Mosfet small resistor on source

[PovTruffe]

Hi,

What is the purpose of the small value resistor R7 inserted on the mosfet source ?
Schematic: http://dl.free.fr/e2ProNRN5

 

[Joerg]

Hi,

What is the purpose of the small value resistor R7 inserted on the mosfet source ?
Schematic: http://dl.free.fr/e2ProNRN5

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

 

[PovTruffe]

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?

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

 

[Joerg]

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

In case someone else wants to look:

http://ww1.microchip.com/downloads/en/AppNotes/01209B.pdf

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.

 

[Grant]

Hi,

What is the purpose of the small value resistor R7 inserted on the mosfet source ?
Schematic: http://dl.free.fr/e2ProNRN5

Where's the rest of the circuit?

Grant.

 

[Martin Riddle]

In case someone else wants to look:

http://ww1.microchip.com/downloads/en/AppNotes/01209B.pdf

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.

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

Cheers

 

[Joerg]

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

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

 

[Joerg]

Wow, what an insanely bad appnote!

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

 

[Joerg]

Reduce R7 to zero and think fundamentals

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.

 

[E]

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

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

 

[PovTruffe]

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

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 :-((

 

[PovTruffe]

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

???

 

[Fred Bartoli]

Joerg a écrit :

But I am not a fundamentalist

I never even have an R7 in mine unless I do current-mode control.

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

Which
I increasingly do because everything has to be so friggin' small these days.

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...

 

[Eeyore]

What is wrong exactly with this schematic and appnote?
I started making a simulation using Proteus/MPLAB and it seemed to work
prettty well.

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

 

[Eeyore]

Where's the rest of the circuit?

http://ww1.microchip.com/downloads/en/AppNotes/01209B.pdf

 

[PovTruffe]

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...

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?

 

[PovTruffe]

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

OK I understand now!
If you read the link above too fast you might see the word "Porn".
I am not going to comment the subconscious issues here...

Dont worry "free.fr" is the second largest ISP in France and "dl.free.fr"
is their file transfer service, such as MegaUpload.

 

[PovTruffe]

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.

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.

 

[Joerg]

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?

Not if it's NiMH cells, those can develop several amps of current. But
R7 would be a rather crummy way to limit power. Unless it was a several
watt resistor type it would go phssssoush and leave a nasty smell
behind. As would the FET unless cooled. Also, it would be sort of "We
are not so sure about the quality of the uC code so let's not trust it" ;-)

The proper way to do this is to write code that is reliable and handles
the start-up situation every time, plus a fuse.

 

[Joerg]

What is wrong exactly with this schematic and appnote?

Not wrong, just not great IMHO. Things like this, quote "When the
microcontroller detects that the output current has dropped below the
required level, it pulses the MOSFET four times in rapid succession to
boost the voltage output. Four pulses are used to generate more current
flow and to speed up the rise time under load."

That's a sledge hammer method, requires a larger inductor than than a
cycle-by-cycle design would need because it might enter continuous
conduction mode (CCM), saturate and the FET goes *PHUT*. Maybe that's
why they have R7 in there.

Even my simplest and cheapest (discrete) converter designs all the way
back to the early 90's have an inner current mode control loop. It's
state of the art but this one doesn't have it.

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.

Watch out for that 1N914, 75V is riding it at the limits.

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

No surprise there. Here is another little issue with this circuit: C2 is
1uF, C6 is only 10nF. So if the load would suddenly become low impedance
C2 rapidly dumps its charge into C6, pin 6 can go above VDD, then into
its parasitic substrate diode with gusto ... *POOF*