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Track widths, current, and maximum temperature rise.

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tracidfish

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Hi

I am designing and building a pwm motor controller.

I am desigining it to run a DC motor with a maximum current of 8 amps and 12 Volts DC supply.

I am using surface mount components and using home etched PCB's.

I am now at the stage of designing the PCB layout. I have been investigating the current caryying capacity of the copper clad board and the track widths needed.

I have found an equation that calculates the minimum track width but this depends on the maximum temperature rise that I allow.

What is a suitable temperature rise?


I will most liley be using FR4 board, with a copper thickness of (1oz) 34uM.
For example if I allow a rise in temperature pf 20 degress C the width is 3.4mm or 136.7 mil
10 degress the track width is 5.22mm or 208 mil.

I will be using mosfet and diodes in this circuit all of wich will have heat sinks. The connection from the board to the motor will be via a cable connected to the pcb by terminal blocks.

I presume it is best practice to have the track between the mosfet and the terminal block as short as possible? Is there any good practice measures I should be aware of here, can the fets and diodes be too close to the terminals or can i come straight from the mosfet pins to the pcb terminals then leaving the board?


Thank you.
 
One thing you need to keep in mind is that your calculation will only tell you the temperature rise from current. You need to add to that the heat that will come from the components themselves. Even if the components have heatsinks, there will still be some heat flowing into the PCB through their leads, as well as conducted and radiated from the bodies/heatsinks.

To answer your question though, you want to use all of the available space for copper when you have power currents flowing and components that generate heat.
 
I remove the silkscreen from high current traces so the traces will have solder on them. This helps lower the resistance a little and helps get the heat into the air better. I learned this trick when we made PC power supplies where the copper thickness is very thin to save money.

I just finished a project with 100A current on the PCB. The 100 amps runs on both sides of the board to cut the resistance in half. I almost went 4 layer to get more copper. I use 'vias' in many places to connect the two layers together. 2oz copper also helps.
 
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