calculating pcb trace width for low duty cycle

Discussion in 'General Electronics Chat' started by Gibson486, Jan 10, 2013.

  1. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    I have PWM pulses at about 4A. However, the pulse is only about 15 micro seconds long, so it is a really low duty cycle. When i calculate the trace width on the PCB, do I still need to calculate it as if it were 4 amps?
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    This doesn´t say anything about the duty cycle, it could be on for 15μs and off for 1μs. You need to state the repetition rate or length of the other half of the pulse.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    Not necessarily. What is the duty cycle? You said the pulse if 15μs but you didn't state the pulse period or frequency.

    At low duty-cycle the concern would be the voltage drop due to the 4A of current, not trace heating from the current (which is the concern for DC current).
     
    Gibson486 likes this.
  4. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    Duty cycle is pretty low... like 5%. I need to measure the actually frequency, though. I think it is 90 Hz, but i may be wrong. I need to check my notes at home.
     
  5. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    The limiting factor is temperature. This is where the duty cycle is important. Gives the track time to cool down before the next pulse.

    But there are other things: Voltage drop, not duty-cycle dependant. Inductance, important with fast pulses.

    Best to have the track as thick and short as possible. Even if it has to narrow at one section, keep the rest as wide as possible.
     
    Gibson486 likes this.
  6. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    Well, I was off on the frequency... it is actually approx 750-800 hz.

    Duty cycle is high for about 15us low for about 1.25ms (1250 us).

    On the supply side, it says current is about 2mA being drawn. not sure if that can be an accurate way of measuring the avg current over time.
     
  7. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    I am really constrained space wise. Could I have really thick traces coming from the connectors, but shrink them the remainder of the way? Or do i have no choice but to make the traces wide enough and have more than a 2 layer board?
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    You can use a thinner trace and solder a wire over it. Really depends how long it is etc.
     
    Gibson486 likes this.
  9. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    The average current is about 50mA. Hence planning for 500mA should be ok.
     
    Gibson486 likes this.
  10. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    Thanks, so when I put this in the trace width calculator, can I amply adjust current to my duty cycle times pulsed current to get avg current? Does that suffice in terms of current calculation?
     
  11. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Like others said, the thermal capacity of the trace and pcb will average the current in terms of carrying capability, but the question is if your circuit can tolerate the higher resistance of a thin trace and thus the voltage drop at the hight current.
     
  12. JMac3108

    Active Member

    Aug 16, 2010
    349
    66
    As others have said ...

    (1) Sizing the traces for average current will ensure that the traces do not get too hot.

    (2) However, in a case like yours where the average current is low, but the peak current is high, traces sized simply for average current may be too small. The result will be a lot of voltage drop acsoss them when the peak current is being drawn. This may or may not be a problem in your circuit.

    As a starting place I would attempt to size the traces for my peak current. If other resitrictions prevented this I would reduce them, but not as small as the average current calculation would indicate.

    .... Kubeek said exactly what I said but more concise! ...
     
    Last edited: Jan 11, 2013
  13. vrainom

    Member

    Sep 8, 2011
    109
    19
    If you absolutely must make a thin trace then fill the trace with a little bit of solder to make it more robust. Or even solder some copper wire to the top of the trace.
     
  14. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Notice that I did not size the trace for average current. I used the average current to set my absolute bottom limit. Now we can look at trace widths:

    500mA 5mil
    1A 12mil
    2A 30mil
    4A 80mil

    Looking at these figures, I would not go lower than 20 mil anyway which is good for 1.4A, assuming 1oz Cu.
     
Loading...