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Change to Shottky?

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revenson(at)comcast.net
Guest

Posted: Tue Jan 14, 2014 7:20 am Post subject: Change to Shottky?

p { margin: 0; }I wired Z11 with the earlier recommended endurance bus diode w/15 watt heatsink.  My loads are 11.5 amps intermittent maximum, 6.1a. typical, and if alternator quits, I could shut things off to get to about 3.3 amps for comfortable endurance use.  I see 13.1-13.3 volts on this bus during flight.  I have had no issues in 350 hours.



Since completing my RV in 2010, I have been absent from the aeroelectric list.  I have noticed that the Shottky diode is now recommended.

Did a search to attempt to 'catch up' with the discussion and have tentatively concluded that this change would be purely optional, and not 'highly recommended' for some electrical reason.



I'm on a weight reduction kick, so my only objective to change to the Shottky would only be to eliminate the 1/2 lb. of the heatsink.



Bob, et al:  I'm looking for some assurance.  Have I missed something pertinent to this decision or am I on track?



Roger

"For I know the plans I have for you," declares the Lord, "plans to prosper you and not to harm you, plans to give you hope and a future."  Jeremiah 29:11



 [quote][b]

jluckey(at)pacbell.net
Guest

Posted: Tue Jan 14, 2014 8:50 am Post subject: Change to Shottky?

Roger,

The advantage of Schottky diodes is that they have less voltage drop than silicon diodes. Silicon is approx 0.7 volts and Schottky about 0.5 volts. The advantage being that Schottkys dissipate a little less energy.

Using your numbers for typical load:

Existing diode:

P = IE

6 * .7 = 4.2W

Schottky diode:

6 * .5 = 3.0W

I agree w/ your conclusion, probably not worth changing anything for a delta of 1.2W.

-Jeff



        From: Roger Evenson <revenson(at)comcast.net>

 To: AeroElectric-List(at)matronics.com 

 Sent: Tuesday, January 14, 2014 7:19 AM

 Subject: Change to Shottky?

  

 

 #yiv1059132124 p {margin:0;}#yiv1059132124 p {margin:0;}I wired Z11 with the earlier recommended endurance bus diode w/15 watt heatsink.  My loads are 11.5 amps intermittent maximum, 6.1a. typical, and if alternator quits, I could shut things off to get to about 3.3 amps for comfortable endurance use.  I see 13.1-13.3 volts on this bus during flight.  I have had no issues in 350 hours.



Since completing my RV in 2010, I have been absent from the aeroelectric list.  I have noticed that the Shottky diode is now recommended.

Did a search to attempt to 'catch up' with the discussion and have tentatively concluded that this change would be purely optional, and not 'highly recommended' for some electrical reason.



I'm on a weight  reduction kick, so my only objective to change to the Shottky would only be to eliminate the 1/2 lb. of the heatsink.



Bob, et al:  I'm looking for some assurance.  Have I missed something pertinent to this decision or am I on track?



Roger

"For I know the plans I have for you," declares the Lord, "plans to prosper you and not to harm you, plans to give you hope and a future."  Jeremiah 29:11



  	  Quote:

teblejw

Joined: 19 Nov 2013
Posts: 14

Posted: Tue Jan 14, 2014 11:21 am Post subject: Change to Shottky?

At Bob's suggestion, I've been looking at double anode - single cathode arrangements in a TO-247 case, wired so that current goes through both diodes.   My average amperage will be 6-7 amps (12 to 14 total) and I can find packages that will have voltage drops of 0.3 volts or less.  The tradeoff is higher leakage, but I don't see how a few tens of mA will cause problems.  With only 4 watts or so, Bob suggested using the aluminum already behind the panel.  Even if you don't have enough sheet aluminum, there are light-weight heat sinks for the TO-247 package that only cost a couple of bucks and weigh an ounce or so.  The TO-247 does have to be solder connected and mounted with an electrically resistive pad, which  adds little to cost or weight.

