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  1. #21
    Eddy's Avatar
    Location unknown. Last Activity: 15-05-2011 Has been a member for 7-8 years. Has a total post count of 3.
    Quote Originally Posted by irving2008 View Post
    Eddy, I didnt miss the point - if its a motor controller is not dissipating 60w continuously, as its almost certainly a PWM arrangement. Its only dissipating that wattage during the 'on' cycle at a high frequency. take for example a common controller chip from ST. Its capable of 30A without a heatsink... but thats because the switching frequency is 10khz. for a single pulse at that frequency the thermal characteristic is <0.1degC/W so a heatsink of only 16sq cm of copper on the PCB will suffice to keep the mean junction temperature below 125degC. You could never run a linear circuit at that sort of load. The actual static chip thermal resistance is typical at 15decC/W... at 60W the jiunction temp would be 15*60deg over ambient = 900deg... it would have long since melted!

    For the record I have designed systems for both miltary (-55 -> +125degC) and space applications (-100 -> +250degC)!
    I assume you are referring to the VNH3SP30. This chip cannot handle 30A without a heatsink. It's thermal protection kicks in when more than 8A continuous is output using the small PCB footprint. It has a PWM input capable of 10K but it is supplied externally, fully under the control of the user. There is no internal PWM capability. This IC switches high and low side power MOSFETs as instructed by the inputs and PWM may be used if you chose to use it.

    Again, my point is that airflow over the chip is much more efficient at dissipating heat than a hunk of metal.

    Eddy

  2. Quote Originally Posted by Eddy View Post
    I assume you are referring to the VNH3SP30. This chip cannot handle 30A without a heatsink. It's thermal protection kicks in when more than 8A continuous is output using the small PCB footprint. It has a PWM input capable of 10K but it is supplied externally, fully under the control of the user. There is no internal PWM capability. This IC switches high and low side power MOSFETs as instructed by the inputs and PWM may be used if you chose to use it.

    Again, my point is that airflow over the chip is much more efficient at dissipating heat than a hunk of metal.

    Eddy
    Eddy, you are right in respect of the specific implementation as done by Pololu, but thats because they didn't provide the board area that ST recommend for full current operation - they say it needs a heatsink, ST's application note say otherwise if the PCB layout is right.

    However, I am not disagreeing with you that airflow is a good dissipator of heat. The effect is still governed by surface area and all the airflow does is effectively increase that surface area. By way of example a typical 20mm x 20mm device fully exposed to a ducted 5m/s airflow (120cu ft/min 100mm dia fan) with no interruptions in the airflow is roughly equivalent to the device being fitted with a 32degC/W heatsink - i.e. better than no heatsink by a factor of 3 or 4, but airflow alone is not the answer to good cooling - good thermal management is the effective use of both properly designed heatsinks (not just hunks of metal) and properly directed airflow.

    We are getting off topic here. The original issue was how to provide a variable voltage, high current, regulated supply. A solution based on linear devices could be made to work but the cooling becomes problematic leading to reduced expectations of what can realistically be achieved. I still contend that the right answer is not to generate the heat in the first place - a switch-mode low loss solution is the right approach to this requirement and need not be significantly more expensive or difficult to make.

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