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I've carefully read Wikipedia article and can't get one important thing - is there galvanic separation in such supply?

The diagram in that article goes like this:

SMPS block diagram

What I see here is that the transformer is only on one path and there's the "output->chopper controller" path that bypasses the transformer. Usually the transformer is the unit that performs galvanic separation.

Does this mean a switched-mode power supply doesn't feature galvanic separation between the input and the output? Is it possible for unlimited current to flow through the power supply?

finnw
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sharptooth
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2 Answers2

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Not all SMPSs provide galvanic separation. DC/DC converters transforming between two low voltages in a circuit often don't. The block diagram, however, shows a mains connected SMPS, and most of those do have galvanic separation.

The feedback often goes via an opto-coupler, which indeed is missing in the article's block diagram. The signal is PWM, so a normal (digital output) opto-coupler can be used.

stevenvh
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    Does this imply there's galvanic separation thanks to the opto-coupler? – sharptooth Jun 06 '11 at 14:43
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    @Sharptooth - Yes. In the forward path it's the transformer, in the feedback it's the opto-coupler (between the chopper controller and the inverter chopper). – stevenvh Jun 06 '11 at 14:52
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    [Schematic showing opto-isolation](http://www.lcdmonitorrepairebook.com/wp-content/uploads/2010/04/smps-schematic.png) [block diagram showing isolation in feedback](http://ac8gy.com/images&_supportinf_files/SMPS/SMPS%20BLK%20Dia.JPG) – endolith Jun 07 '11 at 18:47
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It depends on the supply. The ones used for laptops and mobile phone chargers have an isolated output which is achieved with a transformer. The high frequency used means that it can be very small.

Leon Heller
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