The device has separate GATE control and outputs (DGATE and HGATE) for back-to-back FET driving. An integrated ideal diode controller (DGATE) drives the first MOSFET (Q1) to replace a Schottky diode for reverse input protection and reverse current blocking for output voltage holdup. An integrated charge. In this video I will be explaining MOSFET Transistors. We will look into the basics of how the are made and how to use them in your projects. A MOSFET is a transistor. Coupling to drive the MOSFETs very effective at preventing electrical noise from being conducted back to the controller.

A MOSFET is a transistor. It is a Metal Oxide Field Effect Transistor.
Here are the symbols for FETs and MOSFETs:

The MOSFETS have been drawn in these positions because
that is how they will be located in a bridge arrangement
and you can see how and where the three
terminals will be.

More MOSFET symbols

Here is an animation showing how to turn on an N-channel MOSFET:

MOSFET turns ON when gate-to-source
is more than about 2v (2v to 5v)

The easiest way to understand how MOSFETs work is to compare them with PNP and NPN transistors and show them in similar circuits. The advantage of a MOSFET is this: It requires very little current (almost zero current) into the gate to turn it ON and it can deliver 10 to 50 amps or more to a load.
THE BASE
The BASE (the input lead) of a normal transistor is the GATE for a MOSFET.
A MOSFET can be used in place of an ordinary transistor (called a bipolar junction transistor, or BJT) providing one slight difference is taken into account.
An ordinary NPN transistor will turn ON when the base voltage is about 0.65v more than the emitter but a MOSFET needs the gate terminal to be at least 2v to 5v, (depending on the type of MOSFET) above the source voltage.
This is called the GATE VOLTAGE and the exact value is difficulty to extract from some data sheets.
That's why you need to know the value before using a FET or MOSFET.
Delivering a higher voltage (up to 12v) will not damage the device or cause more gate current to flow but supplying a minimum voltage will alter the current capability enormously. You must not use a MOSFET if you can only just deliver the minimum gate voltage as the MOSFET will act like a high-power resistor and get very HOT.
When a MOSFET is used in a high-current situation, it is important to provide a fast rise-time to the gate so the FET turns on quickly and does not heat up.
If you have a circuit with a fast rise-time voltage in the order of 2v to 10v, a FET device is a good solution.
You can find the GATE VOLTAGE by building the circuit and testing the MOSFET.
Connect a 100k pot to a 12v supply and take the wiper to the gate terminal.
Gradually increase the voltage by turning the pot and measure the voltage across the LOAD. When the voltage is equal to full rail voltage, the FET is fully turned ON. Perform this operation as fast as you can to prevent the FET heating up. Hold your finger on the FET and reduce the voltage slightly until the FET starts to warm up. Measure the voltage on the gate with a digital meter. Now increase the voltage so the FET remains cold. Measure the voltage.
You now have the required gate voltage for the device and a minimum voltage (the gate voltage must be above the minimum voltage).
Here is a comparison between an NPN transistor and N-channel MOSFET:

A zener must be added to the gate of a MOSFET if the gate voltage comes from a supply that is above 20v.
A normal transistor is a current amplifying device.
For a load current of 100mA, the base current for a BC547 will need to be about 1mA.
This means it has a current gain of about 100.
A MOSFET is a voltage controlled device and the current it will handle depends on its physical size and the way it is constructed. You cannot change this parameter.
For a load current up to about 35Amp, the gate current for a IRZ40 will be less than 0.25mA. When the gate voltage is 3v to 4v higher than the source, it turns on and the resistance between source and drain terminals is about 0.028 ohms. It will handle up to 35 amps.
The load determines the current through the MOSFET (not the MOSFET) and if it is less than 35 amps, a IRFZ40 is suitable for the application.
Comparison between a PNP transistor and P-channel MOSFET:

When the gate voltage is 4v LOWER than rail voltage, the MOSFET turns ON. The 10k resistor on the base of the transistor is needed to prevent the base current exceeding the amount of current needed by the transistor to deliver current to the load. However the 10k resistor on the gate of the MOSFET is not needed. Providing the voltage (up to 18v) on the gate rises and falls quickly, the MOSFET will not get hot. The critical period of time is the 0v to 3v section of the waveform as this is when the MOSFET is turning on.
PUSH PULL
MOSFETs can be placed in push-pull mode, just like PNP and NPN transistors.
They must be connected correctly to prevent damage.
In the following circuit you can see the transistors and MOSFETs have been connected incorrectly.
For the PNP/NPN transistor circuit, as the input changes from high to low or low to high, both transistors are turned on during the transition. Only one transistor is turned on when the line is high and only the other transistor is turned on when the line is low, but during the transition, BOTH are turned on.
The same applies with the MOSFETs. When the input is at mid-rail, a voltage between gate and source will be produced for both MOSFETs. Since a MOSFET can handle many amps, this will put a short-circuit across the power rail and will cause a lot of damage.