Tom



[quote]     

   From: Jeff Luckey <jluckey(at)pacbell.net>

 To: "aeroelectric-list(at)matronics.com" <aeroelectric-list(at)matronics.com> 

 Sent: Tuesday, January 14, 2014 9:49 AM

 Subject: Re: AeroElectric-List: Change to Shottky?

  

 

 Roger,

The advantage of Schottky diodes is that they have less voltage drop than silicon diodes. Silicon is approx 0.7 volts and Schottky about 0.5 volts. The advantage being that Schottkys dissipate a little less energy.

Using your numbers for typical load:

Existing diode:

P = IE

6 * .7 = 4.2W

Schottky diode:

6 * .5 = 3.0W

I agree w/ your conclusion, probably not worth changing anything for a delta of 1.2W.

-Jeff



     

   From: Roger Evenson <revenson(at)comcast.net>

 To: AeroElectric-List(at)matronics.com 

 Sent: Tuesday, January 14, 2014 7:19 AM

 Subject: Change to Shottky?

  

 

 #yiv525709406  p {margin:0;}#yiv525709406  p {margin:0;}I wired Z11 with the earlier recommended endurance bus diode w/15 watt heatsink.  My loads are 11.5 amps intermittent maximum, 6.1a. typical, and if alternator quits, I could shut things off to get to about 3.3 amps for comfortable endurance use.  I see 13.1-13.3 volts on this bus during flight.  I have had no issues in 350 hours.



Since completing my RV in 2010, I have been absent from the aeroelectric list.  I have noticed that the Shottky diode is now recommended.

Did a search to attempt to 'catch up' with the discussion and have tentatively concluded that this change would be purely optional, and not 'highly recommended' for some electrical  reason.



I'm on a weight  reduction kick, so my only objective to change to the Shottky would only be to eliminate the 1/2 lb. of the heatsink.



Bob, et al:  I'm looking for some assurance.  Have I missed something pertinent to this decision or am I on track?



Roger

"For I know the plans I have for you," declares the Lord, "plans to prosper you and not to harm you, plans to give you hope and a future."  Jeremiah 29:11



  	  Quote: 	
		  


	




 

 

 [b]

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Jan 15, 2014 12:46 pm Post subject: Change to Shottky?

At 01:20 PM 1/14/2014, you wrote:

  At Bob's suggestion, I've been looking at double anode - single 

cathode arrangements in a TO-247 case, wired so that current goes 

through both diodes.   My average amperage will be 6-7 amps (12 to 14 

total) and I can find packages that will have voltage drops of 0.3 

volts or less.  The tradeoff is higher leakage, but I don't see how a 

few tens of mA will cause problems.  With only 4 watts or so, Bob 

suggested using the aluminum already behind the panel.  Even if you 

don't have enough sheet aluminum, there are light-weight heat sinks 

for the TO-247 package that only cost a couple of bucks and weigh an 

ounce or so.  The TO-247 does have to be solder connected and mounted 

with an electrically resistive pad, which adds little to cost or weight.



     You're not going to find Schottky devices with rated

     for such low voltage drops at their rated current.

     To push the drop down, you use oversized to grossly

     oversized diodes lightly loaded and yes, your

     drop will go down a tad.

The advantage of Schottky diodes is that they have less voltage drop 

than silicon diodes. Silicon is approx 0.7 volts and Schottky about 

0.5 volts. The advantage being that Schottkys dissipate a little less energy.



Using your numbers for typical load:

Existing diode:

P = IE

6 * .7 = 4.2W



Schottky diode:

6 * .5 = 3.0W



I agree w/ your conclusion, probably not worth changing anything for 

a delta of 1.2W.'



    Agreed . . . The only time the diode is 'wasting'

    energy is when the alternator is running and if you're

    running a 40A+ alternator, the energy 'savings'

    is trivial.



