Film-Tech Forum ARCHIVE: Rectifier diode current rating question.

This is topic Rectifier diode current rating question. in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Eric Robinson (Member # 2966) on 03-30-2006, 12:36 PM:

I have read many of the posts about replacing diodes in the forums; however, I am still unclear on one concept.

Supposing I have a xenon lamp which would be run at 85 amps. And supposing I want to install a new set of upgraded diodes in my rectifier. Furthermore, the rectifier has 3 reverse and 3 forward diodes. Would I purchase diodes which can supply 90 amps each or would I buy 30 amp diodes since there are three diodes each providing 30 amps for a total of 90 amps? [Confused]

Posted by Marin Zorica (Member # 1716) on 03-30-2006, 01:57 PM:

You must have all diodes capable to 90A, but not the max current was 90A, so if you gonna have 90A currentthen you need about 120-130A diodes.

You have Semikron SKN70/12 or SKN70/04 diodes, they are cheap and ok.

Also don't forget the diode voltage! Don't go belov 400V!

Posted by Dave Macaulay (Member # 813) on 03-30-2006, 02:37 PM:

It's not entirely simple. Many rectifiers use diodes rated less than the full output current, check an Irem - one of the most reliable types.
The current rating is the maximum continuous current that the diode should carry when properly mounted on a heat sink. A 3-phase bridge will have each diode passing full load current somewhere around 1/3 of the time. Using 1/3 rated diodes would mean they'd be overloaded 300%, more than I would think safe. Using 60 amp diodes for a 3-phase 85A bridge would likely be just fine.
You will be safest using diodes rated at the full output current, but the diode packages limit the maximum current diode that you'll be able to install. The most popular style used is the DO-5 1/4-28 stud mount; this is available up to 85A. A higher current rating means a different package, probably DO-8 with a bigger stud and a big cable attached.
You don't need to use a higher rated diode than the rectifier output rating.

Posted by Eric Robinson (Member # 2966) on 03-30-2006, 02:38 PM:

Hello,

Thanks for the info. I am trying to look these up on allied electronics website, but can not find the specific models you are referencing.

I almost understand the way Semikron specs their parts with the model number.

An SKN70/12 is:

SKN - anode to stud
70 - 70 amp
12 - multiply by 100 = 1200 PIV

is this correct?

If so, what is a SKN71/12

according to Semikron, the "71" is to designate the current; however, Semikron shows this model as 70 Amps?

Also is the SKN - anode to stud a "forward" or "reverse" diode?

Any help is appreciated.

ERIC

Posted by Marin Zorica (Member # 1716) on 03-30-2006, 03:08 PM:

SKN is anode stud, and you gott same caracteristic but SKR types for cathode studs, so if you have 3phase rectifier, you can put 3 SKN on one heat sink, and 3 SKR on other heat sink, because you have 3 anodes and 3 acthodes conected together to get + pole and - pole!

And also i found is some catalogue that SKN70/12 is 95A 1200V

Posted by Eric Robinson (Member # 2966) on 03-30-2006, 03:59 PM:

If a SKN is anode stud then the anode stud is mounted to the heatsink. Would this imply that the output of the diode at the anode would be (+) ?

If so, would this SKN diode be refered to as a reverse or forward diode (in theatre tech terminology?)

Thanks,

ERIC

Posted by Mark Gulbrandsen (Member # 72) on 03-30-2006, 08:49 PM:

quote: Dave Macaulay
The most popular style used is the DO-5 1/4-28 stud mount; this is available up to 85A. A higher current rating means a different package, probably DO-8 with a bigger stud and a big cable attached.
You don't need to use a higher rated diode than the rectifier output rating.

Dave, yer a little out of date on diodes! AAh, er Semikron makes a 140 amp stud mount in a DO-5 package. This diode also has a 1600 piv rating! These diodes are supplied in many new Irems, are available cheap from Allied Electronics in either leaded for Irem, or standard DO-5 stem type. I have yet to have one of them fail and I've got them running in Strong, Christie, and Irem units. There is also absolutely no reason not to use very conservatively rated parts in any high current situation, this can only add to the life span of the semi's.

Posted by Marin Zorica (Member # 1716) on 03-31-2006, 06:45 AM:

Eric, i do that this way:

I use two large heatsink for 3 diodes each! So you put SKN70/12 on first heatsink and on that heatsink you got + pole, on other you put 3 SKR70/12 and on that heat sing you get - pole, also then you got to conect wires from diodes, first diode on + heat sink to first didoe on - heatsink, and like that second and third diode, then you get 3 phase AC input which comes to didoes from transformer secondary.

