Zener SOAR

Started by John Larkin September 11, 2017
Well, there don't seem to be SOAR specs on zeners. I suppose I'll have
to blow some up.


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

On 2017-09-11 13:27, John Larkin wrote:
> > Well, there don't seem to be SOAR specs on zeners. I suppose I'll have > to blow some up. >
Why should there be? A zener either conducts next to nothing or it starts to conduct and then the voltage drop is always very close to its zener voltage or a diode drop when in the other direction. They do give power specs. -- Regards, Joerg http://www.analogconsultants.com/
On Mon, 11 Sep 2017 13:40:36 -0700, Joerg <news@analogconsultants.com>
wrote:

>On 2017-09-11 13:27, John Larkin wrote: >> >> Well, there don't seem to be SOAR specs on zeners. I suppose I'll have >> to blow some up. >> > >Why should there be? A zener either conducts next to nothing or it >starts to conduct and then the voltage drop is always very close to its >zener voltage or a diode drop when in the other direction. They do give >power specs.
The issue is how it survives transient power dissipation. You could say "never exceed the continuous power rating" but then you could say the same for mosfets, and not have SOAR curves. For a zener, it would amount to a power-vs-time or a current-vs-time curve. Transzorbs usually have a SOAR curve or some equivalent. Some SMB packaged surface-mount transzorbs can dissipate 600 watts for a while. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On 2017-09-11 14:38, John Larkin wrote:
> On Mon, 11 Sep 2017 13:40:36 -0700, Joerg <news@analogconsultants.com> > wrote: > >> On 2017-09-11 13:27, John Larkin wrote: >>> >>> Well, there don't seem to be SOAR specs on zeners. I suppose I'll have >>> to blow some up. >>> >> >> Why should there be? A zener either conducts next to nothing or it >> starts to conduct and then the voltage drop is always very close to its >> zener voltage or a diode drop when in the other direction. They do give >> power specs. > > The issue is how it survives transient power dissipation. You could > say "never exceed the continuous power rating" but then you could say > the same for mosfets, and not have SOAR curves. > > For a zener, it would amount to a power-vs-time or a current-vs-time > curve. > > Transzorbs usually have a SOAR curve or some equivalent. Some SMB > packaged surface-mount transzorbs can dissipate 600 watts for a while. >
All you get with zeners is a thermal response graph set like figure 4 here: http://www.onsemi.com/pub/Collateral/1N5333B-D.PDF I am not in any way an expert on this but super-high pulse dissipation is AFAIK not the domain where regular zeners would shine. Whenever I needed that I made active zeners where a FET took over the wrestling work. TVS are ok as well but they often have tolerances that are too high and too much capacitance. -- Regards, Joerg http://www.analogconsultants.com/
On Mon, 11 Sep 2017 14:50:32 -0700, Joerg <news@analogconsultants.com>
wrote:

>On 2017-09-11 14:38, John Larkin wrote: >> On Mon, 11 Sep 2017 13:40:36 -0700, Joerg <news@analogconsultants.com> >> wrote: >> >>> On 2017-09-11 13:27, John Larkin wrote: >>>> >>>> Well, there don't seem to be SOAR specs on zeners. I suppose I'll have >>>> to blow some up. >>>> >>> >>> Why should there be? A zener either conducts next to nothing or it >>> starts to conduct and then the voltage drop is always very close to its >>> zener voltage or a diode drop when in the other direction. They do give >>> power specs. >> >> The issue is how it survives transient power dissipation. You could >> say "never exceed the continuous power rating" but then you could say >> the same for mosfets, and not have SOAR curves. >> >> For a zener, it would amount to a power-vs-time or a current-vs-time >> curve. >> >> Transzorbs usually have a SOAR curve or some equivalent. Some SMB >> packaged surface-mount transzorbs can dissipate 600 watts for a while. >> > >All you get with zeners is a thermal response graph set like figure 4 here: > >http://www.onsemi.com/pub/Collateral/1N5333B-D.PDF > >I am not in any way an expert on this but super-high pulse dissipation >is AFAIK not the domain where regular zeners would shine. Whenever I >needed that I made active zeners where a FET took over the wrestling >work. TVS are ok as well but they often have tolerances that are too >high and too much capacitance.
We're going to add some SOT23 zeners to some power mosfets D-G, to protect from overvoltages. Some fets are avalanche energy rated, but some aren't, so the zeners are an option. But will we blow the zeners? The zener has to supply all the gate charge, numbers like 80 volts and 100 nC. That's only 8 microjoules in this case, so it's probably OK. I'll blow some up to be sure. The composite zener+mosfet winds up with its own avalanche energy rating. That's what I should test. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Mon, 11 Sep 2017 14:50:32 -0700, Joerg <news@analogconsultants.com>
wrote:

>On 2017-09-11 14:38, John Larkin wrote: >> On Mon, 11 Sep 2017 13:40:36 -0700, Joerg <news@analogconsultants.com> >> wrote: >> >>> On 2017-09-11 13:27, John Larkin wrote: >>>> >>>> Well, there don't seem to be SOAR specs on zeners. I suppose I'll have >>>> to blow some up. >>>> >>> >>> Why should there be? A zener either conducts next to nothing or it >>> starts to conduct and then the voltage drop is always very close to its >>> zener voltage or a diode drop when in the other direction. They do give >>> power specs. >> >> The issue is how it survives transient power dissipation. You could >> say "never exceed the continuous power rating" but then you could say >> the same for mosfets, and not have SOAR curves. >> >> For a zener, it would amount to a power-vs-time or a current-vs-time >> curve. >> >> Transzorbs usually have a SOAR curve or some equivalent. Some SMB >> packaged surface-mount transzorbs can dissipate 600 watts for a while. >> > >All you get with zeners is a thermal response graph set like figure 4 here: > >http://www.onsemi.com/pub/Collateral/1N5333B-D.PDF > >I am not in any way an expert on this but super-high pulse dissipation >is AFAIK not the domain where regular zeners would shine. Whenever I >needed that I made active zeners where a FET took over the wrestling >work. TVS are ok as well but they often have tolerances that are too >high and too much capacitance.
Some zeners specify transient power limits, some don't. Some SOT23s can dissipate 30 watts for a millisecond, impressive. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
There's an old Motorola (now On Semi) appnote talking about zener ratings 
for pulse, ESD and surge purposes.