I wired Z11 with the earlier recommended endurance bus diode w/15 

watt heatsink.  My loads are 11.5 amps intermittent maximum, 6.1a. 

typical, and if alternator quits, I could shut things off to get to 

about 3.3 amps for comfortable endurance use.  I see 13.1-13.3 volts 

on this bus during flight.  I have had no issues in 350 hours.



    That heatsink is WAaayyyy bigger than you need for this

    application.



Since completing my RV in 2010, I have been absent from the 

aeroelectric list.  I have noticed that the Shottky diode is now recommended.



    It's an OPTION . . . one of several.



Did a search to attempt to 'catch up' with the discussion and have 

tentatively concluded that this change would be purely optional, and 

not 'highly recommended' for some electrical reason.



    EXACTLY. I would not recommend anyone swap out

    an existing rectifier installation. The Schotty devices

    are attractive for new construction. My product uses

    the airframe for heat management . . . a bit more compact

    than the rectifier . . .



    There are VERY few incarnations of the e-bus that

    demands a finned heat-sink for the rectifier. Either

    the silicon junction rectifier -OR- the Schottky

    device should exist quite happily simply sinked

    to the airplane.



    Products like this



http://tinyurl.com/mfp9k7a



    probably make sense in a composite airplane but I

    suspect that few installations in metal airplanes

    need the extra heat-sink.

I'm on a weight reduction kick, so my only objective to change to the 

Shottky would only be to eliminate the 1/2 lb. of the heat-sink.

    1/2 pound? Which heat-sink are you using?

Bob, et al:  I'm looking for some assurance.  Have I missed something 

pertinent to this decision or am I on track?

    Is this a metal airplane? Have you looked at simply

    bolting the diode to local sheet metal?



   Bob . . .

revenson(at)comcast.net
Guest

Posted: Wed Jan 15, 2014 1:30 pm Post subject: Change to Shottky?

Bob:  To answer your questions, this is an RV7, all metal airplane.   The heat sink I used was the 15 watt one, the larger of the two offered at B&C.  It weighs 7 oz.



I will remove it and install directly to the aluminum subpanel.



Would you change your recommendation if you knew I was in Arizona, occasionally flying in 100+ temperatures?



Thanks.  Roger





From: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com>

To: aeroelectric-list(at)matronics.com

Sent: Wednesday, January 15, 2014 1:43:39 PM

Subject: Re: Change to Shottky?



--> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com>



At 01:20 PM 1/14/2014, you wrote:

  At Bob's suggestion, I've been looking at double anode - single 

cathode arrangements in a TO-247 case, wired so that current goes 

through both diodes.   My average amperage will be 6-7 amps (12 to 14 

total) and I can find packages that will have voltage drops of 0.3 

volts or less.  The tradeoff is higher leakage, but I don't see how a 

few tens of mA will cause problems.  With only 4 watts or so, Bob 

suggested using the aluminum already behind the panel.  Even if you 

don't have enough sheet aluminum, there are light-weight heat sinks 

for the TO-247 package that only cost a couple of bucks and weigh an 

ounce or so.  The TO-247 does have to be solder connected and mounted 

with an electrically resistive pad, which adds little to cost or weight.



     You're not going to find Schottky devices with rated

     for such low voltage drops at their rated current.

     To push the drop down, you use oversized to grossly

     oversized diodes lightly loaded and yes, your

     drop will go down a tad.

The advantage of Schottky diodes is that they have less voltage drop 

than silicon diodes. Silicon is approx 0.7 volts and Schottky about 

0.5 volts. The advantage being that Schottkys dissipate a little less energy.



Using your numbers for typical load:

Existing diode:

P = IE

6 * .7 = 4.2W



Schottky diode:

6 * .5 = 3.0W



I agree w/ your conclusion, probably not worth changing anything for 

a delta of 1.2W.'



    Agreed . . . The only time the diode is 'wasting'

    energy is when the alternator is running and if you're

    running a 40A+ alternator, the energy 'savings'

    is trivial.



