Forums

zener diode pulse current rating

Started by panfilero April 20, 2012
On 4/20/2012 7:47 PM, Jamie wrote:
> mike wrote: > >> On 4/20/2012 7:00 PM, Jamie wrote: >> >>> panfilero wrote: >>> >>>> On Apr 20, 5:46 pm, Bill Martin <w...@wwmartin.net> wrote: >>>> >>>>> On 04/20/2012 03:44 PM, panfilero wrote: >>>>> >>>>> >>>>> >>>>> >>>>> >>>>> >>>>> >>>>> >>>>> >>>>> >>>>>> On Apr 20, 3:36 pm, panfilero<panfil...@gmail.com> wrote: >>>>>> >>>>>>> I need a 30V zener to drop the a high voltage of 100V down to 60V >>>>>>> so I >>>>>>> can use it with a switch mode power supply chip. So while this >>>>>>> circuit is in use the zener will be exposed to current pulses >>>>>>> from the >>>>>>> switching, these current pulses are in the neighborhood of 1A. I've >>>>>>> gone to digikey and pulled up some zeners but I don't see this >>>>>>> rating, >>>>>>> where can I find the rating that tells me whether or not it's ok to >>>>>>> pulse 1A through my zener? >>>>> >>>>> >>>>>>> here's the one I'm looking at >>>>> >>>>> >>>>>>> http://www.nxp.com/documents/data_sheet/NZX_SER.pdf >>>>> >>>>> >>>>>>> much thanks! >>>>> >>>>> >>>>>> I'm guessing it's in the power rating, if it's a 30V zener, and has a >>>>>> power rating of 500mW, then max current is I = .5/30 = 17mA.... weak! >>>>>> but this doesn't tell me about current pulses...spikes... >>>>> >>>>> >>>>> I dunno, but if you are dropping 100V to 60V with a 30V zener, that >>>>> leaves about 10V of "smoke" to dispose of...:-) >>>>> >>>>> bill >>>> >>>> >>>> >>>> good point.... let's say for this thread 100 = 90 >>> >>> why not simply use a transistor to boost the handling of the >>> zener circuit? >>> >>> >>> >>> >>> >>> Cathode equal >>> >>> >>> + >>> | >>> | >>> +---+-----+------------------------+ >>> | | | >>> + | | >>> z | | >>> zener diode A | | >>> + + | >>> | |/ | >>> +-----+-| NPN + >>> | |> | >>> |+ + - Reverse >>> .-. | ^ protect >>> | | + + >>> 10k | | V Protection diode | >>> '-' - | >>> +----+----+------------------------+ >>> | >>> | >>> | >>> + >>> >>> Anode Equal >>> >>> >>> You may not need the reverse protect diode, but to simulate a real zener >>> as a stand alone in a circuit where reverse flow may occur, this >>> is needed. >>> Your zener voltage would be what you want - ~ 1.8 volts. (2 diode drops >>> and saturation voltage of the NPN) >>> >>> The other protect diode in the emitter must be there for fast >>> turn offs that will generate negative voltage in cases where it will >>> zener the Base-Emitter, you don't want that to happen. >>> >>> With this circuit, you can use very small zeners. Select the proper >>> transistor to handle the power. >>> >>> >>> Jamie >>> >> >> Sometimes a solution to the original problem is better than an emulating >> a component of a bad design. >> >> There's something comforting about a pre-regulator circuit that doesn't >> explode instantly when you short the output to ground.
> really, why don't you actually try it instead of assuming it. > > If it really matters to you, I use that circuit in many, many! clamping > protecting circuits for HI-Pot test equipment and HV protection clamps > and regulators to increase the handling current on zeners. > > It works as advertised. Never had one of those fail unless there was > some incoming that exceeded the NPN transistor. If that being the case > then there is something else wrong and shouldn't be doing that. > > Also, that circuit is very common from old school. I am not the first > one to use that and it's been used in countless designs. So go tell them > they are wrong, too! If it would make you feel any better, you could put > a R in series with the zener, but really, all that is going to do is > slow it down a bit. > > > > Jamie > >
