I didn't realize this issue would cause such debate, but it has definitely been educational! I think I am assured that my safety margin is reasonable even without paralleling. Dissipation should not be high. I think the only worry is exceeding the peak half-cycle draw at turn-on, and there are ways to address that.
Thanks again!
Thanks again!
janneman said:
If you don't want to fry the amp with a test signal it should withstand full power for at least 5-10 minutes but if it's OK to have a black hole after some unexpected input, that's fine by me.
Compare NAD the cheapest, this amp will burn if you drive it at party levels. How funny is that?
till said:
I'll go out on a limb and see how right (wrong) I am.
Schottky diodes as a majority carrier device should be closer to ideal in terms of their recovery characteristic. Fast/soft recovery conventional PN are just trying to get closer to this standard. However not all Schottky are equal, especially in terms of reverse leakage current.
http://www.ixys.com/t070299c.pdf
dhaen said:Heck, why worry on the detail?
I just work out the current, then choose a rectifier of 2 to 4 times the rating. They're cheap. It's not as if we are designing for mass production where every cent counts.
Oversize rectifiers also drop slightly less volts.
Schottkys are the coolest
Sure, no problem. Your (or anybody else's) call anyway. I was just trying to answer a specific question as good and as precise as I could.
Jan Didden
This matter of diode specification seems fraught with peril. In my next amp I'm using 5A average forward current, but with a 324000µF CRCRC power filter. The conduction angle with such a filter is difficult to determine, but I know it is roughly 11°-15°. With this duty cycle (3-4%) the peak repetitive forward current will be about 80A!
If you consider a device like IRF MBR4045WT, you see it has an average forward current of 40A and peak repetitive forward current of 80A. However this latter spec is for a 50% duty cycle, I think. With a 3% duty cycle I think I should be okay??? The max. surge current for this device is over 1000A.
Anyone have advice on a similar high-current flat-pack Schottky? I chose Schottky over HEXFRED and similar diodes because the low forward voltage of the Schottky allows me to have higher voltage rails with the same transformer.
If you consider a device like IRF MBR4045WT, you see it has an average forward current of 40A and peak repetitive forward current of 80A. However this latter spec is for a 50% duty cycle, I think. With a 3% duty cycle I think I should be okay??? The max. surge current for this device is over 1000A.
Anyone have advice on a similar high-current flat-pack Schottky? I chose Schottky over HEXFRED and similar diodes because the low forward voltage of the Schottky allows me to have higher voltage rails with the same transformer.
As long as we talk about rectified currents it's a little bit hard to determine how big the diodes should be. One important factor is temperature of the bridge. It should not be too hot and if the smoothing package is bigger than "normal" maybe it's a good idea to have a little larger diodes due to those peak currents.
Bridge diodes
A bridge is typically current rated using the following:
1. Maximum average forward rectified output current, with the ratings based either on a case temperature if heatsinked, or ambient temperature, if it is supported by the leads.
2. Surge current. This is the non repetitive current pulse the unit will survive..usually rated as one 8.3 mSec pulse, but can be rated two pulses.
3. Maximum reverse leakage and Vbr.
4. Case temperature.
Each diode within will see half the average current..a 10 amp bridge will have each diode seeing 5 amps average. A ten amp bridge can be built with 5 amp diodes, but the surge ratings may be too low.. the surge rating for larger die is better, and the vf will be reduced.
Schottky's have much lower VF for the current, but depending on the barrier metal, may leak like a sieve....Shottky's can dissipate as much power in the reverse direction as forward, depending on the application.
Using 4 8 amp diodes for an 8 amp bridge is perfectly fine..But if the cap bank is really oversized, you will be better off with larger diodes just for the surge that the caps will have at turn on. Skimping on this will result in reliability issues quickly (it blow'd up).
For 60 or 50 cycle use, all diodes in production today do not dissipate any significant power during turn off, so the recovery speed of the die is not important for derating. Typical stuff will be 2 microsecond.
Some manu's also provide a fusing rating for the device..the 6 ampere GI device I'm looking at has a fusing rating of 127 amp squared seconds.
PN die will be rated to 150 C typical junction...shottky's 100 C or 125 C depending on manu.
BTW, GI is general instrument...I was an app. eng there for a while.
Cheers, John
A bridge is typically current rated using the following:
1. Maximum average forward rectified output current, with the ratings based either on a case temperature if heatsinked, or ambient temperature, if it is supported by the leads.
2. Surge current. This is the non repetitive current pulse the unit will survive..usually rated as one 8.3 mSec pulse, but can be rated two pulses.
