How to Make a Lamp Chaser Using a Mosfet Circuit
Today, MOSFETs are everywhere. It is a component manufactured in industrial quantities like the others so it has become affordable, like its bipolar precursor. They are easily obtained, a part costs about 10 cents and sometimes barely a penny by buying lots online: $10 for 1000 parts, shipping included, it is close to what is called "electronic dust because if you drop one, it's almost not worth bending down to pick it up.
1. Draw me a MOSFET...
A MOSFET is a component which makes it possible to switch current, ie to let pass more or less electrons between two electrodes according to a third. It can be seen as a switch (like a relay) or a variable resistor.
The maximum switching current depends on the size of the case and other characteristics, provided by the manufacturer. The MOSFET reference is chosen according to the type of application, and there is a very wide variety of models. We will focus on low power applications, with TO92 or SOT23 packages being the cheapest.
The most common references in TO92 housing (with lugs, which can be planted in a solderless plate) are BS170 for the N channel version (equivalent to the NPN), and BS250 for the P channel (the complement of the N channel) . The 2N7000 is another common reference, similar to the BS170, but with a different pinout.
The equivalents in SOT23 (3mm surface mount package) packages are BSS138 (N) and BSS84 (P). At our level, the 2N7002 is more or less equivalent to the BSS138.
Larger and more powerful MOSFETs are available, capable of withstanding hundreds of volts and/or hundreds of amperes. At the opposite extreme, transistors of the CMOS family (miniaturized version of MOSFETs) today constitute the bulk of current computer circuits and a chip can integrate billions of them. Due to their tiny size they can only switch a ridiculously low current and can't handle more than 2V, but in exchange they are super fast.
2. The three-legged beast
A discrete MOSFET transistor, as purchased individually, has three pins, like its bipolar cousin:
The source (equivalent to the emitter of a bipolar transistor) is usually connected directly to the power source.
The drain (equivalent to the collector) is usually connected to the circuit to be switched.
The gate, which is the base, controls the resistance between the source and the drain.
We could therefore almost replace a bipolar by a MOSFET. However, unlike bipolar, no current flows through the gate of a MOSFET*, as it is a type of insulated-gate transistor.
* contrary to what page 90 of Hackable n°7 suggests
3. Anatomy of a MOSFET
The term MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor in English. Let's start by looking at it from the end.
Already this indicates that it is a transistor and it is actually a variable resistor. The word “transistor” is short for “Transfer Resistor” and a bipolar transistor behaves more like a current amplifier.
Field Effect means field effect. It is an electric field that controls the resistance of the component. Much like ancient vacuum tubes, MOSFETs are therefore controlled with a voltage, as opposed to bipolar transistors which are controlled by a current flowing through their base. They therefore have the advantage of consuming less energy and they can amplify with a much higher gain. They dissipate almost no energy when saturated, although it does take some energy to get there.
Metal Oxide Semiconductor represents the succession of layers that make up the transistor. A layer of metal (conductor) is deposited on a very thin layer of oxide (insulator) resting on a semiconductor (usually based on silica doped with impurities). In integrated circuits, the metal layer is now replaced by polycrystalline silicon, but the term Metal has remained.
The thinner the insulating layer, the more intense the electric field at the level of the semiconductor, and therefore the more sensitive the transistor will be and it will switch at a lower voltage. On the other hand, this makes it very sensitive to overvoltages, in particular to static electricity discharges. The insulation is fragile and it often happens that a MOSFET clicks for a yes or a no!
Their rubbing can generate incredible tension, especially in dry weather!
The enriched insulated-gate MOSFET (Metal Oxide Semiconductor Field Effect Transistor) transistor is used in power electronics to supply switching loads ([1],[2]). There are many applications: switching power supply, inverter, motor power supply, class D audio amplifier, etc.
This document shows how to operate a power MOSFET transistor with a PWM (pulse width modulation) control signal. We first consider a one-quadrant chopper made with a single MOSFET transistor, before addressing the case of the two-quadrant chopper, made with two transistors in half-bridge assembly.
These oscillations have the disadvantage of giving an overvoltage which can be very large if the switching is very rapid. In the present case, the tension reaches 18 V. There is also a propagation of these oscillations from one point to another of the circuit, possibly by hertzian way (electromagnetic disturbances). We see here their effect on the control voltage, but the disturbances are also visible on the supply voltages. Their frequency here is about 8 MHz.
3.d. Reduction of switching oscillations
The adjustments described in this paragraph must be made on the final printed circuit of the application, because the parasitic inductances and capacitances depend on the layout of the tracks.
The oscillations, which occur just after the switching of the transistors, are due to the parasitic capacitances and inductances present in the power mesh containing the load. The pseudoperiod of the oscillations is 120 ns. To estimate the parasitic capacitance, a capacitance Ca is added between the output (connected to the load) and the ground, so as to obtain a period twice as large. The required value is Ca=14 μF. Knowing that the natural frequency of an LC circuit is inversely proportional to the square root of C, the capacitance has been multiplied by 4, which means that the parasitic capacitance is one third of the added capacitance, i.e. Cp=4.7 nF .
