The maximum output current of a PICAXE is 20mA. So, it can be seen that hfe is also a significant function of the load current for that particular device. For simple switching applications, you must always use the minimum number quoted in the data sheet.įor a TIP36C, the minimum hfe is listed as 10 for a 15A load and 25 for a 1.5A load. Unfortunatley, even for transistors with the same type number, the hfe value can vary considerably from device to another. This is an important number to know and understand when specifying a bipolar transistor. The ratio of base current to possible collector current is known as "hfe". In the case of the bipolar, it requires higher base current. In the case of the FET, that means larger capacitance. The problem is that higher currents require larger junction sizes. This means that switch on/off times are much faster for the same junction size. However, the bipolar transistor does not have (or at least, very little) base capacitance. This voltage is quite small but when compared to a FET of similar junction size, the volt-drop for the same conducted current will be higher and hence under static conditions, the heat disspiated will be higher than for a FET of the same junction size. The gate has capacitance which means that unless whatever drives it has the ability to both sink and source quite high currents (for a brief moment), the FET will spend a long time in transistion when it opperates linearly and hence will dissipate a large amount of heat.Ī significant advantage of FETs is that they have a low resistance when on and hence dissipate very little power when driven correctly.Ī bipolar transistor requires a certain amount of voltage across it's junction to actually work. In the case of a non-logic level device, the problem of driving it is obvious voltage wise, but there is another issue. What that means is that a logic level FET is designed to be almost fully on at around 4v whilst a non-logic level device will typically require 10v or more to turn it on fully. Logic level switched and non-logic level. ![]() With FETs for example, there are two main categories. How important these differences are comes down largely to which particular device you choose. Not so much pro's and con's but a different way of driving and a different response to different drive methods.Įssentially, a FET requires a voltage on the gate to turn it on whilst a bipolar requires a current into the base to turn it on.
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