Using MOSFETin as a Key

In this article we will see the Use of MOSFETin as a Key. MOSFET with N-channel Development mode works using positive input voltage. Due to the very high gate resistance, we have seen that we can safely parallelize many different MOSFET until we reach the existing carrying capacity we need.

Connecting various MOSFeTs in parallel can allow us to change high currents or high voltage loads, while doing so becomes expensive and practical in both components and circuit board space. To overcome this problem, effective Transistors or power FETs have been developed that take power.

Now we know that there are two main differences between field-effective transistors, exhaustion mode for fets only, and both development mode and exhaustion mode for Mosfets. In this tutorial, we will look at using the development mode MOSFET as a key because these transistors require a positive gate voltage to "turn it on" and zero voltage to "turn it off".

The operation of the development mode MOSFET or enhancement-MOSFET can be best defined using the curves shown below. When the input voltage (VIN) of the transistor's gate is zero, MOSFET transmits almost no current, and the output voltage (VOUT) is equal to the VDD supply voltage. So MOSFET" cutting is working "in the zone" off".

MOSFET Properties Curves

Using MOSFETin as a Key
MOSFET Properties Curves

The minimum open state gate voltage required to ensure that MOSFET remains "on" when carrying the selected drain current can be determined from the V-I transfer curves above. When the VIN is high or equal to vdd, the MOSFET Q point moves to point A along the load line.

Drain current ID increases to its maximum value due to a decrease in channel resistance. ID becomes a VDD-independent constant and depends solely on VGS. Therefore, the transistor acts as a closed switch, the groove opening resistance does not fall completely to zero due to the RDS(on) value, but it becomes too small.

Similarly, when the VIN drops to low or zero, the MOSFET Q point moves from point A to point B along the load line. The channel resistance is very high, so the transistor acts as an open circuit and the current does not flow along the channel. Therefore, if mosfet's gate voltage switches between two values, high and low MOSFET acts as a "single-pole single shot" (SPST) solid state switch, and this action is defined as follows:

Cutting Zone

Zero input gate voltage (VIN), zero discharge current ID and output voltage are VDS = VDD. Therefore, for a development type MOSFET, the conductive channel is closed and the device is "turned off".

Cutting Properties

Using MOSFETin as a Key
Cutting properties

We can then define the cutting zone or "off mode" when using an enhancement-MOSFET as a switch, gate voltage, VGS < vth, böylece ID = 0. For MOSFET, a P-channel development should be more positive than gate potential source.

Saturation Zone

In saturation or linear region, the transistor will be biased so that the maximum gate voltage is applied to the device. This causes RDS channel resistance (as small as possible with the maximum drain current flowing through the MOSFET switch). Therefore, the conductive channel for the development type MOSFET is open and the device is in an "open" state.

Saturation Properties

Using MOSFETin as a Key
Saturation Properties

VGS > vth so ID = maximum. For MOSFET, a P-channel development should be more negative than gate potential source.

When using Mosfet as a switch, we can drive MOSFET to "open" faster or slower, or to pass high or low currents. " And the "OFF" device allows standard bipolar joint transistors to be used as a very effective switch with a much faster transition speed.

Example of Using Mosfet as a Key

Using MOSFETin as a Key
Example of Using Mosfet as a Key

In this circuit layout, a development mode N-channel MOSFET is used to replace a simple lamp "on" and "off" (which can also be an LED).

Gate input voltage vgs are taken to an appropriate positive voltage level to turn on the device, and therefore the lamp load is at zero voltage level, which translates to "on", (vgs = +and ) or the device "off", (vgs = 0V).

If the resistant load of the lamp is to be replaced by an inductive load such as coil, solenoid or relay, a "flywheel diode" will be required in parallel with the load to protect MOSFET from any self-generated back emf.

Above is a very simple circuit to replace a resistant load, such as a lamp or LED. However, when using power Mosfets to replace inductive or capacitive loads, some kind of protection is required to prevent damage to the MOSFET Device. The driving of an inductive load has the opposite effect from the drive of a capacitive load.

For example, a capacitor without an electric charge is a short circuit, which results in a high "sudden" current, and when we remove the voltage from an inductive load, we have a large accumulation of reverse voltage when the magnetic field collapses, resulting in an induced back impurity in the bandages of the inductor.

Then we can summarize the switching properties of both N-channel and P-channel type MOSFET in the table below.

