high current JFETs
high current JFETs
(OP)
Can anyone recommend a manufacturer of high current JFETs (0.5-5 amps)? I've started with the big firms (On, Fairchild) and so far I'm only finding smaller ones (< 100 mA). Does this beast exist?
I have a circuit where a PFET is charging a very large capacitor; the Rds of the PFET keeps the power supply from triggering its overcurrent limit. After it gets charged I'd like a lower Rds than the PFET has. If I can parallel the PFET with a JFET then the JFET would turn on after the initial charge and reduce the overall Rds.
I have a circuit where a PFET is charging a very large capacitor; the Rds of the PFET keeps the power supply from triggering its overcurrent limit. After it gets charged I'd like a lower Rds than the PFET has. If I can parallel the PFET with a JFET then the JFET would turn on after the initial charge and reduce the overall Rds.





RE: high current JFETs
You kick in the bypass to dump the charge limiter. Set the resistor at your pre-charge current requirement.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: high current JFETs
RE: high current JFETs
The PTC is to protect the limiting resistor. If your limiting resistor has got the stones you can ditch it to. Getting much simpler.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: high current JFETs
RE: high current JFETs
One of those ceramic caps can probably take a monstrous blast since it is just a brief inrush current, once in a while. Maybe 10X the watt rating. I think many of them list the maximum current allowed that won't "pop" the resistive trace. And you are even protected from rapid power cycling because brief OFF/ONs will find you with the caps still charged.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: high current JFETs
Z
RE: high current JFETs
What kind of laser? Ion? CO2?
Seems really lame that the laser guy expects you to handle shutdown sequencing. Or are you doing the 'Laser' and the laser guy is just a 'tube' salesman?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: high current JFETs
Dan - Owner

http://www.Hi-TecDesigns.com
RE: high current JFETs
Have you considered using a power inductor to charge the capacitor. This will improve the efficiency tremendously, thereby reducing the waste heat in the charging circuit.
RE: high current JFETs
100F, 2.7V in 17.1 ml! NessCap ESHSR-0100C0–002R7. These supercaps are amazing. I'll put three together with some protection circuitry for 33F.
5V, so only 413 J in the resistor. With a several second time constant I'd think the power inductor would get much larger than my resistor. Luckily I only charge the cap at power up and then let it trickle-charge after that so I don't have to go through this sequence too often.
Z
RE: high current JFETs
Your inductor should not be designed based on the time constant of the charge time but rather on the time constant of your switching time, which is anything you want. You might find an itty bitty transistor, a small inductor and one control IC will do almost everything you need.
jsolar
RE: high current JFETs
I'm charging a 33F cap with a current limit of 14A. A quick simulation shows that to stay below that current I need an 8H inductor that can handle 14A. Does such a creature exist? Or am I reading VisiGoth's post properly that this is the wrong way to think about this? I need to add a switch.
I'm presently planning to charge with a 0.4ohm resistor, this will take about 40 seconds. If I switch to a PWM controller could I get much faster? I'll have to dust off my switching power supply books.
I'm also attempting to charge this cap to 5V off a power supply that I can crank up to 5.5V. I don't have much headroom because this circuit is an afterthought/band-aid.
Z
RE: high current JFETs
Still not sure about this crazy scheme.. Why can you not just yank the power on the laser? I have never seen one you can't with the exception of flowing gas ones that need to cool down with flowing gas briefly.
RF laser? Is a Maser or are you talking about an RF excited gas laser? (I'm presuming.)
How is adding a truck load of caps going to fix this shortcoming?
BTW I believe you must put a resistor across those caps if you hook them in series or you can over stress the one with slightly smaller capacitance.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: high current JFETs
But you do not need an inductor based on your charging time constant. You can create an intermediate time constant based on a clock. i.e. 100 kHz switcher current source would be a small low cost solution. But not less than a 10 W resistor.
jsolar
RE: high current JFETs
Luckily though, as I've shown management how interesting this little band-aid is getting, they've decided to reopen discussions with the laser manufacturer...
RE: high current JFETs
Thanks.
Sounds like a helluva lot of fun anyway you cut it.
Some people have all the fun...
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com