I reread my two sentences over and over. Can't say I understand why you took offense to me being more conservative. I never found fault with your using a zener diode, synthesized or otherwise, in an application where a zener diode was appropriate. I did suggest, in an earlier post, that using a zener, synthesized or otherwise, in this application might not be optimal. If a current spike shorts the zener, synthesized or otherwise, it likely takes out the power supply it's protecting...and maybe it takes out the whole board. Compound that with an attempt to use a 1/2W zener with 1A current pulses. I am NOT comforted by that design. I stand by my assertion that it can be beneficial to take a step back, look at the big picture and try a different topology rather than patch up a brute-force design that has limitations by adding more brute to the force. We can't do that, because the big picture was never presented by the OP. All I can do is suggest caution.
On 4/20/2012 9:00 PM, Jamie wrote:
> panfilero wrote: > >> On Apr 20, 5:46 pm, Bill Martin <w...@wwmartin.net> wrote: >> >>> On 04/20/2012 03:44 PM, panfilero wrote: >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>>> On Apr 20, 3:36 pm, panfilero<panfil...@gmail.com> wrote: >>>> >>>>> I need a 30V zener to drop the a high voltage of 100V down to 60V so I >>>>> can use it with a switch mode power supply chip. So while this >>>>> circuit is in use the zener will be exposed to current pulses from the >>>>> switching, these current pulses are in the neighborhood of 1A. I've >>>>> gone to digikey and pulled up some zeners but I don't see this rating, >>>>> where can I find the rating that tells me whether or not it's ok to >>>>> pulse 1A through my zener? >>> >>>>> here's the one I'm looking at >>> >>>>> http://www.nxp.com/documents/data_sheet/NZX_SER.pdf >>> >>>>> much thanks! >>> >>>> I'm guessing it's in the power rating, if it's a 30V zener, and has a >>>> power rating of 500mW, then max current is I = .5/30 = 17mA.... weak! >>>> but this doesn't tell me about current pulses...spikes... >>> >>> I dunno, but if you are dropping 100V to 60V with a 30V zener, that >>> leaves about 10V of "smoke" to dispose of...:-) >>> >>> bill >> >> >> good point.... let's say for this thread 100 = 90 > why not simply use a transistor to boost the handling of the > zener circuit? > > > > > > Cathode equal > > > + > | > | > +---+-----+------------------------+ > | | | > + | | > z | | > zener diode A | | > + + | > | |/ | > +-----+-| NPN + > | |> | > |+ + - Reverse > .-. | ^ protect > | | + + > 10k | | V Protection diode | > '-' - | > +----+----+------------------------+ > | > | > | > + > > Anode Equal > > > You may not need the reverse protect diode, but to simulate a real zener > as a stand alone in a circuit where reverse flow may occur, this > is needed. > Your zener voltage would be what you want - ~ 1.8 volts. (2 diode drops > and saturation voltage of the NPN)
When does the transistor saturate?
> The other protect diode in the emitter must be there for fast > turn offs that will generate negative voltage in cases where it will > zener the Base-Emitter, you don't want that to happen. > > With this circuit, you can use very small zeners. Select the proper > transistor to handle the power. > > > Jamie >
Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