Also if you are gonna do like this, you must insulate heatsinks from ground and from each other!!!!

Posted by Eric Robinson (Member # 2966) on 03-31-2006, 11:36 AM:

Here are three topics:

1) I noted on Semikrons website that: the SKN70/12 is leaded and SKN71/12 is not leaded, which is opposite logic of the International Rectifier naming convention, furthermore, the "Type Designation System" on Semikrons website mentions nothing which addresses the above system of identification. Why am I finding this so difficult? [Razz]

2) Thanks Marin Zorika for all your input. I will not be creating my own heatsinks. I appreciate your advice though. I am wondering though if the diodes on the heatsink which have anode to stud mounts are called forward or reverse diodes?

3) Thanks Mark G. I was hoping to hear from you. I have read most if not all of your other posts on this topic. I looked on allied and can not find the 140A Semikron diode. Do you perhaps know the model number? The only 140A diodes I could find on Semikrons website were fast recovery 1/2" stud mount.

Best Regards,

ERIC

Posted by Gordon McLeod (Member # 33) on 03-31-2006, 08:31 PM:

Most of the 140 amp I have seen have been 1/2" stud mount
I usually use th 85amp 600v IR diodes in most rectifiers and have had very few failures

Posted by Mark Gulbrandsen (Member # 72) on 03-31-2006, 09:11 PM:

quote: Gordon McLeod
Most of the 140 amp I have seen have been 1/2" stud mount

I believe the Semikron diodes are the the largest capacity made in a DO-5 package. The IXYS high speed diodes in the Strong compact switchers are 120 amp in a TO-247 plastic package. Its typically the part that that you hear go bam when the rectifier heads south. There are just four of these in a 3kw switcher! Semiconductor technology is certaianly moving right along....

Mark

Posted by Eric Robinson (Member # 2966) on 03-31-2006, 10:04 PM:

Yep.

[ 04-01-2006, 01:50 PM: Message edited by: Eric Robinson ]

Posted by Dave Macaulay (Member # 813) on 04-02-2006, 10:55 PM:

Semikron and IR have pretty much the same ratings, Semikron is a bit more conservative - IR's 85HFxx 85A Iav diode is rated at 133A Irms continuous; Semikron SK71/xx 70A Iav has a 150A Irms rating. If there's a part I don't know of or see on the Semikron spec sheets let me know.
So, unless once again mistaken, by the rated power (in the manufacturers part number) IR's 85A diode is the highest power DO-5 diode I can find.
Yes, installing higher capacity devices will enhance longevity. But - excepting the Strong blocking diode and similar - rectifier diodes especially in 3-phase bridges do not carry the full output current all the time and don't need to be rated for the full output current.
Diode failures in passive rectifiers in my experience are caused by 2 things. First, and most popular, lack of cleaning that leads to clogged fans and insulated heatsinks overheating the diodes, or bad installation of replacements with no heatsink compound, loose mounting hardware, and/or cold solder joints. Second, failed HF bypass capacitors that allow overvoltage stress on the diodes during lamp ignition.

Posted by Eric Robinson (Member # 2966) on 04-03-2006, 12:51 AM:

Interesting point about the diode rating and the Irms rating. I'm guessing that Irms is current root mean square? The logic seems backwards to me from speaker ratings which generally provide a peak and an RMS rating. The peak rating being higher than the rms rating.

The diode you speak of "85HFxx 85A Iav diode is rated at 133A Irms continuous" seems to be an 80 or 85 amp diode depending on the naming convention. So it seems confusing to me that it would have a higher rating for Irms.

It just goes to show that I need to do some more learning about rectifier diodes before I move forward.

Posted by Robert Minichino (Member # 3495) on 04-03-2006, 11:24 AM:

The 85A rating is the Iav, or average current, which ends up being 1/pi times peak. Irms or RMS current is half of peak, or pi/2 times Iav. So Irms = Iav * pi / 2 = 85A * 1.57 = 133A.

Don't forget the diode only conducts for half a cycle which is why the peak, average and RMS relationships are different.

Posted by System Notices (Member # 2357) on 09-28-2008, 07:07 PM:

It has been 909 days since the last post.