Transistors and MOSFETs will produce short-circuit

MOSFET output is less than rail voltage

The solution is shown in the diagram below. The transistor configuration will work on ANY rail voltage but the MOSFET 'totem-pole configuration' will only work up to 5v. This is due to the characteristics of a MOSFET. The MOSFETs used in this arrangement have a gate-to-source characteristic of slightly more than 3v and do not turn on when the voltage across these two terminals is 3v. This means the supply can be 6v and when the input is at mid-rail, 3v will be across each gate-to-source and neither will be turned on. That's why TTL logic is limited to 5v operation. The output will be extremely close to rail-to-rail for the MOSFET configuration. Outlook for mac version.

Max voltage for MOSFET arrangement is 5v

For a supply greater than 5v, a different MOSFET configuration must be used to get full rail-to-rail output. The MOSFETs must be turned on individually.

PUSH-PULL USING MOSFETS

PUSH PULL USING MOSFETS
The circuit above sinks up to 35A via the N-channel MOSFET and delivers about 18Amp via the P-channel MOSFET. Input A must rise quickly to prevent the MOSFET heating up during the turning-on period. Input A must rise to at least 4v to guarantee the MOSFET turns ON.
Input B must rise above 0.65v to turn the transistor ON. The voltage on the collector of the transistor will fall and this will provide a gate-to-source voltage for the P-channel MOSFET.
Both inputs must not be HIGH at the same time as this will turn ON both MOSFETs and create a short-circuit on the power rail.

The circuit above is much more complex than meets the eye.
To turn on the top N-channel MOSFET, the gate must be taken at least 3v higher than the source because it is a SOURCE FOLLOWER (similar to an EMITTER FOLLOWER). This is equal to Vin + 3v.
How does pin HG get this high voltage?
It gets it from a voltage doubling circuit made up of the 0.33u, high speed diode D1 and an oscillator in the chip.
The circuit is a buck converter and will reduce any supply voltage to a lower voltage with very high efficiency. It allows a small 'packet of energy' to flow to the Vout terminal via the inductor L1 and this percentage determines the Vout voltage.

Here is an audio amplifier using PUSH PULL mode to drive a speaker:

The top two transistors are in push-pull mode to turn the P-channel MOSFET on and off very quickly. They speed up the incoming waveform and prevent the MOSFET generating heat during the turning-on process.
The two lower transistors do the same thing.
The diodes and resistors connected to the input form a voltage-divider to correctly bias the push-pull transistors.
H-BRIDGE
An H-Bridge can be designed using MOSFETs:

Input A HIGH, Input D HIGH - forward rotation
Input B HIGH, Input C HIGH - reverse rotation
Input A HIGH, Input B HIGH - not allowed
Input C HIGH, Input D HIGH - not allowed
The H-Bridge can be designed with two more transistors so that only two input lines are needed.

PWM MOTOR SPEED CONTROLLER
Here is a circuit from a 12v drill. The MOSFET will deliver up to 30Amps.
The frequency of the oscillator is in the range 550Hz to about 6.5kHz, with an off period of about 2.6us.

PWM 12v CORDLESS DRILL MOTOR CONTROLLER

3-LED CHASER
This circuit let's you see how a FET turns on and how it works.
Remove the connections to the gate of the first FET and the LED will start to illuminate.
The gate will start to get a charge on it and the FET will turn on.
Place a 1M between gate and 0v and the FET will turn off.
This shows the sensitivity of the gate. The charge on the gate must be removed for the FET to turn OFF.
This circuit will show how the FET turns ON slowly as the voltage on the gate increases and turns OFF slowly as the voltage drops:

WHY MOSFETs FAIL
There are quite a few possible causes for device failures, here are a few of the most important reasons:

• Over-voltage:

MOSFETs have very little tolerance to over-voltage. Damage to devices may result even if the voltage rating is exceeded for as little as a few nanoseconds. MOSFET devices should be rated conservatively for the anticipated voltage levels and careful attention should be paid to suppressing any voltage spikes or ringing.

• Prolonged current overload:

High average current causes considerable thermal dissipation in MOSFET devices even though the on-resistance is relatively low. If the current is very high and heatsinking is poor, the device can be destroyed by excessive temperature rise. MOSFET devices can be paralleled directly to share high load currents.

• Transient current overload:

Massive current overload, even for short duration, can cause progressive damage to the device with little noticeable temperature rise prior to failure.

• Shoot-through - cross conduction:

If the control signals to two opposing MOSFETs overlap, a situation can occur where both MOSFETs are switched on together. This effectively short-circuits the supply and is known as a shoot-through condition. If this occurs, the supply decoupling capacitor is discharged rapidly through both devices every time a switching transition occurs. This results in very short but incredibly intense current pulses through both switching devices.
The chances of shoot-through occurring are minimised by allowing a dead time between switching transitions, during which neither MOSFET is turned on. This allows time for one device to turn off before the opposite device is turned on.