While few zeners carry pulse ratings, there doesn't seem to be any reason to 
suspect any average part is exceptionally inferior to those documented in 
the appnote.

TVSs are just big fat zeners with pulse ratings rather than DC ratings.

I don't know of any mechanism that would resemble 2nd breakdown, or give any 
more than a straight linear SOA (i.e., straight downward-sloping lines on 
the log-log plot, with different offsets for different single-shot pulse 
widths).

As a diode, the usual charge storage and recovery business applies, but you 
wouldn't normally use a zener as a rectifier, or under fast recovery or 
drift-recovery conditions, so that should be easy to avoid.

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com

"John Larkin" <jjlarkin@highland_snip_technology.com> wrote in message 
news:1bsdrcd4bhd89notq8k87jioh9mplu2pjj@4ax.com...
> > Well, there don't seem to be SOAR specs on zeners. I suppose I'll have > to blow some up. > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com >
On Mon, 11 Sep 2017 22:36:37 -0500, "Tim Williams"
<tmoranwms@gmail.com> wrote:

>There's an old Motorola (now On Semi) appnote talking about zener ratings >for pulse, ESD and surge purposes. > >While few zeners carry pulse ratings, there doesn't seem to be any reason to >suspect any average part is exceptionally inferior to those documented in >the appnote. > >TVSs are just big fat zeners with pulse ratings rather than DC ratings. > >I don't know of any mechanism that would resemble 2nd breakdown, or give any >more than a straight linear SOA (i.e., straight downward-sloping lines on >the log-log plot, with different offsets for different single-shot pulse >widths). > >As a diode, the usual charge storage and recovery business applies, but you >wouldn't normally use a zener as a rectifier, or under fast recovery or >drift-recovery conditions, so that should be easy to avoid. > >Tim
It looks like even small zeners, SOT23 and SOD323 types, are good for tens of watts for 1 millisecond, which is 10s of millijoules. So I should be OK. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On 09/11/2017 11:36 PM, Tim Williams wrote:
> There's an old Motorola (now On Semi) appnote talking about zener > ratings for pulse, ESD and surge purposes. > > While few zeners carry pulse ratings, there doesn't seem to be any > reason to suspect any average part is exceptionally inferior to those > documented in the appnote. > > TVSs are just big fat zeners with pulse ratings rather than DC ratings. > > I don't know of any mechanism that would resemble 2nd breakdown, or give > any more than a straight linear SOA (i.e., straight downward-sloping > lines on the log-log plot, with different offsets for different > single-shot pulse widths).
That's plausible, since the TC of avalanche current is small and negative, so hot spots would avalanche slightly less. However, depending on the construction of the diode, there may be some fairly significant lateral resistance in the epi. Something like a MELF package would reduce this, because the metal makes contact over the whole surface of the die. Maybe the metal contact covers the whole die in the SMT parts as well, I don't know.
> > As a diode, the usual charge storage and recovery business applies, but > you wouldn't normally use a zener as a rectifier, or under fast recovery > or drift-recovery conditions, so that should be easy to avoid.
I haven't measured it myself, but I doubt that zener recovery is slow at all. In reverse bias there's no stored charge in the junction, and so nothing to shield out the applied field. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On Tue, 12 Sep 2017 07:24:43 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 09/11/2017 11:36 PM, Tim Williams wrote: >> There's an old Motorola (now On Semi) appnote talking about zener >> ratings for pulse, ESD and surge purposes. >> >> While few zeners carry pulse ratings, there doesn't seem to be any >> reason to suspect any average part is exceptionally inferior to those >> documented in the appnote. >> >> TVSs are just big fat zeners with pulse ratings rather than DC ratings. >> >> I don't know of any mechanism that would resemble 2nd breakdown, or give >> any more than a straight linear SOA (i.e., straight downward-sloping >> lines on the log-log plot, with different offsets for different >> single-shot pulse widths). >That's plausible, since the TC of avalanche current is small and >negative, so hot spots would avalanche slightly less. However, >depending on the construction of the diode, there may be some fairly >significant lateral resistance in the epi. > >Something like a MELF package would reduce this, because the metal makes >contact over the whole surface of the die. Maybe the metal contact >covers the whole die in the SMT parts as well, I don't know. >> >> As a diode, the usual charge storage and recovery business applies, but >> you wouldn't normally use a zener as a rectifier, or under fast recovery >> or drift-recovery conditions, so that should be easy to avoid. > >I haven't measured it myself, but I doubt that zener recovery is slow at >all. In reverse bias there's no stored charge in the junction, and so >nothing to shield out the applied field. > >Cheers > >Phil Hobbs
First few zener data sheets that I liiked at, there were just DC power specs. Then I found some others, SOT23 sizes, that have power:time curves, typical value around 30 watts for 1 millisecond. That's pretty impressive. I only need enough to turn a non-avalanche-controlled fet into one that clamps safely in the 100 volt ballpark, which it looks like the smallest zeners can do without a problem. The zener only needs to furnish 50 nC or so of gate charge. Avalanche-rated mosfets can typically clamp tens of mJ. Looks like the external zener version can gulp a lot more. -- John Larkin Highland Technology, Inc lunatic fringe electronics