Mosfet protection diode

[sbirkenstock]

Hi,

another beginners question (I come from tube amps and not too many diodes there....)
I read a few schematics including this one:
http://runoffgroove.com/peppermill.html
about a (zener) diode protecting the gate of a MOSFET.
I have a couple of questions about it:
a) is this only need for MOSFETs or also for JFETs or BJTs?
b) if I use an LED like in the peppermill, the signal would be cut off at the forward voltage of the LED? So with a high enough input (like with a booster) this LED would clip the signal?
c) where does the "danger zone" start with an MOSFET (volts at the gate)? It obviously can take 9 volt if a 9v1 zener diode is used by most schematics I saw.
d) just to make sure: this is protecting from high voltage coming from the input?

Best regards,
Stephan 

[bluebunny]

The LED or Zener is protecting the MOSFET from (mostly) static voltages.  Skidding across the linoleum in your nylon socks will thoroughly zorch the skinny bit of glass insulating the gate (it's about 20V "wide" - to quote R.G.).  Doesn't apply to JFETs or BJTs, which are built quite differently (no skinny glass).

[antonis]

is this only need for MOSFETs or also for JFETs or BJTs?

As Marc said, it doesn't apply to Fets or BJTs but you may notice in some circuits a diode facing from Emitter to Base for reverse B-E overvoltage protection..

[amz-fx]

(b) Using an LED for protection is a bad idea since the protection is depending on its reverse breakdown voltage. The breakdown of an LED can vary from color to color, or even individual device to device, so you cannot be sure what precise voltage you have. Also, you don't know what speed the breakdown happens at; it may be too slow to stop spikes. Lastly, you don't know how the LED reacts to the breakdown - does it gradually begin to be faulty, or leaky or open?  A zener is a much better protection.
(c) The absolute maximum gate to source voltage will be listed on the datasheet for the mosfet, and for the BS170 that is commonly used in effects, it is 20v. The 9v1 is what I put in the mosfet booster when I designed it and the value has become common. Prior to that, a pair of 1N4148 diodes were normally used with mosfets - one is reverse connected from gate to V+ and the other is reverse connected from gate to ground.
(d) It is protecting the gate from damage from over-voltage, whether from a power spike, static discharge or large input signal.

regards, Jack

[Rob Strand]

Just to be clear here, "protection" has a number of meanings:

1) protection against static discharge
2) protection against abuse in the field
3) protection of components where the circuit itself might produce over currents, reverse bias of junctions etc.

For effects the MOSFET protection covers (1).  In some circuits it may be covering case (3).  As a consequence of being there will help with (2) even though the design has no requirement for type (2) protection (and probably doesn't have type (2) protection elsewhere in the design!).

The reverse bias diodes on BE junctions antonis mentioned are often purposefully added handle type (2) protection and in some cases type (3).   As consequence of being there they help with type (1) protection.

Adding  protection may affect the behaviour of the circuit, this might be good or bad!  For example a large negative swing with LED or Zener protection on a MOSFET will have an effect because the diodes then forward biases. The LED will clip at 1.7V whereas the zener will clip at 0.7V - so they behave differently.  Some circuits you don't what this to occur so you might see a normal diode in series with a zener or two zeners in series in opposite directions.

Another "problem" with protection devices is they add capacitance which can affect the circuit.  Fortunately a MOSFET has a high capacitance and the added capacitance isn't such a big deal.   I'd say Zener in general will have higher capacitance than LEDs.

Unlike Frank, I'm not against LED's as protection.  I think they do a good enough job and I doubt they have breakdown voltages over 20V.  Zener's aren't that fast but they are slow because of the capacitance - which actually helps protect against spikes.   

[PRR]

> with a high enough input ...this LED would clip the signal?

The protection is normally selected to "never" matter on normal signal. Long before the diode conducts, the MOSFET has hit max/min conduction and is already clipping the signal.

Gate-ZAP happens somewhere past 25V. Specs tell you to stay to 20V. This is not a problem because we never need over 10V to jam a MOSFET to the max(min). Modern MOSFETs may only need 2V to swing all the way, especially in non-power circuits.

The turn-on/off voltage varies from one MOSFET to the next.

The "problem" with LEDs is they come in several voltages, not closely specified. Yes, some LEDs with some MOSFETs in some circuits may let the MOSFET do its thing. Other LEDs and MOSFETs or other circuits, the LED may conduct before the MOSFET is properly slammed.

If a known-good plan uses LED, use the same model MOSFET and same color and size LED as the original.

A 10V (7V-15V) Zener is good for all small audio MOSFET gate protection.