wrong?
Gareth |
In addition to the comments which have been made already:
First, breadboarding SMPS isn't a good idea. You've got to lay down a board -- i believe that in the case of the LM3524 National Semi provides you with a PCB template. There are all sorts of nasties running around and you have to pay close attention to layout. www.expresspcb.com has just reduced the price of their miniboards and improved their software, btw.
Dead Time instead try decreasing the duty cycle -- the Nat Semi apnote tells you how to do it. after all, the chip does have a voltage reference and an error amp on it -- this is what they are for.
the push-pull transformer supply, one of the 5 supply designs which Claude has referenced in the past, spreadsheet can be found at
http://www.tech-diy.com/smps_xfmr.xls
this should give you an idea of the peak currents, etc. the equations were cobbled together from Linear Tech's application notes.
jack |
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| Bosium |
Ok,
first, thanks for taking the time to answer my questions.
now, I am only breadboarding the smps to see whether the design is sound. I am not planning on drawing any current from it in its current state.
I study at Cape Tech, and we can have boards made here very cheap, ie about R50 for a 10x10cm board (that's about $6), and I have the necessary software. I have been told that layout and track lengths etc play a big part, and I also plan to make a ground plane on the board.
I had a look at the spreadsheet you linked to, and i'm not ashamed to say that it looks a bit complex for my purposes. Tons of people get smps's to work with relative ease. My lecturer says that I should be able to get a reasonable idea of how it works when it's breadboarded, atm it doesn't work at all. I have a funny feeling it's my transformer. I used a single enamel-coated copper wire (about 1mm thick) for my primary, and single strand +-0.4mm for my secondary. This is not how the finished product will be, I haven't even connected the secondary to anything.
I hope the core is suitable, as i said, it's from a power inductor from a pc psu.
anyway, ill try reducing duty cycle in the meantime, you never know.
thanks a lot.
Gareth |
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| hitsware |
| Your core is probably saturating.......You can't just take any core and use in these things ......... |
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| Claude Abraham |
| I would agree that core saturation is very likely the culprit. With a push-pull converter, the transformer core is being subjected to a volt-second (flux) ramp in both directions. If the two FETs possess unequal on resistance, or slightly differing pulse widths due to delays in the PWM controller, the volt-seconds in one direction will be unequal to that in the other direction. The core magnetic flux will creep positively or negatively into saturation. One method to prevent this is to use a series blocking cap. Better yet, a current-mode controller would sense and limit both volt-seconds and amperes, preventing saturation. Unfortunately, the LM3524 is a voltage-mode controller. Also, if the core is from a power inductor, designed to carry dc, it more than likely has a gapped core. This is not desirable in a transformer because the magnetizing current increases as a result, increasing conduction losses. Also, power is lost due to the gap. The right core geometry should be used (low leakage inductance), and the right material should be selected (ferrite, or at 50 kHz, a low frequency for an SMPS, maybe tape-wound nickel-iron is suitable), and no gap should be used (other than the small incidental gap which is inevitable when mating two halves of core material). The windings should be constructed for minimum leakage inductance, and low skin effect. Also, the full bridge rectifier is not used in the push-pull, but rather, the full wave center-tapped, followed by a power inductor, followed by the filter cap, for each output. App notes cover this. I hope this helps. Best regards. |
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| jackinnj |
remember to orient the beginnings and endings of the transformer correctly.
I drew in the error amp circuitry. if you make the drawn in resistor adjustable you can adjust the duty cycle of the LM3524.
I guess I have the inductors where Claude wants them.
