Hoffman Amplifiers Tube Amplifier Forum

Other Stuff => Solid State => Topic started by: kagliostro on March 03, 2017, 06:24:23 am

Title: Mosfet question
Post by: kagliostro on March 03, 2017, 06:24:23 am

On schematics like this the 12v zener purpose is to protect the mosfet


(http://i.imgur.com/iy7VLMN.jpg)


(http://i.imgur.com/mYMPaZE.jpg)


I would like to know, if on the datasheet the mosfet is draw this way


(http://i.imgur.com/qDfZaG2.jpg)

is possible to omit the 12v zener (because the mosfet has it's own internal protection) ??

Thanks

Franco

Title: Re: Mosfet question
Post by: sluckey on March 03, 2017, 07:11:01 am

I would say yes. But I'd like to read a data sheet for a MOSFET that has the internal zeners.

Title: Re: Mosfet question
Post by: kagliostro on March 03, 2017, 08:44:09 am

Ciao Steve

the datasheet is this

http://docs-europe.electrocomponents.com/webdocs/13e4/0900766b813e4677.pdf (http://docs-europe.electrocomponents.com/webdocs/13e4/0900766b813e4677.pdf)

Franco

Title: Re: Mosfet question
Post by: 2deaf on March 03, 2017, 09:14:01 am

The 12V zener is acting as a regulator, not as device protection.  In the middle diagram, the 12V zener is part of a crude current limiter.  There is a lower voltage at the source than at the gate that is only approximately predictable.  Then there is a further voltage drop across the 10R resistor.  When the current becomes large enough, these two voltages combined cause the zener to break over and current begins to flow through the 100K resistor which lowers the gate voltage.  The whole thing will come to an equilibrium at around 700mA with a wide variation between different MOSFETs of the same or different type.

Title: Re: Mosfet question
Post by: kagliostro on March 03, 2017, 11:30:48 am

Thanks

My knowledge is so poor that I didn't recognized that

Franco

Title: Re: Mosfet question
Post by: PRR on March 03, 2017, 02:05:34 pm

> I'd like to read a data sheet

Quote

Table 4  Gate-source zener diode
Symbol Parameter Test conditions Min. Typ. Max. Unit
BVGSO Gate-source breakdown voltage Igs=± 1mA (Open Drain) 30 V

1.1 Protection features of gate-to-source zener diodes
The built-in back-to-back Zener diodes have specifically been designed to enhance not only the device's ESD capability, but also to make them safely absorb possible voltage transients that may occasionally be applied from gate to source. In this respect the Zener voltage is appropriate to achieve an efficient and cost-effective intervention to protect the device's integrity. These integrated Zener diodes thus avoid the usage of external components.

It seems to replace the protection-only Zener we often use.

As said, K's second diagram has 10 Ohms inside the Zener-MOSFET loop, so that plan needs an external Zener. (It may not need the G-S Zener unless you are doing something odd.)

Title: Re: Mosfet question
Post by: kagliostro on March 03, 2017, 03:34:56 pm

Thanks

Franco