Keep in mind that varying the current through the load in a controlled manner is the primary function (the raison d'être, if you please) of any power semiconductor device. But that approach could lead us away from the real point, which is how the device is controlled to vary the load current. PART 1 - Gate or base / Channel or junctionĬhannels or junctions? How many? What type? These and other aspects of the internal device geometry and construction might be one way of looking at power semiconductors, as they are indeed different for the different types of solid-state power devices. You will need to clearly define your design criteria and approach first, then you can begin to evaluate the advantages and trade-offs of the various available power semiconductors. Finall, the operating frequency is important as well. The load power modulation technique will also be important and that could be linear, switching control or static. For example, the area of your application that could be motor control, power supply or maybe audio amplifiers and that will influence your choice. The answer it's valid, nonetheless, as the selection is truly dependent on a very wide variety of factors and aspects for the project you want to design. The answer is not straightforward, I would say: “ It depends.” And yes, taht's not a very informative or satisfying response but let me explain. As a designer you will have to determine whether to use BJTs or MOSFETs in your application power stage? Or should the designer use IGBTs? Would they work in the design? Would they be better? So there are several choices, but which is the best? If you short an output pin at 5V it will likely dissipate 600mW in the output transistor (10 times abs max), probably raising its temperature at something like a few thousand deg C per second - that rate of heating will cause mechanical stress even if only for a few milliseconds - mechanical stress can lead to failure too.If you always wondered which power transistor you should use for your circuits, in this article we will see all the main differences between the IGBT, the BJT and of course, the MOSFFET. ![]() If the manufacturer says the abs max limit is 40mA, stick to that limit if you want reliable operation (unless you are prepared to run your own series of long-term reliability tests to further characterise the chip's response to current pulses!) That's a lot of power for one CMOS FET (it'll be quite a large FET compared to most on the die of course, but it'll still be microscopic), and exceeding the current limit will cause thermal stress, or thermal damage, or in extreme localised melting of the chip and interconnect. That's about 65mW and 90mW dissipated in the output transistor at the abs max current of 40mA (at 5V and 3V respectively. I recommend a gate resistor of atleast 100 ohms since the gate capacitance could potentially damage the pins.įrom the datasheet the output transistors (they are FETs, not gates) on-resistance is about 40 ohms with a 5V supply about 60 ohms with a 3V supply. Each draws 2A at 12v (24 watts per color) and the transistors, even without heat sinks, never even get warm. The FET is connected so as to be "switching ground" to string of LEDS, one each Red Green and Blue. So I'll likely continue to do it the wrong way per usualįor example, I have three of these, with their gate terminal directly connected to a PWM pin, so I can PWM fade via the FETs. Even using as a stop-motion photo strobe, I have not seen any turnoff delays caused by gate charge being held too long. As for the gate to source resistor, I've read it is unnecessary in most microcontroller use because there is enough of a discharge path internally to handle the gate charge dissipation. I know that there is some merit to a gate resistor to limit the current charging the gate, but I just don't see that small a capacitance needing it. ![]() I'm using these straight from the pin with no gate resistance, and also no gate to source resistor, as the microcontroller provides a discharge path for the gate capacitance. ![]() Though MANY times cooler than a BJT or Darlington. As I understand it, it's not actually fully 'ON' until somewhere around 10v under a heavy load, so it provides a bit more resistance at 5v and therefore runs somewhat hotter. ![]() Yes, that's the gate switching threshhold.
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