I wired Z11 with the earlier recommended endurance bus diode w/15 

watt heatsink.  My loads are 11.5 amps intermittent maximum, 6.1a. 

typical, and if alternator quits, I could shut things off to get to 

about 3.3 amps for comfortable endurance use.  I see 13.1-13.3 volts 

on this bus during flight.  I have had no issues in 350 hours.



    That heatsink is WAaayyyy bigger than you need for this

    application.



Since completing my RV in 2010, I have been absent from the 

aeroelectric list.  I have noticed that the Shottky diode is now recommended.



    It's an OPTION . . . one of several.



Did a search to attempt to 'catch up' with the discussion and have 

tentatively concluded that this change would be purely optional, and 

not 'highly recommended' for some electrical reason.



    EXACTLY. I would not recommend anyone swap out

    an existing rectifier installation. The Schotty devices

    are attractive for new construction. My product uses

    the airframe for heat management . . . a bit more compact

    than the rectifier . . .



    There are VERY few incarnations of the e-bus that

    demands a finned heat-sink for the rectifier. Either

    the silicon junction rectifier -OR- the Schottky

    device should exist quite happily simply sinked

    to the airplane.



    Products like this



http://tinyurl.com/mfp9k7a



    probably make sense in a composite airplane but I

    suspect that few installations in metal airplanes

    need the extra heat-sink.

I'm on a weight reduction kick, so my only objective to change to the 

Shottky would only be to eliminate the 1/2 lb. of the heat-sink.

    1/2 pound? Which heat-sink are you using?

Bob, et al:  I'm looking for some assurance.  Have I missed something 

pertinent to this decision or am I on track?

    Is this a metal airplane? Have you looked at simply

    bolting the diode to local sheet metal?;              -Matt Dralle, List Admin.========





 [quote][b]

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Jan 16, 2014 6:15 am Post subject: Change to Shottky?

At 03:29 PM 1/15/2014, you wrote:

 	  Quote: 	
		  Bob:  To answer your questions, this is an RV7, all metal 

airplane.   The heat sink I used was the 15 watt one, the larger of 

the two offered at B&C.  It weighs 7 oz.



I will remove it and install directly to the aluminum subpanel.



Would you change your recommendation if you knew I was in Arizona, 

occasionally flying in 100+ temperatures?


	


   Nope. 100+ isn't 'hot' for a semiconductor.

   The it's only 30F or 16C hotter than what

   people like to exist in. Most semiconductor

   devices are rated to operate up to 175C

   AT THE JUNCTIONS . . . that is hot.



   The only time your 100F hypothesis would put

   a device at risk is if the heat sink were

   marginal in the first place. After operating

   your system loads for say 30 minutes or so

   (time for temps to stabilize) put your fingers

   on the device . . . you may find it 'toasty' but

   it probably wouldn't 'sizzle spit' . . . most

   of the time you'll find it warm but not dangerous

   for the epidermis.



   If you're really curious, you can put a temp

   sensor on the bolt that mounts the device and get

   some real numbers . . . good thing to know

   and share.



   If you don't have a capable remote temperature

   measuring instrument, you might consider this

   one . . .



http://tinyurl.com/lomcyp3



   Comes with two probes and is shipped from US

   location . . . fast.



   No matter how convincing I might be by offering

   advice from experience, there's nothing better

   than to go measure it for yourself and then

   report the results.



   Bob . . .

Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Thu Jan 16, 2014 7:14 am Post subject: Re: Change to Shottky?

The calculation here on the dissipation and voltage drops of Schottkys and P/N diodes are simply being pulled out of the air. What diodes are you referring to? You show me yours and I'll show you mine.

 

A "standard" 15A 50V P/N diode is  1.5 Volts Vf like 1N3208. So at 13V you get 11.5V out.



I use and sell IXYS  DSSX61-0045A. Power_Deuce_Schottkys. At 15A they are 0.45Volts Vf. So at 13VDC you get 12.55VDC out.



I supply these on heatsink. They are isolated and paired so they can be used in Y-configurations or separate or paired. 



