switchmode power supplies 6

[mrdee]

hi, i am looking for a 24v dc power supply unit and do not understand what is meant by a switchmode power supply unit?
can anyone help please,thanks

 

[nbucska]

Switchmode supply generates an AC and usually uses
inductive/capacitive circuit elements or transformers
co convert the voltage with relative high efficiency.

The linear power supply consists of rectifier followed by an ohmic -- i.e. lossy -- attenuator (usually having some
active elements e.g. transistor or MOSFET ).
<nbucska@pcperipherals.com>

 

[MMAshwin]

Hi,

What is a Switchmode power supply ?.

There are many types of switch mode power supply, but basically the principles are the same.

You take the incoming supply (can be a/c or d/c depending upon the design of the smps) and chop it up at very high frequency.

This chopped up dc (now ac) is applied to a transformer for stepping up or down (usually down) to the required voltage.
across the secondary of the transformer will be rectification and smoothing.

The regulation of the supply occurs by a sensing amp on the output of the secondary which monitors the dc output. This is then fed to the original chopper circuit. If it senses the voltage dropping it will change the mark/space ratio of the chopped frequency. This will increase the output voltage.

They tend to be more efficient and smaller than linear supplies. It is also easier to make them multi input voltage.
However they do generate more rf noise and are harder on their capacitors. Usually they go dry and it screws up the supply. However they are now producing special SMPS capacitors these days.
Any help ?, yes no let me know.

Regards

 

[melone]

One other major drawback with the switchmode supplies is that their instantaneous current capacity is terrible with regards to a linear supply. The reason for this is because the linear supply has very large caps on the outputs. These caps can supply a tremedous amount of current for a very short amount of time. This means that if your circuit requests a large surge current from your supply, these caps can provide them.

 

[jbartos]

Suggestion: Visit

http://www.repairfaq.org/REPAIR/F_smpsfaq.html#SMPSFAQ_003

http://www.ferrus-power.co.uk/index.html

http://www.emipower.com/

http://www.ee.uts.edu.au/~venkat/pe_html/pe07_nc8.htm

etc. for more info

 

[ctolbert]

The first link by jbartos is really good. I've used it in the past for troubleshooting PS's.

Carl

 

[walker1]

The main reasons for a switchmode supply/regulator are saved power and saved space.
A switcher is a lot smaller (transformer size), and substituting a linear drop element (like a transistor) with pulse width modulation saves power.

Designing switchmode supplies is a bit more complicated than doing a liniar regulator though, as you have to considder a lot more issues befor you are there.

Switching frequency, magnetic propperties of the iron or ferrite core of the coil/transformer, finding faster simiconductors, recovery time in the rectifiers, and so on.

I once tried to feed a small transformer with a standard iron core from a 100 kHz supply. I didn't get much out on the other side, but due to loss in the core the transformer nearly melted.

 

[koolkid]

Switchmode power supplies are not for beginners. It takes many years to reach a level of decent proficiency. Hundreds of PhD chose it as their specialty, and power electronic centers galore at universities. There're thousands of patents on the subject.

For inexpensive designs go to

http://www.1stquadrant.com/services.htm

For inexpensive products go to HSC electronics.

 

[lyledunn]

In simple terms, why are switch mode poer supplies responsible for currents in earth conductor as a function of their normal operation

 

[electricuwe]

Refering to mrdee's question I gues he didn't want to build a switch mode power supply himself.

For 24 V output there are lots of switch mode power supplies available for currents in range of 0,5A up to 40A since this is the voltage commonly used as power supply for PLCs.

Compared to power supplies with an line frequency transformer the advantages are:

regulated output voltage (avialable on the other type only at additional expense)
smaller size
smaller losses
no low frequency ripple

The only important disadvantage is slightly higher cost.


To lyledunn:
The reason for currents on the earth connection are some small capacitors included in the power supply to suppress RFI (Radi Frequency Interference). If the currents are higher than a few mA there's something wrong.

