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Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
I suppose I am getting a little more serious about electronics now and I feel I need some good test equipment. I just went out and bought an Instek DDS function generator and now I want a nice benchtop switching power supply. I have seen these online for sale, but they seem to run atleast $300 for what I want, and these supplies don't have digital control (you can not "type in" what voltage/current you want).
I figure it might be worth my while to try to design a good smps to meet my needs. Something in the range of 0-50v and 0-5a seems about right. While I have seen many linear power supplies online, few have digital control and even fewer use PWM. Maybe the reason that the kind of supply I want is so expensive is because it is very hard to design one?
Anyway, I wanted to have a stab at it, so here is my design using at TL494, some ADC's and some DAC's. The DAC's set the comparison voltage that the 494 sees for current and voltage feedback. The ADC's are optional but allow for the microcontroller to know in real time what the current and voltage output of the power supply is. I intend to use an AVR micro to control the DAC's so that the user's voltage and current requirements can be met. I would also like to use a cheap LCD module to display the voltage output, current output, and current limit in real time.
My design uses dfferential voltage sensing so that the voltage drop across the current sense resistor does not cause voltage errors that grow with current output.
I have several questions for those more experienced than me:
1. Am I likely to have problems trying to achieve very low output voltages due to the fact that I am trying to use single supply opamps and their outputs will never really get to 0v? How close can a good rail to rail opamp get? Within say 10mv or so? 2. How good is a TL494 at dealing with very small input voltages? If i want my powersupply to output 1v for example, I will be feeding 50mv into the inputs of the opamp inside the 494. I believe I am concerned with the common mode input range here and the error at very small input voltages? 3. Is this design workable or possibly fundamentally flawed?
Maybe an expert knows how a real digitally controlled smps works and can shed some light on their design for me!
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
Hey, I have no idea how the digital SMPS work, but I have one of these: (30W version) and i can take some pictures of the internals if you want. From memory, it seems it uses multi point tapped transformer, and an H-bridge. A relay is used to switch between high power and low power modes, the high power mode being below 16 volts, up to 2.4 amps. low power mode is up to 32v and 1.2 amps. Another nice feature is the fan is always on low, until the current exceeds a certain threshold (300ma for this). Temperature would be better for protection, but this is nice for keeping a constant temperature.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Have you noticed you've asked one of those typical 4HV unanswerable questions - as you haven't specified the requirements of the load, you have no rational basis whatever for your power supply design. Do you need 10W, 1kW, 1MW - you leave it up to your readers to imagine what your requirements are, and respond in the same imaginary vein.
Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
The requirements of my load? Whatever I feel like hooking it up to that day! Perhaps a resistive load that I want a constant current through? or maybe a flyback driver? or perhaps a magnetic levitator even!
A ripple of 50mv or so would be nice as well but likely not required. Oops, I should have said it in my post as well, the supply should output 0-50v at 0-5a with digital control.
rp181: I hate to have you tearing your nice power supply apart just to snap me a few pictures unless you really want to. Does your power supply display a current limit on the screen, or the actual current your load is drawing?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
cjk2 wrote ...
The requirements of my load? Whatever I feel like hooking it up to that day! Perhaps a resistive load that I want a constant current through? or maybe a flyback driver? or perhaps a magnetic levitator even!
A ripple of 50mv or so would be nice as well but likely not required. Oops, I should have said it in my post as well, the supply should output 0-50v at 0-5a with digital control.
rp181: I hate to have you tearing your nice power supply apart just to snap me a few pictures unless you really want to. Does your power supply display a current limit on the screen, or the actual current your load is drawing?
Well, know you've formulated your answer - a variable supply offering 0 - 50V @ 0 - 5A, with a maximum ripple of 50mV, why do you need the help and advice of anyone else?
Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
Well, know you've formulated your answer - a variable supply offering 0 - 50V @ 0 - 5A, with a maximum ripple of 50mV, why do you need the help and advice of anyone else?
For the same reason that just because I want a car that can go 200mph does not mean I know how to build one. Just because I know what I want does not mean I know how to get there.
I am interested in learning how a "real" digitally controlled power supply works, maybe someone here has built or has worked on them before and can enlighten me. I am also interested in any input about the feasibility of building a power supply based on the design I posted. Thankfully, I find this kind of work very interesting. maybe I will just have to play with at TL494 for a while and see how it behaves!
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529
Someone like me doesn't just not take something apart Its quite a clean mechanical design, ile go take some pics now. EDIT: camera charging, will get the pics up tonight. The screen works as follows: When turned on, there is text saying "OUTPUT OFF" as well as the current set current and voltage When the output turns on, a side pixel indicates HI or LO mode. During operation, the supply shows the current being drawn, and the voltage for that current. If the load draws more current then it is set to, it automatically switches to CC, and shows the voltage being used to get to that current. If you want to set the voltage or current in operation, or see what its at, a button can be pressed to see the current, then voltage, and changed accordingly. Hope that makes sense. The supply has an RS232 interface, so that may lend to the complexity. The discerning sections seem to be half bridge (two sinks, each with one FET, and what looks like a diode) Power conditioning, line filter, and output of the transformer Display control main microcontroller firmware IC
EDIT (again): here are the pics. Front panel Overview(note: this pic came out very well, and you can see the part no. on most all of the IC's, if you want the 10MP version, pm me your email and ile send it over) One side of the bridge (?) Firmware IC next to microcontroller (if they are not the same) Main board (power control?)
Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
Thanks very much for sharing. I take it that supply uses linear regulation because I don't see any big inductors that would be used in a filter. I wonder if those fets are being used in some sort of chopper or (more likely by the size of the heat sinks) are being used in linear mode.
I suppose I will play around with the 494 when time permits and see how it behaves with the low input voltages, etc. I have also requested some ADC's and DAC's from maxim so when (if) they arrive I might just have a go at building something similar to my original circuit to see how it behaves.
It looks like I will need to do some reading on stability, frequency compensation, etc.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
> It looks like I will need to do some reading on stability, frequency compensation, etc.
Definitely. I'd particularly look at the control theory / stability / frequency compensation stuff. Also look into how you will test your PSU when you've designed and built it. Things like line and load regulation tests are essential, and load-step testing and s/c recovery are very important tests too.
For example if the load on your +5V supply suddenly goes from 10A down to 1A, does the voltage overshoot by only 100mV or as much as 2 volts!?!? How long does it take to recover before the output is back within "spec"?
It's also worth bearing in mind how cheap the far-east manufactured workbench supplies are these days. If you just want something to use rather than spending 6 months learning about SMPSUs and loop compensation, then buy one and start using it today in the knowledge that someone else has already done the tedious reading and difficult calculations for you!
Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
An update on all of this:
I have decided to build a slightly more traditional power supply to start. I am planning to build a switching supply with current and voltage feedback with an AVR controlled display to show voltage and current information. I want to use a voltage mode controller like an SG3525, MC33025, or UC3825 to control this supply. I will use pots like a normal bench top supply does to set max voltage and current.
I intend to document my progress here and eventually work my way up to a fully digital design if time permits. Lets get started then!
First, Is there an easy way to figure out how many volts per turn my core can take without saturating? I have a function generator, inductance meter etc. If it turns out to be hard to get a real number, ill probably just use 5v per turn or so.
Second, If i wanted an unused core, say ETD39 and bobbin, where could I buy one? All of the suppliers I have seen only sell large quantities or charge a $25 export fee, etc.
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Digital benchtop power supply design
Do you need 10W, 1kW, 1MW - you leave it up to your readers to imagine what your requirements are, and respond in the same imaginary vein.
Its quite a clean mechanical design, ile go take some pics now.
