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Minimum clearance

AnalogKid

Well-Known Member
#2
Probably not. The main issue isn't damage to the FET, it's arcing on the pc board that could expose low voltage circuit areas to high voltages where the user might come in contact. IEC60950 spells out minimum creepage (over a surface) and clearance (through the air) distances for various voltages and relationships with the AC mains. Some power transistors have part numbers with appendices to indicate pre-formed leads in a stagger pattern to increase the distances. Another method is to route a slot in the pc board between two pins.

ak
 
Thread starter #3
Probably not. The main issue isn't damage to the FET, it's arcing on the pc board that could expose low voltage circuit areas to high voltages where the user might come in contact. IEC60950 spells out minimum creepage (over a surface) and clearance (through the air) distances for various voltages and relationships with the AC mains. Some power transistors have part numbers with appendices to indicate pre-formed leads in a stagger pattern to increase the distances. Another method is to route a slot in the pc board between two pins.

ak
Then how are these guys selling it. Imean the mosfet is rated for 800V
 
#4
Hello!
I am using FCPF220N80 MOSfet for 400 Volts operation. it is rated for 800Volts. I have one doubts though, The clearance(distance) between the drain and source pins of the mosfet is only 2.5 mm. Will it be able to sustain without any damage?
datasheet:https://www.onsemi.com/PowerSolutions/product.do?id=FCPF220N80
Your OK provided you use narrow pads and conformal coating, you only require functional insulation and 1mm is sufficient for that on a coated pcb (top and bottom). I use exactly the same mosfet, BTW make sure you use tranzorbs on it as it's very sensitive to overvoltage (doesn't gently avalanch but self destructs).
 
#6
Nope not the gate that's ok its drain to source voltage I am talking about, many mosfets will avalanche and dissipate a bit of power in an overvoltage situation (>800V) but this guy in my experience just prefers to put its legs up in the air and die!! (lookup 1.5KE series of transorbs, most surface mounts are not man enough for the job).

Anyway the F package is brilliant for high voltage as its difficult to get enough creepage distance around the insulating bush etc for high voltage operation for plain TO220. I agree with analoguekid that slotting would do the job too but on my high density high voltage pcb's I would end up slotting so much the board would fall apart :)

You could go TO247 but it makes everything relatively huge! TO220 is fine up to a point at high voltage you just have to take care with the detail, when I say coated I mean a proper high voltage coating not just any old muck (for example standard hotmelt burns at those sorts of voltages!).
 

AnalogKid

Well-Known Member
#7
Are you talking about back-to-back zener protection?
Yes.

A tranzorb is a specially built zener diode. It is a below-average voltage regulator, but can handle hundreds or thousands of amps for a brief time period without blowing like a fuse. The single diode type is for DC. The back-to-back zener types are for AC voltages.

ak
 

unclejed613

Well-Known Member
Most Helpful Member
#9
most sources list the breakdown voltage of air as 3kv/mm, so at 800V and 2.5mm spacing you have a long way to go. what you might want to do however is to not use PCB pads with points (use round pads, not square) and use high voltage soldering technique (a little bit of extra solder to give the connection a spherical profile, no points, no inclusions and no bubbles in the solder). clean all flux residue off the PCB. at 800V, PCB traces might have some corona discharge around the edges, so painting them with HV dope will keep that from being a problem. the other thing that HV traces on a PCB do that can eventually cause arcing over, is they gather dust. again HV dope painted over the traces will help keep the trace isolated from the dust that collects (it will collect because there's an electrostatic field).
 

dknguyen

Well-Known Member
Most Helpful Member
#10
most sources list the breakdown voltage of air as 3kv/mm, so at 800V and 2.5mm spacing you have a long way to go. what you might want to do however is to not use PCB pads with points (use round pads, not square) and use high voltage soldering technique (a little bit of extra solder to give the connection a spherical profile, no points, no inclusions and no bubbles in the solder). clean all flux residue off the PCB. at 800V, PCB traces might have some corona discharge around the edges, so painting them with HV dope will keep that from being a problem. the other thing that HV traces on a PCB do that can eventually cause arcing over, is they gather dust. again HV dope painted over the traces will help keep the trace isolated from the dust that collects (it will collect because there's an electrostatic field).
Huh. Never thought about the extra solder to make a spherical joint thing.
 

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