3300uF is probably overboard -- I suppose that since you haven't used the "regulator"portion of the LM3524 there may be a reason for this. at any rate, such a high capacitance value just swamps the ability of the error amp to resond to changes in line voltage, etc. |
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| hitsware |
| Without the whole circuit, what are you doing to simulate the feedback? When fully operational you should not see anything near a squarewave with a light load. Just narrow pulses and some ringing............Looking at the the drains |
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| jackinnj |
i should have specified that the resistive voltage divider going from V+ to the LM3524's error pin should provide enough voltage for the duty cycle you want. I use these chips for driving a bigger H-Bridge for DC motors so use an adjustable pot in this position.
if you have a scope you should also make sure that the FET's are really turning off |
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| Bosium |
In reply:
I think you're right about the core saturating. That would explain why the fets are dissipating so much. I tried turning down the dut cycle, that worked a bit, purely becuase the fets were turning on less often. This doesn't help me though..
I'm going to get a small toroidal core in the next few days which should work if that is ideed the problem. btw, for reference, what exactly do you mean by transformer orientation? Do you mean i must use the correct wires to energise the correct windings? fair enuogh.
As far as output inductors go, i'll use the inductor toroin core i'm using as a transformer as a core for my power inductor(s). When i wnd it. do i wind both positive and negative windings in the same direction? I'm going to use one core for both.
re the filter caps, i specified 3300uF caps because I intend on drawing about 250Wrms maximum continous power from each (one) board to power a leach amp at +-58V. The large value allows good bass response, or so i understood. can anyone back me up on this? The output won't need to respond much, its input will be more or less constant.
Thanks
Gareth
btw, if anyone wants it, here's the cad .sch file as it is atm (ie without inductors etc.)
ps: damn. won't let me upload anything but an image. oh well. |
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| subwo1 |
| My impression is to wind them in opposite directions so that the magnetic flux in the core does not try to cancel from each polarity. I am not sure if if that is right though. Or just experiment to see what works better. |
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| EnvisionAudio |
I have successfully designed SMPS for car audio both with AND without output inductors. They're not necessary expenses for this "low power" application. I've found that IRF540s are not good parts for SMPS switchers as their Rds(on) is too high. An IRFZ44 or 48V or N is great for this app. Even an IRFZ34 is OK, but the 48s are pretty inexpensive these days. Also, the speedup transistors are just NOT necessary for two phases of two MosFets, even for fast switching. Just take them out and add 75-100ohm gate resistors. For more than two per phase, use the speed-up circuit...but it's just complicating things at this point. If you are concerned with shoot-through, move up to a SG3525 regulator which includes shutdown logic to prevent shoot through upon failure of the chip/oscillator.
For your overheating problem - I'll bet your core is saturating. OR the windings are incorrectly wound ie: out of phase. OR, the FETs are entering a linear mode (ie: oscillation). Usually this is due to stray capacitance - put those gate resistors close to the Gate pins and keep your traces to and from the FETs reasonably large and short. You can even pull the strands off the toroid and connect within mm's of the FET - no long traces! ;)
Hitsware: feedback is not necessary in this free running SMPS. Check out a few hundred car amps over the past ten years and get back to me on that. Most have NO feedback comp, though I disagree with that method. :whazzat: |
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| jackinnj |
| quote: | Originally posted by EnvisionAudio
I have successfully designed SMPS for car audio both with AND without output inductors. They're not necessary expenses for this "low power" application.
Hitsware: feedback is not necessary in this free running SMPS. Check out a few hundred car amps over the past ten years and get back to me on that. Most have NO feedback comp, though I disagree with that method. :whazzat: |
Nope, you need the inductors. Perhaps you should take a look at the output on a scope. There are spikes generated because of the inductance leakage in the transformer in a push-pull design. The equation is:
L=0.5Vo / (I*f)
Feedback -- I showed voltage feedback to the error amplifier -- you should remember that in an automotive environment you can have some significant swings -- obviously a good amplifier design will accomodate variation in the supply voltage, but since this design calls for more than 4X stepup through the transformer, any variation in primary voltage is multiplied by the same factor. You can damp the resonse of the error amplifier.
There is a current limit function on the LM3524 -- not the "Current Mode" which Claude referred to, but if it's there, why not use it.?