I've looked and don't see any better diodes for general aircraft purposes. If the difference was as small as some here have erroneously stated, we wouldn't be having this conversation. Most modern battery operated equipment have no p/n diodes. They waste too much power.



Remember: With diodes the measurements are often at 50% duty cycle. Also remember that Vf is not reduced when diodes are in placed parallel (unlike R for resistors).

Power_Deuce_Schottky_ Manual.pdf

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Power_Deuce_Schottky_ Manual.pdf

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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

teblejw

Joined: 19 Nov 2013
Posts: 14

Posted: Thu Jan 16, 2014 2:10 pm Post subject: Change to Shottky?

Eric,

I think that I misinterpreted the use of the word "average."  (My aero, mechanical, and civil engineering degrees were not big on diodes.)  Say, for the specific diode STPS60L30CW: http://www.st.com/en/resources/technical/document/cd00001857.pdf  ,

Fig. 1 provides "average forward power dissipation versus average forward current (per diode)."  I now understand that the term "average" is the average for the cycle (not the average between the two diodes), i.e., the average amperage for a square-wave cycle with a delta of 0.5 would be half the peak to peak amperage.  So, if I understand you correctly, the voltage drop with both diodes  connected to the same source should be calculated using the total current, as if there were only one diode connected. For this particular diode assembly and my system (14A max. at 13V), the voltage drop from Fig. 9 at a Tj of 125 degC is 0.31 to 0.32, for a power dissipation of slightly more than 4W.  I would only use this diode if I were confident that I could keep the Tj low, because it has high leakage at high temperature and because this diode is only rated to 150 degC.  But the leakage versus voltage drop is the tradeoff I see in the selection process.  Not clear to me that we need extremely low leakage.

I don't understand why the voltage drop should be calculated as if all the current goes through only one diode.  This implies that the current doesn't substantively split, because if it did, the voltage drop in each diode would be lower.  If there is a good  authoritative reference that you can suggest, I would appreciate it.  I've Googled and found different views and opinions.

Thanks.





Tom



 



 	  Quote: 	
		       

   From: Eric M. Jones <emjones(at)charter.net>

 To: aeroelectric-list(at)matronics.com 

 Sent: Thursday, January 16, 2014 8:14 AM

 Subject: Re: Change to Shottky?

  

 

 --> AeroElectric-List message posted by: "Eric M. Jones" <emjones(at)charter.net (emjones(at)charter.net)>



The calculation here on the dissipation and voltage drops of Schottkys and P/N diodes are simply being pulled out of the air. What diodes are you referring to? You show me yours and I'll show you mine.

 

A "standard" 15A 50V P/N diode is  1.5 Volts Vf like 1N3208. So at 13V you get 11.5V out.



I use and sell IXYS  DSSX61-0045A. Power_Deuce_Schottkys. At 15A they are 0.45Volts Vf. So at 13VDC you get 12.55VDC out.



I supply these on heatsink. They are isolated and paired so they can be used in Y-configurations or separate or paired. 



I've looked and don't see any better diodes for general aircraft purposes. If the difference was as small as some here have erroneously stated, we wouldn't be having this conversation. Most modern battery operated  equipment have no p/n diodes. They waste too much power.



Remember: With diodes the measurements are often at 50% duty cycle. Also remember that Vf is not reduced when diodes are in placed parallel (unlike R for resistors).



--------

Eric M. Jones

http://www.periheliondesign.com/

113 Brentwood Drive

Southbridge, MA 01550

(508) 764-2072

emjones(at)charter.net





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=417007#417007





Attachments: 



http://forums.matronics.com//files/power_deuce_schottky_manual_109.pdf

=                    &n-->

Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Fri Jan 17, 2014 3:11 pm Post subject: Re: Change to Shottky?