>why not simply use a transistor to boost the handling of the >zener circuit?
Then he will just post asking about transistor pulse current ratings. Extra annoying when he links an excellent datasheet with clear abs max current, power, junction temperature limits, and transient thermal impedance graphs.
On Fri, 20 Apr 2012 13:36:49 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

>I need a 30V zener to drop the a high voltage of 100V down to 60V so I >can use it with a switch mode power supply chip.
Why not use a different switcher, rated for 100 V ? It does not make sense to burn 30 % (or should it be 40 %:-) of the available power in a resistor (zener) and then use a switcher to go down to whatever voltages are needed ? The total system efficiency will be better with a 100V switcher :-)
On Sat, 21 Apr 2012 17:32:49 +0100, nospam <nospam@please.invalid>
wrote:

>Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote: > >>why not simply use a transistor to boost the handling of the >>zener circuit? > >Then he will just post asking about transistor pulse current ratings. > >Extra annoying when he links an excellent datasheet with clear abs max >current, power, junction temperature limits, and transient thermal >impedance graphs.
--- Bingo! -- JF
On Fri, 20 Apr 2012 13:36:49 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

>I need a 30V zener to drop the a high voltage of 100V down to 60V so I >can use it with a switch mode power supply chip. So while this >circuit is in use the zener will be exposed to current pulses from the >switching, these current pulses are in the neighborhood of 1A.
. . . --- What's the quiescent current through the Zener, how long do the pulses last, and how often do they occur? -- JF
On 4/21/2012 2:08 PM, John Fields wrote:
> On Fri, 20 Apr 2012 13:36:49 -0700 (PDT), panfilero > <panfilero@gmail.com> wrote: > >> I need a 30V zener to drop the a high voltage of 100V down to 60V so I >> can use it with a switch mode power supply chip. So while this >> circuit is in use the zener will be exposed to current pulses from the >> switching, these current pulses are in the neighborhood of 1A. > . > . > . > > --- > What's the quiescent current through the Zener, how long do the pulses > last, and how often do they occur? >
That's the problem with most questions asked here...lack of context. A person is certainly free to ask a specific question and get a specific answer. BUT You can often judge the experience (I was gonna say competence. That's the right word, but would ignite the ethernet and we don't want that.) of the person asking the question by the form and words used. When the big red flags go up, it can be helpful to the person to elaborate...whether they like it or not. The side effect is that we turn on each other, but that's just the way of the web. In my opinion, there is NO pulse width that allows 40W of peak power in a 1/2W device to be considered an acceptable design...none. And it's difficult to filter the current pulses. You start adding caps and discover that something else blows up when you crowbar the 100V supply. Are we comforted by the fact that most designers wouldn't consider that a problem? Without context, we're left with only common sense to guide us. Ignoring the 10V discrepancy in the question, someone stated that the zener could sustain 17mA if you didn't derate it. For many single chip switchers, the duty factor is closely related to the ratio of input and output voltages. Narrow 1A spikes seem out of place. I would prefer a design that didn't have 'em. Yes, it's possible for a system integrator to be in a situation where they have to mate two bad designs. That's why you need your BEST engineers doing the integration. One of the jobs of a project manager is to stamp out tunnel vision. Everybody needs to be aware of the big picture...not an expert, but aware. Communication is key. I used to tell engineers at the first project meeting that I was gonna personally test the design before the second prototype phase. The list of abuses would contain Crowbar the power supplies. Any connector that could be put on the wrong socket or backwards or off-by-one on the right socket would be tried and the power turned on. Bunch of other single-point failures that could happen in manufacturing. You could have heard the whimpering and wailing in the next county. Probably increased the cost a few pennies too. But the concept propagated to other parts of the design so engineers considered more than just what happened when everything was working right. Yes, you can easily come up with single-point failures that can't be protected against and will reduce the project to a smoldering lump. The key point is that MANY, MOST can be mitigated by paying attention. Enough rant... Back to the current situation. More context would be helpful. If we were voting based on what's been divulged, I'd bet that this is a disaster waiting to happen. I refuse to be comforted by the fact that most designs are faulty in ways that could have been easily avoided by paying attention. Excuses are cheap. Field failures aren't.
"mike" <spamme9@gmail.com> wrote in message 
news:jmvlsl$ood$1@dont-email.me...
> In my opinion, there is NO pulse width that allows 40W of peak power > in a 1/2W device to be considered an acceptable design...none.
