Diode Voltage Drop Question...

[ViReN]

Hello All,

Its very simple (perhaps a dumb) question...

Following is the setup ....
----------------------
Using a Inductor based Step Up Boost Circuit... (typical efficiency of 50 - 60%)
I/P = 4.5 - 6 Volts @ 100 - 800 mA
O/P = 45 - 70 Volts @ 5 - 40 mA
LED's are connected in Series (depending upon the configuraiton)

Case Condition
--------------
Schottky Diodes are expensive (as compared to Standard Diode)

Question
--------
--> Is it OK to use a Standard P-N Junction Diode (which has a drop of about .6 Volts) instead of using a Shottky Junction Diode (dont know exact voltage drop.. but definately less than P-N Junction one) ? Especially when the cost difference is like 1 : 35 (p-n : Shottky) ...

--> What would be disadvantage of using p-n Junction... Is it possible to use a more ampere rating diode (may be 2 Amp etc)... or standard one amp will do ?

What I Feel
-----------
Since the drop is just of 0.6 Volts, as compared with the output voltage of 45 - 70 volts, is a insignificant drop... But, at the same time, since inductors are involved, spikes in o/p voltage are there and hence there might be a problem ... I am not sure and i doubt it.. & thats why.. this post /ubbthreads/images/graemlins/smile.gif

Thanks & Regards,
ViReN

 

[Steve K]

good question!

balancing costs is one key to good engineering. The disadvantages of the silicon diode are lower efficiency due to the higher Vf, and slower switching speed (depends on whether you use a plain diode, or a fast recovery diode).

the big question is: will using a cheaper diode cost you more in terms of efficiency? Lower efficiency means you'll need more heatsinking, and more input power.

and if this was a low-volume production job, and you were getting paid for it, will the cost of the engineering time required to fiddle around exceed the savings in the diode cost?

just a few ideas to consider... /ubbthreads/images/graemlins/grin.gif


Steve K.

 

[WildRice]

Schottky Diodes have a faster recovery rate ie they operate better at higher frequencies. They will turn on/off much quicker, allowing more of the energy to pass. I have tried both in a circuit and have found that regular 4001 diodes dont work as well for 1.5v step up circuits, HOWEVER, a regular diode might work OK in a higher voltage setup like you are describing. Try them both and see what the differences are.

Jeff

 

[ViReN]

Hello,

Kool... I think I should try it.... hope that it wont damage the circuit or blow off the transistors etc or some thing...

ViReN

 

[MrAl]

Hello there,

If you have very low current and the voltage isnt too
high you can try the 1N4148 type diodes as they are
very fast too. You'll have to check the rating.

Take care,
Al

 

[WildRice]

it shouldent pop anything. Just be sure that your output cap is rated high enough. I would recomend a .1uF and 10-47uF in parallel, that should smooth out the output.

Jeff

Also, i picked up 100 schotchy diodes on ebay for about 5bucks a whils ago.

 

[ViReN]

WHAT... 100 Schottky for 5 Bucks !!!.... Greattttttt

Where ... whereeeeeeee /ubbthreads/images/graemlins/smile.gif .... (ooh .. u live in USA.. i dont /ubbthreads/images/graemlins/grin.gif ).... i will have to add $25 to it /ubbthreads/images/graemlins/frown.gif

but i still am interested to know /ubbthreads/images/graemlins/smile.gif from where...

Also, I would like to ask... What is a "Vodka" Diode ? is it better than Schottky

Mr Al... Thanks... I do have a couple of 1N4148's with me... I will try with these too...

Thanks & Regards,
ViReN

 

[MrAl]

Hi again,

Just a quick word regarding Schottky's...

If you are dealing with a higher voltage output the
extra drop of a 'normal' switching diode doesnt matter.
It's what the output voltage to diode voltage
ratio is that makes the difference in efficiency, not the
absolute value of the voltage drop itself. In low
output voltage converters it still makes a big difference
of course, but not in high output voltage ones.

'Rectifier' type diodes really bite in any case --
probably even at 1kHz switching frequency.
This means dont use 1N4000 series diodes :-(

Take care,
Al

 

[vicbin]

Hi Viren,

I can understand your frustation, I also live in what they called "less developed country" like you do. Here "some" of my rants, e.g: the s/h + import tax alone for my last Arc LS "second" , yep its a second, and this "indirect stuff" cost me more than the latest Arc4 itself ! /ubbthreads/images/graemlins/frown.gif /ubbthreads/images/graemlins/broke.gif

Actually, I believe in your country, India isn't it ? You can get Schottky diode for "free", or a lot more cheaper than you expected, but of course with some efforts, patience + some energy needed. /ubbthreads/images/graemlins/wink.gif

I know this sound silly or weird to some of you who live in US which has the luxury of "Digikey", but this an unvoidable option for CPFer like me. /ubbthreads/images/graemlins/frown.gif

Simple, just be prepare one small corner of your room or table as a junkyard for un-used/broken electronic boards such as computer mother board, old broken cell phone, computer cards etc..etc. All of this "junk" sometime cost nothing. /ubbthreads/images/graemlins/naughty.gif

Now the best part is in those junks is you can find valuable items for toying switching or led circuits such as schotky diode, exotic low VGS + low RDS MOSFET, SMD inductors and my best favourites is low ESR capacitors such as high value SMD ceramic or hundredths uF SMD tantalum caps.