 

[melone]

I must disagress with electricuwe on the only important disadvantage of switch mode power supplies. One MAJOR drawback is instantaneous current capablitity. Switch mode supplies cannot supply as much instantaneous current as a linear power supply. If your connected circuit does not need large surge currents, then a switch mode supply is the probably the most sensible choice. However, I have encountered several situations (while performing testing on automotive grade components), that a switch mode supply is simply useless!

Otherwise, electricuwe has summarized the major differences between the two supply philosphies very well.

 

[schwarz]

hi,
Switchmode power supplies basic features have been already mentioned. I would like to add that besides small volume SMPS because of its high efficiency >80% for 1000W extends backup hours for batteries. SMPS is the technology used in telecom rectifiers/chargers.

 

[TheDOG]

What about inrush current of SMPS'?

 

[electricuwe]

To melone:

not all SMPS can supply large surge current but some can :

visit

http://www.puls-power.de

Also inrush current is quite different on different units.

Buying the suitable SMPS is not as easy as it seems to be because not all important properties are specified in the data sheets, but with some care you can solve the surge current problem as well as the inrush current problem.

 

[sputnik]

Try Ferrus Power ( see jbartos 23rd Dec )if you want an SMPS repaired. We now use them all the time and the service is excellent !

 

[ionutnusoreanu]

Hello to all of you! I'm trying to design my own smps and i don t know how to design/calculate the magnetic core . I want you to help me to design that. It is a half bridge config. to obtain 2 x 30V/6A output. The basic schematic is like a PC power supply. I've bought the magnetic core and the pricipal parameters are:
AL 5100;f=25...300khz;Ae=233mm;Br=0,2T. I want the comut. frequency to be...almost 150khz. Vin=220V/50Hz Sent me an e-mail or a letter pls. I m appologiseing you for my poor english. I m from Romania. Yours Ionut.N

 

[Warpspeed]

Lyledunn,

Switchmode power supplies mostly use square-wave switching waveforms that can have very fast turn on and turn off times. The peak voltages and currents reached can sometimes be very high as well, compared to the rated average output power. The problem is to enclose all this electrical violence in a metal box that must be connected to the ground wire for electrical safety reasons.

There are several ways around these problems, but capacitance between the high frequency switching components, and the enclosure can result in high frequency noise currents in the ground wire. For example a switching transistor may be thermally connected to a large heatsink, but electrically insulated from it. There will still be capacitance between the transistor and heatsink, so noise currents could be induced into the heatsink. That is just one example of how it could possibly happen, but there are plenty of other possibilities.

These problems can all be solved, but often the requirement is to keep the physical size and manufacturing cost to an absolute minimum. It is a very competitive business, and it sure ain't easy. The sales and marketing people usually define what is required not the engineers unfortunately. As long as it meets some generally recognised EMC standard it is good enough.

With regard to output surge current capability, any specification within reason could be met with a suitable design. But most design requirements are for constant load and minimal surge (inrush) current capability. In fact you usually deliberately want to limit fault current as fast as possible as a design goal. A power supply that rapidly shuts down with a dead shorted output is excellent in most applications.

If all you want is to power some sort of delicate electronic instrument or circuit, voltage regulation and and noise would be high on the list of requirements.

For automotive applications something a bit more robust but with poorer regulation and noise specifications may be in order. It is pretty difficult to duplicate the low output impedance of a lead acid battery. If you want to be able to deliberately create short circuit fault currents of hundreds or even thousands of amps, it's easy with a battery. Your switchmode power supply could be designed to duplicate it, but it would be a massive and expensive exercise, and probably quite pointless in the end.

 

[automatic2]

a few points that haven't been simplified. In a linear supply, the pass element(s) dissipate the power difference from supply to output. In a switchmode supply, the pass element(s) are full on or full off (duty modulated) which are it's two most efficient states. The average output is had largely without the power dissipation that occurs in the linear type.

Step down DC to DC converters (supplies) with small to moderate current demands make good applications for linear supplies. A single device regulator is a good example. You can start inserting pass elements into this simple circuit to increase current capabilities, but at some point your going to say, hey, Why don't I modulate this element to increase it's efficiency. By applying duty modulation (switch mode) you'll reduce it's heat generation.