When using diodes in parallel, the Vf drop will stay the same as the lowest diodesâ€™ Vf but reverse leakage Ir (and capacitance) will add. But it is often cited as a bad idea because as one diode heats up, its Vf will decreaseâ€¦and it will draw more current, etc. This is called a thermal runaway failure. You can avoid this by placing the diodes in tight thermal contact with each other, or using a small resistor in series with each, or making certain the Vfs start and thermally track each other as closely as possible (and the diodes were made at the same time, same batch, etc.).



Schottky diodes especially trade off forward Vf with reverse leakage. So always look for this when trying to get a very low forward Vf. I canâ€™t say reverse leakage is always a bad thing; it depends on the use. The spec sheet for STPS60L30 states:



Dual center tap Schottky rectifier suited for Switch Mode Power Supply and high frequency DC to DC converters. Packaged in TO247, this device is intended for use in low voltage, high frequency inverters, free-wheeling and polarity protection applications.



This statement by STM is a good indication that this part has a huge reverse leakage Ir, since HF inverter transformers in power supplies can allow for that reverse DC leakage easily while they benefit from the very low Vf. And it does. Ir(max) Tj=125C, 500 mA! Can your application accept that?



The spec sheet for IXYS DSSX61-0045A Ir (max) is 20 mA.



I donâ€™t want to specify or analyze specific diode choices, but I think IXYS DSSX61-0045A represents a good compromise for power blocking diodes in price/performance /Vf/Ir mostly because it is packaged in an isolated SOT-227 package. There are lower Vf diodes, and lower Ir diodes, and diodes that are far pricier. But this is a great compromise and very versatile diode. Remember, Schottkys dissipate less and often require no heatsink compared to a similarly-rated P/N diode. 



 	  Quote: 	
		  >>From: Thomas Blejwas <tomblejwas>

Subject: Re: AeroElectric-List: Re: Change to Schottky?



-Eric, I think that I misinterpreted the use of the word "average."

- (My-aero, mechanical, and civil engineering degrees were not big on diodes.) -Say, for the specific diode STPS60L30CW: http://www.st.com/en/resources/technical/document/cd00001857.pdf- , Fig. 1 provides "average forward power dissipation versus average forward current (per diode)."- I 

now understand that the term "average" is the average for-the cycle (not 

the average between the two diodes), i.e., the average amperage for a square-wave cycle with a delta of 0.5 would be half the peak to peak amperage.

>>- So, if I understand you correctly, the voltage drop with both diodes connected to the same source-should be calculated using the total current, as if there were only one diode connected. 	




Right, if I get what you're saying. Think of Vf standing for â€œVertical fallâ€ (waterfall). Adding waterfalls in parallel, doesnâ€™t make each Vf waterfall taller. You can get more current, but you canâ€™t reduce the Vf potential loss. It is what it is. At the top of the waterfall you have a potential; at the bottom you have lower potential. 



 	  Quote: 	
		  >>For this particular diode assembly-and my system (14A max.-at 13V), the voltage drop from Fig. 9 at a Tj of 125 degC is 0.31 to 0.32, for a power dissipation of-slightly more than 4W.- I would only use this diode if I were confident that I could keep the Tj low, because it has high leakage at high temperature and because this diode is only rated to 150 degC.- But the leakage versus voltage drop is the tradeoff I see in the selection process.-

	


True. But even at a Tj of 125 degC, used as an isolation diode, Iâ€™d be careful because it doesnâ€™t isolate much. This might matter.



 	  Quote: 	
		   >>Not clear to me that we need extremely low leakage. I don't understand-why the voltage drop should be calculated as if all the current goes through only one diode.- This implies that the current doesn't substantively split, because if it did, the voltage drop in each diode would be lower.- If there is a good authoritative-reference that you can suggest, I would appreciate it.- I've Googled and found different views and opinions. 	




There is a substantial body of work on using Schottkys. Try: http://www.onsemi.com/pub_link/Collateral/AND9038-D.PDF or other application notes from major manufacturers.



