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Need some help making a PCB for a full bridge

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profdc9

Mon Jan 15 2018, 05:30AM

Registered Member #58522 Joined: Tue Mar 15 2016, 08:33PM
Location:
Posts: 50

Ok I am going to try and learn something. I laid out a board for a full bridge. I am not sure how to get the traces to be very wide, because they become wider than the spacing between the pins on the TO-247 package. Nevertheless, this is what I've come up with so far. I have the trace width 5 mm, but this is probably going to be too narrow for DRSSTC unless the copper can be made very thick. As you can see, I tried to get the loop size for the DC bypass, snubbers, and the distance between the low and high side IGBTs as small as possible.

Anyways I welcome comments, I thought I would throw my hat into the ring.

Dan

|)4+80!|-|\\/

Mon Jan 15 2018, 10:54AM

Registered Member #61803 Joined: Thu Sept 21 2017, 09:18AM
Location: Bhopal,India
Posts: 34

Blackcurrant wrote ...

Try about 2.5mm for your clearance

What's holding your mains connector and heat sinks on?
Is the blue image the top layer?
It's not clear what side components are on.
Will the PCB copper get too hot?

Well the AC jack is just for show I won't really be using it
Red's the top layer
SO, the 2.5 mm clearance should be between the supply rails and the Primary right??

Blackcurrant

Mon Jan 15 2018, 12:36PM

Registered Member #2989 Joined: Sun Jul 11 2010, 12:01AM
Location: UK
Posts: 94

The clearance is just the space between anything at different potentials.
If you have 240VACrms mains then say about 345DC.
From the link, the table recommends 500v 2.5mm etc.
I would think 1mm with a coated board would be fine, but it was just an example.
Best to give things extra space if you can.

Profdc9 try using power planes :)

Here's one I destroyed earlier

dexter

Mon Jan 15 2018, 01:57PM

Registered Member #42796 Joined: Mon Jan 13 2014, 06:34PM
Location:
Posts: 195

|)4+80!|-|\\/ wrote ...

SO, the 2.5 mm clearance should be between the supply rails and the Primary right??

here's the pcb (copper only) for a small SSTC i built
on the low voltage side the clearance is 1.27mm or 100 mill
on the HV side the clearance is as wide as the soldering pads of the TO247IGBT
my job was easy because i built a half bridge and didn't used electrolytics

|)4+80!|-|\\/

Mon Jan 15 2018, 02:13PM

Registered Member #61803 Joined: Thu Sept 21 2017, 09:18AM
Location: Bhopal,India
Posts: 34

OOH
I SEEE

I've increased the isolation to about 1.27mm
Is that Fine??

profdc9

Mon Jan 15 2018, 03:48PM

Registered Member #58522 Joined: Tue Mar 15 2016, 08:33PM
Location:
Posts: 50

I kind of deliberately avoided power planes and copper pours because I didn't think it would be a good idea to have possible up to 600 volts potential difference over the entire board. It seems to invite arcing at some part of the board between some close components somewhere. I suppose I could make it a four layer board with generous clearance around the pours, but I was hoping to keep it to be two layers. Do you think that the EMI or the loop inductance is going to be too great just routing traces manually? The entire board is at mains potential so I'd prefer not to make the power planes over the entire board and make the entire board "hot."

Dan

profdc9

Mon Jan 15 2018, 05:34PM

Registered Member #58522 Joined: Tue Mar 15 2016, 08:33PM
Location:
Posts: 50

Ok, I am going to try to get this right. I added copper pours below the central part of the board where the high current is. I left 2.5 mm clearance between pads, so hopefully 600 volts will not cross it.

I also added copper pours for the output pads, so hopefully the area of the copper is maximized throughout.

Dan

Weston

Tue Jan 16 2018, 11:14PM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

Looks decent. Whats it for / what are the RC snubbers for?

profdc9

Wed Jan 17 2018, 02:10AM

Registered Member #58522 Joined: Tue Mar 15 2016, 08:33PM
Location:
Posts: 50

When I built a half-bridge before, I found I needed snubbers to prevent inductive spikes from destroying the transistors. I was using my half bridge to draw arcs for fun and the disruption of the arc can cause inductive spikes. The snubbers were something like 3-5 ohm resistors (2 W) and 100 nF 600 V film capacitors. So I included them in this design, because I figured that if I could reasonably fit them, they could be included as long as they didn't increase the loop size too much.

I used an on-line calculator, and it seems for 1 oz copper, the temperature rise should be about 20 C for the copper filling I'm using, so I think it should work ok. I would just be afraid of frying the spot directly around the solder contact. I added spots for small TVS diodes on the gates as well. The result is below. I may make another version of this board which is a half bridge with the other side with spots for a capacitor divider instead. The size of this is under 10 cm by 10 cm so it can be made at Seeedstudio for their $5 board charge. For them to make the board with 3 oz copper, it is $140!

Now I just have to get the universal DRSSTC driver made, but right now I'm just learning about board design. I have a half bridge driver I've had fabbed as well. I can release some Gerber files if anyone's interested.

Dan

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