The maximum voltage across the device Vds is 60V, so our 12V supply will not be an issue. A quick look at the specs shows some key parameters. We will use a FET to do the heavy lifting. Since the micro has a maximum sink and source limits of 25ma, we fall short on the current side as well. Our 5V micro cannot directly switch 12V without risk of damage. The resistance of the relay coil is 360 ohms. The problem is to control a 12V from the microcontroller output pin. We will revisit that problem but this time use a FET as the switch. In section 7 we used an example of a bipolar transistor to switch a relay. Once the capacitor is charged, no further current will flow until the state of the micro’s output pin changes. The only current that flows is the amount needed to charge or discharge this capacitance. The gate, the FET control pin, looks like a small capacitor between the gate and source pins. The pin must drive current through this diode. To the microcontroller output pin, the BJT base pin looks like a diode. Normally we will not want the FET to be in the linear range in switching applications. If Vgs, sometimes referred to as just the gate voltage, is in between these limits the resistance will be somewhere between low and high. The data sheet will refer to this value as Rds (Resistance drain-source). If Vgs is above a certain level, the resistance will be very low (a few ohms or less). If the voltage is zero volts, the resistance will be very high (several million ohms), and essentially be an open circuit. The value of the resistor will depend on the voltage between the gate and source (Vgs). The resistor is between the source and drain pins. The best way to look at an FET is as a voltage controlled variable resistor. Unless otherwise noted, whenever the term FET is used, it will refer to an N channel, enhancement mode MOSFET. These are the most commonly used FET in microcontroller based circuits. We will limit the discussion to N channel, enhancement mode MOSFET. The most common type of FET in switching circuits is the MOSFET (Metal Oxide Semiconductor Field Effect Transistor). Then there are enhancement mode and depletion mode variations. The gate is where the controlling voltage is applied. Essentially the collector current of a BJT is the base current multiplied by the gain of the transistor. These transistors are known as current controlled devices. Part 7 described the operation of bipolar junction transistors (BJT). This section shows how to use a different type of transistor, the Field Effect Transistor (FET) which can have advantages in some circuits. Actual measurement examples are shown below.Previous sections showed how to use bipolar transistors to switch loads with higher currents and/or voltages than can be handled by the microcontroller output pin directly. These switching times tend to increase slightly as the temperature rises, but since a temperature increase of 100☌ results in a switching time increase of about 10%, the temperature dependence can be thought of as almost nil. Temperature Characteristic of the Switching Characteristic For example, the turn-on time may be interpreted as “the time from when V GS has risen to 10%, until the MOSFET is 10% turned on”. Here, the expressions “rise/fall” of V DS may seem reversed when thinking of a waveform, but the output is inverted, and so these expressions are used. >Fall time: Time from 90% to 10% of the fall of V DS >Turn-off delay time: Time from 90% of the fall of V GS until 90% of the fall of V DS >Rise time: Time from 10% to 90% of the rise of V DS >Turn-on delay time: Time from 10% of the rise of V GS until 10% of the rise of V DS In addition, there are time-related parameters as well these parameters may stipulate the time from one event to another. The suggested measurement circuit adopts the stipulated conditions. In general, as in the example above, the conditions for V DD, V GS, I D, R L, and R G are indicated on the datasheet. Hence, reference measurement conditions are stipulated and a measurement circuit is suggested. These parameters related to switching are greatly affected by the signal source impedance and drain load resistance R L of the measurement circuit.
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