• No free-wheel current path:

When switching current through any inductive load (such as a Tesla Coil) a back EMF is produced when the current is turned off. It is essential to provide a path for this current to free-wheel in the time when the switching device is not conducting the load current.
This current is usually directed through a free-wheel diode connected anti-parallel with the switching device. When a MOSFET is employed as the switching device, the designer gets the free-wheel diode 'for free' in the form of the MOSFETs intrinsic body diode. This solves one problem, but creates a whole new one..

• Slow reverse recovery of MOSFET body diode:

A high Q resonant circuit such as a Tesla Coil is capable of storing considerable energy in its inductance and self capacitance. Under certain tuning conditions, this causes the current to 'free-wheel' through the internal body diodes of the MOSFET device. This behaviour is not a problem in itself, but a problem arises due to the slow turn-off (or reverse recovery) of the internal body diode.

MOSFET body diodes generally have a long reverse recovery time compared to the performance of the MOSFET itself.
This problem is usually eased by the addition of a high speed (fast recovery) diode. This ensures that the MOSFET body diode is never driven into conduction. The free-wheel current is handled by the fast recovery diode which presents less of a 'shoot-through' problem.

• Excessive gate drive:

If the MOSFET gate is driven with too high a voltage, then the gate oxide insulation can be punctured rendering the device useless. Gate-source voltages in excess of +/- 15 volts are likely to cause damage to the gate insulation and lead to failure. Care should be taken to ensure that the gate drive signal is free from any narrow voltage spikes that could exceed the maximum allowable gate voltage.

• Insufficient gate drive - incomplete turn on:

MOSFET devices are only capable of switching large amounts of power because they are designed to dissipate minimal power when they are turned on. It is the responsibility of the designer to ensure that the MOSFET device is turned hard on to minimise dissipation during conduction. If the device is not fully turned on then the device will have a high resistance during conduction and will dissipate considerable power as heat. A gate voltage of between 10 and 15 volts ensures full turn-on with most MOSFET devices.

• Slow switching transitions:

Little energy is dissipated during the steady on and off states, but considerable energy is dissipated during the times of a transition. Therefore it is desirable to switch between states as quickly as possible to minimise power dissipation during switching. Since the MOSFET gate appears capacitive, it requires considerable current pulses in order to charge and discharge the gate in a few tens of nano-seconds. Peak gate currents can be as high as 1 amp.

• Spurious oscillation:

MOSFETs are capable of switching large amounts of current in incredibly short times. Their inputs are also relatively high impedance, which can lead to stability problems. Under certain conditions high voltage MOSFET devices can oscillate at very high frequencies due to stray inductance and capacitance in the surrounding circuit. (Frequencies usually in the low MHz.) This behaviour is highly undesirable since it occurs due to linear operation, and represents a high dissipation condition.
Spurious oscillation can be prevented by minimising stray inductance and capacitance around the MOSFETs. A low impedance gate-drive circuit should also be used to prevent stray signals from coupling to the gate of the device.

• The 'Miller' effect:

MOSFET devices have considerable 'Miller capacitance' between their gate and drain terminals. In low voltage or slow switching applications this gate-drain capacitance is rarely a concern, however it can cause problems when high voltages are switched quickly.

A potential problem occurs when the drain voltage of the bottom device rises very quickly due to turn on of the top MOSFET. This high rate of rise of voltage couples capacitively to the gate of the MOSFET via the Miller capacitance. This can cause the gate voltage of the MOSFET to rise resulting in turn on of this device as well ! A shoot-through condition exists and MOSFET failure is certain if not immediate.
The Miller effect can be minimised by using a low impedance gate drive which clamps the gate voltage to 0 volts when in the off state. This reduces the effect of any spikes coupled from the drain. Further protection can be gained by applying a negative voltage to the gate during the off state. eg. applying -10 volts to the gate would require over 12 volts of noise in order to risk turning on a MOSFET that is meant to be turned off !

• Conducted interference with controller:

Rapid switching of large currents can cause voltage dips and transient spikes on the power supply rails. If one or more supply rails are common to the power and control electronics, then interference can be conducted to the control circuitry.
Good decoupling, and star-point earthing are techniques which should be employed to reduce the effects of conducted interference. The author has also found transformer coupling to drive the MOSFETs very effective at preventing electrical noise from being conducted back to the controller.

• Static electricity damage:

Antistatic handling precautions should be used to prevent gate oxide damage when installing MOSFET or IGBT devices. But are very reliable once they are soldered in place.

There are many more fact and circuits using MOSFETs on the web. This discussion is only a starting-point.

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, V_gs and V_ds must both be within limits. The same for current, I_d. 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 (V_GS) 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.

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. Office 2012 for mac os.

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.

Mosfet Bidirectional Switch

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

Mosfet Back To Back Switch

ON Semiconductor
onsemi.com

Mosfet Back To Back Pain