I have a low power 1400 volt power supply and I'd like to add a bleeder, and have an LED light up if there's voltage present on the caps. I was thinking about a series stack of three LND150 depletion fets and the LED, with source resistors to set the currents to about 150 uA or so. That would keep the LED brightness constant down to 10 volts maybe, and bleed linearly. At least one of the fets will avalanche, maybe two. I think you said that this is OK. I could also use a single 2SK4177 as the current sink, but I'd have to get some gate voltage from somewhere.
ping Win LND150 avalanche
Started by ●November 21, 2015
Reply by ●November 22, 20152015-11-22
John Larkin wrote...> > > I have a low power 1400 volt power supply and I'd like > to add a bleeder, and have an LED light up if there's > voltage present on the caps. > > I was thinking about a series stack of three LND150 > depletion fets and the LED, with source resistors to > set the currents to about 150 uA or so. That would > keep the LED brightness constant down to 10 volts > maybe, and bleed linearly. > > At least one of the fets will avalanche, maybe two. > I think you said that this is OK.In general MOSFETs are perfectly happy in avalanche; in fact most power parts have avalanche ratings. You just have to stay within the power dissipation limit. (If you analyze a datasheet "avalanche rating" you'll see it's simply derived from junction-temp limit and thermal mass from the transient thermal impedance plots, using the inductor value, voltage and currents in question, so there's nothing special about avalanche.) 150uA at say 550 or 600 volts breakdown is 90 mW. That's fine for the wimpy LND150. Note, avalanche voltage can go up another 10% if the junction heats towards 125 or 150 deg C.> I could also use a single 2SK4177 as the current sink, > but I'd have to get some gate voltage from somewhere.That part has a sensible power rating. If you want it's easy to use MOSFETs in series, while avoiding avalanche, with a stack of resistors. See AoE III, Figure 9.110 on page 696. The inset shows how you use one LKND150 to set the current, with additional series MOSFETs, even ordinary enhancement-mode types. The resistor stack divides the voltage for the gates. You'll need five resistors, if you limit them to 300V. You could run the LND150 at say 300V, the 2SK4117 can have the remaining 1100V, a relaxed value for both of them. -- Thanks, - Win
Reply by ●November 22, 20152015-11-22
On 22 Nov 2015 06:58:35 -0800, Winfield Hill <hill@rowland.harvard.edu> wrote:>John Larkin wrote... >> >> >> I have a low power 1400 volt power supply and I'd like >> to add a bleeder, and have an LED light up if there's >> voltage present on the caps. >> >> I was thinking about a series stack of three LND150 >> depletion fets and the LED, with source resistors to >> set the currents to about 150 uA or so. That would >> keep the LED brightness constant down to 10 volts >> maybe, and bleed linearly. >> >> At least one of the fets will avalanche, maybe two. >> I think you said that this is OK. > > In general MOSFETs are perfectly happy in avalanche; > in fact most power parts have avalanche ratings. You > just have to stay within the power dissipation limit. > (If you analyze a datasheet "avalanche rating" you'll > see it's simply derived from junction-temp limit and > thermal mass from the transient thermal impedance plots, > using the inductor value, voltage and currents in > question, so there's nothing special about avalanche.) > > 150uA at say 550 or 600 volts breakdown is 90 mW. > That's fine for the wimpy LND150. Note, avalanche > voltage can go up another 10% if the junction heats > towards 125 or 150 deg C. > >> I could also use a single 2SK4177 as the current sink, >> but I'd have to get some gate voltage from somewhere. > > That part has a sensible power rating. If you want > it's easy to use MOSFETs in series, while avoiding > avalanche, with a stack of resistors. See AoE III, > Figure 9.110 on page 696. The inset shows how you > use one LKND150 to set the current, with additional > series MOSFETs, even ordinary enhancement-mode types. > The resistor stack divides the voltage for the gates. > You'll need five resistors, if you limit them to 300V. > > You could run the LND150 at say 300V, the 2SK4117 > can have the remaining 1100V, a relaxed value for > both of them.OK, thanks. I guess I'll use three LNDs. https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/Bleeder_1400.JPG If I let two of them avalanche, they will hog the power dissipation, but that's OK. Again, the problem with using an enhancement fet is that it needs gate voltage from somewhere guaranteed to be available always. Deriving that from +1400 volts is a nuisance. A battery would be silly. Here's my HV regulator. It has almost zero quiescent dissipation and fast programmable rise/fall times, about 1KV/ms up or down. https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/HV_Reg_T840.jpg
