Output clamping diodes:When do we need them???

[Leolabs]

Saw many high power amp,many do have them but some aren't,why???Because of the design????

[Elvee]

Hi,

They are useful in case of highly inductive loads: they recycle the reactive power dumped by the load to the power supply rails.
An example of such a load is a lightly loaded transformer in a 100V line PA system.
LV

[richie00boy]

They are usually employed when VI limiting of the output stage is used as a protection measure, as the sudden cutting off of the output stage and inductive loads cause flyback spikes which have to be absorbed somehow. With the output stage being high impedance at this point the diodes return the current back to the power supply and clamp the voltage at one diode drop above the rails thus not exceeding the Vce rating of the transistors.

[Piercarlo]

Originally posted by Leolabs
Saw many high power amp,many do have them but some aren't,why???Because of the design????

Left alone these diodes are useless. In order to comply with their claimed nominal function, they should be helped *at least* by another pair of reverse biased diodes which protect the base-emitter junctions of output devices.

However their protective function is just limited in the occurrence of hard-clipping on inductive load. If protection have to be assured also in occurrence of sudden speaker cable disconnection with high but not full drive, other circuits and remedies are to be implemented.

Hi
Piercarlo

[unclejed613]

i've seen them in amps that use protection relay circuits as well..... they also protect output transistors against inductive kickback..... especially at clipping, which is another sharp transition that can take place in the output stage

[unclejed613]

it's also generally accepted design practice to put the clamping diodes across any BJT that might possibly drive an inductive load. they're not needed with most MOSFETs, since the diode is part of the transistor substrate already. with some designs where a driver section might be used with BJTs in one model and MOSFETs in another model with almost the same circuitry and board layout, the diodes might be included with the MOSFET amp, just because they're there. that way the production line doesn't need additional setup time.

[AndrewT]

Left alone these diodes are useless. In order to comply with their claimed nominal function, they should be helped *at least* by another pair of reverse biased diodes which protect the base-emitter junctions of output devices.

However their protective function is just limited in the occurrence of hard-clipping on inductive load. If protection have to be assured also in occurrence of sudden speaker cable disconnection with high but not full drive, other circuits and remedies are to be implemented.

I have never seen this view before.
Any corroboration?

[Piercarlo]

Originally posted by AndrewT
I have never seen this view before.
Any corroboration?

Is simple: imagine a power amp throttling about to half maximum output voltage and playng consistent bass content which usually require current beefing from output devices. In case of sudden disconnection of output cables, especially if long and connected to complex crossover, a voltage spike that may occur may be insufficient to activate the usual clamping C-E diodes ma enough to reverse breakdown the B-E junction of output devices and damage it (usually shorting it).
I've meet, many years ago, a mobile P.A. amplifier which mounted *in series*with the emitters of output devices (two old 2N3055) a 1N5401 power diode just for preventing this occurrence (worsened, in that circustamces, by the presence of an output transformer).

Hi
Piercarlo

PS - However this kind of fault is not "mandatory" Many is related to the path available for discharging the spike's energy. A 100 Ohm resistor placed in parallel to the output often may be enough in preventing serious damage to the active devices. In the most of cases the accident don't cause any damage and the amplifier can continue his work blissfully... But in the case were is not too recommendable counting on luck (professional amplification at first) some kind of preventing measures must be taken in account.

[teemuk]

Originally posted by Piercarlo
In the most of cases the accident don't cause any damage and the amplifier can continue his work blissfully...

I remember reading somewhere that even though a transistor would survive reverse biasing the junction each such occurence will weaken it's structure and contribute to addition of noise. The process is so slow that it's difficult to perceive. So, damage may have happened after all. After several such occurences the transistor will eventually die. Better be safe than sorry.

[Piercarlo]

Originally posted by teemuk
Better be safe than sorry.

I agree fully with you! .

Hi
Piercarlo