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UC3854 datasheet shows mystery zener diode in circuit?

cupoftea

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Hi,
Page 13 of UC3854 Datasheet

..shows a Zener diode D5. In the attached LTspice sim of LT1248.....you can see that a zener here stops power going above a certain level......but the UC3854 has peak current limiting anyway, so why bother with D5. What fault mode would D5 be expected to serve? Also, what value would D5 typically be?

EDIT....ok , Apologies found it, page 11 of slua172..

slua172
 

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  • LT1248 UC3854 Boost PFC 1000W 200vac D5.zip
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d123

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Treez,

I'm really sorry to say this, a tiny bit only 'though:

Try posting images sometimes for your endless questions, it's the considerate thing to do. I mean, so many attached files all the time just for what are usually schematics of your dodgy homebrew malfunctioning circuits? You would think it's all top secret work you do, not doubtful cheapskate products.

And I would hate to get malware in my laptop by having once made the mistake of having downloaded one of your LTSpice zip + schematic pdf etcs when I was trying to help you. It would make me very angry at you, more than a person might imagine.

And, you should do the legwork, not expect others to do it for you in a knee-jerk reaction all the time.

(fair comment)
 

danadakk

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I think its for the inductive transients that occur driving the MOSFET gate C and its parasitic L.

Clamps the controller driver output to 20V and the Vgs breakdown limitation of the MOSFET.

Note that its a Schottky.






Regards, Dana
 
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Rahul_d

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Hi,
Page 13 of UC3854 Datasheet

..shows a Zener diode D5. In the attached LTspice sim of LT1248.....you can see that a zener here stops power going above a certain level......but the UC3854 has peak current limiting anyway, so why bother with D5. What fault mode would D5 be expected to serve? Also, what value would D5 typically be?

EDIT....ok , Apologies found it, page 11 of slua172..

slua172
The Zener diode in this application prevents saturation of the error amplifier which helps / prevents current overshoots during zero crossing or power line dropout condition
 

danadakk

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I just caught an error, its symbol on schematic is a Zener, but the 1N5820 is a
Schottky rectifier. It is connected to the driver output so maybe Rahul_d can
illuminate what the schematic is doing......

Something aint right ?


Regards, Dana.
 

Rahul_d

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I just caught an error, its symbol on schematic is a Zener, but the 1N5820 is a
Schottky rectifier. It is connected to the driver output so maybe Rahul_d can
illuminate what the schematic is doing......

Something aint right ?


Regards, Dana.
I was referring to the TI Appnote SLU172 & zener as shown at the location below
1659729489341.png


Diode D4 is positioned at the output of the controller, possibility is they are trying to limit negative going swing on UC3854 drive pin to prevent damage to the controller IC.

if they use zener they should place it close to MOSFET gate to source to limit / clamp the Gate voltage excursions.

"the D4 symbol is correct it shows schottky symbol."
--- Updated ---

Hi,
Page 13 of UC3854 Datasheet

..shows a Zener diode D5. In the attached LTspice sim of LT1248.....you can see that a zener here stops power going above a certain level......but the UC3854 has peak current limiting anyway, so why bother with D5. What fault mode would D5 be expected to serve? Also, what value would D5 typically be?

EDIT....ok , Apologies found it, page 11 of slua172..

slua172
The Symbol D5 in LTSPICE schematic is a Schottky diode symbol, in CCM PFC you need to use SiC Schottky diode for boost diode. - everything looks normal.
 
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cupoftea

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LTSpice zip + schematic pdf etcs when I was trying to help you. It would make me very angry at you, more than a person might imagine.
Thanks, i do wonder about this.......i mean, i dont put virus's in them, but is it possible for a virus to kind of "jump in" to someone's laptop when they download something? Its interesting to me too as i have lost two laptops to virus's.
Some of the files in my current laptop were in these virus'd laptops. and i do wonder if the virus's still lurk?

I worked in a co where i used my USB stick in there laptops, and in mine...there IT guy was able to tell me, every day, what music i had listened to on youtube the night before.
--- Updated ---

And, you should do the legwork, not expect others to do it for you in a knee-jerk reaction all the time.
Thanks, this is interesting, because i have had this question brewing for some time....now seems like a good time if i may....isnt it a good thing for the forum the more questions people ask on it?.......since it increases the chance that google searchers will "google in" to this forum......i always make the question title well descriptive (eg not just "help")....so that people can "google in" to this forum from the same words in my question title..........i thought the more hits to this forum, the better? Is that not so? The more forum traffic, the better for the forum owners?

I do do the legwork aswell......but i am sure you would agree, when you get an engineering problem, one great way to sort it out in your mind is to make out a question saying what you dont understand.....it helps in the process of forming the understanding....and if it brings people to this forum, then all the better if that helps the forum.

Dont get me wrong, i've a mate who got pulled in front of his gaffer for asking a question about a chargepoint........which you woudl assume was just "everyday electronics", since a chargepoint is basically a relay on a stick......so a lot of engineers dont use this forum for fear of being "pulled up"....this is a shame, i , and many others, would like to see loads more questions on this forum, from other engineers...after all, nobody "has" to answer them, so it doesnt seem like imposing?

I do answer qu's aswell, for others, ive answered quite a few in the last week, on various forums.

I believe a lot of company owners dont understand electronics, and mistakenly believe that there IP is being given away by people asking questions about their PCBs etc....this i dont believe is true....company owners are doing themselves out of wealth by thinking that.....if its general everyday electronics, then it can only be a help to the company concerned.....all my questions, are , i believe "general everyday electronics".

I believe company owners should get all their electroncis engineers to go onto the forums and ask questions about the companys circuitry......wherever its general electronics...like all my questions.....this will vastly improve the wealth of these company owners.

Bt the way, here is a free SMPS course for anybody who wants it.........

SMPS course_Big folders
https://drive.google.com/file/d/17ij_pQLkAzzYeL_TRNci86sXHMirKHmD/view?usp=sharing

SMPS course_little folders
https://drive.google.com/file/d/1WvElhDNfHI0EBei4rouSNEdWWTO1oCtJ/view?usp=sharing
 
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