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Surge capacity.

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In Reply to: RE: Yes, they have a higher forward voltage drop. posted by kalido on March 15, 2009 at 16:48:41

When you throw the power switch on your amp (to turn it on from being off for a long time), the power transformer and rectifier bridge see the filter caps as a dead short. The peak current that flows into them depends on exactly when in the AC power wave the switch closes, and is limited only by the series resistance in the transformer, rectifier bridge, and associated wiring.

The caps charge up rapidly and the current drops to the normal idle current once the caps reach full charge. The turn-on pulse is what applies over-current stress to the rectifiers. It is difficult to predict what level this stress reaches. Big amps, with big aggregate filter capacity and large transformers, may generate very high surge current levels upon start-up. Some amps include "soft-start" provisions that limit the surge current at start-up, then get out of the way for normal operation.

Conventional silicon junction rectifiers have large surge capacities, but the refined fast-recovery devices have little or no surge capacity beyond the rated current. Improving the heat sinks will not do much to increase their surge capacity, as it is controlled by the internal resistances of the active regions in the diodes. Improving the heat sinks will allow the diodes to run cooler when the amp demands a lot of current during operation.

Is your amp a big one, with a heavy power transformer? Does it have a soft-start feature, or do the room lights dim briefly when you turn it on? If it seems to have a large surge at start-up, I would seriously think about a soft-start circuit to protect the rectifiers.

Follow Ups:

RE: Surge capacity. - kalido 23:27:27 03/16/09 (7)
Hi Al,
thanks for the lengthy explanation. The amp is a rotel 60watter integrated and the transformer is just a decent toroidal, definitely not the huge ones and never had light dimming issue before.
The stock rectifier had a surge rating of 125A whereas the new Dyn that i swapped in had a surge rating of 200A. SO this part should be fine shouldnt it ?

As u've mentioned earlier the Dyn does seem to have a higher forward voltage as the overall DC on each main cap drops around 2V and the datasheet confirms this. ( DYN has max forward of 2.5v vs 1.1v of the stock rectifier ).
- You should be OK. - Al Sekela 16:30:04 03/17/09 (6)
  The increased forward voltage is a consequence of how the rectifiers are made. They have to minimize the electrical charge stored in the junctions during forward conduction, and it takes more voltage drop across the minimized regions to push the current through.
  
  Don't push the amp to the point where you hear some distortion. The voltage margin has been reduced by the lower rail voltage, and you don't want to harm your speakers by driving the amp into clipping.
  - RE: You should be OK. - kalido 20:20:30 03/17/09 (5)
    Al, strangely , when i first heard the distortion , the amp was in an idle condition with vol all the way down ? That was the first i noticed it and it became louder as i increased the volume and even any movement on the volume pot cuased it to distort a bit.
    And all these was after a while of playing music.
    - Might be a separate problem. - Al Sekela 15:02:32 03/18/09 (4)
      If you get about normal rail voltages with your new rectifiers, and can check that the rail voltages stay the same when you get the distortion, then it is not the rectifiers but something else. It may be collateral damage from your rectifier replacement project.
      - RE: Might be a separate problem. - kalido 21:57:12 03/18/09 (3)
        
        In Reply to: RE: Might be a separate problem. posted by Al Sekela on March 18, 2009 at 15:02:32
        
        hi Al, i tried replacing it with the stock 4 pin bridge, and there're no distortion at all now. it's looking more and more like heat sinking issue on the DYNs to me... what u reckon ?
        
        Your experiments are making me think it is that or - Al Sekela 10:25:04 03/19/09 (2)
        
        In Reply to: RE: Might be a separate problem. posted by kalido on March 18, 2009 at 21:57:12
        
        something related to the wiring of the fancy rectifiers.
        
        RE: Your experiments are making me think it is that or - kalido 15:04:01 03/19/09 (1)
        
        In Reply to: RE: Your experiments are making me think it is that or posted by Al Sekela on March 19, 2009 at 10:25:04
        
        i'm sure i got it rite the first time as i spend considerable amount of time actualy sketcing the connections just to avoid any mistakes...but i think i'll spend more time on it tomm just to make sure....
        
        basiclly , i had pin 1( of DYN 1 ) soldered to pin 1 ( DYN 2 ) , pin 3 soldered to pin 3. These were then the +/- out. For +AC supply , it was pin 2 of DYN 1 and -AC , pin 2 of DYN 2. I had these drawn up correctly and compared it against the stock 4 pin case and it was identical and i worked fine when i turned it on. But i could be missing something somewhere....
        
        Just be sure there is no backside electrical connection. - Al Sekela 16:06:41 03/20/09 (0)
        
        In Reply to: RE: Your experiments are making me think it is that or posted by kalido on March 19, 2009 at 15:04:01
        
        I don't know about the IXYS Isoplus-220 package. If it is all plastic then you are OK. If there is a metal plate on the back, the metal may be connected to one of the pins inside the package. This (or the heat sink) has to be insulated from any other metal parts.

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