If you or other CPFer interested in the details, I can start a new thread on sharing the tips & tricks how to "harvest" this kind of stuffs minus the pain that I've been experienced in the past. /ubbthreads/images/graemlins/banghead.gif

Vic

 

[ViReN]

Hi vicbin...

Yep I DO live in InDiA /ubbthreads/images/graemlins/smile.gif ... and in some regards .. especially when it comes to Posting / Shipping Charges + Custom's ... really hurts the pockets way too much... ... Rest is relatively kool over here...

Sure I would like to know about harvesting the things... I do have a couple of Old Electronics at my disposal... I look forward for your post... please do post Pictures /ubbthreads/images/graemlins/smile.gif

Mr Al, Thanks for the great information & Guidance... You always have been my guide through the journey .... And of course... thanks to everyone... because it counts equaly too /ubbthreads/images/graemlins/smile.gif

currently, i am searching for 1N4148's specs on the net.. & Datasheets for various other diodes ... (just for knowledge).. and to know more about Switching Frequencies that a rectifier diode is capable of...

ViReN

 

[djpark]

I learnt that a P-channel FET can be used instead of a diode by connecting gate to ground and use source and drain in place of anode and cathode respectively.

If not considering the cost, how is the technical side of the effect especially in the switching circuit if we use one with very low RDS(on) such as below 0.1 ohm?

-- dj

 

[Doug S]

[ QUOTE ]
djpark said:
I learnt that a P-channel FET can be used instead of a diode by connecting gate to ground and use source and drain in place of anode and cathode respectively.

If not considering the cost, how is the technical side of the effect especially in the switching circuit if we use one with very low RDS(on) such as below 0.1 ohm?

-- dj

[/ QUOTE ]
Better unlearn that. It will have a forward drop equal to the Vgs turn on voltage [assuming here that gate and source are connected] and will also conduct through the body diode in the reverse direction.

 

[MrAl]

Hi there,

Yes, transistors can be used in place of diodes, but
the biasing has to be exactly correct for turn on and
turn off points or you could end up with a much higher
voltage drop then any simple diode would provide :-)

Also, the gate drive has to be capable of a short high
current pulse for fast switching.

Also, normally you'll have to find a part with low Ron and
no body diode.

Even these three simple requirements usually means the
circuit has to be designed very carefully, to say the
least.

Take care,
Al

 

[WildRice]

here is one I found on EBAY

SMT barrier diodes

Jeff

 

[ViReN]

Jeff...

Thats GREAT Link.... In fact.. i found out loads of other components too....

Like the Transistors, Capacators... Resistors etc....

Great Link in deed...

How about Inductors ? (any good link that you might have ? .. on ebay) ....

Mr Al... Can i Put MosFET's instead of Transistors ... (in order to reduce the losses in the transistor based Switching Circuit?)


Thanks & Regards,
ViReN

 

[MrAl]

Hi Viren,

What circuit are you working with here?
I lost track :-)

Take care,
Al

 

[ViReN]

Same Transistor Based Switching Circuit.... ... while its still time before actually get SMT Components... I am playing around with this circuit, trying to make different 'mods' putting LED's in everything that uses batteries... & emmits Light /ubbthreads/images/graemlins/smile.gif .... and i am also thinking of ordering more components from ebay...

Once I get Zetex... I will do the same with that circuit... /ubbthreads/images/graemlins/smile.gif

Thanks & Regards,
ViReN

 

[MrAl]

Hi Viren,

In order for Q2 of that circuit to turn off it depends
on a certain gain sometimes called the Beta of the
transistor. What happens is the beta limit allows the
transistor to be forced out of saturation for
high inductor (charge) current and that's what starts
the turn off.
There's no 'beta' for a MOSFET because it works entirely
differently, so there's no easy way to replace Q2 with
a MOSFET. There may be another way of doing it, but
it would take finding a way to turn off the MOSFET when
the inductor current starts to climb high. Perhaps using
the 'old' bipolar device to turn off the MOSFET, or
another idea. Maybe i'll play with the idea a little
at some point.

Take care,
Al