Builders new to electronics often wonder if there is a particular â€œbestâ€ component. There usually isnâ€™t Furthermore sometimes (amazingly!) the manufacturer hides certain factsâ€¦like price, which is a really important characteristic unless you work for the military, or the 56-week lead-time, (happened to me!) or the fact that the parts are counterfeit. (I will send you some fake MC33030P parts if interestedâ€¦straight from 4-Star Electronicsâ€¦who wouldnâ€™t return my money because it took me 32 days to come to the hard-to-believe conclusion that their parts were fakes.)

_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Jan 17, 2014 4:56 pm Post subject: Change to Shottky?

At 09:14 AM 1/16/2014, you wrote:

 	  Quote: 	
		  



The calculation here on the dissipation and voltage drops of 

Schottkys and P/N diodes are simply being pulled out of the air. 

What diodes are you referring to? You show me yours and I'll show you mine.


	


   Okay, our MBR4060 mounted for convenient installation

   heat-sinked to the airframe and provided with terminals

   for attachment of wires.



   With the pair carrying 10A I measure a drop of 0.45

   volts, 20A drops 0.68 volts.



   While one cannot expect two diodes to accurately

   parallel and share a load, the fact that the voltage

   drops at all when the second diode is added demonstrates that

   SOME load sharing takes place. This offers a benefit,

   (perhaps tiny but quantifiable) for spreading the thermal

   stresses over the device's heat-sink surface combined

   with a small drop in voltage. In no way is the practice of

   paralleling expected to offer a critical performance

   jump . . . the second diode was there in the package

   . . . might as well do something useful with it . . .

 	  Quote: 	
		  

A "standard" 15A 50V P/N diode is  1.5 Volts Vf like 1N3208.

So at 13V you get 11.5V out.


	


   I just measured one of the RS 25A bridge diodes

   at 10A and got 0.84 volts, 20A gave me 0.93 volts



 	  Quote: 	
		  I use and sell IXYS  DSSX61-0045A. Power_Deuce_Schottkys. At 15A 

they are 0.45Volts Vf. So at 13VDC you get 12.55VDC out.


	


   Don't know where the 13v comes from. The normal feedpath

   diode takes a feed from the main bus which also drives

   the voltage sense pin of the regulator. Hence, this bus

   is expected to be at 14.2 +/0 0.2 volts any time the

   alternator is operating normally. If the alternator is

   running, the bus is >14v . . . if the alternator is

   not running the system voltage is  <12.5



 	  Quote: 	
		  I supply these on heatsink. They are isolated and paired so they can 

be used in Y-configurations or separate or paired.



I've looked and don't see any better diodes for general aircraft 

purposes. If the difference was as small as some here have 

erroneously stated, we wouldn't be having this conversation. Most 

modern battery operated equipment have no p/n diodes. They waste too 

much power.


	


   Kumquats and watermelons. Diodes used inside

   a piece of electronics have little in common

   with design goals for steering diodes in

   power distribution systems. There are some really

   fat P/N diodes used throughout the Lear, Cessna

   and Beech fleets (and no doubt others) that cannot

   be Shottky due to requirements for lightning and abnormal

   surge voltage requirements. Requirements that

   are not levied onto the internal workings of an

   appliance.



   I think the Hawker 4000 had some Shottky power

   steering in their DC system (Low current stuff

   driven by TR-sets off wild frequency 208, 3-

   phase) and they had to add a lot of monkey-

   motion in the form of Transorbs, etc. to make

   it through lightning testing.



   We're discussing the return on investment in

   light airplanes for saving a watt or two in

   a system powered with a 500+ watt engine driven

   power source is not quantifiable. Energy 'squandered'

   is a fraction of a percent of the whole energy

   bucket no matter what kind of diode you use.



   At the same time, the 'rule of thumb' for selection

   of wires says 5% voltage drop is an acceptable

   compromise between loss of electrical performance

   and unnecessary addition to aircraft empty weight.