Reply by ●November 22, 20152015-11-22
John Larkin wrote...> > Here's my HV regulator. It has almost zero quiescent dissipation > and fast programmable rise/fall times, about 1KV/ms up or down. > >https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/HV_Reg_T840.jpgWhy are there two opto-couplers in parallel? What's the output current spec? Lots of load capacitance? -- Thanks, - Win
Reply by ●November 22, 20152015-11-22
On 22 Nov 2015 14:29:55 -0800, Winfield Hill <hill@rowland.harvard.edu> wrote:>John Larkin wrote... >> >> Here's my HV regulator. It has almost zero quiescent dissipation >> and fast programmable rise/fall times, about 1KV/ms up or down. >> >>https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/HV_Reg_T840.jpg > > Why are there two opto-couplers in parallel? What's the > output current spec? Lots of load capacitance?Gawd-awful schematic color choices :-( ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●November 22, 20152015-11-22
On 22 Nov 2015 14:29:55 -0800, Winfield Hill <hill@rowland.harvard.edu> wrote:>John Larkin wrote... >> >> Here's my HV regulator. It has almost zero quiescent dissipation >> and fast programmable rise/fall times, about 1KV/ms up or down. >> >>https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/HV_Reg_T840.jpg > > Why are there two opto-couplers in parallel? What's the > output current spec? Lots of load capacitance?It's a dual coupler, used elsewhere on the board, so why not? If the CTR is too high, I can always change the 100 ohm drive resistor. But I want a bunch of drive current. The load is 22 nF, which I hope is enough C to bypass the output pulser stage. The customer wants to be able to change the voltage fast, up or down. The average DC load current is tiny.
Reply by ●November 22, 20152015-11-22
On Sun, 22 Nov 2015 16:10:49 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:>On 22 Nov 2015 14:29:55 -0800, Winfield Hill ><hill@rowland.harvard.edu> wrote: > >>John Larkin wrote... >>> >>> Here's my HV regulator. It has almost zero quiescent dissipation >>> and fast programmable rise/fall times, about 1KV/ms up or down. >>> >>>https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/HV_Reg_T840.jpg >> >> Why are there two opto-couplers in parallel? What's the >> output current spec? Lots of load capacitance? > >Gawd-awful schematic color choices :-( > > ...Jim ThompsonMy layout guy likes those colors. I don't.
Reply by ●November 23, 20152015-11-23
Winfield Hill <hill@rowland.harvard.edu> writes:> John Larkin wrote... >> >> >> I have a low power 1400 volt power supply and I'd like >> to add a bleeder, and have an LED light up if there's >> voltage present on the caps. >> >> I was thinking about a series stack of three LND150 >> depletion fets and the LED, with source resistors to >> set the currents to about 150 uA or so. That would >> keep the LED brightness constant down to 10 volts >> maybe, and bleed linearly. >> >> At least one of the fets will avalanche, maybe two. >> I think you said that this is OK. > > In general MOSFETs are perfectly happy in avalanche; > in fact most power parts have avalanche ratings. You > just have to stay within the power dissipation limit. > (If you analyze a datasheet "avalanche rating" you'll > see it's simply derived from junction-temp limit and > thermal mass from the transient thermal impedance plots, > using the inductor value, voltage and currents in > question, so there's nothing special about avalanche.)I was testing some opto-fets last week for this - there was nothing in the datasheet. But an application note says they must not be subjected to transient voltages outside their rating. Customers might connect them to a wire with inductance, or a relay coil perhaps. I started with 100uH but they seemed to be quite happy switching a couple of Henrys at 0.5A all day. (It was supposed to be a test to destruction but I felt sorry for them so stopped there!). I will be using a TVS anyway now but was curious how much there is to worry about, really. Then there is ESD.