2. One quadrant chopper
2.a. Study set-up
The following assembly makes it possible to study the switching of a MOSFET.
The load is a 6.8 Ω resistor (25 W power wirewound resistor) powered by a voltage-regulated laboratory power supply (12 V).
The signal generator delivers a square voltage (of cyclic ratio 1/2) taking the values 0 and 12 V. When the gate G is at 12 V with respect to the source S, the transistor is in the on state. The Drain-Source dipole is then equivalent to an RDSON resistance of the order of 1 Ω. When the gate-source voltage is lower than a threshold (generally of the order of 5 V), the transistor is off and the Drain-Source resistance is practically infinite.
The chopper thus produced is said to have one quadrant because the voltage across the terminals of the load is always positive and the current in the load is always positive (for a resistive load).
The resistor R is needed because the gate-source dipole is equivalent to a capacitance (on the order of several hundred pF). It limits the current when charging this capacitor. The current in R is obtained at the oscilloscope via a differential amplifier.
The tests below are made with an Infineon IPA50R500 MOSFET (the diode is incorporated). The maximum continuous drain current is IDmax=5.4 A, which is more than enough for the 6.8 Ω load supplied at 12 V. During the conduction phases of the transistor, the current in the load is 1.8 A The resistance RDSON=0.5 Ω for VGS=13 V. The gate-source voltage must not exceed 20 V. Here are the drain-source current-voltage curves for this transistor:
For a drain current of 5 A, a gate-source voltage of at least 7 V must be applied in order to be in the resistive zone. Our assembly applies a voltage of 12 V, which is more than sufficient.
In this case, the gate charging current is supplied by the two push-pull transistors. When the control voltage is at the low level (0 V or less), transistor T0 is off and the base of T1 and T2 is at around 12 V. Transistor T1 is on while T2 is off: the gate is charged by resistor R and quickly takes on a 12 V potential with respect to the source. Conversely, when the control voltage is at high level, the base of T1 and T2 is grounded: T1 is blocking while T2 is on and the gate discharges until it has zero potential with respect to the source. This assembly can provide a current in the gate resistor R up to 1 A, which may be necessary to quickly switch a high-power MOSFET transistor.
How to Make a Lamp Chaser Using a Mosfet Circuit
Today, MOSFETs are everywhere. It is a component manufactured in industrial quantities like the others so it has become affordable, like its bipolar precursor. They are easily obtained, a part costs about 10 cents and sometimes barely a penny by buying lots online: $10 for 1000 parts, shipping included, it is close to what is called "electronic dust because if you drop one, it's almost not worth bending down to pick it up.
1. Draw me a MOSFET...
A MOSFET is a component which makes it possible to switch current, ie to let pass more or less electrons between two electrodes according to a third. It can be seen as a switch (like a relay) or a variable resistor.
The maximum switching current depends on the size of the case and other characteristics, provided by the manufacturer. The MOSFET reference is chosen according to the type of application, and there is a very wide variety of models. We will focus on low power applications, with TO92 or SOT23 packages being the cheapest.
The most common references in TO92 housing (with lugs, which can be planted in a solderless plate) are BS170 for the N channel version (equivalent to the NPN), and BS250 for the P channel (the complement of the N channel) . The 2N7000 is another common reference, similar to the BS170, but with a different pinout.
The equivalents in SOT23 (3mm surface mount package) packages are BSS138 (N) and BSS84 (P). At our level, the 2N7002 is more or less equivalent to the BSS138.
Larger and more powerful MOSFETs are available, capable of withstanding hundreds of volts and/or hundreds of amperes. At the opposite extreme, transistors of the CMOS family (miniaturized version of MOSFETs) today constitute the bulk of current computer circuits and a chip can integrate billions of them. Due to their tiny size they can only switch a ridiculously low current and can't handle more than 2V, but in exchange they are super fast.
2. The three-legged beast
A discrete MOSFET transistor, as purchased individually, has three pins, like its bipolar cousin:
The source (equivalent to the emitter of a bipolar transistor) is usually connected directly to the power source.
The drain (equivalent to the collector) is usually connected to the circuit to be switched.
The gate, which is the base, controls the resistance between the source and the drain.
We could therefore almost replace a bipolar by a MOSFET. However, unlike bipolar, no current flows through the gate of a MOSFET*, as it is a type of insulated-gate transistor.
* contrary to what page 90 of Hackable n°7 suggests
3. Anatomy of a MOSFET
The term MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor in English. Let's start by looking at it from the end.
Already this indicates that it is a transistor and it is actually a variable resistor. The word “transistor” is short for “Transfer Resistor” and a bipolar transistor behaves more like a current amplifier.