Using MOSFETin as a Key

Note that the gate terminal is caused by the flow of transmission holes through the p-channel MOSFET, unlike the N-channel MOSFET, which must be made more positive (attracting electrons) than the source to allow the current to flow through the channel. That is, the gate terminal of the P-channel mosfet must be made more negative than the source and will only stop the conductivity (cutting) until it is more positive than the gate source.

Therefore, in order for the development Type Power MOSFET to work as an analog switching device, it is necessary to switch between the "cutting zone". Where: VGS = 0V (or vgs = -and) and "Saturation Zone": vgs(on) = +and. The power deployed in MOSFET (PD) depends on the current flowing from the channel ID at saturation, as well as the "open resistance" of the channel, which is supplied as RDS (on).

Power MOSFET Engine control

Due to the extremely high input or gate resistance that MOSFET has, very fast switching speeds and ease of drive make them ideal for interface with op-amps or standard logic gates. However, care should be taken to ensure that the gate source input voltage is selected correctly, since when using the mosfet as a switch, the device must achieve a low RDS(ten) channel resistance in proportion to this input gate voltage.

Low threshold Type Power Mosfets may not "open" until a minimum of 3V or 4V is applied to their gate, and if exiting the logic gate is only +5V logic, it may be insufficient to fully drive the mosfet to saturation. It is possible to use lower threshold Mosfets designed to interface with TTL and CMOS logic gates with thresholds from 1.5 V to 2.0 v.

Power Mosfets can be used to control the movement of DC motors or brushless stepper motors directly from computer logic or using pulse width modulation (PWM) type controllers. Since a DC motor offers high starting torque and is also proportional to the fixture current, mosfet switches together with a PWM can be used as a very good speed controller to keep the engine running smoothly and quietly.

Simple Power MOSFET Motor Controller Circuit

Using MOSFETin as a Key
Simple Power MOSFET Motor Control Circuit

Since the engine load is inductive, a simple flywheel diode is connected along the inductive load to distribute any back impediction produced by the engine when the MOSFET is "turned off". A compression network serialized by zener diode with diode can also be used to provide faster switching and better control of peak reverse voltage and fall time.

For greater safety, when inductive loads such as motors, relays, solenoids, etc. are used, an additional silicone or zener diode D1 can also be placed in the channel of an MOSFET switch to provide extra protection to the MOSFET switch to suppress overvoltage switching transitions and noise. If necessary. Resistance RGS is used as a tensile resistance to help reduce the TTL output voltage to 0V when MOSFET is "off".

P-channel MOSFET Switch

So far, we've looked at the N-channel MOSFET as a key where the mosfet is placed between the load and the soil. This also allows the MOSFET gate drive or switching signal (low side pass) to be referenced to the location.

Using MOSFETin as a Key

However, in some applications, we require the use of mosfet, the P-channel development mode, where the load is directly connected to the soil. In this case, the MOSFET switch is connected between the load and the positive feed rail (high-side switching), as with PNP transistors.

In a P-channel device, the flow of the traditional drain current is negative. Therefore, a negative gate source voltage is applied to "turn on" the transistor.

This is obtained because the p-channel MOSFET is "upside down" with the source terminal connected to the positive feed +VDD. Then, when the key is low, MOSFET "turns on" and MOSFET "turns off" when the key is high.

This upside-down connection of a p-channel development mode MOSFET switch allows us to serially connect with an N-channel development mode MOSFET to produce a complementary or CMOS switching device, as shown in a pair of feeds.

Complementary MOSFET Motor Controller

Using MOSFETin as a Key

The two MOSFET are configured to produce a double-feed duplex switch with the motor connected between the common drain connection and the soil reference. When input is low, the p-channel MOSFET is opened. Because the gate -source connection is biased negatively. So the engine rotates in one direction. Only positive +VDD feed rail is used to start the engine.

When the input is high, the p-channel device turns off and the n-channel device is turned on. Because the gate source connection is positively biased. The engine now rotates in the opposite direction because the motor terminal voltage is now reversed because it is fed by the negative-VDD feed rail. Then the p-channel MOSFET is used to change the positive feed to the motor (high side switching) for forward direction, while the n-channel MOSFET is used to change the negative feed to the motor (low side switching) for the opposite direction.

There are several configurations for driving the two Mosfet with many different applications. Both P-channel and N-channel devices can be driven by a single gate drive IC, as shown.

However, to prevent cross-transmission with both MOSFET, which are simultaneously conductive throughout the two polarities of the double feed, fast switching devices need to provide some time difference between "off" and the other "on". One way to overcome this problem is to drive both MOSFET gates separately. This then produces a third "stop" option to the engine when both MOSFET are "off".