Minimum slot width?

Hi, all,

My fave Shenzhen PCB vendor, PCBway, is being a bit difficult, and I'm trying to figure out how to respond.

I've designed an adjustable APD power supply for a customer, and am trying to preserve the creepage and clearance requirements despite using small SMT parts, so I need to be able to sneak an L-shaped slot between the pins of a SOT-89 FET (a 500V LND150).

Problem is, their minimum slot width is 0.8 mm (about 31.5 mils), which is wider than the space between the pads.

I can probably sacrifice the middle lead and just use the power pad, but it's unaesthetic to have the lead hovering over a slot.

Anybody got a low-cost PCB vendor that can do narrower slots? 0.3 mm would be great.

Thanks

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs
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onsdag den 25. juli 2018 kl. 02.07.55 UTC+2 skrev Phil Hobbs:

I doubt that is something you can get cheap, an 0.3mm endmill long enough to cut through a pcb is going to be very fragile

Reply to
Lasse Langwadt Christensen

Understood. Laser cutting is what I mostly had in mind.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

Laser cutting of epoxy-glass is very difficult, the beam is not absorbed, but is reflected by the glass.

A lead over a slot won't conduct. Add some plastic.

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Reply to
Winfield Hill

When a laser does cut it leaves carbon.

Reply to
tabbypurr

CO2 lasers can cause charring, but UV lasers (such as excimers) and picosecond-to-femtosecond lasers can cut essentially any material cleanly.

I don't know how far down the food chain those sorts of techniques have reached, though.

Cheers

Phil Hobbs

Reply to
pcdhobbs

The surface creepage distance on the fet itself is about 43 mils, which is acout 11 v/mil at 500 volts. That's not bad. Your PCB wouldn't be much worse even with pads for all the pins.

Do you really need a slot?

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

My main concern is underneath the package. This design is being licensed to the customer, so I'll have no control over the soldering/cleaning technology, and the LND150 is an important part of the protection system (both for the people and the equipment).

I also use slots e.g. under the feedback resistors for sensitive TIAs for the same reason--a 100M 1206 resistor with a slot and a guard ring on the summing junction is pretty well immune to board contamination.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

I'm not sure whether you need the slot so that the product will actually work better, or for compliance with a standard. If the latter, then possibly a narrow slot will not be useful. For example slots narrower than 1mm are ignored for pollution degree 2 in IEC60950, but for pollution degree 1 you can get credit for slots down to 0.25mm. Depending on which standard you want to comply with, there might be a slot width below which you don't want to go.

Reply to
Chris Jones

Hi, Phil -

Take a look at this:

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Scroll down to find they can do edge milling of slots down to 0.2mm. I don't know what edge milling is, so I don't know if it will satisfy you.

Good Luck to you.

John

Reply to
John S

Thanks. There's a +/- in front of that 0.2 mm, so I think that's just the tolerance. I emailed them for a copy of their design rules document.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

AFAIR creepage slots below a certain number are NOT considered to be a slot anyway

For example, when you do transformers, you cannot just make a U formed slot like you please. The distance of the faces of the U must be a certain size, or else the creepage is just shorted

Cheers

Klaus

Reply to
Klaus Kragelund

Why not try the SOT-23 version. The minimum distance between the source and drain pads is 1.2mm, or 47 mils. You should be able to run a 0.8 mm drill between the pads with plenty of clearance. It has lower current, 13 mA agai nst 30 mA, but do you need 30 mA in an APD?

Cheaper, too

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Reply to
Steve Wilson

Flunks on power dissipation. 300V times 1.6 mA is half a watt.

Thanks

Phil Hobbs

Reply to
pcdhobbs

Put three in parallel. They are cheap.

Reply to
Steve Wilson

They're what's doing the current limiting, so that just ups the dissipation to 1.5 W.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

Fine, series. :^)

Which works out just fine if you don't mind a little added resistance, and you do a resistor chain on the gates to equalize them in "cascode".

Can get away with more common transistors for the upper parts, then, too. (Not that you'd bother actually adding another BOM item, at least unless some serious cost reduction is called for.)

Don't LND150s come in SOT89 and such..?

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
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Reply to
Tim Williams

Put resistors in the source leads. The VGS(off) varies from -1V to -3V, and the IDSS varies from 1 o 3 mA, so you need some kind of stabilization anyway.

Now that you have stated the power requirement, it makes it much easier to find a match. There are pleny to choose from. Google high voltage depletion mosfet.

How about a nice IXTY02N50D, 500V, 200 mA, in a TO-252 package. Plenty of room to route clearance slots. $1.96 at Mouser. There are lots more.

It needs a source resistor to stabilize the short circuit current, but you pretty much need that anyway. Now you can set the current to whatever you want.

Reply to
Steve Wilson

Or maybe just use a movie-house rheostat, circa 1960. ;)

That's what I'm using. Problem is, there's no room between the pads for a 31-mil slot. For now we'll just sacrifice the pad area and see.

Thanks

Phil Hobbs

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Reply to
Phil Hobbs

I_DSS is IME always within 1.6 mA +- 200 uA. The power supply is supposed to drive a lot of APD front ends in big biomed instruments.

Thanks. Problem is, I need something much closer to 20 mA than 200, and I don't really want to have to uprate the HV generator to accommodate the production spread of I_D vs R_GS for those big FETs.

If I placed that much faith in subthreshold NMOS models, sure. We'll probably just delete the drain lead of the SOT89 and let it hang 5 over the slot. ;)

Cheers

Phil Hobbs

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Phil Hobbs

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