FET circuit configurations are the common source, common gate, and common drain formats. Each have their own characteristics of voltage and current gain as well as input and output impedance. The choice of the FET circuit configuration or topology is one of the key design parameters on which the overall circuit design is based. Could someone please recommend the part numbers of some typical common FET's and MOSFET transistors. Kind of like the 3904 and 3906 NPN, PNP transistors. Not that the FETs have to be exact replacements for the 3904 or anything like that, I would just like to know what are the most common types of FETs people usually use.
by Majeed Ahmad
High temperatures and operating conditions outside the safe operating area can sabotage MOSFETs used in switching circuits.
The MOSFET (metal-oxide-semiconductor field-effect transistor) is a primary component in power conversion and switching circuits for such applications as motor drives and switch-mode power supplies (SMPSs). MOSFETs boast a high input gate resistance while the current flowing through the channel between the source and drain is controlled by the gate voltage. However, if not appropriately handled and protected, the high input impedance and gain can also lead to MOSFET damage caused by over voltage or too-high current.
First a few basics about avoiding MOSFET damage. Obviously, Vgs and Vds must both be within limits. The same for current, Id. There is also a power limit given by the maximum junction temperature. Basic values for the upper maximum on these parameters are given in the safe operating area (SOA) graph in the MOSFET datasheet. But it turns out, other thermal limits can apply. The SOA graph, for example, generally assumes an ambient temperature of 25° C with a specific junction temperature, usually below 150° C. But there are a variety of conditions that may cause high thermal gradients that may lead to expansion and cracking of the MOSFET die.
One factor to consider in this regard is that MOSFET thermal resistance is an average; it applies if the whole die is at a similar temperature. But MOSFETs designed for switch-mode power supplies can experience a wide temperature variation over different areas of their die. Optimized for on/off switching, they typically don’t work well in their linear region.
A typical failure mode for a MOSFET is a short between source and drain. In this case, only the source impedance of the power source limits the peak current. A common outcome of a direct short is a melting of the die and metal, eventually opening the circuit. For example, a suitably high voltage applied between the gate and source (VGS) will break down the MOSFET gate oxide. Gates rated at 12 V will likely succumb at about 15 V or so; gates having a 20-V rating typically fail at around 25 V.
All in all, exceeding the MOSFET voltage rating for just a few nanoseconds can destroy it. Device manufacturers recommend selecting MOSFET devices conservatively for expected voltage levels and further suggest suppressing any voltage spikes or ringing.
Too little gate drive
MOSFET devices are designed to dissipate minimal power when turned on. And the MOSFET must be turned on hard to minimize dissipation during conduction, otherwise it will have a high resistance during conduction and will dissipate considerable power as heat.
MOSFET devices are designed to dissipate minimal power when turned on. And the MOSFET must be turned on hard to minimize dissipation during conduction, otherwise it will have a high resistance during conduction and will dissipate considerable power as heat.
Generally speaking, a MOSFET passing high current will heat up. Poor heat sinking can destroy the MOSFET from excessive temperature. One way of avoiding too-high current is to parallel multiple MOSFETs so they share load current.
Many P- and N-channel MOSFETs are used in topologies involving an H- or L-bridge configuration between voltage rails. Here, if the control signals to the MOSFETs overlap, the transistors will effectively short-circuit the supply. This is known as a shoot-through condition. When it arises, any supply decoupling capacitors discharge rapidly through both MOSFETs during every switching transition, causing short but large current pulses.
The way to avoid this condition is to provide a dead time between switching transitions, during which neither MOSFET is on.
Over-currents even for a short duration can cause progressive damage to a MOSFET, often with little noticeable temperature rise before failure. MOSFETs often carry a high peak-current rating, but these typically assume peak currents only lasting 300 µsec or so. It is particularly important to over-rate MOSFETs for peak current when they switch inductive loads.
When switching inductive loads there must be a path for back EMF to freewheel when the MOSFET switches off. Freewheeling is the sudden voltage spike seen across an inductive load when its supply voltage is suddenly interrupted. Enhancement mode MOSFETs incorporate a diode that provides this protection.