   That figures out to 0.7 volts per power feeder;

   but that too is a 'rule of thumb' . . . were it

   twice that amount, no airplane is going to fall

   out of the sky and no pilots are going to be

   squinting out into the dark because 'violation'

   of rule by a factor of 2 or even 3 has dimmed

   the lights. No dollars will be saved. So the choice

   of components is driven by FMEA and human

   factors for reducing risk.



   For example, we could replace the normal feedpath

   diode with a relay that is controlled by an

   aux battery management module . . . relay closed

   only when the bus voltage exceeds 13.0 volts.

   Hard contacts, lower voltage drop, more complexity.

   Or put a switch in. But then, the whole purposed

   for the diode was to prevent inadvertent back-feeding

   the main bus from the e-bus due to improper operation

   of switches.

I've looked and don't see any better diodes for general aircraft 

purposes. If the difference was as small as some here have 

erroneously stated, we wouldn't be having this conversation. Most 

modern battery operated equipment have no p/n diodes. They waste too 

much power.



   It's easy to avoid getting wrapped around

   the weight/energy-savings axles in this application.

   Energy savings is trivial . . . weight savings can

   be as much as 1/2 pound if the builder discovers

   that THEIR implementation of the e-bus normal feed

   diode doesn't really need that big heat sink.

   If some diode is dropping enough BTU's to require

   a hefty heat-sink, then it's not driving an e-bus.



   The Endurance Bus was conceived to offer a

   narrowly defined benefit . . . electrical endurance

   that exceeded fuel endurance. With the electrically

   dependent engine needing something on the order

   of 100W power for operation, the concept of an

   E=Bus is no longer useful. It's unlikely that

   anyone will opt to carry enough battery to run

   an engine for 3 hours.  At the same time,

   diodes used to steer power can STILL be sized

   in the 8-12 amp range without extra-ordinary

   heat-sinking in an all metal airplane.



   But in any case, there's no substitute for

   confirmation of one's decisions by direct

   measurement of effects and benefits. Toward that

   end, I supported an RS bridge in air with no heat-sink

   and loaded it to 10A. After 20 minutes, the mounting

   surface stabilized out at just over 100C. Given

   the very limited ability of this surface to reject

   heat, the stress across the internal thermal resistance

   would be low the diode junction was probably not much

   hotter . . . perhaps 110-120C.



   But the thing didn't self-destruct. Then I bolted

   it to an airplane . . . well . . . a part of

   an airplane .040 x 5 x 5 inches. Didn't use

   heat-sink grease under the part but I did

   put a fender-washer on the back side to stiffen

   the aluminum around the mounting hole with

   a notion of bringing the rectifier into better

   contact with the aluminum sheet.



   If one intends to run loads in the 10A class

   through a device like this, some heat-sink grease

   or Sil-Pad underneath plus a backside 'stiffener'

   is probably a good thing to do.



   The second experiment was rather profound.

   After 20 minutes, the metal around the

   rectifier was running 40C.  15C over ambient

   for a rise of about 2C/watt. Assuming I had

   a lousy thermal interface of . . . say

   1.5C/W. These devices will typically

   offer a to case resistance 1.5C/W. So the

   8w dissipated in the diode was taking

   the junction to 8 x 3 or 24C hotter

   than the 'airplane' at 40C. This minimalist

   approach to heat-sink gave us a junction

   temperature about 40C over ambient or 65C

    . .  . . . .WAaaay less than the rated

   150C maximum.



   If your airplane isn't metal, then you can

   use the same part-of-an-airplane that I just

   demonstrated. 25 square inches of aluminum

   goes a long way toward cooling things off.



   If that were a Schottky running 5 watts

   of loss, then the temperature of the junction

   can be expected to drop into the neighborhood

   of 25C over ambient . . . and a 'savings'

   of 3 watts.



   The point of this discussion is to put the

   worries to bed for application of any diode

   technology for steering power to either an

   E-bus or M-P bus of the variety we're

   discussing today.



   Further, note that 'losses' associated with

   these diodes can be eliminated from the

   concerns for battery-only operations. That's

   what the Z-07 discussion is all about.



   Bob . . .

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