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Topic: darlington/mosfet question (Read 5934 times) previous topic - next topic

magnethead794

Jun 21, 2011, 08:02 am Last Edit: Jun 21, 2011, 08:21 am by magnethead794 Reason: 1
How exactly would I use a darlington to power a circuit using an arduino, and what are (besides the obvious) usage differences between a mosfet and a darlington? If I were using an arduino to switch a 12VDC @ 10 amp load at a high frequency (up to 150 times per second), which would I want to use? And how do you heat-sink a darlington?

Found this (MOSFET) but still would like an answer to question anyways. http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1273592301/all

Also found this (which is almost idential to what I'm looking at) which pretty much secures use of MOSFET. http://forums.parallax.com/showthread.php?71139-Darlington-Transistor-vs.-MOSFET



And it's settled. Amazing what Google can do.

www.onsemi.com/site/pdf/PSDE_0411.pdf

Quote
The role of power MOSFETs in modern fuel delivery systems
Power MOSFETs are normally used in fuel delivery to switch the injector on/off per commands from the ECU (Engine Control Unit). One MOSFET will be employed for each injector. As fuel delivery systems have evolved, so have the MOSFET's requirements. This has meant that semiconductor companies have been forced to develop new generations of devices that have enhanced performance in a number of key metrics.
1.   Operating voltage - some MOSFET parameters for direct-injection will be notably higher than for standard injection systems. Direct- injection systems generally require higher pull-in current (in order to get the solenoid operating against high cylinder pressure). This high current must happen quickly (due to limited time available to inject fuel in the engine cycle, which decreases as the RPM rises), therefore a higher voltage rail is required. Direct- injection systems can have rail
voltages >100 V (as opposed to around 50 V for standard injection systems). Thus higher voltage MOSFETs need to be specified, along with voltage boost power supply circuits. Piezoelectric injectors also require high voltages to be activated, typically 200 V or above.

westfw

A darlington has a semiconductor-issue enforced voltage drop across the switch of a volt or two.  That means that its power dissipation will be directly proportional to the switched current.  10 to 20W for a 10A load.  (which is pretty high.)  In general, you can't improve the power dissipation by using a better darlington; you only make it easier to dissipate the power (ie by making it easier to attach to a heatsink.)

A power MOSFET acts like a low-value resistance, and so has a power dissipation proportional to I2, but better MOSFETs have  lower Rds-on  A MOSFET with a 50 milliohm resistance (which is NOT "state of the art) will only be dissipating 5W when switching a 10A load.

150 times a second is not considered "high speed" for either technology.

Many MOSFETs require about 10V to turn on fully, making them somewhat more difficult to connect to a modern microcontroller.  While a darlington will have no problems even being switched by 3V devices.

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