TO-220 PCB spacing for HV MOSFETs

SpreadSpectrum · 20th July 2010, 03:57 AM

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?

Sch3mat1c · 20th July 2010, 05:07 AM

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

Wavebourn · 20th July 2010, 05:18 AM

Quote:

Originally Posted by SpreadSpectrum

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 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.

SpreadSpectrum · 21st July 2010, 02:09 AM

I think I will spread leads a bit. TO-247 looks like a good lead spacing to use, thanks Tim.

jcx · 21st July 2010, 02:24 AM

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

Ian444 · 21st July 2010, 12:24 PM

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.

SpreadSpectrum · 21st July 2010, 02:30 PM

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

tubelab.com · 21st July 2010, 05:17 PM

Quote:

"Yeah I do that all the time and have no problems."

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.

richie00boy · 21st July 2010, 05:33 PM

Quote:

Originally Posted by jcx

the package/lead spacing itself doesn't meet several common standards for creepage, clearance distance at the rated Vds

I have pondered this quite heavily in the past.

TheGimp · 21st July 2010, 05:51 PM

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.)

20th July 2010, 03:57 AM	#1
SpreadSpectrum diyAudio Member Join Date: Nov 2007	TO-220 PCB spacing for HV MOSFETs 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?

20th July 2010, 05:07 AM	#2
Sch3mat1c diyAudio Member Join Date: Jan 2003 Location: Milwaukee, WI	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 __________________ See my Electronics webpage -- the home of Vacuum Tube Drag Racing. The key to being a successful Audiophile: "I reject your reality and substitute my own!"

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21st July 2010, 02:09 AM	#4
SpreadSpectrum diyAudio Member Join Date: Nov 2007	I think I will spread leads a bit. TO-247 looks like a good lead spacing to use, thanks Tim.

21st July 2010, 02:24 AM	#5
jcx diyAudio Member Join Date: Feb 2003 Location: ..	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

21st July 2010, 12:24 PM	#6
Ian444 diyAudio Member Join Date: Nov 2008 Location: Brisbane QLD	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.

21st July 2010, 02:30 PM	#7
SpreadSpectrum diyAudio Member Join Date: Nov 2007	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

21st July 2010, 05:51 PM	#10
TheGimp diyAudio Member Join Date: Aug 2009 Location: Johnson City, TN	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.)

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