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  1. #1
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    1KW Variable SMPS University Project

    Hi all,

    I wish to build a variable lab style SMPS with output voltage from around 1V up to 100V and variable current from 0-10A. I have quite a lot of experience from doing electronics as a hobby for many years as well as studying the subject. I haven't tried anything like this before though. I am building this as part of a university project so this will actually happen and hopefully this thread will be open the next year or so as I progress.

    First of all does anyone know of any existing designs which can be modified to achieve the output I want (I've checked I am allowed to do this). I haven't been able to find any so far?

    Failing this I have already been looking into designing my own from scratch. I am struggling in choosing a topology, I know flyback is more suited to variable voltage but not suitable for high power and ideally I want to achieve 1KW or at least 500W. At the moment I am going with half bridge but could anyone tell me if this is good idea for variable voltage? I need to have isolation as it will be mains powered.

    I have various ideas so far on how to achieve such a wide voltage range from an SMPS with pulse width limitations. So far I am looking at doing one of the following after reaching my lower voltage limit, presumably of 5-10V.

    1. Have a second converter running in series off the output of the first which will rechop the waveform a second time and easily reach down to 0V

    2. The same about but have two running in parallel.

    3. Have a linear regulator stage running from the SMPS secondary which cuts in at a given voltage which will then go down to almost 0V

    4. Lower the switching frequency once a given voltage is reached so that a higher mark-space ratio can be realised.

    5. Feed the disable pin of a driver chip with a clock signal so that i get a duty cycle of the duty cycle, effectively missing out some pulses.

    If anyone could advise which direction to go on with this element that would be great.

    I am also struggling with choosing a base switching frequency as I want it to be as low as possible so that I can get the narrowest possible pulses, could anyone advise a starting point?

    I also have no idea if any of the 5 suggestions will be needed? It has been suggested that it will not be possible to achieve a 1% duty cycle to give a down to nearly 0V at any reasonable frequency. From my initial calculations this seems likely as I would need a td + tr of around 100nS at a low 40KHz switching freq. There are many Mosfets around this speed or faster, but I'm assuming I would never be able to reach this and get proper pulses in reality?

    I haven't looked into it yet but a head start on the best way to have a variable current limit from 0-10A would be great.

    I'd really appreciate any help as these questions are really holding me up from making progress with calculations and have done for some time. I probably already have most of the components and a suitable core from being a hobbyist for so many years but I will get on to this later.

    As the post is so long I have made questions bold to make it much more readable.

    Many Thanks,

    Matt.
    Last edited by matt09; 26th October 2012 at 17:35.

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    Re: 1KW Variable SMPS University Project

    Hi all,

    I wish to build a variable lab style SMPS with output voltage from around 1V up to 100V and variable current from 0-10A. I have quite a lot of experience from doing electronics as a hobby for many years as well as studying the subject. I haven't tried anything like this before though. I am building this as part of a university project so this will actually happen and hopefully this thread will be open the next year or so as I progress.

    First of all does anyone know of any existing designs which can be modified to achieve the output I want (I've checked I am allowed to do this). I haven't been able to find any so far?

    Failing this I have already been looking into designing my own from scratch. I am struggling in choosing a topology, I know flyback is more suited to variable voltage but not suitable for high power and ideally I want to achieve 1KW or at least 500W. At the moment I am going with half bridge but could anyone tell me if this is good idea for variable voltage? I need to have isolation as it will be mains powered.

    I have various ideas so far on how to achieve such a wide voltage range from an SMPS with pulse width limitations. So far I am looking at doing one of the following after reaching my lower voltage limit, presumably of 5-10V.

    1. Have a second converter running in series off the output of the first which will rechop the waveform a second time and easily reach down to 0V

    2. The same about but have two running in parallel.

    3. Have a linear regulator stage running from the SMPS secondary which cuts in at a given voltage which will then go down to almost 0V

    4. Lower the switching frequency once a given voltage is reached so that a higher mark-space ratio can be realised.

    5. Feed the disable pin of a driver chip with a clock signal so that i get a duty cycle of the duty cycle, effectively missing out some pulses.

