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Mosfet

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what I was looking for specifically was an equivalent circuit, and some ideas on how do not destroy them with heat or over voltage. Lets try this:

1.) What voltages are MOSFETs used for (most of the time)?

2.) Are MOSFETs designed for use in AC circuits?

3.) What is the equivalent circuit for a MOSFET?

4.) What is the equivalent circuit for a JFET?
 

QuietMan

Member
Most MOSFETs require at least 10V on the gate to turn fully on. When conducting (fully on) they can have some impressively low ohmage values, and carry some awesome currents. The 10V requirement is a real let down for many uses, so there are another class of MOSFETs that use 5V on the gate, called logic level MOSFETs. Most of these units have a Z in the part number (such as IFZ34). Someone on another forum was telling about units that go even lower gate voltages, but I have no experience with those.

MOSFETs have a relatively high gate capacitance, so you need a driver that can handle high surge currents for them. Once charged, the gate draws little or no current.

If have heard about (but never seen) circuits that use MOSFETs instead of diode in a bridge. They are not diodes of course, but if you turn them on at exactly the right time you can get a rectified signal with no voltage drop and no heat (a result of the really good conductance).

Generally a resistor is used on the gate to prevent ringing. Since their usual state is digital (on/off) you don't want a ringing signal on the gate putting them in the linear region.

My suggestion is get a cheap unit from Radio Shack or where ever, and play with it using 12V.
 

Gary B

New Member
To add a little to the QuietMan, There are two types of MOSFET devices, depletion mode and enhancement mode. QM is referencing the enhancement mode type.

JFET is probably best understood as a reverse biased UJT. As the bias is increased, the electrostatic field increases to the pinch-off point. Use a current limiting resister in the source or drain to keep the device within its maximum current limit and you can play with the bias from zero up to max rated without worrying about destroying the device.
 

crutschow

Well-Known Member
Most Helpful Member
what I was looking for specifically was an equivalent circuit, and some ideas on how do not destroy them with heat or over voltage. Lets try this:

1.) What voltages are MOSFETs used for (most of the time)?

2.) Are MOSFETs designed for use in AC circuits?

3.) What is the equivalent circuit for a MOSFET?

4.) What is the equivalent circuit for a JFET?
1.) MOSFETS are often used in power applications where the voltage is typically 100V or less. But some can be used up to several hundred volts.

2.) The can be used to amplifiy AC signals. If you mean in power line applications, then only in specific type circuits.

3.) 4.) MOSFET and JFET basic DC equivalent circuit is a voltage controlled current source. For AC circuits you need to add the various parasitic capacitances from the gate, source, and drain. The difference between them is that MOSFETS are nomally-off with zero gate bias and JFETS are nomally-on with zero gate bias.

The power they can dissipate varies greatly with the design of the transistor. They go from less than a watt for small devices to tens of watts for large power devices (with suitable heat sinking of course)
 

smanches

New Member
Generally a resistor is used on the gate to prevent ringing. Since their usual state is digital (on/off) you don't want a ringing signal on the gate putting them in the linear region.

I keep hearing people say this, but I think it's only specific to amplifiers, not switching circuits.

If you charge a cap, is there ringing? Why would there be ringing by charging a gate? I've looked with a scope and the only ringing is a bit from the driver itself, but it gets smoothed out by the gate capacitance usually.

I have googled it to no end, and the only references to using a gate resistor like this was in making UHF/VHF amplifiers. The issue is because the mosfets are usually driven by coils, which can then create a tank circuit with the gate.

Does anyone have any other references for this gate resistor?
 

jpanhalt

Well-Known Member
Most Helpful Member
Re: Gate ringing

See:

AN-937 (IR)

Care should be exercised not to exceed the gate-to-source maximum voltage rating. Even if the applied gate voltage is kept below
the maximum rated gate voltage, the stray inductance of the gate connection, coupled with the gate capacitance, may generate
ringing voltages that could lead to the destruction of the oxide layer. Overvoltages can also be coupled through the drain-gate
self-capacitance due to transients in the drain circuit. A gate drive circuit with very low impedance insures that the gate voltage
is not exceeded in normal operation. This is explained in more detail in the next section.

https://www.electro-tech-online.com/custompdfs/2009/12/slup169.pdf

The most difficult task in direct gate drives is to
optimize the circuit layout. As indicated in Figure
8, there might be considerable distance between
the PWM controller and the MOSFET. This
distance introduces a parasitic inductance due to
the loop formed by the gate drive and ground
return traces which can slow down the switching
speed and can cause ringing in the gate drive
waveform.

There is also an application note from Fairchild, but I couldn't find it right off. The Fairchild note calculates the optimum resistor value based on experimental data.

John
 

MikeMl

Well-Known Member
Most Helpful Member
I keep hearing people say this, but I think it's only specific to amplifiers, not switching circuits.

If you charge a cap, is there ringing? Why would there be ringing by charging a gate? I've looked with a scope and the only ringing is a bit from the driver itself, but it gets smoothed out by the gate capacitance usually.

Have you ever heard of "Miller Effect"?
 
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