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In conjunction with my independent study of switching converters using textbooks, I am more or less obsessively tearing apart all the low-ish power (say 5 - 25 Watt) switching converters I can find. I'm finding myself increasingly able to explain what's happening inside and generally learning a lot about the practicalities of the topic:

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I was surprised to discover that all but one of the offline converters I've explored so far use discrete rectifier diodes to form the mains voltage rectifier bridge. This has a few downsides I can see:

  • increased PCB area
  • increased component population effort
  • perhaps increased parasitic C and L, although I don't know that for a fact nor whether anyone would care at that particular part of the circuit

I have a couple hypotheses about why these downsides might be outweighed:

  • Lower cost?
  • Increased thermal coupling for better dissipation without a heat sink?

Anyway, can someone with experience with these designs help me understand the design reasoning that might explain this apparently common choice?

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2  
I'm going with "cheap cheap cheap". – William Brodie-Tyrrell Feb 15 at 0:09
    
I've got 3 different open frame AC-DC converters laying about. They are rated between 40 to 200W. All use an integrated bridge IC. – Nick Alexeev Feb 15 at 0:27
2  
I'm voting for cost & increased heat dissipation (esp. since many of the ICs have a plastic case with lower surface area vs 4 cylindrical, often glass, individual diodes). When purchased in full reels, many discrete rectifier diodes fall to <0.01 USD/ea – Robherc KV5ROB Feb 15 at 1:27
2  
Freedom of layout may or may not also play a role. But cheap is always a good bet. – Ecnerwal Feb 15 at 1:44
up vote 6 down vote accepted

The installed cost of 4 diodes is less than the cost of a packaged rectifier bridge.

Cost of a 1N400x diode in quantity is maybe 0.2-0.3 cents.

That's the main reason- switching power supplies in consumer products are ruthlessly cost-reduced and any fat gets squeezed out.

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On the face of it you would think that an intergrated bridge is the answer .In 1992 I saw 4 solder paste button diodes being used where one side was soldered to a heatsink and the other side was soldered to a wire going to the PCB .It seamed crazy to me at the time .The place where I saw this was a total sweat shop and I was told that it was cheaper to do this than buy a bridge .I was also told that the hand soldering process "pre aged the diodes giving them a lower foward drop " .The pre aging could be an urban myth and the cost stuff may apply in the third world .There is one engineering reason to use 4 diodes and that is to get a better surge current rating so you have more rugged PSU when there are mains transients.In fact I have used 4 diodes sometimes.

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