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Old 25th November 2015, 12:12 PM   #1
SGK is offline SGK
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Default Thermal considerations for Fairchild SDIP bridge rectifiers

Howdy

I need a little low current power supply to produce +- 12V. The 5VA 2x18V transformer will be sitting on the PCB and so generally space will be very limited.

I was thinking about using either the Fairchild MDB6S or DF10S SMD bridge rectifiers rather than a bank of discrete diodes to conserve space. I am, however, a bit baffled by the thermal considerations descriptions in the dottiest with the DF10S being the most confusing.

The data sheet for the MDB6S says the device only needs 35mm^2 of board space (the device is much smaller) while the thermal characteristics are given for a "device mounted on FR-4 PCB with board size = 76.2 mm x 114.3 mm (JESD51-3 standards)."

My total board size can only be about 8x10cm, on which I need to fit the transformer, 2 rectifiers, filter caps, regs, connectors etc, and so I will need to cram things in. (It may not even be possible.)

Can anyone provide guidance regarding interpreting the data sheet thermal considerations for the rectifiers?

Regards

Steve
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Old 25th November 2015, 11:39 PM   #2
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Perhaps the more understandable thermal spec is with 13x13 lands for each device leg. As you are using each diode in the bridge, then the total average power loss of all four die needs to be estimated, and the multi-die measurement spec used for comparison.

Staring with say 50C/W Rja, are you going to be well below the 150C Tj max at max ambient temp around the local pcb environment, and at max loading of the equipment?

You could use PSUD2 to determine rms current levels in each diode, and use forward voltage curves to estimate an average on voltage, given that instantaneous pulse power is high, but will average out in the thermal mass and thermal resistance.

Once you have an estimate for Tj max, then you can assess how your pcb compares with the 13x13 land scenario. Perhaps you are using thermal vias to move heat to the other side of the pcb, or have other ways for heat to transfer away.
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Old 26th November 2015, 06:34 PM   #3
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Hi. I did a rough 'n ready sim in LTspice. I just used 4 1N4148 diodes to simulate the bridge (0.3A continuous current rating but built in LTspice models). The trafo I will be using has a secondary current rating of just 167mA per secondary. (Each secondary will have its own bridge.) The real load won't get close to this but for the sake of investigation I loaded the sim circuit (typical LM317 reg circuit set for 12V) with a resistive load of just under 80mA equating to just under 200mA RMS at the secondary. Power dissipation for each bridge diode is about 46mW - let's say 200mW for 4. Vf for the MDB6S is lower than for the 1N4148 and so this is even more conservative.

I think that implies being under the Tj 150C limit for even relatively high ambient temperatures. Even more so with more realistic load assumptions.

But then I found I had board space, chickened out and slotted in a bunch of 1N4007 in DO41.
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Old 27th November 2015, 08:58 AM   #4
AndrewT is offline AndrewT  Scotland
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As the Tj rises the Vf falls.
So the power dissipated in the diode rises less fast than the increase in current would make you believe.

1A bridge, or 1A diodes, would do equally well for a 80mA supply.
300mA continuous is a lot more and then the diodes will be running hotter.
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Old 27th November 2015, 12:18 PM   #5
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Indeed. At 125C Vf of the MDB6S falls to about 0.8V at 0.8A and closer to 0.7V at 0.2A. (I assume forward current in these charts is RMS.)

Maybe I shouldn't chicken out. There will be fat traces connected to the pins of the MDB6S (76mil), a good sized PCB and I could always add a few vias to GND underneath the device to help with cooling also? Also, any final post reg load will be lower than 80mA (which would place a demand on the transformer secondary which exceeds its rating). Less than half that would be a better assumption.
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Old 27th November 2015, 01:14 PM   #6
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Fairchild have a Spice model for the MDBS10SV which is in the same class of devices. It has a higher current rating and is less well optimised for Vf. I dropped this into my quick 'n dirty LM317 reg model. I then loaded the reg output to achieve approx 170mA AC current at the input to the bridge (slightly higher than the transformer secondary rating). Modelled dissipation of the MDB10SV is 96mW. Taking the dual dice junction to ambient spec of the MDB6S of 250C/W means a conservative - I think - 25C rise above ambient with nothing special done to the board. I think I will give them a go.
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Old 27th November 2015, 05:14 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by SGK View Post
Indeed. At 125C Vf of the MDB6S falls to about 0.8V at 0.8A and closer to 0.7V at 0.2A. (I assume forward current in these charts is RMS.)

Maybe I shouldn't chicken out. There will be fat traces connected to the pins of the MDB6S (76mil), a good sized PCB and I could always add a few vias to GND underneath the device to help with cooling also? Also, any final post reg load will be lower than 80mA (which would place a demand on the transformer secondary which exceeds its rating). Less than half that would be a better assumption.
the current is continuous DC.
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Old 28th November 2015, 01:15 PM   #8
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Here's what I have done at this point. What do you guys think? Good, stupid, dangerous, can be improved?

I used fat traces into / out of the two MDB6S. BR1 sits above the (masked) ground plane on the bottom of the board (blue) and I placed a few via underneath the MD6S to allow heat to flow to the bottom layer more readily. Underneath the second MDB6s I placed an unmasked solder pad, and again placed some vias underneath the MDB6S. I am getting overlap DRC errors for this latter setup but surely this is okay.

Thoughts? Any guidance welcomed!
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Last edited by SGK; 28th November 2015 at 01:24 PM.
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Old 28th November 2015, 03:34 PM   #9
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Mind the creepage distance from Power Line!

You have plenty of free space. Secondary snubbers can be SMD. But I would use a much bigger R7, R8 (in case of an overload on output), or protect them with parallel diodes.
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Old 28th November 2015, 05:43 PM   #10
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Hi Pafi. Thanks for the response.

I will move the trace going from the secondary to C11 to the right. The distance between the right fuse pad and the pads at the top of C11 and C12 is just over 200mil. Not enough? I understood the requirement is 120mil for products protected by earthing (Class I; 230V).

I was wondering about R5/R6 and R7/R8. Will change R5/R6 to SMD - 0.25W resistors ok? - and move C11, C12 and BR1 down.

What would you suggest for R7/8? In normal use (up to and beyond the rating of the transformer secondary) the dissipation is tiny but I take your point re overload although I fear the transformer would be in dire shape in such a situation.

Any views on how I have attempted to keep BR1 and BR2 cool?
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