Schottky rectifiers in linear PSU - Page 1
Forum section: electronics / general

author: NewBeginner
author_rank: Regular Contributor
timestamp: September 10, 2012, 09:10:10 am
content: Hi,I've been searching for more information on schottky rectifiers but all I could find was that they are good for switching power supplies, because of their near zero switching speed and very low forward voltage drop.But how about using them in alinear power supply?Are there any disadvantages / advantages?If I understand correctly schottky rectifiers will have a higher reverse leakage current than a standard PN junction rectifier. Will this be a problem in a linear PSU?Thank you

author: jahonen
author_rank: Super Contributor
timestamp: September 10, 2012, 09:20:00 am
content: Not a problem otherwise than if the leakage gets too high, then thermal runaway (and subsequent self destruction) might occur.Regards,Janne

author: NewBeginner
author_rank: Regular Contributor
timestamp: September 10, 2012, 09:54:45 am
content: Quote from: jahonen on September 10, 2012, 09:20:00 amWouldn't thermal runaway also be a problem in standard PN junction diodes?I mean, in both cases (schottky or standard pn junction diode) if the diode gets too hot thermal runaway might occur. Is this correct?

author: madires
author_rank: Super Contributor
timestamp: September 10, 2012, 10:02:21 am
content: And take care about the reverse voltage! A spike from the transformer could easily kill a schottky diode with a low reverse voltage. Or add a bidirectional TVS diode across the AC input of the rectifier. Had some fun with that and a low-loss linear regulated power supply a while ago.

author: jahonen
author_rank: Super Contributor
timestamp: September 10, 2012, 01:57:49 pm
content: Quote from: NewBeginner on September 10, 2012, 09:54:45 amBasically yes, but schottkys have much higher leakage to start with and leakage grows exponentially along with the temperature, so problem is much more pronounced for schottkys. For example, normal silicon diode might have a leakage of 10 µA at 175°C and similar schottky might leak 10 mA at 125°C. Basically silicon diodes can almost melt solder and still have no problems with self heating induced thermal runaway. Thus schottkys are much more prone to thermal runaway. But if you can keep schottkys cool, there is probably no problem.Regards,Janne

author: grenert
author_rank: Frequent Contributor
timestamp: September 10, 2012, 03:49:18 pm
content: An advantage of Schottkys is the lower voltage drop, which can help you if your transformer voltage is marginal by giving you maybe an extra volt compared to silicon.

author: Fraser
author_rank: Super Contributor
timestamp: September 10, 2012, 05:12:47 pm
content: Hmmmm, I may be talking total b*ll*cks here but in my world of low noise radio receiving equipment powered via linear supplies, I have to concern myself with the RF noise that the power supply diodes generate through their fast switching. The RF noise is suppressed with small bypass capacitors across each diode in the bridge. If the Schottky diodes are even faster switching devices I would definitely add suppressor capacitors to them.Kind RegardsFraser

author: madires
author_rank: Super Contributor
timestamp: September 11, 2012, 11:36:26 am
content: Quote from: Aurora on September 10, 2012, 05:12:47 pmThat's a good question! What I know is that Schottky diodes offer soft recovery, so there should be less RF problems as with silicon ones. Also some vendors are claiming that their soft-recovery rectifier diodes don't need any suppression capacitors. Most good quality stuff got 100nF caps in parallel to silicon rectifier bridges/diodes. Maybe somebody can enlighten us ;-)

author: Psi
author_rank: Super Contributor
timestamp: September 11, 2012, 12:02:13 pm
content: When spec'ing a schottky you will notice that ones with low forward drop, like 0.1-0.4V, have the lowest reverse voltage.Some of the ultra low forward drop diodes are as low as 12V for their reverse voltage.The 1N5819 is a common schottky to use in low voltage power supplies, It has a reverse voltage of 40V and forward voltage of 0.6V at 1AIn the same range there's the 1N5817, it gets you a lower drop of 0.4V at 1A but its reverse voltage is only 20V.So, as said above, with a low reverse voltage you have to be aware of spikes on the supply and deal with them with a TVS or something.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 12, 2012, 09:56:54 am
content: Any value in using a low shottky diode in a linear supply is marginal at best. For the same transformer, all it does is make your regulator dissipate a bit more power. In principle you could get away with a lower voltage transformer, but generally there isn't such a fine grained selection. Even if you wound a custom transformer the improvement would be minimal. A linear supply should be designed to operate over the allowable tolerance for line voltage -- generally the diode losses are a drop in the bucket.Compared to the risk associated with leakage current and thermal runaway, I don't think it is worth considering.

author: Alana
author_rank: Frequent Contributor
timestamp: September 12, 2012, 03:02:03 pm
content: If you have some spare shottky diodes from like old PC PSUs then it may be worth to use them, especially if you want your linear PSU to work closely to your transformer output voltage. I needed that several years ago when i was making my first lab PSU and it works fine without diode problems since then.Otherwise - don't bother.

author: NewBeginner
author_rank: Regular Contributor
timestamp: September 12, 2012, 03:45:54 pm
content: Thank you all.For the moment I just want to better understand the advantages/disadvantages of this two types of diodes.So if I understand this correctly the main advantages of a schottky diode over a silicone one are the low forward voltage drop and the lower switching noise because of their soft recovery.But on the other hand schottky diodes with lowVfwill also have a lower reverse voltage.Also because of the higher reverse leakage current the schottky diodes are more prone to thermal runaway.Is all this correct?Are there any other advantages/disadvantages?Thank you

author: Alana
author_rank: Frequent Contributor
timestamp: September 12, 2012, 04:11:54 pm
content: http://www.radio-electronics.com/info/data/semicond/schottky_diode/power-rectifier.php