Standard bridge rectifier or Schottky diodes for mains rectification - Page 1
Forum section: electronics / general

author: akis
author_rank: Frequent Contributor
timestamp: February 07, 2015, 01:29:46 pm
content: Why would you use one over another for a standard mains AC rectification power supply? I have currently used four of STPS20SM100STN (SCHOTTKY, 20A, 100V) that have low forward drop and so I think would waste less energy as heat. I draw around 1.5 A. Thjey cost around 50 pence each, so you'd need £2.00 for four. Alternatively I could use a bog standard bridge likehttp://uk.farnell.com/multicomp/vsib410/bridge-rectifier-4a-100v-gsib/dp/1861531that costs around 20 pence!

author: AndyC_772
author_rank: Super Contributor
timestamp: February 07, 2015, 01:34:26 pm
content: I'll jump in and say this before anyone else does... a 100V diode on the mains side = crackle, burning, bad smell, etc.If you mean it'll be used on the low voltage side of a linear, transformer-based supply, then a Schottky diode will indeed get a bit less warm than a silicon diode.

author: Dave
author_rank: Super Contributor
timestamp: February 07, 2015, 01:54:34 pm
content: Schottky diodes have the benefit of having a lower forward voltage, therefore lower losses.They are much faster at switching and can be used at much higher frequencies, which sometimes poses a problem. Faster switching means it produces more EM noise. If the circuit you are trying to power is sensitive or if you are designing a product that is going to need certificates, you may have trouble passing an EMC test.

author: c4757p
author_rank: Super Contributor
timestamp: February 07, 2015, 01:57:10 pm
content: Bit of RC damping across the diode can fix that problem nicely.

author: digsys
author_rank: Unknown
timestamp: February 07, 2015, 02:01:23 pm
content: If you're using a transformer, and NOT going straight into a high efficiency DC-DC converter, you already have a LOT of loss !!Mains is specified at +/- 5% at min, up to + or - 7.5%If you ARE using a DC-DC converter and still want to reduce losses, go for an electronic bridge. A bit more $ but a heck of lot more efficient !!

author: DanielS
author_rank: Frequent Contributor
timestamp: February 07, 2015, 02:39:26 pm
content: Quote from: digsys on February 07, 2015, 02:01:23 pmEven if you eliminate rectifier losses altogether, we are talking about 2-3W for standard diodes and less than 1W for Schottky. Synchronous rectification would be quite a bit of extra complexity and cost to save less than 1W.

author: akis
author_rank: Frequent Contributor
timestamp: February 07, 2015, 03:22:07 pm
content: I am rectifying low voltage AC from a mains transformer. Voltages are around 18-25V AC and currents are about 1.5A RMS.The TO-220 Schottkys, all four of them, heat up to 60C (no heatsinks).That means that a conventional bridge, utilising silicon diodes, would heat up much more, especially in a smaller, plastic package (smaller than the surface ares of four TO-220s plus their total of 12 pins vs 4 pins for the bridge).Am I understanding it right? If I am right, if cost is not an obstacle, everyone should prefer the lower loss Schottkys for rectifying the transformer secondaries.

author: akis
author_rank: Frequent Contributor
timestamp: February 07, 2015, 03:25:08 pm
content: Quote from: digsys on February 07, 2015, 02:01:23 pmI have tried using ready made SMPS units, so AC-DC converters, but they leak a ton into the DC output and give me jolts. There have been a few threads around discussing the hazards of typical SMPS units like phone chargers and laptop adapters and so on.If you know of one, I would be very interested in an electronic rectifier circuit that eliminates the heavy iron transformer but at the same time is as "safe" as the transformer rectifiers.

author: akis
author_rank: Frequent Contributor
timestamp: February 07, 2015, 03:26:07 pm
content: Quote from: c4757p on February 07, 2015, 01:57:10 pmWe have a problem rectifying 50Hz mains? Or are we talking about switcher circuits?

author: c4757p
author_rank: Super Contributor
timestamp: February 07, 2015, 03:30:51 pm
content: The problem Dave mentioned. Schottky diodes turn offfast, which can produce some EMI if you're concerned about that. Doesn't matter that you're only switching at 50 Hz, the edges are quick.Try a capacitor roughly an order of magnitude above the diode's junction capacitance and a resistor of some 5-50 ohms, if you want. Or don't, it's probably gilding the lily a bit.

author: amyk
author_rank: Super Contributor
timestamp: February 07, 2015, 04:36:44 pm
content: Quote from: AndyC_772 on February 07, 2015, 01:34:26 pmAn indication that the times have changed when you automatically think of the bridge rectifier as being on the mains side.

author: TMM
author_rank: Frequent Contributor
timestamp: February 07, 2015, 05:29:14 pm
content: Drawing 1.5A from the capacitor smoothed DC side will see somewhere around 6-7A peaks through the rectifier diodes. In the STPS20SM100S datasheet that corresponds to about 0.65-0.7V Vf. For the 20p bridge its about 1.05-1.15V. If you budget £2 for a bridge you can get one that will do around 0.9VThe bridge packages make a lot of sense from a product design/layout point of view. If you have to heatsink them (probably do if you're having to use TO220 package diodes) then having to line up four TO220 packages costs a lot PCB and heatsink space.

author: mariush
author_rank: Super Contributor
timestamp: February 07, 2015, 05:48:31 pm
content: It's more expensive, but on the other hand it's less waste and no need for heatsinks... the pcb copper would suffice if you choose the right mosfets.Have a look at something like LT4320 paired with 4 mosfets with low Rds on :http://uk.farnell.com/webapp/wcs/stores/servlet/Search?catalogId=15001&langId=44&storeId=10151&gs=true&st=lt4320Datasheet for LT4320 :http://cds.linear.com/docs/en/datasheet/4320fb.pdf