Reverse voltage spec for half way rectification diode - Page 1
Forum section: electronics / projects

author: jyrgen
author_rank: Contributor
timestamp: December 22, 2016, 06:09:31 am
content: Hi, I have picked the diode FSV360FP in a circuit board where it should half wave rectify a 24VAC input voltage.I now wonder if it is OK to use this diode like the attached schematic shows. The worst case peak reverse DC voltage will be 2*37,4V=74,8VDC (when input 24VAC have negative peak value). Or should I use the RMS reverse voltage in this case, which should be 37,4V? From the datasheet (also attached) it says that "maximum recurrent peak reverse voltage" is 60V, but "RMS reverse voltage" is 42V. Vill it be OK to use this diode?

author: Someone
author_rank: Super Contributor
timestamp: December 22, 2016, 07:08:24 am
content: Quote from: jyrgen on December 22, 2016, 06:09:31 amThat the peak, from the absolute maximum ratings. If you exceed it for any period of time youmightdestroy the device.Quote from: jyrgen on December 22, 2016, 06:09:31 amThe RMS voltages are suspiciously 2^0.5 of the peak and DC, so its likely just ashorthandshortcut for people who are only working in AC.

author: salbayeng
author_rank: Frequent Contributor
timestamp: December 22, 2016, 09:16:12 am
content: NO NO NOFor several reasons(a) It is a schottky diode , that should ring alarm bells, with schottky diodes you need to consider reverse current as it can be significant heating source, and the diode can go into thermal runaway, rule of thumb is to run ~ 60% of the max peak voltage to avoid thermal runaway. see figure2 , at 100C expect 1mA x 60v fro half the time = 30mW, thats OK, but what if its a warm day + you are drawing a heavy load and the mains voltage is high, then the temperature gets to 125C, and the power lost in reverse bias will be ~120mW, which will cause the temperature to increase, which will cause the reverse leakage to increase, etc,etc,etc, POP! Non-schottky diodes are more rugged.(b) it's in a tiny package, no way can you get 3A through a SOD123 (excluding 1mm thick copper busbars in liquid nitrogen) , you really need a SMA package , with pad area totalling 1sqcm to get more than 1A through(c) small transformers have very poor regulation, typically expect the unloaded voltage to be 20% higher. The mains could be 10% higher, there could be ~20% transients on the mains, distant lightning , fridge plop, someone welding etc .So your design peak voltage needs to be 1.414 x 24v x 120% x 110% x 120% = 54v , so with a half wave rectifier, peak reverse voltage is 108v ... So you might get lucky with a 100v rated device, a 60v device might last a day.(d) the 20% regulation of small transformers means a significant drop in voltage under load, so while your transformer might put out 24v RMS as a sine wave with a resistive load, it will be very flat topped with a half wave rectifier, the transformer might still put out 24V RMS but this could be a 24v peak square wave, so your voltage might only be about 25VDC at the peaks, then decay quite noticably during the other halfwave. So your output voltage might vary from 54v unloaded on a bad day to 26v with heavy ripple when fully loaded ( a bit more than your expectation of 30 to 37v)(Note that most 3 terminal regulators (and many IC's) are only rated to 40v so your power supply will easily exceed this at least several times per year; So you need to use 60v semis in the rest of your design).If you want to use 40v parts, you need to start with a 15VAC Tfr.(e) half wave rectifiers are generally a poor design choice, and should only be used for lightly loaded pilot supplies. Also you need to design using the "worn out" value of a capacitor (half of it's declared capacitance) so buy a 1500uF capacitor so it will be 780uF in 5000hrs time.(f) what Someone said, the absolute maximum ratings are exactly that, diodes are generally better than the average semi so you might get 70v Vb on a 60v part .A much better design for your application would be be a full bridge, where 60v rectifiers would be suitable, I'd use something like 4 x PMEG6045ETPX , these are high temp qualified, and SMA sized. Note that a through hole non-schottky bridge rectifier has a smaller footprint than SMD diodes with the necessary sized pads for cooling. And it's 1/4 of the price.

author: Gyro
author_rank: Super Contributor
timestamp: December 22, 2016, 10:12:22 am
content: Quote from: salbayeng on December 22, 2016, 09:16:12 amWhydo some people insist onthe shouting and headbanging, it gives me a headache. Can't you just present your opinion calmly?

author: jyrgen
author_rank: Contributor
timestamp: December 22, 2016, 11:47:55 am
content: Thanks for your replies.QuoteGood points, I will select a new capacitor based on 108V.QuoteI agree, altough in this application it need to fit with older devices which uses common ground for signals so I can't make a full wave rectification.

author: KerryW
author_rank: Regular Contributor
timestamp: December 22, 2016, 12:20:56 pm
content: As long as the other devices aren't using the same transformer for power, you can use full wave and just connect the grounds together.

author: Seekonk
author_rank: Super Contributor
timestamp: December 22, 2016, 02:21:08 pm
content: Quote from: salbayeng on December 22, 2016, 09:16:12 amMy experience is quite different. A general purpose diode within the product series is generally built to the higher spec voltage. Popular lower values are whatever comes off the line to fill demand. If I test 100 1N4002 diodes 3 will not meet at least 800V. However, Schottky diodes are a totally different beast with no extra margin.

author: Zero999
author_rank: Super Contributor
timestamp: December 22, 2016, 11:40:21 pm
content: Transformers can overheat when a half wave rectifier is connected to the secondary and a significant current is drawn from it. It causes DC flux to be induced in the core, resulting in saturation, excessive current flow and heating.As I think you've hinted, the maximum reverse voltage across the diode will be double the AC peak voltage. This is because the output capacitor charges up to the peak voltage, then the AC voltage reverses, leaving double the voltage across the diode.Use a bridge rectifier, rated to 100V or more.

author: KhronX
author_rank: Frequent Contributor
timestamp: December 22, 2016, 11:50:07 pm
content: Why exactly is half-wave rectification necessary, in this application?If it's absolutely mandatory, worst case you could put two of those diodes in series, if you have several, and/or would want to avoid placing and waiting for another order.