High-Q resonant circuits can store considerable energy in their inductance and capacitance. Under certain conditions, this high energy causes the current to freewheel through the internal body diodes of the MOSFETs as one MOSFET turns off and the other turns on. (An intrinsic body diode is formed in the body-drain p-n junction connected between the drain and source. In N-channel devices, the body diode anode connects to the drain. The polarity is reversed in P-channel MOSFETs.) A problem can arise because of the slow turn-off (or reverse recovery) of the internal body diode when the opposing MOSFET tries to turn on.
MOSFET body diodes generally have a long reverse recovery time compared to the performance of the MOSFETs themselves. If the body diode of one MOSFET conducts when the opposing device is on, a short circuit arises resembling the shoot-through condition. The solution to this problem involves a Schottky diode and a fast-recovery diode. The Schottky diode connects in series with the MOSFET source and prevents the MOSFET body diode from ever being forward biased by the freewheeling current. The high-speed (fast recovery) diode connects in parallel with the MOSFET/Schottky pair. It lets the freewheeling current bypass the MOSFET and Schottky completely. This ensures the MOSFET body diode is never driven into conduction.
Transitions
A MOSFET dissipates little energy during its steady on and off states, but it dissipates considerable energy during times of a transition. Thus, it is desirable to switch as quickly as possible to minimize power dissipated. Because the MOSFET gate is basically capacitive, it requires appreciable current pulses to charge and discharge the gate in a few tens of nanoseconds. Peak gate currents can be as high as an ampere.
A MOSFET dissipates little energy during its steady on and off states, but it dissipates considerable energy during times of a transition. Thus, it is desirable to switch as quickly as possible to minimize power dissipated. Because the MOSFET gate is basically capacitive, it requires appreciable current pulses to charge and discharge the gate in a few tens of nanoseconds. Peak gate currents can be as high as an ampere.
The high impedance of MOSFET inputs can lead to stability problems. Under certain conditions, high-voltage MOSFETs can oscillate at high frequencies because of stray inductance and capacitance in the surrounding circuit (frequencies usually in the low megahertz range). Device manufacturers recommend that a low-impedance gate-drive circuit be used to prevent stray signals from coupling to the MOSFET gate.
References
ON Semiconductor
onsemi.com
onsemi.com
You may also like:
Let’s talk about the basics of MOSFET and how to use them. This tutorial is written primarily for non-academic hobbyists, so I will try to simplify the concept and focus more on the practical side of things.
However if you are into how MOSFET work, I will share some useful academic articles and resources at the end of this post. MOSFET has some advantage and disadvantage over BJT, so choose carefully base on your application.
You can buy MOSFET’s for Arduino Projects on Amazon: http://amzn.to/2Gk6ruW
MOSFET stands for metal-oxide semiconductor field-effect transistor. It is a special type of field-effect transistor (FET).
Unlike BJT which is ‘current controlled’, the MOSFET is a voltage controlled device. The MOSFET has “gate“, “Drain” and “Source” terminals instead of a “base”, “collector”, and “emitter” terminals in a bipolar transistor. By applying voltage at the gate, it generates an electrical field to control the current flow through the channel between drain and source, and there is no current flow from the gate into the MOSFET.
A MOSFET may be thought of as a variable resistor, where the Gate-Source voltage difference can control the Drain-Source Resistance. When there is no applying voltage between the Gate-Source , the Drain-Source resistance is very high, which is almost like a open circuit, so no current may flow through the Drain-Source. When Gate-Source potential difference is applied, the Drain-Source resistance is reduced, and there will be current flowing through Drain-Source, which is now a closed circuit.
In a nutshell, a FET is controlled by the Gate-Source voltage applied (which regulates the electrical field across a channel), like pinching or opening a straw and stopping or allowing current flowing. Because of this property, FETs are great for large current flow, and the MOSFET is commonly used as a switch.
Okay, let me summarize the differences between BJT and MOSFET.
- Unlike bipolar transistors, MOSFET is voltage controlled. While BJT is current controlled, the base resistor needs to be carefully calculated according to the amount of current being switched. Not so with a MOSFET. Just apply enough voltage to the gate and the switch operates.
