Mosfet small resistor on source

Hi,

What is the purpose of the small value resistor R7 inserted on the mosfet source ? Schematic:

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Reply to
PovTruffe
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source ?

Hard to say without seeing the whole schematic. Looks like a chunk of a simple boost converter. R7 doesn't make a lot of sense but since there is a 1N4148 for the diode efficiency seems not to matter. Maybe they wanted to prevent oscillations?

Or maybe it is there to reduce the chance of ice formation in winter :-)

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Reply to
Joerg

source ?

Yes this is a boost converter. Full schematic is on Microchip application note #AN1209.

Reply to
PovTruffe

snipped-for-privacy@mid.>>> Hi,

source ?

#AN1209.

In case someone else wants to look:

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R7 doesn't make a lot of sense. Other than to be able to measure the inductor current. It also protects (to some extent) against inductor saturation but why they put it in the source, beats me.

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Regards, Joerg

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Reply to
Joerg

source ?

Where's the rest of the circuit?

Grant.

Reply to
Grant

Probably to limit the inductor current, and keep the supply from collapsing during startup. (ports are high by default)

Cheers

Reply to
Martin Riddle

They should most certainly not be high. Did the uC architects screw that up? 0.5ohms would draw quite some current.

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Regards, Joerg

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Reply to
Joerg

snipped-for-privacy@mid.>>>> Hi,

source ?

note #AN1209.

Wow, what an insanely bad appnote!

John

Reply to
John Larkin

snipped-for-privacy@mid.>>>>> Hi,

source ?

note #AN1209.

It sure ain't a pretty one, and that's when I really take schematics with a grain of salt.

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Reply to
Joerg

snipped-for-privacy@mid.>>>>>> Hi,

source ?

note #AN1209.

Reduce R7 to zero and think fundamentals :-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

snipped-for-privacy@mid.>>>>>>> Hi,

mosfet source ?

note #AN1209.

But I am not a fundamentalist :-)

I never even have an R7 in mine unless I do current-mode control. Which I increasingly do because everything has to be so friggin' small these days.

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Reply to
Joerg

"Martin Riddle" kirjoitti viestissä:i549hj$plv$ snipped-for-privacy@news.eternal-september.org...

Datasheet says they are high-impedance inputs as default. PIC are always that way.

Reply to
E

"Joerg" a écrit :

What is wrong exactly with this schematic and appnote? I started making a simulation using Proteus/MPLAB and it seemed to work prettty well. It generated up to 75V with with 6V input and a 5k load.

The output will be current regulated and voltage limited.

I am just stuck now with the 18F14K22 A/D converter that always returns a null conversion result :-((

Reply to
PovTruffe

source ?

I do not think that we appreciate an ad for sports TV...

Reply to
Robert Baer

source ?

???

Reply to
PovTruffe

snipped-for-privacy@mid.>>>>>>>> Hi,

mosfet source ?

note #AN1209.

You mean your designs never ever had more than 6 resistors? I knew you were cheap, but not that much :-)

R7 is a pretty good and cost effective way of limiting startup & fault currents (uC controled, no current mode, boost,...) Of course that makes the MOSFET working linear under these conditions and it has to withstand the power...

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Thanks,
Fred.
Reply to
Fred Bartoli

And thereby hangs a tale ( or is it tail ? ). Relying entirely on simulations is very unwise and doesn't prevent today's 'design engineers' creating bad / unreliable products.

As for the function of R7? it is in a location that's typically used to monitor or limit current ( but there don't seem to be the relevant connection to other circuitry ). It would also limit peak current into a capacitive load but if the load is inductive it's redundant since current rises from zero when switching into an inductor.

Graham

Reply to
Eeyore

source ?

formatting link

Reply to
Eeyore

"Fred Bartoli" a écrit :

currents (uC controled, no current mode,

has to withstand the power...

The system will be powered by 4 x AA batteries. Is their internal resistance not sufficient to limit the startup current, making the resistor redundant?

Reply to
PovTruffe

"Eeyore" a écrit :

is very unwise and doesn't prevent

Who said he relies entirely on simulation here? In fact this is the first time I use simulation. My goal was to simulate as much of the circuit as possible and see how far I could go. Now I am stuck with the A/D converter and about to build the hardware.

This is also a learning experience as I am not very familiar with PIC peripherals and have not programmed in C in more than 20 years.

monitor or limit current ( but there don't

peak current into a capacitive load but

switching into an inductor.

Reply to
PovTruffe

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