However, because of the physics involved, the P-channel types cannot match the low on resistance of the N-channel type. MOSFETs come in two flavors: P-channel and N-channel. This switching time determines the efficiency of the system and will be examined in more detail shortly. However, the key point is that it takes some time to go from a very high resistance to a very low resistance. (Next time, we'll look at linear/non-switching applications.) For most practical purposes, the power MOSFET can be considered a switch: either it's on or it's off. Instead, they typically define the breakdown voltage as when 250 mA of current flows through the part. The resistance in the off state is so high that it is usually not stated in the datasheet. That's fairly expensive for a power MOSFET. The IRFB-4410 has these specifications and costs about $4.50. So, if the voltage is 100 volts at 10 amps, then 1,000 watts of power are switched with only one watt lost. Since many MOSFETs come in TO-220 packages, no heatsink is needed in this instance. Modern MOSFETs can have on resistances of less than 10 milliohms.Ī little math shows that this device can handle 10 amps with one watt converted into waste heat (power = current2 x resistance). When on, the part exhibits a low resistance without any non-linear forward voltage drop as seen in bipolar transistors. Typically, about eight volts will turn on the device completely. This is like the charge on one capacitor plate affecting the other plate.
Instead, the charge on the gate affects the conductivity between the drain and source. No significant DC current can flow from the gate to either the source or drain. This is because the gate is completely isolated from the rest of the device. They have three leads like a transistor but are voltage in, current out devices. Power MOSFETs (HEXFETs are International Rectifier's trademarked name for their products) are typically used in power switching applications and are classified as transistors. The basic cost for parts will be about $15 for each project. Over the next two issues, we'll examine these useful devices and show how they can be employed to build: 1) a simple, transformerless voltage doubler that can supply amps of current and 2) a 20 amp constant current source. Power MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) have a lot of nice features that seem to be overlooked.
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