3. Maximum reverse leakage and Vbr.
4. Case temperature.
Each diode within will see half the average current..a 10 amp bridge will have each diode seeing 5 amps average. A ten amp bridge can be built with 5 amp diodes, but the surge ratings may be too low.. the surge rating for larger die is better, and the vf will be reduced.
Schottky's have much lower VF for the current, but depending on the barrier metal, may leak like a sieve....Shottky's can dissipate as much power in the reverse direction as forward, depending on the application.
Using 4 8 amp diodes for an 8 amp bridge is perfectly fine..But if the cap bank is really oversized, you will be better off with larger diodes just for the surge that the caps will have at turn on. Skimping on this will result in reliability issues quickly (it blow'd up).
For 60 or 50 cycle use, all diodes in production today do not dissipate any significant power during turn off, so the recovery speed of the die is not important for derating. Typical stuff will be 2 microsecond.
Some manu's also provide a fusing rating for the device..the 6 ampere GI device I'm looking at has a fusing rating of 127 amp squared seconds.
PN die will be rated to 150 C typical junction...shottky's 100 C or 125 C depending on manu.
BTW, GI is general instrument...I was an app. eng there for a while.
Cheers, John
Great info John. So would you say that the current ratings for a diode are based mostly on junction/barrier temperature? The Schottky I'm pondering is rated to 150°C operating, and I estimate the Rθjs at 2.5K°/W. So do I merely need to analyze the heat transfer to keep the peak junction temp within the ratings, or is there another limit to peak repetitive current?
As for the surge rating, I'm dodging that problem by charging the filter initially through a resistor.
As for the surge rating, I'm dodging that problem by charging the filter initially through a resistor.
tiroth said:John,
Then maybe you can tell me why those GI data sheets are so impossible to locate. (Just kidding!
Because I took them all..
Do you need any info?
jwb said:
The current ratings are derived by the junction temperature..If you look at the forward current derating curve of any diode, you will see it go to zero current (usually straight line) at some temperature..Most manu's will do so at 125 C or 150 C. Below the break temp, anywhere from 40C to 100C, it is constant current . So a 3 amp diode will be three amps up to the break, say 40 C...then they draw a straight line to the highest temp and zero current. The intent of that graph is to depict a constant junction temperature, although it is rather crude. VF lot variation will change that curve, and the manu's make that line conservative.
That graph is actually just an approximation..the reason is the current/voltage curve, which give a non linear power/current relation. But you won't get in trouble if you follow it..
Using theta Jc calcs with heat transfer will work fine.
The GI page I'm looking at also has a graph of peak forward current vs number of cycles at 60 hz..a 3 amp unit is about 150 amps 1 cycle, to about 100 cycles at 27 amps. Note that that is a half sine wave, as per a jedec method. Your diodes look to be rated for other than 60 hz operation, hence the 50% number. If you have transient thetaJc graph of the diode, you can estimate the junction rise max during each cycle.
In a supply, the average will be the only concern. The junction temp fluctuations during each cycle will be only in the degrees range, regardless of the amount of capacitance it is charging..
Try to stay as low in temp as possible..lifetime doubles every 10C drop in junction...and shottky's have some major leakage hot, and you may have to calculate the power loss as a result of the leakage current during the reverse half cycle.
If you find surge is a problem, then you could go with a bigger die size, with less heat sinking required.
Cheers, John
A word of caution.
I looked up your devices..note that the reverse leakage at 100 C is 50 milliamps..
At 45 volts, that is 2.25 watts dissipation. If you run hotter, that will really skyrocket. They are capable of IR runaway if you don't consider that power dissipation. If your running 10 amps, that is about 6 watts forward power, 2.25 reverse..about 1/3 of the expected dissipation added.
The reverse leakage runaway used to be a major issue with schottky's and gold doped fast recovery diodes. But I haven't built or tested a small diode in 11 years. My experience of late is with 6000 amp 2.5 inch diameter diodes..
Cheers, John
I looked up your devices..note that the reverse leakage at 100 C is 50 milliamps..
At 45 volts, that is 2.25 watts dissipation. If you run hotter, that will really skyrocket. They are capable of IR runaway if you don't consider that power dissipation. If your running 10 amps, that is about 6 watts forward power, 2.25 reverse..about 1/3 of the expected dissipation added.
The reverse leakage runaway used to be a major issue with schottky's and gold doped fast recovery diodes. But I haven't built or tested a small diode in 11 years. My experience of late is with 6000 amp 2.5 inch diameter diodes..
Cheers, John
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