"Switch mode". What's it mean?

[VicS]

\"Switch mode\". What\'s it mean?

Battery chargers are often described as "switch mode". I realise there is a gap in my knowledge there. Can someone explain what that means. In simple terms first then in a bit more depth please.

 

[Vara]

Re: \"Switch mode\". What\'s it mean?

If you go through this site http://www.mpoweruk.com/glossary.htm#switcher,and work your way through switch mode and linear chargers it might give you some clues.Means nothing to me.

 

[Black Sheep]

Re: \"Switch mode\". What\'s it mean?

Switched mode power supplies are all solid state, and don't have hefty transformers in them. This means they tend to be lighter, and can often work from a greater range of mains voltages.

Wikipedia has a useful article if you're feeling technical!

 

[Viscount]

Re: \"Switch mode\". What\'s it mean?

I thought it was "switched mode" but anyway, basically in a simple charger the 240Vac is transformed to around 12Vac and rectified. The transformer can be quite large and heavy depending on the power requirements; a way to reduce the weight of the transformer is to first increase the frequency of the input voltage from 50Hz to a few kHz and then transform down to a lower voltage and finally rectify; the higher frequency accounts for the whistle or whine that sometimes is heard from switched mode supplies. I don't think modern ones have this problem, but older ones used to need a certain load before their output came within stated specification.

 

[steverow]

Re: \"Switch mode\". What\'s it mean?

Switch mode refers to the "Chopper" or oscillator transistor which is used to generate a higher frequency AC waveform (usually a square or distorted sine wave). Oscillators are basically "switches" which "switch" from positive to negative a given number of times per second.

Steve.

 

[William_H]

Re: \"Switch mode\". What\'s it mean?

Switch mode in a battery charger can mean 2 different things.
Firstly a lot of power supplies even the tiny wall plug types as already described find it cheaper and lighter to rectify the mains to 380volts DC then switch this at sometimes 100s of kilohertzs and feed into a transformer which is tiny for the power involved. Note it is not a nice sine wave Ac but rather square wave switched pulses with variable on to off ratio. The output of the transformer is rectified and smoothed with a capacitor. Using this circuit it is easy to feed back a sample of the output voltage to the control circuiit so that the output is either precisely what is required (regulated) or can be easily regulated to steps depending on charge state etc.
Using this method it is often easy to make the charger or power supply work on 260volt down to 110 volts AC input without change and this is reflected on the specification label.

Switch mode can also be used to describe the regulation circuiit of a battery charger which uses initially a standard transformer at 50hertz. Indicated by very heavy weight. To explain, if a linear regulator is used a transistor is set up as a variable resistance.( old style used an actual resistance wire and switch) If the transistor is requiired to drop eg. 4 vollts at 20 amps this means the transistor dissipates 80 watts which gets very hot and needs lots of cooling. however if a transistor is arranged to switch on and off at a high rate but with variable on to off ratio then power can be switched into a capacitor and eventually into your batteries etc without so much heating of the switch transistors simply because they are either switched fully on high current but small voltage drop or fully off with low current. Hence the attraction of switch mode.

So the switch mode is very complex but surprisingly cheaper than a transformer. Transformers are in themselves very reliable and seldom fail while the switch mode is complex electronics which possibly are not as reliable. Certainly hard to fault find because different parts of the circuit are interdependant. The switch mode are usually very tightly controlled in output.

Incidentally the switch mode is often used in inverters where the 12volts is switched at high frequencies and transformed to 380volts DC which is then switched to fabricate AC 250 volts at 50 hertz. (or 60 if you live in that other bushy place) The 380 volts is either switched just on or off to make a square wave inverter or switched in a variable fashion to produce something near sine wave.
regards olewill

 

[brianrunyard]

Re: \"Switch mode\". What\'s it mean?

Switch Mode Power Supplies are the current state of the art in high efficiency power supplies. Conventional series-regulated linear power supplies maintain a constant voltage by varying their resistance to cope with input voltage changes or load current demand changes. The linear regulator can, therefore, tend to be very inefficient. The switch mode power supply, however, uses a high frequency switch (in practice a transistor) with varying duty cycle to maintain the output voltage. The output voltage variations caused by the switching are filtered out by an LC filter.

Example: 20volts DC, switched on for 50% of the time = 10volts DC, without the inefficiency of disipating the other 10volts.

 

[VicS]

Re: \"Switch mode\". What\'s it mean?

Thanks for the replies. I get the idea now, although I'm still digesting the Wikipedia article. I knew the heavy mains transformer had been done away with but I did not realise that by raising the frequency the transformer could be so much lighter.