> 150uA at say 550 or 600 volts breakdown is 90 mW. > That's fine for the wimpy LND150. Note, avalanche > voltage can go up another 10% if the junction heats > towards 125 or 150 deg C. > >> I could also use a single 2SK4177 as the current sink, >> but I'd have to get some gate voltage from somewhere. > > That part has a sensible power rating. If you want > it's easy to use MOSFETs in series, while avoiding > avalanche, with a stack of resistors. See AoE III, > Figure 9.110 on page 696. The inset shows how you > use one LKND150 to set the current, with additional > series MOSFETs, even ordinary enhancement-mode types. > The resistor stack divides the voltage for the gates. > You'll need five resistors, if you limit them to 300V. > > You could run the LND150 at say 300V, the 2SK4117 > can have the remaining 1100V, a relaxed value for > both of them.-- John Devereux
Reply by ●November 23, 20152015-11-23
Le dimanche 22 novembre 2015 20:22:44 UTC+1, John Larkin a �crit�:> On 22 Nov 2015 06:58:35 -0800, Winfield Hill > <hill@rowland.harvard.edu> wrote: > > >John Larkin wrote... > >> > >> > >> I have a low power 1400 volt power supply and I'd like > >> to add a bleeder, and have an LED light up if there's > >> voltage present on the caps. > >> > >> I was thinking about a series stack of three LND150 > >> depletion fets and the LED, with source resistors to > >> set the currents to about 150 uA or so. That would > >> keep the LED brightness constant down to 10 volts > >> maybe, and bleed linearly. > >> > >> At least one of the fets will avalanche, maybe two. > >> I think you said that this is OK. > > > > In general MOSFETs are perfectly happy in avalanche; > > in fact most power parts have avalanche ratings. You > > just have to stay within the power dissipation limit. > > (If you analyze a datasheet "avalanche rating" you'll > > see it's simply derived from junction-temp limit and > > thermal mass from the transient thermal impedance plots, > > using the inductor value, voltage and currents in > > question, so there's nothing special about avalanche.) > > > > 150uA at say 550 or 600 volts breakdown is 90 mW. > > That's fine for the wimpy LND150. Note, avalanche > > voltage can go up another 10% if the junction heats > > towards 125 or 150 deg C. > > > >> I could also use a single 2SK4177 as the current sink, > >> but I'd have to get some gate voltage from somewhere. > > > > That part has a sensible power rating. If you want > > it's easy to use MOSFETs in series, while avoiding > > avalanche, with a stack of resistors. See AoE III, > > Figure 9.110 on page 696. The inset shows how you > > use one LKND150 to set the current, with additional > > series MOSFETs, even ordinary enhancement-mode types. > > The resistor stack divides the voltage for the gates. > > You'll need five resistors, if you limit them to 300V. > > > > You could run the LND150 at say 300V, the 2SK4117 > > can have the remaining 1100V, a relaxed value for > > both of them. > > OK, thanks. I guess I'll use three LNDs. > > https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/Bleeder_1400.JPG > > If I let two of them avalanche, they will hog the power dissipation, > but that's OK. > > Again, the problem with using an enhancement fet is that it needs gate > voltage from somewhere guaranteed to be available always. Deriving > that from +1400 volts is a nuisance. A battery would be silly. > > Here's my HV regulator. It has almost zero quiescent dissipation and > fast programmable rise/fall times, about 1KV/ms up or down. > > https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/HV_Reg_T840.jpgHi John, How could you master the current through the opto ACPL's when the TLP191 drives the MOSFET VGS ? How could master the power dissipation on MOSFET and opto ACPL ? Don't understand how it works ... may be some math would be more explicit ? Habib.
Reply by ●November 23, 20152015-11-23
habib.bouaziz@gmail.com wrote...> > Don't understand how it works ...The TLP191B acts like a battery. It has a stack of diodes and runs continuously, providing about 7V (at 10uA) of gate bias. https://octopart.com/search?q=TLP191 http://datasheet.octopart.com/TLP191B%28U%2CC%2CF%29-Toshiba-datasheet-13727684.pdf The optocouplers drive the FET source, in common-gate fashion. -- Thanks, - Win