Here's a graph snipped from Pressman's book ("Switched Power Supply Design") on the inductance leakage spikes: |
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| Eva |
The core used in coupled inductors in AT and ATX power supplies are made of iron powder
They have very low permeability and are intended to make power-storage inductors to be used in buck or boost regulators [or in the secondary side of the transformer if you want regulated output]
These cores are not suitable to be used as transformers in simple magnetic coupling circuits, for that purpose you must use a ferrite core
MOSFETs heat a lot simply because they are conducting lots of current due to low core inductance but very little power is drawn from the 12V supply as current is cycled in both directions from/to core and supply capacitors
Simple magnetic-copupling SMPS need no inductors in the secondary side, both sides are simply coupled like primary and secondary in a 120V or 230V AC transformer with its output rectified
Altough inductors are necesary only if supply has to be regulated, a pi filter is recommended to filter the ripple caused by diode switching and capacitor discharge during deadtime on the output |
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| BeanZ |
The core is saturating. If you are using a iron powder core for the transformer, you are definitely using the wrong material. When you design the transformer follow faraday's law. You must know the DC input voltage that you are switching, the operating frequency, the cross sectional area of the core, and from that you will calculate the number of turns and choose the operating flux. At 50kHz operating frequency and some sort of push-pull topology, you should design the operating flux to be around 800-1000Gauss so that the core losses are minimized. It is also possible even with a soundly designed core to saturate it over time. By using a topology that traverses the core's B-H curve in the first and third quadrants, the core can "walk" up the curve if each half cycle is not an equal and opposite Volt-second product. You can get in touch with me if you want help in designing the transformer. I also have some ferrite toroids that I could sell to parties interested in the USA.
BeanZ |
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| jackinnj |
| quote: | Originally posted by BeanZ
The core is saturating. If you are using a iron powder core for the transformer, you are definitely using the wrong material. |
OK, I am going to assume that we are going to use a proper core -- the Amidon website can help here --
I put a spreadsheet on my website for calculating the turns ratio, inductance of the primary, making assumptions such as the input voltage, output voltage, current, permissible ripple. It does work, by the way!
the physics can be found on Linear Tech's website for the LT1683,
http://www.tech-diy.com/smps_xfmr.xls |
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| hitsware |
>Hitsware: feedback is not necessary in this free running SMPS. Check out a few hundred car amps over the past ten years and get back to me on that. Most have NO feedback comp, though I disagree with that method.
I didn't follow the circuit close enough to tell if it required the loop to be closed or not. I know switchers can be configured without regulation, BUT ....... Most of the car amps I've seen do use it. I guess I've looked at a differant 100 than you have :) |
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| subwo1 |
Or, for winding the core, if you are not good at math, you can place a light bulb in series with the power input to the circuit or the transformer as Eva suggested, and add turns to the primary until it can idle without anything heating up.
Bean Z and Claude Abraham, is there something about this area of Ohio where we just seem to be interested in SMPS design? I can't send an email because my browser is dysfunctional. You can reply that way if you feel like saying "Hi". |
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| jackinnj |
| quote: | [i]
Bean Z and Claude Abraham, is there something about this area of Ohio where we just seem to be interested in SMPS design? I can't send an email because my browser is dysfunctional. You can reply that way if you feel like saying "Hi". [/B] |
You have "Boss Kettering" on the Brain ;) , at least those of you from the Dayton area.
and how's about those 15 foot swells on Lake Erie? That's what sunk the Edmund Fitzgerald! |
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| subwo1 |
| quote: | Originally posted by jackinnj
and how's about those 15 foot swells on Lake Erie? That's what sunk the Edmund Fitzgerald! |
That makes three others now.
That area must be in the Great Lakes Triangle.
