Hi All,
Please allow me to introduce myself. My name's Matt, I'm a scientist by trade and I've been into hobby electronics for about 3 years now, having spent about the last 2 years reading and SPICE simulating various SMPS and DC-DC converters. So I'm just starting to feel comfortable with building one for the first time. I'm glad I've found somewhere where amateurs are discussing this sort of thing, last time I came to this site I believe the SMPS section was decidedly empty.
I've been reading around here at all the "I have no electronics knowledge, please give me a 1kW phase shifted full bridge converter schematic so that I can blow myself up" (paraphrasing) threads and I find these both mildly amusing and quite scary. So I'd like to propose a more realistic first project and I'd like some input from the more experienced members. I'm a member at AX84.com and I like their approach to useful beginner projects, which I think we could emulate.
I'd like to build a bench power supply for general low voltage use. The sort of thing you could easily buy but the point is to learn something.
It should be:
-Offline input (an isolating topology) or a post-regulator for an AT/ATX PSU.
-universal input voltages (i.e. somethign like 90-250V ac)
-Adjustable output something like 5-30V dc, 5A.
-Efficiency is secondary to simplicity, and cost should be reasonable considering the lack of economies of scale.
-Should be a through-hole design which people could more easily make by hand than SMD, although the trade off is increased parasitics and EMI.
-I'd like to make it with discrete components rather than controller ICs for the sake of education, but I accept that this goes against the simplicity case and adds unnecessary parasitics (still a TL431 is often used as a discrete secondary side error amplifier and that seems to work OK) any input on this?
I'm thinking this is probably in the range of a forward converter. I have read that the linear transfer function of buck-derived topologies makes them more suited to adjustable out/in voltages than boost or buch-boost derived topologies. I have also read that voltage-mode control is better for this than current-mode control, (especially with feedforward of the input voltage to the PWM ramp voltage) but I don't know really which is more appropriate.
I've also put a lot of thought into running it off the 5V or 12V output of an AT/ATX power supply with a buck/forward converter post-regulator (depending on whether the required voltage output range is above 12V) People frequently hack these into "lab bench power supplies" but in really rubbish ways and I thought we could do better and give it a variable output and current limiting. Even if a forward converter is used, isolation is not necessary and this would remove the complexity of an optoisolator (which I'd like to avoid if possible, optos need to be tested for stability as their bandwidth etc widely varies between devides and amateurs don't have the tech for this), and the input voltage would be a steady low voltage, the universal input voltage range is taken care of by the ATX PSU.
I think the output current would probably have to come down from 5A if a forward converter was used after an AXT PSU as the peak input current on the input is much higher than the average output current. This could be partially accomodated by adjusting the Nr:Np for a higher duty cycle and therefore lower Ns:Np, so lower peak primary current for a given load current. I was wondering if there is likely to be any problem with the pulsatile current draw on the ATX PSU, whether this would be a problem in various synchronisation states with the ATX PSU's switching, or if it might work with some ATX PSUs but not others.
If the PSU is a standalone offline one, how realistic is it to have a wide input and output voltage range? Everything I've read so far has talked about a fixed output voltage and duty cycle adjustment to account for the input voltage variation, but if both are variable, that seems to be quite a wide Vout:Vin range to try to accomodate. Any comments on this?
There's a lot for you to chew over, hopefully some useful discussion will come of this.
Cheers,
Matt
Please allow me to introduce myself. My name's Matt, I'm a scientist by trade and I've been into hobby electronics for about 3 years now, having spent about the last 2 years reading and SPICE simulating various SMPS and DC-DC converters. So I'm just starting to feel comfortable with building one for the first time. I'm glad I've found somewhere where amateurs are discussing this sort of thing, last time I came to this site I believe the SMPS section was decidedly empty.
I've been reading around here at all the "I have no electronics knowledge, please give me a 1kW phase shifted full bridge converter schematic so that I can blow myself up" (paraphrasing) threads and I find these both mildly amusing and quite scary. So I'd like to propose a more realistic first project and I'd like some input from the more experienced members. I'm a member at AX84.com and I like their approach to useful beginner projects, which I think we could emulate.
I'd like to build a bench power supply for general low voltage use. The sort of thing you could easily buy but the point is to learn something.
It should be:
-Offline input (an isolating topology) or a post-regulator for an AT/ATX PSU.
-universal input voltages (i.e. somethign like 90-250V ac)
-Adjustable output something like 5-30V dc, 5A.
-Efficiency is secondary to simplicity, and cost should be reasonable considering the lack of economies of scale.
-Should be a through-hole design which people could more easily make by hand than SMD, although the trade off is increased parasitics and EMI.
-I'd like to make it with discrete components rather than controller ICs for the sake of education, but I accept that this goes against the simplicity case and adds unnecessary parasitics (still a TL431 is often used as a discrete secondary side error amplifier and that seems to work OK) any input on this?
I'm thinking this is probably in the range of a forward converter. I have read that the linear transfer function of buck-derived topologies makes them more suited to adjustable out/in voltages than boost or buch-boost derived topologies. I have also read that voltage-mode control is better for this than current-mode control, (especially with feedforward of the input voltage to the PWM ramp voltage) but I don't know really which is more appropriate.
I've also put a lot of thought into running it off the 5V or 12V output of an AT/ATX power supply with a buck/forward converter post-regulator (depending on whether the required voltage output range is above 12V) People frequently hack these into "lab bench power supplies" but in really rubbish ways and I thought we could do better and give it a variable output and current limiting. Even if a forward converter is used, isolation is not necessary and this would remove the complexity of an optoisolator (which I'd like to avoid if possible, optos need to be tested for stability as their bandwidth etc widely varies between devides and amateurs don't have the tech for this), and the input voltage would be a steady low voltage, the universal input voltage range is taken care of by the ATX PSU.
I think the output current would probably have to come down from 5A if a forward converter was used after an AXT PSU as the peak input current on the input is much higher than the average output current. This could be partially accomodated by adjusting the Nr:Np for a higher duty cycle and therefore lower Ns:Np, so lower peak primary current for a given load current. I was wondering if there is likely to be any problem with the pulsatile current draw on the ATX PSU, whether this would be a problem in various synchronisation states with the ATX PSU's switching, or if it might work with some ATX PSUs but not others.
If the PSU is a standalone offline one, how realistic is it to have a wide input and output voltage range? Everything I've read so far has talked about a fixed output voltage and duty cycle adjustment to account for the input voltage variation, but if both are variable, that seems to be quite a wide Vout:Vin range to try to accomodate. Any comments on this?
There's a lot for you to chew over, hopefully some useful discussion will come of this.
Cheers,
Matt