Surface mount heat sinks

tracidfish · Feb 15, 2013

Hello

I am building a DC PWM board.

I am designing my own board on diptrace that I intend to etch myself. and would like to use surface mount components.

I need to use an 8 ampsurface mount diode ( https://www.electro-tech-online.com/custompdfs/2013/02/0900766b80af50a6.pdf)

I understand that i will need to use a heatsink but i do not now how to do this with surface mount devices.

Can any one explain how this is done?

Thanks.
Charlie

Joe G · Feb 15, 2013

What I did, Was when laying out my pcp. I added two through holes for a heatsink that would cover my LM317 regulators. after assembling the brd I mounted the heatsink over the tops of them w/ thermal paste and used 0-80 screws through the brd to retain them. I then added a cooling fan, both the fan and heat sink were scavanged from a computer mother brd. I also allowed for the recommended thermal pad requirements from the data sheet just to be safe. Hope that helps how I did it.

ronv · Feb 17, 2013

Usually the PCB is used as the heat sink, but the best you can do is about 1 square inch - bigger than that doesn't help much. If that is not enough a different package might be best. Give us the current the diode will actuall see.

https://www.electro-tech-online.com/custompdfs/2013/02/an-994.pdf

tracidfish · Feb 18, 2013

Hi

Thanks for the link.

I am trying to understand its contents.

I am using
moset IRLR3714Z
Diode 8EWF10S

and will be driving 8 amps, 12v load.

MrAl · Feb 18, 2013

tracidfish said:
Hello

I am building a DC PWM board.

I am designing my own board on diptrace that I intend to etch myself. and would like to use surface mount components.

I need to use an 8 ampsurface mount diode ( https://www.electro-tech-online.com/custompdfs/2013/02/0900766b80af50a6.pdf)

I understand that i will need to use a heatsink but i do not now how to do this with surface mount devices.

Can any one explain how this is done?

Thanks.
Charlie

Hi,

How did you determine that you need this somewhat special kind of diode?

The heatsink would normally be 1 square inch of clad on a FR-4 or similar board.

Note that the rating is not a straight 8 amps, it's only 8 amps at 50 percent duty cycle.

tracidfish · Feb 18, 2013

hi

I chose it as it was fast recovery, cheap and in surface mount. The application not says it uses are; Output rectification and freewheeling diode in inverters,
choppers and converters. The use for this one is as a free wheeling diode.

I did not notice the 8 amps at 50% duty cycle. Please could you show me one that is more suitable for use?

It needs to be at least 8 amp and surface mountable and suitable for 0 - 100% duty cycle.

You say the surface mount is normally 1 square inch of fr4. I think you may be wrong as the heat dissipated by this amount of
power will be too much for the 1 inch.

Most I have seen have a heat sink on them.

ronsimpson · Feb 18, 2013

AE10774TR
I have this type. It needs to connect to copper and the copper connect to the hot part.
7109DG
I like this one.

tracidfish · Feb 18, 2013

Thanks ron

ronsimpson · Feb 18, 2013

Those part numbers should get you into catalogs that will have more pictures

ronv · Feb 18, 2013

That diode has a high voltage drop so high power disapation. Take a look at something like this.

https://www.mouser.com/ds/2/427/vbt1045bp-85668.pdf

Are you driving a motor? Tell us more about its current and resistance.

RCinFLA · Feb 18, 2013

For the forward voltage drop at 8 amps from the spec you will generate over 9 watts of heat (assuming your requirement is 100% duty cycle). That is a very large heatsink. You would be better off finding a diode with less voltage drop at 8 amps, assuming it meets your switching speed.

ronsimpson · Feb 18, 2013

8EWF10S
That is a 1000 volt diode. It will have a very bad Vf. 1.3V
MBR1045
This diode is 45 V at 10A with a Vf that is much smaller. 0.47V

tracidfish · Feb 19, 2013

Hi

Yes it is driving a motor.

Unfortunatly I will not now the motor charactorisitcs as it will be used on different motors from different manufacturers.

All I can now is that they will all be 12V and 8 amps.

Thanks for your help everyone.

tracidfish · Feb 19, 2013

Okay so I have now learnt about the forward volt drop charactoristics of a diode.

I have also learnt how to work out the heat sink required. For others benefit here is a very good document. https://www.electro-tech-online.com/custompdfs/2013/02/diodes.pdf

Can someone explain to me what i need to consider with regards to switching speed and duty cycle?

Thanks

ronsimpson · Feb 19, 2013

what type of motor? DC with brush?

ronv · Feb 19, 2013

tracidfish said:
Can someone explain to me what i need to consider with regards to switching speed and duty cycle?

Click to expand...

Usually schottky diodes are used for low voltage applications like yours. In addition to having low voltage drop they are very fast so switching speed is not an issue. Sometimes if the FET you are using in your PWM is very fast you may need to worry a little about reverse recovery time. Again it is usually not a concern with schottky diodes.
The reason I ask about the motor was for duty cycle power rating on your diode. Since it is unknown we must assume that the spec is 8 amps maximum. This would mean that the motor resistance is 1.5 ohms and it would draw 8 amps at 12 volts if it weren't allowed to turn and maximum voltage was applied. If this is the case the average diode current will be 1/2 of the motor current. This is because for 50% of the time the current is thru the FET and then the other 50% thru the diode.

schmitt trigger · Feb 19, 2013

ronsimpson said:
AE10774TR
I have this type. It needs to connect to copper and the copper connect to the hot part.
7109DG
I like this one.

I've used similar heatsinks, and they are very good at removing heat from SMT devices.

So good, in fact, that they are almost impossible to solder with your soldering iron......it will suck all the heat out.
You'll have to use a hot-air gun to preheat the heatsink and surrounding pad area before soldering it.

tracidfish · Feb 19, 2013

They are self excited pumps.

So can I use reflow to solder the heatsinks?

Do I need some sort of compund between the diode junction and the copper board or can I use solder paste?

schmitt trigger · Feb 19, 2013

Yes, you can reflow the heatsinks with hot air. Do these first before you populate the rest of the board.

alec_t · Feb 19, 2013

All I can now is that they will all be 12V and 8 amps.

Is that running current or the much higher stall current?

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