Switching power supplies
Switching power supplies
(OP)
Hello,
Does anyone have a recommendation where I can find some good free information on all of the parameters dealing with switching power supplies. I am also interested in any good books on the subject.
Thanks,
sparkyyyyy
Does anyone have a recommendation where I can find some good free information on all of the parameters dealing with switching power supplies. I am also interested in any good books on the subject.
Thanks,
sparkyyyyy





RE: Switching power supplies
http://www.national.com/appinfo/power/
The technical papers and app notes will keep you busy for quite a while.
Other manufacturers like Unitrode and Motorola also have good websites.
RE: Switching power supplies
"Switching Power Supply Design" by Abraham Pressman is more in depth but less intuitive to the uninitiated. There is also a good book by Keith Billings.
RE: Switching power supplies
How much power? Volts , amps ect.
If low power most sites have software for their chips.
National,Fairchild,Ti,Motorola,lineartech.,ect.
Heres some links I use;
Good software for simulating, but very expensive.
http://www.intusoft.com/psbook.htm
This site has alot of good links
http://www.smps.us/
Ridley is very good training and the software is also
very good.
http://www.ridleyengineering.com/
http://www.switchingpowermagazine.com/
Worked with this guy , very good book on using spice
to simulate power supplies. Has alot of tips on convergence
problems.
http://www10.pcbcafe.com/book/parse_book.php?article=SMPS/SMPS.htm
Some free training from TI/Unitrode.
http://focus.ti.com/general/docs/training/trainingevents.tsp?familyId=2
RE: Switching power supplies
A colleague of mine is saying they can be damaged by a light load. Has anyone else ever heard of this? If so, what would fail? I realize more details may be needed but in general, has anyone seen a switcher fail (DC-DC) by too small a load? Thanks in advance.
RE: Switching power supplies
RE: Switching power supplies
RE: Switching power supplies
Yes, a switching power supply will most likely be messed up due to too little load current (this is true for pretty much ALL switching power supplies that do not occasionally employ discontinuous mode). There are various failure modes associated with insufficient loading.
A good example is with a boost DC-DC converter with insufficient loading. The inductor of a boost DC-DC converter acts as a constant current source feeding constant current (with some non-zero ripple current) to a filter capacitor and your load. With insufficient loading the capacitor will increase it's voltage until it breaks down.
Your power supply is most likely either some kind of a forward DC-DC converter or some kind of a fly-back DC-DC converter. Newer off-the-shelf DC power supplies will have controls that will employ discontinuous mode operation to avoid the problem I mentioned in the paragraph above. It is possible your power supply will require a minimum load.
So,...what to do...If your power supply requires a minimum load (quite often 10% of rated output current for a given rail), you will need to determine that from the data sheets and then apply the minimum load. If the data sheets have no mention of a minimum load, then you must have a different problem.
RE: Switching power supplies
Your explanation makes a little sense to me. I have not looked at switching design to know exactly what is going on with light loads. Our application certainly is lightly loading the supply (150A rated and pulling 20A). I will be getting the part numbers, etc soon to figure out if this is a possible cause of the supply failures. Sounds like it could definately be an issue. Of course, its hard to decipher without a data sheet. Thanks for the info.
RE: Switching power supplies
In my experience at light loads the power supply
may not preform to the specs. (load regulation,ripple & noise, and transiant response),but it should not be damaged.
RE: Switching power supplies
RE: Switching power supplies
RE: Switching power supplies
I have seen several switching power supplies that spent a good amount with 120VAC applied to the input and the output running open circuit fail early. The failure mode was the output voltage was floating between 16V to 20V instead of the intended 12V. Inexpensive switching power supply IC's one might use as part of a larger design might not survive if the intended load is completely removed (or never there).
Lastly, a number of newer supplies already have a 'dummy load' on the PWB. I have two supplies I purchased in the last 3 months sitting next to me. Looking at the traces associated with the 'main rail', i.e. the 5V rail or, for the sake of the other supply, the 24V rail, I see power resistors across the rail to ground. I would guess these resistors serve several purposes, amongst which would be when the power supply first turns on and begins transferring power. If there is no load on the output and the supply is trying to transfer the rated power, the voltage across the output filter caps will probably continue to increase. The resistors will probably 'bleed' off the extra stored energy, allowing the supply's controls to adapt to the current load (no load, light load, etc...).
The supplies I mentioned at the top of this particular post were supposed to have loads that drew at least 10% of the rated current. I had to dissipate a minimum of 9 watts (12V * 7.5A * 0.10W). This project was 'mobile' (think shipping abuse), so I had to 'batten' everything down which created quite a challenge.
RE: Switching power supplies
RE: Switching power supplies
RE: Switching power supplies
RE: Switching power supplies
- Define the topology. Buck, boost, fly-back, etc. Also, specifications like isolation, output power, voltage input range, power factor, efficiency, true-hole or surface mounting components and applicable standards.
- Choose the controller that fit your needs: Voltage or current mode, built-in or external switcher, seeking info into the Application Notes and Prototype models at the main semiconductor manufacturers, for instance, Unitrode (Texas), ST, Maxim, National, IR, LT, etc.
- Define the operation mode of your controller: In general, discontinuous mode is preferred over continuous mode. However, this is not always the case, in PC power supplies (for example the PS of my PC doesn't start unloaded.) This operation mode is dependant on the controller, the inductor and load conditions.
- Search for suppliers and/or manufactures for the main components, named: low impedance capacitors, switching transistors, fast rectifier diodes, magnetic materials, coil forms and heatsinks.
- Prepare a worksheet for calculations of the several sections of the SMPS: Primary switcher, snubber circuits, inductors and transformers, rectifier section, feedback section. Make what-if scenarios.
- Simulate in PSPICE, if possible, the primary switching for several inductances, turn ratios, capacitances, etc. Do not forget nor disregard your calculations when manufacturing or requesting samples. Fortunately, the behavior of magnetic materials is close to reality.
- Technology and new products appear every day, so check for manufacturer’s advising on new designs.
- Search the Internet on specific studies about snubber calculations, transformer design and PSPICE models. Unless you are designing for hobby, it is difficult to found ready made circuits.
- Printed circuit design is another important issue.
- Built and test your circuit by sections in open loop mode first, then in closed loop, making the necessary adjustments.
Good luck.
RE: Switching power supplies
It may not be worth designing a power supply if you can use a "standard" unit designed for powering a PC. These normally share out the total power with the highest power to the 5V output, but a 150W unit normally has 12V outputs rated at 2A and a standard 250W power unit could run to 3 or 4A on the 12V rails.