I'm still iterating on my high-voltage pulse generator. Rev C looks very good. I've been pushing it up voltage and frequency (like, 1200 volts and 4 MHz) and seeing what parts fry, and working on them. I have a dpak schottky diode in a sort of snubber place. According to Spice, it shouldn't get too hot, and it's soldered to all sorts of copper pours and thermal vias and stuff. It's an SBRD10200 and it's frying. I happened to have some smaller, lower current diodes around, UPS5100E3, which is a sawed-off sub-dpak thing. I figured I might squeeze in two of those in parallel... but just one runs cool. https://www.dropbox.com/s/6zv36c4d6v7dsbn/Two_Diodes.JPG?raw=1 Weird. I'm guessing that the big one has a p-n guard ring that makes it have reverse recovery current right after a big forward current spike. Data sheets never talk about that. I heat sinked things too well, so it's a bear to desolder/resolder. https://www.dropbox.com/s/bg0399wzzi4k3ze/T850C_D1_Kluge.jpg?raw=1 -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
guard ring?
Started by ●October 12, 2018
Reply by ●October 12, 20182018-10-12
On 12-Oct-18 10:01 PM, John Larkin wrote:> > > I'm still iterating on my high-voltage pulse generator. Rev C looks > very good. I've been pushing it up voltage and frequency (like, 1200 > volts and 4 MHz) and seeing what parts fry, and working on them. > > I have a dpak schottky diode in a sort of snubber place. According to > Spice, it shouldn't get too hot, and it's soldered to all sorts of > copper pours and thermal vias and stuff. It's an SBRD10200 and it's > frying. I happened to have some smaller, lower current diodes around, > UPS5100E3, which is a sawed-off sub-dpak thing. I figured I might > squeeze in two of those in parallel... but just one runs cool. > > https://www.dropbox.com/s/6zv36c4d6v7dsbn/Two_Diodes.JPG?raw=1 > > Weird. I'm guessing that the big one has a p-n guard ring that makes > it have reverse recovery current right after a big forward current > spike. Data sheets never talk about that. > > I heat sinked things too well, so it's a bear to desolder/resolder.Heat gun/ + heat pad under... --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
Reply by ●October 12, 20182018-10-12
On Fri, 12 Oct 2018 23:06:07 +0100, TTman <kraken.sankey@gmail.com> wrote:>On 12-Oct-18 10:01 PM, John Larkin wrote: >> >> >> I'm still iterating on my high-voltage pulse generator. Rev C looks >> very good. I've been pushing it up voltage and frequency (like, 1200 >> volts and 4 MHz) and seeing what parts fry, and working on them. >> >> I have a dpak schottky diode in a sort of snubber place. According to >> Spice, it shouldn't get too hot, and it's soldered to all sorts of >> copper pours and thermal vias and stuff. It's an SBRD10200 and it's >> frying. I happened to have some smaller, lower current diodes around, >> UPS5100E3, which is a sawed-off sub-dpak thing. I figured I might >> squeeze in two of those in parallel... but just one runs cool. >> >> https://www.dropbox.com/s/6zv36c4d6v7dsbn/Two_Diodes.JPG?raw=1 >> >> Weird. I'm guessing that the big one has a p-n guard ring that makes >> it have reverse recovery current right after a big forward current >> spike. Data sheets never talk about that. >> >> I heat sinked things too well, so it's a bear to desolder/resolder. > >Heat gun/ + heat pad under...I don't want to melt everything else, and there's stuff on the back side of the board, too. My manufacturing people could do it for me, but they're busy. I applied my biggest Metcal tip and waited about 5 minutes for the big diode to finally let loose. I suspect that the solder didn't fully flow under the new tiny diode, but it runs cool. Anyhow, that's a cute little schottky without a guard ring problem. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●October 12, 20182018-10-12
TTman <kraken.sankey@gmail.com> wrote in news:ppr5sh$3mq$1@dont-email.me:> On 12-Oct-18 10:01 PM, John Larkin wrote: >>snip>> >> I heat sinked things too well, so it's a bear to desolder/resolder. > > Heat gun/ + heat pad under... > > --- > This email has been checked for viruses by Avast antivirus software. > https://www.avast.com/antivirus > >I agree. Even a chamber is a good idea. Bring the assembly UP in temperature so that the transition to reflow the part is less of a jump. It is easier to desolder a part that is already sitting at 200�C when one starts adding the reflow heat source (hot air, etc.). A 200 degree transition on a hot part is far easier to impart than a 330 degree jump from ambient.