Field Effect means field effect. It is an electric field that controls the resistance of the component. Much like ancient vacuum tubes, MOSFETs are therefore controlled with a voltage, as opposed to bipolar transistors which are controlled by a current flowing through their base. They therefore have the advantage of consuming less energy and they can amplify with a much higher gain. They dissipate almost no energy when saturated, although it does take some energy to get there.
Metal Oxide Semiconductor represents the succession of layers that make up the transistor. A layer of metal (conductor) is deposited on a very thin layer of oxide (insulator) resting on a semiconductor (usually based on silica doped with impurities). In integrated circuits, the metal layer is now replaced by polycrystalline silicon, but the term Metal has remained.
The thinner the insulating layer, the more intense the electric field at the level of the semiconductor, and therefore the more sensitive the transistor will be and it will switch at a lower voltage. On the other hand, this makes it very sensitive to overvoltages, in particular to static electricity discharges. The insulation is fragile and it often happens that a MOSFET clicks for a yes or a no!
Their rubbing can generate incredible tension, especially in dry weather!
The enriched insulated-gate MOSFET (Metal Oxide Semiconductor Field Effect Transistor) transistor is used in power electronics to supply switching loads ([1],[2]). There are many applications: switching power supply, inverter, motor power supply, class D audio amplifier, etc.
This document shows how to operate a power MOSFET transistor with a PWM (pulse width modulation) control signal. We first consider a one-quadrant chopper made with a single MOSFET transistor, before addressing the case of the two-quadrant chopper, made with two transistors in half-bridge assembly.
These oscillations have the disadvantage of giving an overvoltage which can be very large if the switching is very rapid. In the present case, the tension reaches 18 V. There is also a propagation of these oscillations from one point to another of the circuit, possibly by hertzian way (electromagnetic disturbances). We see here their effect on the control voltage, but the disturbances are also visible on the supply voltages. Their frequency here is about 8 MHz.
3.d. Reduction of switching oscillations
The adjustments described in this paragraph must be made on the final printed circuit of the application, because the parasitic inductances and capacitances depend on the layout of the tracks.
The oscillations, which occur just after the switching of the transistors, are due to the parasitic capacitances and inductances present in the power mesh containing the load. The pseudoperiod of the oscillations is 120 ns. To estimate the parasitic capacitance, a capacitance Ca is added between the output (connected to the load) and the ground, so as to obtain a period twice as large. The required value is Ca=14 μF. Knowing that the natural frequency of an LC circuit is inversely proportional to the square root of C, the capacitance has been multiplied by 4, which means that the parasitic capacitance is one third of the added capacitance, i.e. Cp=4.7 nF .
2. One quadrant chopper
2.a. Study set-up
The following assembly makes it possible to study the switching of a MOSFET.
The load is a 6.8 Ω resistor (25 W power wirewound resistor) powered by a voltage-regulated laboratory power supply (12 V).
The signal generator delivers a square voltage (of cyclic ratio 1/2) taking the values 0 and 12 V. When the gate G is at 12 V with respect to the source S, the transistor is in the on state. The Drain-Source dipole is then equivalent to an RDSON resistance of the order of 1 Ω. When the gate-source voltage is lower than a threshold (generally of the order of 5 V), the transistor is off and the Drain-Source resistance is practically infinite.
The chopper thus produced is said to have one quadrant because the voltage across the terminals of the load is always positive and the current in the load is always positive (for a resistive load).
The resistor R is needed because the gate-source dipole is equivalent to a capacitance (on the order of several hundred pF). It limits the current when charging this capacitor. The current in R is obtained at the oscilloscope via a differential amplifier.
The tests below are made with an Infineon IPA50R500 MOSFET (the diode is incorporated). The maximum continuous drain current is IDmax=5.4 A, which is more than enough for the 6.8 Ω load supplied at 12 V. During the conduction phases of the transistor, the current in the load is 1.8 A The resistance RDSON=0.5 Ω for VGS=13 V. The gate-source voltage must not exceed 20 V. Here are the drain-source current-voltage curves for this transistor:
For a drain current of 5 A, a gate-source voltage of at least 7 V must be applied in order to be in the resistive zone. Our assembly applies a voltage of 12 V, which is more than sufficient.
In this case, the gate charging current is supplied by the two push-pull transistors. When the control voltage is at the low level (0 V or less), transistor T0 is off and the base of T1 and T2 is at around 12 V. Transistor T1 is on while T2 is off: the gate is charged by resistor R and quickly takes on a 12 V potential with respect to the source. Conversely, when the control voltage is at high level, the base of T1 and T2 is grounded: T1 is blocking while T2 is on and the gate discharges until it has zero potential with respect to the source. This assembly can provide a current in the gate resistor R up to 1 A, which may be necessary to quickly switch a high-power MOSFET transistor.
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