Just thought of something, though, I don't think email will work either because of anti-spam filters. |
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| Claude Abraham |
| quote: | | Bean Z and Claude Abraham, is there something about this area of Ohio where we just seem to be interested in SMPS design? I can't send an email because my browser is dysfunctional. You can reply that way if you feel like saying "Hi". |
Well, my interest in SMPS design is necessary as I earn my living designing electronic products. The input power source I've designed around over the last 25 years has been the 12 and/or 24 volt automotive bus, 115 and/or 230 volt 50/60 Hz commercial power, 28 volt military bus, full-wave-rectified 12 & 24 volts ac, and batteries. Output voltages range from 1.8 volts to 450 volts. With an SMPS, the options are numerous. One can convert an input to just about any value of output with high efficiency and small size, especially in ac offline applications where 50/60 Hz transformers used in non-SMPS are large and heavy. In addition, I happen to have a personal fascination with SMPS, including design of the custom magnetics. With SMPS design, virtually every aspect of EE comes into play. Analog circuits, digital logic, magnetics, e/m fields, automatic control loops, feedback & stability, math, component parameters & limitations, rf & emi filtering, all come into play with SMPS design. I am basically a nerd. I really enjoy this stuff. |
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| subwo1 |
| quote: | Originally posted by Claude Abraham
One can convert an input to just about any value of output with high efficiency and small size, especially in ac offline applications where 50/60 Hz transformers used in non-SMPS are large and heavy |
This advantage is my main motivation. |
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| JOE DIRT® |
Claude if you can spare the time please enlighten us on your concepts
DIRT® |
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| jackinnj |
| quote: | Originally posted by Claude Abraham
One can convert an input to just about any value of output with high efficiency and small size, especially in ac offline applications where 50/60 Hz transformers used in non-SMPS are large and heavy. In addition, I happen to have a personal fascination with SMPS, including design of the custom magnetics. With SMPS design, virtually every aspect of EE comes into play. Analog circuits, digital logic, magnetics, e/m fields, automatic control loops, feedback & stability, math, component parameters & limitations, rf & emi filtering, all come into play with SMPS design. I am basically a nerd. I really enjoy this stuff. |
I am disappointed that neither of you guys got the reference to Kettering -- the inventor of all the electronics which went into GM vehicles from the '20s I believe -- his life story is as worthy reading as Alfred P. Sloane's "My Years with General Motors", and he was from Ohio. |
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| subwo1 |
| quote: | Originally posted by jackinnj
I am disappointed that neither of you guys got the reference to Kettering -- the inventor of all the electronics which went into GM vehicles from the '20s I believe -- his life story is as worthy reading as Alfred P. Sloane's "My Years with General Motors", and he was from Ohio. |
Sorry, I thought the knowledge was presupposed on your part. Kettering was the inventor of the electric automobile starter at that. |
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| EnvisionAudio |
Interesting. I just moved to Kettering just a few months ago after living all my life in Des Moines, IA. Number of reasons for that including all the great industrialization history around here. Plus, wow - the locals know and love electronics. It's simply amazing.
I've attempted ONE offline powered SMPS, and needless to say I didn't have enough time to complete it - or actually make it work properly. :confused: I would enjoy learning offline SMPS in more detail.
I'm no math wiz by any means, but I'm capable and willing to learn - I guess I'm just intuitive about this stuff so I stick with it. ;)
I'd love to hear from other Dayton area folks about SMPS. Maybe we can start a little club or something...:rolleyes:
Regards,
Aaron Hammett
aaron@envisionelectronics.com |
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| jackinnj |
an OFFLINE Smps is daunting -- I started making flyback converters with the LM3524D about 6 years ago, then used the Nat Semi Simple Switchers for buck, boost and inverting, and now tinkering around with very low noise push-pull -- the idea being to get microvolts of noise instead of the millivolts -- just got in a bunch of LT3439's from Linear today along with LT1964's, LT1761's -- I will go blind soldering these thin-SOT devices (actually "toaster oven reflow" !) So we are straying a bit from car-audio since the ambient noise levels in the car and tens of decibels different from those in my den.
