Forums

How long does it take for a diode to conduct?

Started by Tim Wescott April 9, 2012
Is it reasonable to expect that the turn-on (not off -- _on_) time of a 
diode should be pretty much consistent with the inductance of the 
package, and not much else?

A customer just sent me some O-scope traces of a circuit where a voltage 
at a catch diode anode is (at least apparently) going several hundred 
volts above its cathode for a microsecond before settling out to the 
nominal anode voltage of the diode.

We're not sure if it's a measurement artifact because the coil has some 
700A in it, or if it's a real event; I wouldn't be willing to even 
believe it's a real event except that I know that funny things can happen 
at high currents.

-- 
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
Tim Wescott wrote:

> Is it reasonable to expect that the turn-on (not off -- _on_) time of a > diode should be pretty much consistent with the inductance of the > package, and not much else? > > A customer just sent me some O-scope traces of a circuit where a voltage > at a catch diode anode is (at least apparently) going several hundred > volts above its cathode for a microsecond before settling out to the > nominal anode voltage of the diode. > > We're not sure if it's a measurement artifact because the coil has some > 700A in it, or if it's a real event; I wouldn't be willing to even > believe it's a real event except that I know that funny things can happen > at high currents. >
What the customer should do is this.. Set the scope at that peak as the trigger point and expand the scan at that point. If there is some descending ripples( ringing), well then we know we are dealing with a high Q reactive circuit. If all you see is a single peak with no descending ripples, then I take that as a slow transition in switching. Jamie
Tim Wescott wrote:
> > Is it reasonable to expect that the turn-on (not off -- _on_) time of a > diode should be pretty much consistent with the inductance of the > package, and not much else? > > A customer just sent me some O-scope traces of a circuit where a voltage > at a catch diode anode is (at least apparently) going several hundred > volts above its cathode for a microsecond before settling out to the > nominal anode voltage of the diode. > > We're not sure if it's a measurement artifact because the coil has some > 700A in it, or if it's a real event; I wouldn't be willing to even > believe it's a real event except that I know that funny things can happen > at high currents.
Slow turn-on is a fairly common diode pathology. 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 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
On Mon, 09 Apr 2012 22:17:40 -0400, Phil Hobbs wrote:

> Tim Wescott wrote: >> >> Is it reasonable to expect that the turn-on (not off -- _on_) time of a >> diode should be pretty much consistent with the inductance of the >> package, and not much else? >> >> A customer just sent me some O-scope traces of a circuit where a >> voltage at a catch diode anode is (at least apparently) going several >> hundred volts above its cathode for a microsecond before settling out >> to the nominal anode voltage of the diode. >> >> We're not sure if it's a measurement artifact because the coil has some >> 700A in it, or if it's a real event; I wouldn't be willing to even >> believe it's a real event except that I know that funny things can >> happen at high currents. > > Slow turn-on is a fairly common diode pathology.
Augh. So -- what to do? Are there diodes that are specifically rated for quick turn-on? If not, are there diodes (or diode types) that are noted for turning on rapidly? Can multiple diodes in parallel be used to speed up the turn-on (this seems like all sorts of wrong to me, by the way -- but I feel I have to check). -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
On Apr 9, 10:17=A0pm, Phil Hobbs
<pcdhSpamMeSensel...@electrooptical.net> wrote:
> Tim Wescott wrote: > > > Is it reasonable to expect that the turn-on (not off -- _on_) time of a > > diode should be pretty much consistent with the inductance of the > > package, and not much else? > > > A customer just sent me some O-scope traces of a circuit where a voltag=
e
> > at a catch diode anode is (at least apparently) going several hundred > > volts above its cathode for a microsecond before settling out to the > > nominal anode voltage of the diode. > > > We're not sure if it's a measurement artifact because the coil has some > > 700A in it, or if it's a real event; I wouldn't be willing to even > > believe it's a real event except that I know that funny things can happ=
en
> > at high currents. > > Slow turn-on is a fairly common diode pathology. > > 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 > 845-480-2058 > > hobbs at electrooptical dot nethttp://electrooptical.net
What's the physics involved? Would increasing the temperature help? George H.
George Herold wrote:
> > On Apr 9, 10:17 pm, Phil Hobbs > <pcdhSpamMeSensel...@electrooptical.net> wrote: > > Tim Wescott wrote: > > > > > Is it reasonable to expect that the turn-on (not off -- _on_) time of a > > > diode should be pretty much consistent with the inductance of the > > > package, and not much else? > > > > > A customer just sent me some O-scope traces of a circuit where a voltage > > > at a catch diode anode is (at least apparently) going several hundred > > > volts above its cathode for a microsecond before settling out to the > > > nominal anode voltage of the diode. > > > > > We're not sure if it's a measurement artifact because the coil has some > > > 700A in it, or if it's a real event; I wouldn't be willing to even > > > believe it's a real event except that I know that funny things can happen > > > at high currents. > > > > Slow turn-on is a fairly common diode pathology. > > > > Cheers > > > > Phil Hobbs
> > What's the physics involved? > Would increasing the temperature help? > > George H.
I don'g actually know why that is, but some diodes overshoot a lot. There's more info in one of Jim Williams's app notes, http://www.linear.com/docs/27403 . 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 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Tim Wescott <tim@seemywebsite.com> wrote in
news:T_OdnWGfG_V6BB7SnZ2dnUVZ_gqdnZ2d@web-ster.com: 

