![]() ![]() Then we can summarise the switching characteristics of both the N-channel and P-channel type MOSFET within the following table. Driving an inductive load has the opposite effect from driving a capacitive load.įor example, a capacitor without an electrical charge is a short circuit, resulting in a high “inrush” of current and when we remove the voltage from an inductive load we have a large reverse voltage build up as the magnetic field collapses, resulting in an induced back-emf in the windings of the inductor. But when using power MOSFETs to switch either inductive or capacitive loads some form of protection is required to prevent the MOSFET device from becoming damaged. If the resistive load of the lamp was to be replaced by an inductive load such as a coil, solenoid or relay a “flywheel diode” would be required in parallel with the load to protect the MOSFET from any self generated back-emf.Ībove shows a very simple circuit for switching a resistive load such as a lamp or LED. The gate input voltage V GS is taken to an appropriate positive voltage level to turn the device and therefore the lamp load either “ON”, ( V GS = +ve ) or at a zero voltage level that turns the device “OFF”, ( V GS = 0V ). In this circuit arrangement an Enhancement-mode N-channel MOSFET is being used to switch a simple lamp “ON” and “OFF” (could also be an LED). Therefore for an enhancement type MOSFET the conductive channel is closed and the device is switched “OFF”. Here the operating conditions of the transistor are zero input gate voltage ( V IN ), zero drain current I D and output voltage V DS = V DD. So if the gate voltage of the MOSFET toggles between two values, HIGH and LOW the MOSFET will behave as a “single-pole single-throw” (SPST) solid state switch and this action is defined as: 1. Cut-off Region The channel resistance is very high so the transistor acts like an open circuit and no current flows through the channel. Likewise, when V IN is LOW or reduced to zero, the MOSFET Q-point moves from point A to point B along the load line. Therefore, the transistor behaves like a closed switch but the channel ON-resistance does not reduce fully to zero due to its R DS(on) value, but gets very small. I D becomes a constant value independent of V DD, and is dependent only on V GS. The drain current I D increases to its maximum value due to a reduction in the channel resistance. When V IN is HIGH or equal to V DD, the MOSFET Q-point moves to point A along the load line. The minimum ON-state gate voltage required to ensure that the MOSFET remains “ON” when carrying the selected drain current can be determined from the V-I transfer curves above. So the MOSFET is “OFF” operating within its “cut-off” region. When the input voltage, ( V IN ) to the gate of the transistor is zero, the MOSFET conducts virtually no current and the output voltage ( V OUT ) is equal to the supply voltage V DD. The operation of the enhancement-mode MOSFET, or e-MOSFET, can best be described using its I-V characteristics curves shown below. In this tutorial we will look at using the Enhancement-mode MOSFET as a Switch as these transistors require a positive gate voltage to turn “ON” and a zero voltage to turn “OFF” making them easily understood as switches and also easy to interface with logic gates. We now know that there are two main differences between field effect transistors, depletion-mode only for JFET’s and both enhancement-mode and depletion-mode for MOSFETs. To overcome this problem Power Field Effect Transistors or Power FET’s where developed.V While connecting together various MOSFETS in parallel may enable us to switch high currents or high voltage loads, doing so becomes expensive and impractical in both components and circuit board space. ![]() We also saw that due to this very high input (Gate) resistance we can safely parallel together many different MOSFETS until we achieve the current handling capacity that we required. We saw previously, that the N-channel, Enhancement-mode MOSFET (e-MOSFET) operates using a positive input voltage and has an extremely high input resistance (almost infinite) making it possible to interface with nearly any logic gate or driver capable of producing a positive output. ![]()
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