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MOSFET or Schottkey Diode?

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cdcll

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fetdioda

Can anybody tell me which one is better for parallel DC/DC modules(5v output, 30A, for redundancy), MOSFET or Schottkey Diode? and any reference design available?
 

flatulent

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hot spare

Are you going to have a hot spare system where both are on and connected to the load, or is only one connected to the load at a time?

I would suggest that you have the first option which will cut the heat down by half in the converters which will greatly improve their reliability. This will require an external management circuit which must have a failure rate less than 10% of either supply otherwise the system reliability is not improved.

If you want a dumb system, you can use the diodes and sense the voltage on the load side of them. This will share the load between the converters, but not equally. This method assumes that the converters fail in the off and not in the higher voltage condition.
 

pool_77

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fet rectifier for small frequency:

xxx.hw.cz/design/fet_dioda/fetdioda.html

pool_77
 

cdcll

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Dear flatulent,

Thank you for your suggestion.
I am going to have a hot spare system where both are on and connected to the load, that's why I need the or'ing doide.
The problem is I have no solution to cut the heat down if I use the low Vf schottky doides(Vf is about 0.4v), but I am not familiar with the power MOSFET, I found that most MOSFETs with very low Rds(on) have the internal inverse parallel diode. So if one DC/DC module fails, the fail one will become the load of the good DC/DC module through that internal inverse diode, could you please give me more detail suggestion? eg, MOSFET part number, external management circuit part number, or schematic.

Regards,
cdcll
 

sigint

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Most power supply manufacturers that support hot swap include schottky diode oring in their supplies - I would trust their judgement and if necessary use schottky diodes.

SiGiNT

I have used these in the past with good results. How much current are you supplying?
 

cdcll

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The load can be up to 30A at 5V, the oring schottky diode will have about more than 12 Watt heat dispension, but there is no fan allowed in my design, could you recommend suitable solutions to me?
 

shanren

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in fact,you problem is not so easy.but if you want low power dispension,maybe MOSFET is the only way.you can use four 20V 0.005 oum mosfet parallel,get a very low instance, only about 1W dispension,and four SO8 chip mosfet can work well with about 30 degree temprature rise.do warry about others,if you find a error in one power modle,you should shut down it in the primary side of it.
best wishes.
 

flatulent

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fets are bidirectional

Another problem is that FETs are bidirectional (the ordinary direction works better than the other, but the other direction still conducts) even if you get the ones with no internal protection diode. You will probably have to have some external management circuit to connect one or the other.

Use "high side switches" type FET circuits. Search for this key word (actually three words) when you go to IC manufacturer's sites. (Also search this site for the three word pharse for more information.) This switch type circuit IC goes between each supply + side output to the load. Your external management circuit will have to be able to tell when the main supply is not working and switch the two switches to the opposite state to connect the other power supply.

The diode is the foolproof method. Can you look inside your supplies? They may have some diode type circuits which will allow you to electrically parallel the supply outputs directly. Ask the manufacturer for suggestions. They may have application notes on this very subject. The supplies may have external control pins that you can use to switch them on and off. Do they have fault output indicators? These can be taken advantage of in your design.
 

cdcll

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Dear all,

If I choose a DC/DC module with parallel operation allowed, eg, PH150F48 from Densei-Lambda, which has GOOD/NO GOOD signal output, do I still need the the oring schottky diode for redundant operation?

Regards,
cdcll
 

cdcll

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Does anyone know if the schottky diode 85CNQ015 is suitable for the oring diode?
but it is very expensive, how can I find a cheaper one?
 

flatulent

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looks good

The data sheet on the web indicates that they are specifically designed for your type aplication of being redundant. Follow the directions that come with the unit and it looks like they can be directly paralleled and share the current load for further reliability from lower temperatures.
 

cdcll

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Dear flatulent,

Thank you for the reply. I do not have any experience with the DC/DC module from Densei-Lambda, do you mean the Lambda DC/DC module can be used without the oring diode for redundant operation to cut heat down? I am not sure the parallel operation is only for doubling power or for redundant and current share both.

Best wishes,
cdcll
 

flatulent

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N+!

Note the equation N+1 in the web data sheet. This is a standard code word for multiple redundant units where you have 1 more unit than you really need and that way when one fails the others fulfill your needs.

The application note from Lamda which will explain the wiring details: http://www.densei-lambda.com/products/sps/ps_pm/ph/ph_par.pdf

It looks like you would need the diodes but the reliability would be improved by the load sharing.
 

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