Selfoscillating class D amplifier

[vogor]

Originally Posted by 81bas

Yes, I have seen oscillating 78XX very often. They can even heat from such reason, when there is nothing connected to the output!

For my startup/shutdown schematic is important, that the driver vcc will fall down AFTER the main power supply. Disconnecting the fans from your 18V supply would provide this effect easily.

Ok, I will continue experiments later.

[vogor]

Originally Posted by 81bas

are you sure, that the amp was started in this case? you wrote, that it was needed to give some small input signal, before it starts to work

I have reduced 200K the resistor to 100K now with it everything is all right

[81bas]

Originally Posted by vogor

I have reduced 200K the resistor to 100K now with it everything is all right

Then why you needed the startup/shutdown circuit at all? Only because of small PUP at shutdown??

[vogor]

Originally Posted by 81bas

Then why you needed the startup/shutdown circuit at all? Only because of small PUP at shutdown??

Yes, I wish to eliminate this PUP at shutdown, then still protection and a limiter will need to be added

[81bas]

Originally Posted by vogor

I have reduced 200K the resistor to 100K now with it everything is all right

Also, did you used the Zener diode parallel to C13? With 100K resistor at driver idle there can be produced too high voltage also!

Also, I think I understand the reason of this failure... In my startup/shutdown schematic I have seen in simulations, that the 100uf cap will charge to 15V in 2-3 seconds via 100K resistor (providing SD delay) with 65V rails, and if you connected my startup/shutdown circuit, then it has produced the startup delay, and your 47uf C13 cap have charged via 100k resistor in 2-3 seconds to about 30 Volts!!! (at idle the driver consumes very small current) Sure it has killed your upper mosfet after startup! And I have recommended you to use the Zener diode parallel to C13 already in previous posts!

[81bas]

Originally Posted by 81bas

Then why you needed the startup/shutdown circuit at all? Only because of small PUP at shutdown??

LOL! I have seen in IR2110 datasheet now, that after active SD, the high and low side will be NOT reenabled, if the HIN or LIN is active So delaying the SD signal at startup will not help to start amp oscillation! (have AP2 said about it already?)
Also, connecting the 100K resistor between +65V and the amp OUTPUT will help to start oscillation in the same way, as connecting this resistor to Vb! It works because 100K resistor produces POSITIVE DC offset at output during startup (via charging C13 or directly) and the input integrator needs to initiate the oscillation with NEGATIVE pulse

Shame on me, that I did not realised it previously

[vogor]

Originally Posted by 81bas

LOL!...
Shame on me, that I did not realised it previously

Yes, similar top mosfet has burnt down for this reason, zener in parallel с13 could rescue the scheme... So what your general result taking into account result of my last experience? your scheme of deenergizing will work if to use the separate rectifier for 7812?

[81bas]

Originally Posted by vogor

Yes, similar top mosfet has burnt down for this reason, zener in parallel с13 could rescue the scheme... So what your general result taking into account result of my last experience? your scheme of deenergizing will work if to use the separate rectifier for 7812?

yes, this will work! The startup delay seems to be useless, but since you have implemented this already, it will not interfere.

[vogor]

Originally Posted by 81bas

yes, this will work! The startup delay seems to be useless, but since you have implemented this already, it will not interfere.

One more moment, what elements in your scheme are responsible for operation and occurrence SD border, at amplifier "PUP" deenergizing appeared almost at once, I think it is necessary to raise a threshold to 55V, what it is necessary to change for this purpose?

[81bas]

Originally Posted by vogor

One more moment, what elements in your scheme are responsible for operation and occurrence SD border, at amplifier "PUP" deenergizing appeared almost at once, I think it is necessary to raise a threshold to 55V, what it is necessary to change for this purpose?

It is needed to play on R4 and R2. But 35-40V threshold is OK too, I think...