There is a well thought-out design in the ARRL handbook with details on winding the transformers. Perplexingly they use bipolar switching transistors AND the LM3524 (which isn't a current-control chip.)
The HamVention is staying in Dayton, btw. |
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| BeanZ |
I also make my living designing electronic products. Most of which are custom high reliability aerospace application products. I, myself, am primarily an analog / power engineer. I have designed a few switchers including a ZVS and an active power factor correction front end with universal input. If anyone has any questions about switchers or wants some incite or questions you can email me at BeanZAudio@columbus.rr.com. I will try to help as much as I can. Just bear be patient for a reply.
BeanZ |
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| Claude Abraham |
| quote: | | Claude if you can spare the time please enlighten us on your concepts |
Originally posted by Joe Dirt
Well, Joe, it's a very broad question, but I'll give it a try.
The biggest challenge when designing an SMPS, is accomodating a wide input voltage range. This places high demands on the components used. The automotive input voltage spans the range from 9 to 16 volts, or a ratio of 1.78:1. Likewise with the 24 volt automotive range in some parts of the world (18 to 32 volts, 1.78:1 ratio). If the SMPS is to accomodate either input, 9 to 32 volts, the ratio becomes 3.56:1, making the design more difficult. The power MOSFETs, capacitors, and rectifiers must withstand the highest voltage when off, but must carry the largest current at minimum input voltage.
My first choice as far as topology goes, is a non-isolated (transformerless) dc-dc converter. A simple inductor is a catalog item, obtainable in a day from several suppliers. A simple inductor carries continuous current, which results in the lowest losses. A transformer, OTOH, is usually a custom wound item. With home-wound transformers, low leakage inductance is hard to obtain. I read posts from hobbyists who add a few turns here, remove a few there, etc. At the high frequencies used in SMPS, leakage inductance seriously impairs reliability, performance, and efficiency. The leakage inductance stores energy, which must be absorbed as heat with snubbers, or saved, and recycled. Without snubbers, the energy stresses the components. Also, in order to minimize leakage, all windings should span a full layer. This assures maximum flux coupling. This may mean parallelled multifilar windings to fill the breadth of the window. Interleaving the windings may be necessary if more layers are needed. Also, with a transformer, the current is not continuous. It flows for part of the cycle, and is zero the rest of the cycle. This means more ac ripple current for a given average (load) current. More ripple current means more increased conduction losses due to skin effect. A simple inductor conducts current continuously (in a continuous conduction mode design), and losses are minimized, as well as skin effect, due to lower ac ripple current. With a simple inductor, leakage inductance, and the circuitry needed to deal with it, is not present. If isolation is not needed, one should strongly consider a simple inductor, without a transformer. There are applications, however, when a transformer is a good idea, even when isolation is not needed. I'll cover that shortly.
As far as control methods go, I use both voltage-mode control (VMC), as well as current (CMC). I definitely prefer CMC, but I can get results nearly as good with VMC depending on the application. CMC produces quicker transient response, better line and load regulation, is easier to stabilize, and offers better control of maximum current. The VMC control chips do include peak current limiting, but the tolerances are generally looser as far as the peak limit threshold goes. There seems to be some confusion regarding current limiting. Adding peak current limiting to a voltage mode controller essentially makes it a voltage mode controller with peak current limiting, *NOT* a current mode controller. A CMC has two feedback loops, one for output voltage (outer), and one for inductor current (inner). A VMC has one loop, for output voltage control. With CMC, the error amp output is always being compared to the sensed current, and when the current exceeds the error signal, the FET is turned off by the comparator. This is true at all levels of load current. With VMC, an artificial ramp is generated with an internal oscillator, and the error signal is compared to this ramp. When the ramp exceeds the error, the FET is turned off by the comparator. The peak current limit on VMC takes place after the error amp and comparator. A logic gate has an input for the peak current limit signal. When active, the gate is switched off, disabling the FET. With peak-limited VMC, the sensed current is an active part of the circuit only during output overcurrent conditions, and has no effect during normal operation. With CMC, OTOH, the current signal actively controls the FET always, even at light load currents.