    If anyone could advise which direction to go on with this element that would be great.

    I am also struggling with choosing a base switching frequency as I want it to be as low as possible so that I can get the narrowest possible pulses, could anyone advise a starting point?

    I also have no idea if any of the 5 suggestions will be needed? It has been suggested that it will not be possible to achieve a 1% duty cycle to give a down to nearly 0V at any reasonable frequency. From my initial calculations this seems likely as I would need a td + tr of around 100nS at a low 40KHz switching freq. There are many Mosfets around this speed or faster, but I'm assuming I would never be able to reach this and get proper pulses in reality?

    I haven't looked into it yet but a head start on the best way to have a variable current limit from 0-10A would be great.

    I'd really appreciate any help as these questions are really holding me up from making progress with calculations and have done for some time. I probably already have most of the components and a suitable core from being a hobbyist for so many years but I will get on to this later.

    As the post is so long I have made questions bold to make it much more readable.

    Many Thanks,

    Matt.
    Hi Matt
    What kind of PSU ? isolated or not isolated ? where your input voltage is came from ? from another source directly ? how much is it's value ?
    Best Wishes
    Goldsmith
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  3. #3
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    Re: 1KW Variable SMPS University Project

    Hi Goldsmith,

    I would like to use isolated topology as the input voltage will be rectified mains (240Vac here). This is why the topologies I mentioned are isolated ones.

    Thanks



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    Re: 1KW Variable SMPS University Project

    The dynamic range of your duty cycle will depend greatly on the speed of your switching devices and gate drivers. I've built converters which can operate effectively with on times down to 10ns, but you won't be able to achieve that with a 1KW design. 100ns might be possible, but you will need very good driver design, and good layout. A 40dB (100:1) duty cycle ratio is quite large, and you will probably need to allow frequency to vary in order to accomplish that.

    Having two stages of regulation may make achieving your dynamic range easier. But it will make the system control more challenging, since cascaded switching converters have a tendency to go unstable. Or you could try using a SMPS transformer with multiple taps, and use a relay to switch between them, effectively giving you a selectable turns ratio. I've seen it done before...

    The output current limit is another degree of complexity, since primary-side current sensing won't be accurate over a wide dynamic range. You'll need an isolated secondary side current sensor.



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    Re: 1KW Variable SMPS University Project

    I am aiming for 1KW but would anything over about 500W would be good, would this make reaching narrower pulses far easier as I'm trying to avoid things like frequency variation if possible.

    I really like your idea of multiple taps on a transformer, a relay could switch in somewhere around 30V or 50V and also switch anything required in the control circuit so that as a result pulse width suddenly increases again to compensate for the lost turns, yes? If this is correct this sounds the best solution so far as it would be really simple to implement, the only thing I ask is would it be more challenging to design my own transformer with 2 windings and get proper flux linkage in both.

    I have some little lem transducers already which give an isolated voltage out based on current but I was wondering what the best design system would be for this kind of project to limit current over a range (roughly).

    The question I most would like to know at the moment though is the basic topology to go with and starting frequency to try?

    Many thanks.



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    Re: 1KW Variable SMPS University Project

    Quote Originally Posted by matt09 View Post
    I am aiming for 1KW but would anything over about 500W would be good, would this make reaching narrower pulses far easier as I'm trying to avoid things like frequency variation if possible.
    Probably not. If you're working from 240VAC, then you'll be using ~500V FETs, which will be relatively slow regardless of the load. I'd guess you're looking at 200ns pulse widths at least.
    I really like your idea of multiple taps on a transformer, a relay could switch in somewhere around 30V or 50V and also switch anything required in the control circuit so that as a result pulse width suddenly increases again to compensate for the lost turns, yes? If this is correct this sounds the best solution so far as it would be really simple to implement, the only thing I ask is would it be more challenging to design my own transformer with 2 windings and get proper flux linkage in both.
    Putting more taps on shouldn't really be an issue. Each tap may add a little bit more leakage, but doubt it will matter much overall. The leakage introduced by the relay itself is another question. Also you will likely need to be careful when switching the relay in order to protect components from huge surge currents, and reduce output glitches. Controlling it with a MCU would probably make sense.

