1KW Variable SMPS University Project

[matt09]

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.

 

[matt09]

51 views 0 replies?

 

[MicrosiM]

Info

Hi

Welcome, May be this project is rare, and i dont know if other folks have some info.

I got some information's for you to start with your project, could you please take a look at the attached files, and let me knoe what do you think

this is a 0-50V 0-5A SMPS, you have to work around this design to get your goal


check this project also:- http://uzzors2k.4hv.org/index.php?page=benchsmps1


regards

 

[matt09]

Hi there,

Thanks very much for that, I have tried contacting the author of the circuit but not heard anything back. I have found a more up to date revision of the circuit with a balance coil on the input which apparently results in stable CC operation. It would be great if someone with experience such as yourself could assist with modifications to the circuit for 0ish-100V operation, or at least as wide a voltage range as possible. I will try a address each section of the design which will make it easier for feedback also.

EMI Filter: Since I am planning on operating at 50KHz I assume the EMI filter section can be left untouched.

Switching Transformer: I realise this circuit is designed for US voltage so this will require a change in turns ratio. I have used the Transformer spreadsheet on this site which gives me a turns ratio of 28 on the primary and 20 on the secondary. I have tried to understand many other formulas for wire gauge etc but keep getting lost in the calculations because of the variable duty cycle so some help here with worked calculations would be great. I was trying to work though the ones posted here http://www.diysmps.com/forums/showthread.php?302-My-first-build-smps-1kw-design-by-microsim..&p=5596&viewfull=1#post5596

I will spec parts for 1kw so that it is easy to fallback to 500w if I run into problems. For this I think I need to use ETD49 core which will support well over 1KW in half bridge at 50KHz with and N87 or N27 core (I dont think it matters which really?). Out of curiosity could the gate drive transformer also be interchanged with a high power opto-isolator?

Feedback section: I'm aware the gains of opamps will need changing to compensate for the higher voltages/currents although I'm not entirely sure what each of them are doing with regards to the overall design.

UC3825 Chip: I haven't spent much time looking into this yet, I see the duty cycle is rated down to 0% but surley this is impossible since is would require an infinitely small pulse width. With regards to achieving 0ish-100V will I simply be able to change circuit values and maybe manage it without any other components or will a whole redesign and other stages be necessary, again I'm a bit lost in the direction of this since there is so little available on what I'm trying to do. I also can't make sense of the choice of components for the frequency at 50KHz as on the datasheet the graph shows a different CT and RT values.

Cheers.

 

[matt09]

Anyone? I've seen people asking far more open ended questions on the forum than this

 

[Kanwar]

What you need exactly is a H-bridge stage configured as bidirectional converter, in which you can get voltages and currents of both polarities. You can do this by using class-d pwm technique which can enable you to get 0 to 50V range easily in either polarity.