As far as topologies go, the buck converter is the best choice for non-isolated applications. It is a member of the forward family. It is the quickest, offers lowest output noise, places minimum stress on output caps, and easiest to stabilize. The buck can only step the voltage down, so the desired output voltage value must be less than the minimum input. This is the principal limitation.
If the output is greater than the minimum input, the buck cannot be used. The remaining choices are members of the flyback family. They are the boost, buck-boost (inverting and non-), SEPIC, and Cuk converters. The boost can only step the voltage up. It places minimum stress on the input capacitor. The non-inverting buck-boost and the SEPIC, can step up or down. If the output value lies in between the min and max input, these are the circuits to use. For negative outputs, the inverting buck-boost and Cuk ("C" pronounced like the "ch" in "chair", and it rhymes with "fluke", i.e. "chuke"), resp. are the choices. These flyback circuits should not be used for very high power outputs (75 to 300 watts depending on voltage and current values). Flybacks are the noisiest, and stress the parts the most, and have sluggish transient response. They are not a problem as long as they only need to convert a low level of power. If higher power is needed, and output voltage exceeds the input, the buck cannot be used, so a transformer-isolated converter is needed. I've already covered them previously, and I'll add more on these in a later post.
I use power MOSFETs exclusively, Schottky rectifiers, continuous conduction mode inductor current, and large-valued X5R ceramic input and output capacitors. These newest X5R caps have super-low esr, and give large capacitance value in a compact size. My latest products feature 22 uf, 6.3 volt X5R caps in a 1206 SMD body. I use the newest PWM controllers, with external frequency compensation, allowing me to extract peak performanc, with any type of output cap. For logic and microprocessors, I use hysteretic PWM controllers, for quick transient response. For other applications, I use fixed frequency PWM for lower noise.
I've designed maybe 40 to 50 SMPS over my 25-year EE career, and 25 or so custom magnetic parts for SMPS, including filter chokes, flyback transformers, forward xfmrs, push-pull xfmrs, half-bridge xfmrs, common mode chokes, & rf chokes. My creations can be found on everything from defense and aerospace equipment to consumer electronics. One final caveat. I've had people confront me with a design, component choice, or technique which I found to be bad practice, and urged them not to use it. The response I usually get is "Hey, it works!" My respone is "How well?" Many ideas "work". It is really difficult to obtain small size, affordable cost, cool temperatures, high efficiency (long battery life), and low noise. Those who can really know SMPS. I've given drum lessons for years. I'll never forget what Susan, a former student of mine said. "You can't fake a roll." If you've played drums, you know that if you haven't been practicing, the roll fades quickly, and there's not much you can do to disguise a poor roll. Here's Claude's analogy to Susan's drum statement. *You can't fake efficiency*. I've really been running on. I hope you've found this to be valuable. Best regards. |
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| jackinnj |
| quote: | Originally posted by Claude Abraham
[I]"You can't fake a roll." If you've played drums, you know that if you haven't been practicing, the roll fades quickly, and there's not much you can do to disguise a poor roll. Here's Claude's analogy to Susan's drum statement. *You can't fake efficiency*. I've really been running on. I hope you've found this to be valuable. Best regards. |
when you say "roll" I think of some guy or gal up there in an F-16 --- and you have to think of your triple-redundancies. There is no margin for failure even in consumer systems -- like ABS braking.