Reply by ●October 12, 20182018-10-12
"John Larkin" <jjlarkin@highland_snip_technology.com> wrote in message news:b782sdhv8gtok4b8br9pjpik68m6rf15qt@4ax.com...>>Heat gun/ + heat pad under... > > I don't want to melt everything else, and there's stuff on the back > side of the board, too. My manufacturing people could do it for me, > but they're busy. I applied my biggest Metcal tip and waited about 5 > minutes for the big diode to finally let loose. I suspect that the > solder didn't fully flow under the new tiny diode, but it runs cool.> Anyhow, that's a cute little schottky without a guard ring problem. >Schottky can be pretty bad for snubbing, depending. The peak current can hit the guard ring diodes and you get big recovery loss. The capacitance is huge. You'd have to add a shunt to be sure what was going on with the other one. Regular diodes can have faster recovery than schottky-guard-ring recovery, but they also cost forward as well as reverse recovery. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
Reply by ●October 12, 20182018-10-12
On Friday, October 12, 2018 at 6:38:33 PM UTC-4, John Larkin wrote:> On Fri, 12 Oct 2018 23:06:07 +0100, TTman <kraken.sankey@gmail.com> > wrote: > > >On 12-Oct-18 10:01 PM, John Larkin wrote: > >> > >> > >> I'm still iterating on my high-voltage pulse generator. Rev C looks > >> very good. I've been pushing it up voltage and frequency (like, 1200 > >> volts and 4 MHz) and seeing what parts fry, and working on them. > >> > >> I have a dpak schottky diode in a sort of snubber place. According to > >> Spice, it shouldn't get too hot, and it's soldered to all sorts of > >> copper pours and thermal vias and stuff. It's an SBRD10200 and it's > >> frying. I happened to have some smaller, lower current diodes around, > >> UPS5100E3, which is a sawed-off sub-dpak thing. I figured I might > >> squeeze in two of those in parallel... but just one runs cool. > >> > >> https://www.dropbox.com/s/6zv36c4d6v7dsbn/Two_Diodes.JPG?raw=1 > >> > >> Weird. I'm guessing that the big one has a p-n guard ring that makes > >> it have reverse recovery current right after a big forward current > >> spike. Data sheets never talk about that. > >> > >> I heat sinked things too well, so it's a bear to desolder/resolder. > > > >Heat gun/ + heat pad under... > > I don't want to melt everything else, and there's stuff on the back > side of the board, too. My manufacturing people could do it for me, > but they're busy. I applied my biggest Metcal tip and waited about 5 > minutes for the big diode to finally let loose. I suspect that the > solder didn't fully flow under the new tiny diode, but it runs cool.I was going to suggest a good size iron, and solder sucker. Then an exacto, under the tab. 5 min is much too long. George H.> > Anyhow, that's a cute little schottky without a guard ring problem. > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
Reply by ●October 12, 20182018-10-12
> "John Larkin" <jjlarkin@highland_snip_technology.com> wrote in message > news:b782sdhv8gtok4b8br9pjpik68m6rf15qt@4ax.com... >>>Heat gun/ + heat pad under... >> >> I don't want to melt everything else, and there's stuff on the back >> side of the board, too. My manufacturing people could do it for me, >> but they're busy. I applied my biggest Metcal tip and waited about 5 >> minutes for the big diode to finally let loose. I suspect that the >> solder didn't fully flow under the new tiny diode, but it runs cool. >Hot air machines are super cheap, and don't melt anything but what they're pointed at. Assuming the only thing melting is solder. You probably shouldn't have melty things like electrolytics and film caps nearby on an SMT board, anyway, but a heat shield (aluminum foil) keeps that feasible. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
Reply by ●October 13, 20182018-10-13
John Larkin wrote>I'm still iterating on my high-voltage pulse generator. Rev C looks >very good. I've been pushing it up voltage and frequency (like, 1200 >volts and 4 MHz) and seeing what parts fry, and working on them. > >I have a dpak schottky diode in a sort of snubber place. According to >Spice, it shouldn't get too hot, and it's soldered to all sorts of >copper pours and thermal vias and stuff. It's an SBRD10200 and it's >frying. I happened to have some smaller, lower current diodes around, >UPS5100E3, which is a sawed-off sub-dpak thing. I figured I might >squeeze in two of those in parallel... but just one runs cool. > >https://www.dropbox.com/s/6zv36c4d6v7dsbn/Two_Diodes.JPG?raw=1 > >Weird. I'm guessing that the big one has a p-n guard ring that makes >it have reverse recovery current right after a big forward current >spike. Data sheets never talk about that. > >I heat sinked things too well, so it's a bear to desolder/resolder. > >https://www.dropbox.com/s/bg0399wzzi4k3ze/T850C_D1_Kluge.jpg?raw=14 MHz at 1000V, I did higher frequencies with tubes in the sixties, with 1000V supply at .5 A peak. Without seeing the circuit it is hard to tell, but Schottky diodes would not be my choice, there are nice fast si diodes or at least there were. Schottkys leak, get very hot because current flows both directions that way. Thermal runaway results. A few pF from a fast si diode .. helps tune the circuit ;-) Anyways no way to tell without seeing what that diode is doing. Snubber = flyback diode here?
Reply by ●October 13, 20182018-10-13
"Tim Williams" <tiwill@seventransistorlabs.com> wrote in news:pprker$6d5$1@dont-email.me:> Hot air machines are super cheap, and don't melt anything but what > they're pointed at. Assuming the only thing melting is solder.With PCBs that have heavy via and high via counts near heavy parts even a single hot air jet can be detrimental to a board. Bringing the entire assembly up in temperature OVER ambient means less thermal WORK has to be done to do the job, AND less overall stress occuurs on the whole thing. One brings the entire assembly temp up (at least the area surrounding the rework location), THEN use a direct hot air jet to reflow the part. They have benchtop setups for this. http://www.zeph.com/
Reply by ●October 13, 20182018-10-13
<DecadentLinuxUserNumeroUno@decadence.org> wrote in message news:ppscr9$145q$1@gioia.aioe.org...> "Tim Williams" <tiwill@seventransistorlabs.com> wrote in > news:pprker$6d5$1@dont-email.me: > >> Hot air machines are super cheap, and don't melt anything but what >> they're pointed at. Assuming the only thing melting is solder. > > With PCBs that have heavy via and high via counts near heavy parts even a > single hot air jet can be detrimental to a board.Well, you set it too high then (somewhere north of 260C). And yeah, if you have too much thermal mass for this to work, you need a preheater. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