- Because they are voltage controlled, MOSFET have a very high input impedance, so just about anything can drive them.
- MOSFET has high input impedence.
To use a MOSFET as a switch, you have to have its gate voltage (Vgs) higher than the source. If you connect the gate to the source (Vgs=0) it is turned off.
For example we have a IRFZ44N which is a “standard” MOSFET and only turns on when Vgs=10V – 20V. But usually we try not to push it too hard so 10V-15V is common for Vgs for this type of MOSFET.
However if you want to drive this from an Arduino which is running at 5V, you will need a “logic-level” MOSFET that can be turned on at 5V (Vgs = 5V). For example, the ST STP55NF06L. You should also have a resistor in series with the Arduino output to limit the current, since the gate is highly capacitive and can draw a big instantaneous current when you try to turn it on. Around 220 ohms is a good value.
This page shows some detail explanation how a MOSFET works as a switch. This page shows some advanced usage of MOSFET.
MOSFETs come in four different types. There are three main categories we need to know.
- N-Channel (NMOS) or P-Channel (PMOS)
- Enhancement or Depletion mode
- Logic-Level or Normal MOSFET
N-Channel – For an N-Channel MOSFET, the source is connected to ground. To turn the MOSFET on, we need to raise the voltage on the gate. To turn it off we need to connect the gate to ground.
P-Channel – The source is connected to the power rail (Vcc). In order to allow current to flow the Gate needs to be pulled to ground. To turn it off the gate needs to be pulled to Vcc.
Depletion Mode – It requires the Gate-Source voltage ( Vgs ) applied to switch the device “OFF”.
Enhancement Mode – The transistor requires a Gate-Source voltage ( Vgs ) applied to switch the device “ON”.
Despite the variety, the most commonly used type is N-channel enhancement mode.
There are also Logic-Level and Normal MOSFET, but the only difference is the Gate-Source potential level required to drive the MOSFET.
![Circuit Circuit](/uploads/1/1/9/3/119372387/233474233.jpg)
I will try to explain it in the simplest way I can, for more detail or if you are in doubt, check the references and links I provide at the bottom of the post.
Most Common Mosfets
MOSFET is a voltage controlled field effect transistor that differs from a JFET. The Gate electrode is electrically insulated from the main semiconductor by a thin layer of insulating material (glass, seriously!). This insulated metal gate is like a plate of a capacitor which has an extremely high input resistance (as high as almost infinite!). Because of the isolation of the Gate there is no current flow into the MOSFET from Gate.
When voltage is applied at the gate, it changes the width of the Drain-Source channel along which charge carriers flow (electron or hole). The wider the channel, the better the device conducts.
The MOSFET are used differently compared to the conventional junction FET.
- The infinite high input impedance makes MOSFETs useful for power amplifiers. The devices are also well suited to high-speed switching applications. Some integrated circuits contain tiny MOSFETs and are used in computers.
- Because the oxide layer is so thin, the MOSFET can be damaged by built up electrostatic charges. In weak-signal radio-frequency work, MOSFET devices do not generally perform as well as other types of FET.
Where to put the load to a MOSFET? Source or Drain?
Because load has resistance, which is basically a resitor. For N-channel MOSFET the reason we usually put the load at the Drain side is because of the Source is usually connected to GND.
If load is connected at the source side, the Vgs will needs to be higher in order to switch the MOSFET, or there will be insufficient current flow between source and drain than expected.
Heat Sink connected to the Drain?
Typically the heat sink on the back of a MOSFET is connected to the Drain! If you mount multiple MOSFETs on a heat sink, they must be electrically isolated from the heat sink! It’s good practice to isolate regardless in case the heat sink is bolted to a grounding frame.
What is the Body Diode For?
Common Mosfets
MOSFETs also have an internal diode which may allow current to flow unintentionally. The body diode will also limit switching speed. You don’t have to worry about it if you are operating under 1Mhz.
Mosfet Common Source
- Theory behind MOSFET (Youtube Video Lecture)