As a singer, I know that you can't fake an "A above high C" (unless you are a contra-tenor and someone checks your shorts!). good post ! |
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| JOE DIRT® |
Claude very good post.......ever think of joining WATT>??...you could get a recognised paper;)
Cheers The DIRT® |
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| EnvisionAudio |
Eh, I'm glad to be learning this stuff from the best. :nod:
My feeble experiences are in push-pull supplies for 12V apps. I started with them about six years ago after seeing a bazillion car amplifiers with the same type of supply. I figured "Hey, if they can do it - I should be able to figure it out, I guess..." I read as much as I could find on the subject at the time then built my first Toroid transformer from a reject core out of a really junky car amp. I "litz" wound several strands of RadioShack enamel wire and proceeded to wrap them, paying close attention to phase and wrap style. It produced +/- 25V. I designed it to work with a Class D amplifier I'd designed then from a HIP4080 motor driver IC that could handle some nasty 0.5 ohm loads that car audio heads were into at the time. The toroid could deliver 600W of power before saturation, which was pretty remarkable for my "first time". I still have the tranny and use it for "testing" supplies I built.
Someone earlier mentioned that a pushpull supply MUST have inductors at the output or you'll get ringing...well, I get ringing if the layout's poor, but some basic snubbers have always worked well for me. I get a VERY clean square wave pulse up to around 80% of full power output, then things kind of go to heck - but it's been awhile since I've built one. I'm completely interested in returning and making some truly nice (ie: well designed) supplies.
Got a good source for enameled wire? My fingers are getting calloused just thinking about it. :) |
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| jackinnj |
| quote: | Originally posted by EnvisionAudio
Someone earlier mentioned that a pushpull supply MUST have inductors at the output or you'll get ringing...well, I get ringing if the layout's poor, but some basic snubbers have always worked well for me. I get a VERY clean square wave pulse up to around 80% of full power output, then things kind of go to heck - but it's been awhile since I've built one. I'm completely interested in returning and making some truly nice (ie: well designed) supplies.
Got a good source for enameled wire? My fingers are getting calloused just thinking about it. :) |
Inductors and layout -- don't forget that there is a large di/dt on some traces, thus the chip manufacturers all recommend "fat" traces. What may appear to be an innocuous ground return will carry a very large current for a very brief period of time, so it is also storing energy, has a high impedance.
Transformers -- unless you are going to buy 600 of them on a reel from you wind up with home-wound and leakage -- to deal with it
Magnet/enamel wire -- an interesting problem in linear programming -- fatter is better but you pretty soon wind up with no area with which to wind turns. I guess the next thing I will have to put on my website is an Excel spreadsheet which optimizes wire size given the type of core, current etc. I am going to have to wait for a rainy or snowy day, however. |
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| EnvisionAudio |
| quote: | | Magnet/enamel wire -- an interesting problem in linear programming -- fatter is better but you pretty soon wind up with no area with which to wind turns. I guess the next thing I will have to put on my website is an Excel spreadsheet which optimizes wire size given the type of core, current etc. I am going to have to wait for a rainy or snowy day, however. |
I just looked at "what everyone else is doing" in terms of wire gauge. I have some wound with 16 and 14 gauge while others are several smaller strands paralleled. I've also gotten used to winding the primary with lower gauge (step up) like a standard AC mains transformer. That really saves space.
I've got a question, though. (well, more than a few) What determines the amount of wraps on a given core? I've always thought there was a fine balance between fewer wraps and multiple wraps, but is it related more to efficiency or overall power output? Say, for example, the turns ratio is 1:3 and you choose 6 turns primary, 18 secondary (CT). In my experience, this is pretty low, but I don't know why - it just looks bare. :rolleyes: But I've seen SMPS with just that - very few turns. Others are chock full of turns, like 12:36 (CT) or more - even those using power MosFets (IRFZ44), where it would seem inefficient to do that...? I'll get some pics posted with examples if that would help.
