I am getting ready to design a board for a tube circuit and am planning on using FQPF2N90 as source followers to drive some tubes. I could potentially have almost 800V from drain to source and am a bit worried about the spacing between pins of the TO-220 package. Spacing between exposed traces (the solder holes) will be about 40 mils.
I have read on the 'Net that breakdown voltage between exposed traces across FR4 is 40V/mil. This gives about 1600V, a margin of 2:1. However, most sources recommend having much more spacing than this for these voltage levels. I am probably going to conformal coat these if it will help.
I guess I am looking for a warm fuzzy like, "Yeah I do that all the time and have no problems." Anyone else use HV TO-220 MOSFETS on PCBs in tube circuits?
I have read on the 'Net that breakdown voltage between exposed traces across FR4 is 40V/mil. This gives about 1600V, a margin of 2:1. However, most sources recommend having much more spacing than this for these voltage levels. I am probably going to conformal coat these if it will help.
I guess I am looking for a warm fuzzy like, "Yeah I do that all the time and have no problems." Anyone else use HV TO-220 MOSFETS on PCBs in tube circuits?
If you don't feel comfortable with it, you could edit the footprint to stagger the pins, or spread them out (or use a TO-247 sized footprint instead).
I've seen slots carved in the board around high voltage transistors, too.
Tim
I've seen slots carved in the board around high voltage transistors, too.
Tim
I use for 400V stabilizer, from 800V B+. It sees all 800V DS voltage when switched on. No special measures used: no slots, no coating. Just legs arranged in a triangle.
the package/lead spacing itself doesn't meet several common standards for creepage, clearance distance at the rated Vds
I plan on coating the lower body, leads, and pcb - and lead forming for larger pcb pad clearance when using TO-220 with 600 V
that doesn't mean other people don't ship smps that violate those standards
I plan on coating the lower body, leads, and pcb - and lead forming for larger pcb pad clearance when using TO-220 with 600 V
that doesn't mean other people don't ship smps that violate those standards
You can spread the legs, but the 3 stubs coming from the TO-220 package before you do the leg-spreading is where the action might happen, or not. Conformal coating will help reduce arcing under moist or humid conditions. Consider it takes 16,000V or so to create a good solid spark across a 0.030" gap (spark plug). 0.030" = 0.75mm.
Yeah, I'm kind of frustrated that they don't use a different package for a 900V Fet. It has really low Crss, though. 5.5pF
OK, I'll say that. In truth I have only been to 650 volts. That's the end of my power supply knob.
I would assume that you are referring to a professionally made PC board with a solder mask coating. The home made stuff that is etched in ferric chloride does have stray copper particles and ionic contaminants all over and imbedded into the surface. Add some south Florida humidity and some fireworks should happen.....but it hasn't yet.
I did find some nice TO-247 Fuji mosfets on sale at Allied last year. I will probably use them in big voltage experiments, but my TO-220's havent blown yet.
Air breakdown is around 15,000V per inch depending on humidity.
I measured the lead to lead distance of a TO-220 at 0.060, so the breakdown is around 900V not counting creepage. Then there are those SMT parts like TO-252 where it is closer to 700V.
I think there is a need for a new package (and leadframe) for the HV parts. The only parts I see in TO-247(AD or S) are high power devices.
They really need to get rid of the drain off the leadframe and just have the gate and source coming out the lower body with the tab as the Drain. (This wouldn't work on an Isolated-Tab package.)
I measured the lead to lead distance of a TO-220 at 0.060, so the breakdown is around 900V not counting creepage. Then there are those SMT parts like TO-252 where it is closer to 700V.
I think there is a need for a new package (and leadframe) for the HV parts. The only parts I see in TO-247(AD or S) are high power devices.
They really need to get rid of the drain off the leadframe and just have the gate and source coming out the lower body with the tab as the Drain. (This wouldn't work on an Isolated-Tab package.)
I am using an IRFIB5N65 as a electronic choke, and that as a source follower. It handles extremely well in startup and in shutdown.
There is 44k with a voltage divider of 1 meg to earth, to 20 muF; then 100k to 0,47 muF and subsequently 220 ohm to the gate. It has a residual hum of 3 mV (measured B&K). Not bad isn't it.
HV is max. 450V raw. No precautions on the transistor. The legs fan out. Note, there is no inductive load neither on input nor on output. If so, there would of course be the risk of sparking.
albert
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