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Transistor VS MOSFET for linear power supply

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xavier60:
In hard switching applications, MOSFETs generally require large Gate drive current because the Gate driver has to fight the large current  of the Miller capacitance being charged and discharged due to the Drain being made to slew so quickly.
As a series pass element, although there can be rapid load current fluctuations, the voltage across Drain and Source doesn't change all that quickly so the Miller current is very small.
MOSFETs in linear power supplies are typically driven by an ordinary op-amp.

MiDi:

--- Quote from: not1xor1 on December 07, 2018, 08:30:35 am ---I had saved a few tips from IRF, regarding switching mosfets use in linear region. Unfortunately the links are dead.

--- End quote ---

Is AN1155 one of those notes?

Jwillis:

--- Quote from: Hero999 on December 06, 2018, 01:12:21 pm ---
Are you sure? BJTs are typically more susceptible to secondary breakdown than MOSFETs. Look at a few data sheets and you'll see what I mean.

--- End quote ---

That is true but power MOSFETs have parasitic PN and BJT elements within the structure and as the internal geometry shrinks due to hot spotting beyond the thermal runaway stage some power MOSFETs can exhibited a failure mode resembling secondary breakdown.I was simply trying to be brief in my explanation.
Not to say it can't be done but I would use a mosfet that is rated much higher than the BJT your attempting to replace.Or ,as suggested by others, to use a Mosfet designed for linear applications .

not1xor1:

--- Quote from: MiDi on December 07, 2018, 11:20:35 am ---
--- Quote from: not1xor1 on December 07, 2018, 08:30:35 am ---I had saved a few tips from IRF, regarding switching mosfets use in linear region. Unfortunately the links are dead.

--- End quote ---

Is AN1155 one of those notes?

--- End quote ---

they were short web pages.
it looks like infineon deleted lots of old IRF web pages and app notes.

In any case here is Using HEXFET MOSFETs as Linear Amplifiers
BTW I've got also Using HEXFET MOSFETs as variable loads, just let me know if anybody is interested.

Answer ID 214  |   Updated 04/08/2009 12:43 AM
Question
We would like to design a linear power amplifer with N channel HEXFET - Power MOSFETs.

In various forms of literature we find many designs with complementary HEXFET - Power MOSFETs but only N channel devices are employed.

The output voltage is +200V/-30V and the current is 0.5A. Because, the N channel devices are better than P channel devices for high voltage applications we want to use the N channel FETs in the output stage. But the drive stage is difficult to design. Help.
Answer
IR HEXFET® MOSFETs are designed to be used as Switches. This does not preclude their use in linear applications but as the device technology advances to provide ever better performance as switches with lower Rds(0n) and gate charge figures, this makes the devices ever less suitable for linear applications. A lower Rds(on) equates to an output characteristic with a steep slope, which makes it difficult to maintain a suitable gate bias to keep the drain current operating point stable. As the drain current is deliberately varied in response to an applied input voltage, this changes the dissipated power within the part, changing the junction temperature and thus the value of Rds(on), i.e the slope of the output characteristic. If the device is being used in audio amplifier applications, other than class D for which most are ideally suited, the frequency of Rdson variation will not be seen as a fast variation as the thermal inertia will average them out to produce an average value for Rdson. This is the value you would use to solve the thermal equlibrium equation to determine if the Tj achieved in the application will be within permitted limits or not.

As higher voltage MOSFETS have higher Rds(on) values and thus a more shallow output slope these will be more suitable for linear applications.
All our current Application Notes are written around switching operations and we have none dealing directly with linear amplifier circuits.

With N channel devices, as the MOSFET is turned on by taking the gate voltage positive with respect to the source, you will have to arrange a gate bias supply capable of going positive of +200V by around a maximum of 10V if you want to control the MOSFET completely between full on and full off. As in your application the required output voltage also has to go negative you will have to use a split supply so that the effective gate voltage range is 0V to +210VDC for full range control.

P channel devices are controlled by taking their gates negative with respect to their sources, but the unavailability of high voltage P channel HEXFET® MOSFETs means they are not available with Vds(br) ratings above around 150V.

joeyjoejoe:
As a beginner, I found this video a very nice overview on MOSFETs in linear mode :

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