[b]Questions regarding Switched-Mode Power Supply[/b]

[Irwin]

What makes a switched-mode power supply a switch-mode? or What with the switched-mode in switched-mode power supply? How does this differ from the conventional linear power supply? Badly need the answers for these questions now I expect the help of the experts here. Thank you!  ;D

 

[Hero999]

In a SMPS, the output transistor is either on or off.

In a linear power supply, the output transistor always operates in its linear region, hence the name linear power supply.

 

[indulis]

Well, you could have an "off-line" or a DC-DC converter... both are considered SMPS. In an off-line switcher, the AC is rectified & filtered (then a PFC stage in most) and then fed to a DC-DC converter. The "switch mode" part is that the DC voltage is "chopped up" (switched on and off) producing a square wave whose duty cycle can be varied and filtered to produce an other DC voltage. A switch mode power supply is generally more efficient than a linear supply. For example using a linear regulator to reduce 12VDC to 5VDC @ 1A the regulator would dissipate (12-5)*1 watts, in this case 7 watts. That's an efficiency of ~41% where as with a SMPS, say a buck converter, you could make the same voltage reduction at an efficiency in the 90's. That's a big reduction in wasted power (heat).

It's "not really" the output transistor that's switched on/off and there is always an LC of some type after the "switching device".

 

[Kevin Weddle]

Indulis, do commercial power supplies utilize switch mode? Most operate directly from the power outlet, and even the cheapest AC to DC adaptors are used to power laptop computers.

 

[Hero999]

Yes, lots of commercial power supplies are now switched mode, although one does occasionally bump into the odd linear power supply; most commonly in audio PSUs and AC adaptors.

 

[indulis]

The power density of switchers is quite high... for example my (work) bench supplies are Xantrex a XHR 100- 10 (100V @ 10A) and a XHR 40 25 (40V @ 25A). Both are 1 KW supplies and measure ~3"x8.5"x16". In a linear supply of the same power, the transformer would probably bigger than that by itself. Although, one problem that I have run into on occasion is powering a SMPS with a SMPS... nasty things can start to happen, particularly if some noise gets in to the sense lines.

 

[Kevin Weddle]

Also, power supplies that are not integrated into a single product are somewhat general purpose. Whether they waste too much power is not as important as their purpose.

 

[indulis]

Also, power supplies that are not integrated into a single product are somewhat general purpose.

What???

 

[www.electronworks.co.uk]

switched mode power supplies rely on 2 priciples:

a switch does not dissipate any heat:

heat = volts x current

in a switch the current is zero when switch is open or voltage is zero when switch is closed. therefore heat dissipation is always zero (theoretically).

Also inductors and capacitors do not dissipate heat
If you want to smooth out the switched waveform (from the switched ac to dc), use inductors and capacitors.

Thus you can achieve power regulation with theoretically zero heat loss. Heat is energy from your input supply, so zero heat means zero power loss means longer battery life.

Bill Naylor
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[indulis]

Also inductors and capacitors do not dissipate heat

That is not correct. Inductors have resistance (the wire itself) and capacitors have ESR both of which will and do dissipate power (heat).

 

[www.electronworks.co.uk]

I appreciate inductors and capacitors have resistance and will dissipate heat. I was talking theoretically to make the point easier to understand.

Considering the wider picture, you need to look at switching times (passing through the linear region of the FET will dissipate heat). You will also need to consider track length with its impact on resistance and cross coupling

You should also consider the proximity of the switching components to the inductor to avoid radiation. You need to look at the value of any feedback resistors as making them too high may cause the inductor to induce fields into them causing instability.

You will also have radiated and conducted emissions from the inductor. There will be switching losses in the controller itself.

Then you have stability issues by picking the wrong output cap and inductor values. That is a whole different arguement...

I tried to keep it simple... ;D

 

[indulis]

in a switch the current is zero when switch is open or voltage is zero when switch is closed...

Only in a ZCS or ZVS supply... the other topologies don't have that luxury.

In terms of SMPS, the number one thing to get a good understanding of is the concept of Pulse Width Modulation (PWM)... a concept you might want to include in the "Technical Articles" that pertain to DC-DC's on your website.

With a solid basic supply design, things like switching losses, parasitic resistance & inductance are really considered higher order effects.

Then you have stability issues by picking the wrong output cap and inductor values.

Those two components are basic to any SMPS design... the design dictates their value, you don't just arbitrarily "pick" them... be it a simple buck converter, or a more complicated isolated topology. Certainly, good layout practices should be followed to minimize the "loop areas" in the feedback and minimize any proximity effects. As for the "high current paths", short and fat is always preferred.