Protection of Mosfet Amps from input Overdrive...

[Workhorse]

Hi everybody,

Everybody knows about clipping in amplifiers, which happens when input signal exceeds its maximum limits and output starts to flatten between the power supply rails...If input drive exceeds, then the mosfet might fail instantly due to gate rupture.
But one thing...what would be the best protection implemented to protect the mosfet amps form overvoltage drive at inputs.
1.Zener diode at gate of mosfets[everyone uses it]
2.back to Back Zeners at input of amp.[some of us might use it.]
3.You guys tell me.

Waiting for your nice suggesstions....

Kanwar

[vAD]

It only idea, therefore your comments are interesting to me, pin @to input@ soft limiter.

[Bensen]

This could be an intersting thread for me, as I'm in the middle of designing an amp and and looking for protection circuits.

This next URL shows in point 7 a current limiting protection. But I wonder if this has maybe influence on the sound colour/quality?

http://www.ee.und.ac.za/coursemain/D...wrAmpNotes.htm

The ETI466 has also a protection circuit thas goes out from the voltage over the emittor resistors.
But using this kind of protection isn't perfect, because the maximum amount off powerdissipation isn't produced when the voltage over the Re's is max.

The powerdissipation is also affected by the phase angle at the output of the amp.
Example:
PSU voltage +-48V / 190Wrms/4Ohm
*0° --> max Pd=130W --> Vgs=25V / ID=4.9A

*50° --> max Pd=340W --> Vgs=49.12V / ID=6.9A
--> by ID=4.9A --> Pdmax=39W

So, with this kind of protection you can never get the full output power.

[Bensen]

Sorry gentleman, I've made a big mistake.
I've mistaken the phase angle at one certain moment with the output power.


BUT, with that exaple: (3x2SK1530 /55°C)
50° --> Imax is 7A to not cross the SOA curve
0° -->Imax is 10A to not cross the SOA curve

So you need to know the inductance/capacitance (phase angle) of the speaker system to get maximum output power without exceding the max power dissipation.

Greetz
Ben

[Bensen]

Phase angle 0degrees 190W in 4Ohm

[Bensen]

Phase angle 50degrees 190W in 4Ohm

[Workhorse]

Bensen,
Please stick to the topic, we are here to discuss overdrive clip protection not the overcurrent protection...

vAD....thanks ...any suggestions to offer....

Overdrive certainly damages the amps if it exceeds certain limits....

regards,
Kanwar

[ilimzn]

Kanwar, this actually is on topic as any overload of the output will cause gate overdrive due to NFB trying to force the amp to do something it cannot. Besides, zeners are often used as current limiters, and if present, will quite simply protect the MOSFET from gate overdrive.

That being said, amps WILL clip. Either by current limit, or voltage limit (trying to go 'over' the power rail voltages). In both cases there is potential for gate overdrive. As far as protecting the MOSFET, a zener is perfectly adequate. The choice of using one, using one + a diode, or two anti-series, has little to do with which one is better, as the putpose is voltage limiting, but is a consequence of the difference in Vgs_bias and Vgs_limit. For complementary SF configurations, if (2xVgs_bias + 0.6V) > Vgs_limit, you can use one zener diode. In other cases, you have to use a zener + regular diode. Reason is simple, you do not want the forward biassed zener of one polarity MOS to limit the other polarity MOS, or seriously upset Vgs_bias. For complementary SF configurations, negative Vgs on the N channel part is automatically limited by limiting the maximum negative (i.e. MOS turned on) Vgs of the P part and vice versa - the negative voltage can at most be Vgs_limit - 2*Vgs_bias. Because of this I have found anti-series zeners to be unnecessary, but of course, they will work just fine, except that one of each anti-series zeners will really be working as a normal diode.
All the above examples imply no source resistors on the output transistors. If there are some, Vgs_limit include the voltage drop on these at the desired maximum current. This also means that Vgs at maximum current + voltage drop on the series source resistors (=Vgs_limit ~~= zener voltage), must be less than Vgs_breakdown, as the same zener limits gate drive for cases when output current is 0, so there is no voltage drop on the source resistors.

The real question is, do you want to protect your ears from gate overdrive - in other words, when an amplifier clips, given there is likely a NFB in place, ehat happens to the front stages? Are there any overhang effects? Any saturation and associated carrier storage problems in the front end transistors? Etc, etc. Unless the amp front end is designed for this not to be a problem (an interesting example of some of that can be found in the Stochino non-slew-limited amplifier), the only real solution is clipping the input signal when the gate gets close to the treshold - i.e. 'pre-clipping'. I think vAD is on the right track, though the idea needs serious polishing (sharper and better defined threshold, speed issues on the optocoupler...)

[Workhorse]

Hi ilimzn,

I have already mentioned the zener protection for gate overdrive in my first post....i think you misunderstood my topic....
I already know what you have said in your earlier post....
I just want to say that, if an amp [output load capability of driving 2 ohms]is loaded by only 16 ohms and input signal limit which is 1 volt for full output exceeds to somewhat 15 volts in input than the output starts to hard clip.....just finding the right solution to this problem.......
One way to go is using VCA voltage control amplifier, or using an Opto coupled FET, or optocoupled LDR to solve this problem....other i dont know yet.

Though i have seen some pro-amps using back to back zeners at input to clamp the over voltage at inputs and thus protects the output also......

regards,
Kanwar

[sam9]

I've used zeners in a headphone amp where I had concerns that inputs of a higher voltage than normal line-level might be applied. While a simiulation will tell you that they cause some distortion at levels below the hard limit, the amount is actually so small (.000x%!) that you won't hear and can't easily measure it. Simple and effective. I suggest you not worry and just use the zeners -- no need to come up with something novel.

For a power amp they are used used to protect input circuitry especially in pro-amps where being "idiot proof" is a real concern. If the concern is with protecting the output devices perhaps a more satifying approach would be to reduce the closed loop gain. In the pro-world there is a sort of custom that the gain is set so that rated power is achieved with a 1V input. This is often cited in a "sensitivity" spec. Consumer audio sometimes used lower sensativity/gain because rated power is aimed more at increased headroom rather that average volume.