Scaling up constant current buck converter power supply - Page 1
Forum section: electronics / projects

author: e100
author_rank: Frequent Contributor
timestamp: December 15, 2014, 04:44:49 am
content: I have an idea for a salt water chlorinator power supply that has the following specsInput: 24vdc (from bench top SMPS)Output: approx 8.5vdc at 20 ampsMax ripple: 1 amp ppA chlorinator is a non-linear, non-inductive load. Once the voltage climbs above about 7v there is a rapid rise in current.I haven't been able to find a suitable off the shelf product with a continuously rated output current greater than about 12 amps.All the example designs I've seen are for relatively low current applications such as driving LEDs.If I were to scale up something like the 'poor mans buck converter'http://www.instructables.com/id/Poormans-Buck/?ALLSTEPSwould it still work reliably? Presumably switching 20 amps at several 100 KHz is going to be generating a lot of RFI which could affect other parts of the circuit?The construction will be point to point wiring on a proto board.Are there any basic traps that I should consider when up scaling?Mike

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 15, 2014, 10:49:44 am
content: Funny, I built this years ago,http://seventransistorlabs.com/tmoranwms/Circuits_2010/High_Current_Supply_p1.pnghttp://seventransistorlabs.com/tmoranwms/Circuits_2010/High_Current_Supply_p2.pnghttp://seventransistorlabs.com/tmoranwms/Circuits_2010/High_Current_Supply_p3.pnghttp://seventransistorlabs.com/tmoranwms/Circuits_2010/High_Current_Supply_p4.pnghttp://seventransistorlabs.com/tmoranwms/Circuits_2010/High_Current_Supply_p5.pngsome comments athttp://seventransistorlabs.com/tmoranwms/Circuits_2010/index.htmlTim

author: e100
author_rank: Frequent Contributor
timestamp: December 15, 2014, 12:03:12 pm
content: Quote from: T3sl4co1l on December 15, 2014, 10:49:44 amThis looks like a complete switch mode power supply complete with custom wound transformer and dangerous voltages.This is quite a leap from a simple buck converter.http://cdn.instructables.com/FPV/KX88/H2BMNHFK/FPVKX88H2BMNHFK.LARGE.gifFor not-wanting-to-be-electrocuted reasons I opted for an off the shelf 240vac to 24vdc switch mode power supply as a pre-regulator. These are cheap, readily available, provide transformer isolation and the 24v can power the control electronics. Now I only have to worry about fire, explosions and RFIHopefully I'll be able to use commonly available components without having to wind custom transformers and inductors.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 15, 2014, 12:05:16 pm
content: Yes, though the purpose is the same: I was doing a lot of chlorate electrolysis at the time, and wanted something a bit nicer than a rewound MOT and some diodes.Ironically, I never got to use it, as I ran out of materials (graphite anodes) and time and space to do it anymore.As for that buck circuit, it's surprisingly usable, given its simplicity. Do mind that the shunt resistors will get quite hot -- it could stand to be scaled for a lower sense voltage. But, lower voltage means more susceptibility to switching noise, so it's a tricky business. If you don't mind using a beefier resistor, then by all means.For ideas on inductor selection and waveforms and stuff, try:http://schmidt-walter.eit.h-da.de/smps_e/smps_e.htmlthe difference is, the ripple current is fixed by the controller. You'll probably want a ferrite cored inductor, because powdered iron types do not handle ripple very well.Tim

author: MrAl
author_rank: Super Contributor
timestamp: December 15, 2014, 02:53:11 pm
content: Quote from: e100 on December 15, 2014, 04:44:49 amHi,Generally if you have a load that draws a lot of current you would use what is usually called a "slow start circuit". This kind of circuit is usually simple because all it does is limit the rate of rise of output voltage (and thus current) when you turn the unit on. So instead of banging the output with say 10v within 10 microseconds, it will force the output to climb slowly a little at a time which gives the load more time to react and the power elements only increase the power delivered a little at a time. This can make a big difference.The slow start mechanism varies greatly for each controller chip, but an example would be a capacitor that charges when the power supply is first turned on, and this regulates the output to a higher and higher level until it reaches the target level. If this capacitor was across the reference voltage for example this would mean it would ramp the output up little by little as it charged up. Once it gets up to normal level it would not have any additional effect. when the power supply is switched off the cap must be discharged, and this is often accomplished with a diode from the positive terminal of the cap to the input positive terminal.There are so many variations though you really have to look at the actual chip you are using. Some chips even have a way to do this with the addition of just one additional capacitor.