> On Mon, 09 Apr 2012 22:17:40 -0400, Phil Hobbs wrote: > >> Tim Wescott wrote: >>> >>> Is it reasonable to expect that the turn-on (not off -- _on_) time >>> of a diode should be pretty much consistent with the inductance of >>> the package, and not much else? >>> >>> A customer just sent me some O-scope traces of a circuit where a >>> voltage at a catch diode anode is (at least apparently) going >>> several hundred volts above its cathode for a microsecond before >>> settling out to the nominal anode voltage of the diode. >>> >>> We're not sure if it's a measurement artifact because the coil has >>> some 700A in it, or if it's a real event; I wouldn't be willing to >>> even believe it's a real event except that I know that funny things >>> can happen at high currents. >> >> Slow turn-on is a fairly common diode pathology. > > Augh. So -- what to do? Are there diodes that are specifically rated > for quick turn-on? If not, are there diodes (or diode types) that are > noted for turning on rapidly? Can multiple diodes in parallel be used > to speed up the turn-on (this seems like all sorts of wrong to me, by > the way -- but I feel I have to check). >
(part)cure: never turn the laser completely off, just slightly below or above lasing, should mean a big decrease in capacity. Somebody have a forward voltage versus capacity plot?
On Mon, 09 Apr 2012 20:25:28 -0500, Tim Wescott <tim@seemywebsite.com>
wrote:

>Is it reasonable to expect that the turn-on (not off -- _on_) time of a >diode should be pretty much consistent with the inductance of the >package, and not much else?
Not a PN junction diode. They take a while to conduct, the phenom being called, strangely, "forward recovery." We did a pulse generator that used power diodes in DSRD (drift step recovery) mode. We applied 48 volts in the forward direction, and waited about 100 ns for the current to ramp up to 50 amps. The rampup looked pretty linear. Then we applied -400 volts to get them to snap.
> >A customer just sent me some O-scope traces of a circuit where a voltage >at a catch diode anode is (at least apparently) going several hundred >volts above its cathode for a microsecond before settling out to the >nominal anode voltage of the diode.
That can happen. You may need a high-voltage schottky, SiC or GaN or whatever, or a few silicon schottkies in series. John -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On 4/9/2012 7:24 PM, Tim Wescott wrote:
> On Mon, 09 Apr 2012 22:17:40 -0400, Phil Hobbs wrote: > >> Tim Wescott wrote: >>> >>> Is it reasonable to expect that the turn-on (not off -- _on_) time of a >>> diode should be pretty much consistent with the inductance of the >>> package, and not much else? >>> >>> A customer just sent me some O-scope traces of a circuit where a >>> voltage at a catch diode anode is (at least apparently) going several >>> hundred volts above its cathode for a microsecond before settling out >>> to the nominal anode voltage of the diode. >>> >>> We're not sure if it's a measurement artifact because the coil has some >>> 700A in it, or if it's a real event; I wouldn't be willing to even >>> believe it's a real event except that I know that funny things can >>> happen at high currents. >> >> Slow turn-on is a fairly common diode pathology. > > Augh. So -- what to do? Are there diodes that are specifically rated > for quick turn-on? If not, are there diodes (or diode types) that are > noted for turning on rapidly? Can multiple diodes in parallel be used to > speed up the turn-on (this seems like all sorts of wrong to me, by the > way -- but I feel I have to check). >
There is always the issue of current hogging with paralleling diodes. Given your posts in the past, I think you knew that but were in need of a cup of coffee prior to hitting the send button. ;-)
On Mon, 09 Apr 2012 20:28:45 -0700, miso wrote:

> On 4/9/2012 7:24 PM, Tim Wescott wrote: >> On Mon, 09 Apr 2012 22:17:40 -0400, Phil Hobbs wrote: >> >>> Tim Wescott wrote: >>>> >>>> Is it reasonable to expect that the turn-on (not off -- _on_) time of >>>> a diode should be pretty much consistent with the inductance of the >>>> package, and not much else? >>>> >>>> A customer just sent me some O-scope traces of a circuit where a >>>> voltage at a catch diode anode is (at least apparently) going several >>>> hundred volts above its cathode for a microsecond before settling out >>>> to the nominal anode voltage of the diode. >>>> >>>> We're not sure if it's a measurement artifact because the coil has >>>> some 700A in it, or if it's a real event; I wouldn't be willing to >>>> even believe it's a real event except that I know that funny things >>>> can happen at high currents. >>> >>> Slow turn-on is a fairly common diode pathology. >> >> Augh. So -- what to do? Are there diodes that are specifically rated >> for quick turn-on? If not, are there diodes (or diode types) that are >> noted for turning on rapidly? Can multiple diodes in parallel be used >> to speed up the turn-on (this seems like all sorts of wrong to me, by >> the way -- but I feel I have to check). >> >> > There is always the issue of current hogging with paralleling diodes. > Given your posts in the past, I think you knew that but were in need of > a cup of coffee prior to hitting the send button. ;-)
That was a big part of the "all sorts of wrong" that I was referring to. Presumably if one did it one would need current balancing resistors, and one might leave blood on the floor before one got things working. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com