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Do switching power supplies use transformer as well?

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matrixofdynamism

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I understand that there are multiple topologies of switched mode power supplies and the primary reasons for using them is their efficiency when compared with linear ones.

Is it true that the switched mode power supply portion merely replaces the linear regulator bit of an AC-DC converter and not the transformer? If this is the case then how come my smart phone's USB charger is so very tiny. I know transformers as being big, I can't image any fitting in a single plug.

By the way, I know that unregulator power supplies only have a smoothing capacitor after the bridge rectifier stage. They do not have a linear regulator stage. This causes the output voltage to drop the more it is loaded. Why does the voltage drop within the power supply when we load it? Is it dropped across the transformer and the diodes?
 
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E-design

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For safety, you have to maintain isolation with a transformer in most applications. With the switching supply, the transformer can be made very small or thin when using a high switching frequency.
 

KlausST

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Hi,

the higher the switching frequency the smaller the transformer.

Mains frequency is 50 or 60 Hz, in modern switch mode supplies the frequency is some 100000 Hz.

Klaus
 

Audioguru

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I know that unregulator power supplies only have a smoothing capacitor after the bridge rectifier stage. They do not have a linear regulator stage. This causes the output voltage to drop the more it is loaded. Why does the voltage drop within the power supply when we load it? Is it dropped across the transformer and the diodes?
The transformer windings have resistance that causes a voltage loss when loaded. The forward voltage of the rectifiers increases when they are loaded. The ripple voltage at the capacitor increases when it is loaded.
 

KlausST

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Hi,

why do unregulated power supply drop voltage when loaded?
One ppoint is the capacitor. with rectified 50Hz mains input it is charged every 10ms. But only for 1..3 ms.
With no load it doesn´t discharge ==> the voltage is constant.
With load it is discharged and thus the voltage drops until it is charged again..

Klaus
 

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Reduction of the ripple voltage across the main filter capacitance needs a huge filter capacitor for the rectified mains low frequency but a fairly small capacitor for the high frequency of a switch mode supply.
 
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