    The question I most would like to know at the moment though is the basic topology to go with and starting frequency to try?
    Half bridge should work well, but it's one of the more difficult topologies to get working. You might also consider using a two switch forward converter topology.

    40KHz is probably a good starting point. I would design your transformer based on that number, then adjust the exact frequency once you know the actual parameters of the transformer you built.



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    Re: 1KW Variable SMPS University Project

    Thanks, I have looked into forward converters and found the following.

    The major disadvantages of two-switch forward are hard switching and large switching.

    Low efficiency is due to several reasons. First, two-switch forward is a hard

    switching converter; the switches are hard turn-on and turn-off. This will increase

    the switching loss for high frequency operation. Second, two-switch forward has

    higher conduction loss compare with half bridge and full bridge converters. This

    is because the energy transfer only happens during two switches are on. Because

    of transformer reset requirement, the maximum duty cycle can only reach 0.5.

    Which means at best, only half of the time this converter can transfer energy to

    the output; this will increase the RMS current through the primary switch. With

    same reason, the voltage-second on output inductor is much higher in two-switch-

    forward converter compared with half bridge and full bridge converter.

    Because of these penalties of two-switch forward, it is not so widely used for

    this application now.
    If you were to build this supply yourself would you still choose the forward topology over the half bridge and if so why as this would be very useful to understand since I can't see much else distinguishing them. Also in what way is is more difficult to get working.

    Would you still recommend going with switched multiple windings rather than a switched lower frequency introducing a lower feasible duty cycle (although this then requires a variable frequency driver chip since I don't want to have to design that too)?

    Many thanks.
    Last edited by matt09; 26th October 2012 at 21:43.



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    Re: 1KW Variable SMPS University Project

    A commercial supply and its specification.
    http://www.mastechpowersupply.com/dc.../prod_111.html
    Last edited by ALERTLINKS; 27th October 2012 at 20:05.
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    Re: 1KW Variable SMPS University Project

    Thanks, that would be perfect if they showed a schematic which they don't. It is a requirement by the University that I must build my own supply and design my own PCB with a few circuit modifications at least for it to not be classed as plagiarism.



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  10. #10
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    Re: 1KW Variable SMPS University Project

    Quote Originally Posted by matt09 View Post
    Thanks, I have looked into forward converters and found the following.



    If you were to build this supply yourself would you still choose the forward topology over the half bridge and if so why as this would be very useful to understand since I can't see much else distinguishing them. Also in what way is is more difficult to get working.
    I would use the half bridge topology, because I've been able to use it in the past successfully. But it's overall a more complicated design, due to issues like flux balancing, current sensing, etc. The two topologies are about the same in terms of dynamic range. If you truly cared about dynamic range more than efficiency, then a flyback would probably be the way to go.
    Would you still recommend going with switched multiple windings rather than a switched lower frequency introducing a lower feasible duty cycle (although this then requires a variable frequency driver chip since I don't want to have to design that too)?

    Many thanks.
    For the switched windings to work you would have to plan how the control loops would handle the sudden change. If you are using a MCU for control, then you can easily build in some nice control scheme to handle the transitions (like making the instantaneous duty inversely proportional to turns ratio). If you use a strictly analog PWM controller, then you have to worry about things like changing loop dynamics, transients on the input and output, etc.



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    Re: 1KW Variable SMPS University Project

    what about using a full bridge SCR controlled rectifier as first stage



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    Re: 1KW Variable SMPS University Project

    You should consider UCC3895 and PSM (Phase Shift Modulation). This way you shoud be able to achieve your SMPS to go from 0V to your designed max voltage.
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