TIA,
Aaron |
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| jackinnj |
the number of turns is a function of the inductance needed AND the permeability of the material.
you can use my plug n play Excel calculator to determine the required inductance -- it's just the equations from Linear Tech's website -- you have to know the switcher frequency, nominal current, ripple and a couple other things. The calculator just spills out the inductance and turns ratio for you.
http://www.tech-diy.com/smps_xfmr.xls
there's good information at the Bytemark website www.bytemark.com |
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| Tensop |
Hi,
i'm currently playing around with a pcb for use with a computer powersupply to bring 240 volts down to 12v, then up to 35v.. and one area of the design is annoying me a little bit.. the trace length on the mosfets.
I have the layout symmetrical so the trace resistances(althoug minimal) will be identical on each side.
Also the trace length to the gates will be as short as possible.. however i cant get it that short.
In a computer motherboard the trace distance between the mb SMPS PWM controller and the gate of a mosfet is around 2cm at most..however its all surface mount which makes mounting easy.. even easier as they use 4 or even 6 layer PCBs
the current trace distance from the output of the SG3525 and the input on the mosfet, is.. as measured at around 8cm for the right mosfet, and around 6cm for the right mosfet. this is one side. it is identical to the other 2 mosfets.. as i said its symmetrical.
Included is a picture, your thoughts/opinions please.
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| EnvisionAudio |
What is the question here? I didn't see one.
If your question is: "Is it going to matter is one gate trace length is 2cm longer than the other?" then the answer is "no" with consideration of the switching frequency. The addition of gate resistors pretty much stops parasitic oscillations from ever reaching the FET gates. What's your switching frequency? |
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| raidfibre |
| quote: | Originally posted by Bosium
Hi there
Quick question. I have breadboarded a smps I have designed by picking bits up here and there. It is unregulated, but I'm not too worried because it's for car use. I plan to build two of these smps's and use them to drive a leach amp each, which I have already built (I can't praise this amp enough).
The problem is, I get an acceptable waveform from the push-pull stages, and the FETs switch, but they get so hot I could fry an egg on them. I tried increasing dead-time by increasing the timing cap like it says in the app notes of the sg3524, but still nothing. Have I wound the transformer wrong?
My osc runs at 50KHz btw. The outputs overlap at ground (or near ground) potential, but there is a fair amount of deadtime between the high states of each output. I've used the core from the current choke on the +5v line in a PC smps as my transformer core.
I have wound 5+5 turns on the primary and 10+10 on the secondary in the opposite direction. I know this will give me the wrong output voltage for leach amps, but this is just testing atm, not the "real deal". Can anyone help?
Thanks in advance.
Gareth |
This thread is really old but nobody answered the original question. The SG3524 has a single uncommitted NPN transistor on each output. You cannot drive a totem pole follower without a pull-down resistor. |
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| funberry |
The pulldown resistors are definitely the answer here.
But I noticed another problem area: your XF core.
Using surplus or recovered parts is a great satisfacton for a hobyist.
However, using an unknown core for the power X-former in a hi-powered SMPS is the worst idea. It's just like using an unmarked capacitor or resistor, and having no way to measure it's value, but throwing it in the circuit anyways.
Most core materials you're likely to find in recovered inductors are not made to the stringent requirements needed for a hi frequency power transformer core.
You'll find that ferrite cores for really powerful high frequency transformers (200-500 W and higher) are not easy to come by.
Furthermore, Push-pull designs are prone to core walk-up if even the smallest asymmetry exists between the driver transistors, saturating even a well-calculated core.
Your best bet, if you want surplus, is to cannibalize an existing SMPS, and use the power x-former from that circuit as your core (strip the windings, and rewind). You need to make sure that:
1. the SMPS you're stripping was rated for at least the same power output as your design.
2. know what topology the surplus SMPS was using, ie if it was flyback, expect an air gap in the transformer core. If your topology is push-pull or forward, then the gap won't do. Pay attention to that. You may be able to change your design to a flyback just to use a desirable gapped core, if that's what's available.
But don't use any cores that served a function other than of power XF in an SMPS, because those cores can and will screw around with your head. |
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