Well, why not? Back in the toob days, horizontal sweep outputs were actually rated for this service. Typical datasheet reads 30W plate dissipation (abs. max., continuous service), 260W short circuit during startup only. This rating today would be labeled as something like "single pulse non-repetitive", with a maximum pulse width of a leisurely 60 seconds or so. Admittedly that's a peak-to-average ratio of not quite 10, rather than 80. Tubes are essentially limited by dumb joule heating until voltages cause vacuum breakdown or ion bombardment destroys the cathode; the peak could be much higher for shorter periods, but the cathode will saturate before you reach the actual pulsed thermal limit of the electrodes themselves. There are plenty of other examples in semiconductors, anyway. If your statement were unconditionally true, real switching circuits would essentially never work. A transistor (optimistically) rated for ~500W, will easily see, under hard inductive switching, power pulses over 20kW peak. That's a peak-to-average ratio of 40 right there. If you count against the actual continuous power rating, it could be 100W (ratio of 200) for typical heatsinking, all the way down to just 1-2W without any heatsink at all (for a ratio of a whopping 1000!). Clearly, there is SOME pulse width that allows a peak-to-average ratio over 80 to be considered an acceptable design, for some component types. Real engineers don't go by hearsay, they actually run experiments and think for themselves. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
On 4/22/2012 12:50 AM, Tim Williams wrote:
> "mike" <spamme9@gmail.com> wrote in message > news:jmvlsl$ood$1@dont-email.me... >> In my opinion, there is NO pulse width that allows 40W of peak power >> in a 1/2W device to be considered an acceptable design...none. > > Well, why not? > > Back in the toob days, horizontal sweep outputs were actually rated for > this service. Typical datasheet reads 30W plate dissipation (abs. max., > continuous service), 260W short circuit during startup only. This rating > today would be labeled as something like "single pulse non-repetitive", > with a maximum pulse width of a leisurely 60 seconds or so. > > Admittedly that's a peak-to-average ratio of not quite 10, rather than > 80. Tubes are essentially limited by dumb joule heating until voltages > cause vacuum breakdown or ion bombardment destroys the cathode; the peak > could be much higher for shorter periods, but the cathode will saturate > before you reach the actual pulsed thermal limit of the electrodes > themselves. > > There are plenty of other examples in semiconductors, anyway. > > If your statement were unconditionally true, real switching circuits > would essentially never work. A transistor (optimistically) rated for > ~500W, will easily see, under hard inductive switching, power pulses > over 20kW peak. That's a peak-to-average ratio of 40 right there. If you > count against the actual continuous power rating, it could be 100W > (ratio of 200) for typical heatsinking, all the way down to just 1-2W > without any heatsink at all (for a ratio of a whopping 1000!). > > Clearly, there is SOME pulse width that allows a peak-to-average ratio > over 80 to be considered an acceptable design, for some component types. > > Real engineers don't go by hearsay, they actually run experiments and > think for themselves. > > Tim >
So, you're saying that it's OK to use a device designed and specified for such use??? Couldn't agree more. Apparently, you've never got the panicked call from production, "hey mikie, your product is exploding. The parts have the right part number, but they're a different color. Incoming inspection says they meet the spec. Lead time on the old parts is 20 weeks. We've got enough of the old parts in rework to last 'till Wednesday. FIX IT!!!??" Or the call from purchasing, "hey mikie, your idiot engineer wants to add a 40W pulse power spec on a jellybean 500mW part. The vendor just laughed at me. Whatdayawannado???" Or you've never had to explain safe operating area to an engineer who wanted to use a transistor at max voltage and current simultaneously. I have a lot of experience with what engineers do. Product designers expect the product to sail through manufacturing and be trouble free in the customer's hands. Sometimes, that means overruling what "engineers" "think". Go back a couple of weeks and review the thread on "slightly reverse biased tantalums". Perfect example of allegedly rational engineers arguing that it's OK to do something irrational. Abusing a part WAY, WAY beyond its specification when there are conservative alternatives is bad business.
On Sun, 22 Apr 2012 02:14:20 -0700, mike wrote:

> Or you've never had to explain safe operating area to an engineer > who wanted to use a transistor at max voltage and > current simultaneously.
Apparently a usage of the word "engineer" with which I am unfamiliar. Where I come from, engineers are experienced graduates who design and develop equipment that works, and is suitable for purpose. They know about SOA, and where to find the relevant data. "Product designers" are people who decide what sort of pretty box it's to go in. -- "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)