Undervoltage and shut off detector for Class D prototype

[Andy F]

Hi:

I am working on a class D proto based on IRaudamp3 application note from IR.
By now the prototype is working and I get 150w rms of clean audio (more refinements are on the way).
Now I want to add some protection (overcurrent sensing is made using a sense resistor in series with the upper mosfet and another one in series with the lower one, the voltage sensed is used to trigger a NE555 as monoastable to produce a shutdown of 3 seconds on the mosfet driver chip.
Also, I use the same NE555 to produce a a delayed soft startup of 3 seconds when the amp is turned on.
Now I try to implement a similar circuit to avoid the nasty pop noise when the amp is shut down so I am thinking in a kind of undervoltage detector to detect the voltage ramp fast enough to activate the shutdown pin on the driver before the noise gets the amp output.
Can someone give me some directions or ideas to proceed? (I am using the IR IRS20124S driver, it has an active low SD pin).

[classd4sure]

How are you shutting down the amp currently?

[Baldin]

If you are using the IRS20124S, then why do you need an extra circuit for current protection? I thought this was one of the very nice builtin features of this driver IC!

Are you byeing the drivers online? If so then where ... I would like to try them out.

AC fall out can be detected by placing 2 diodes directly on the fransformer (before your rectification). These will serve as half wave rectifiers. This will then be detected by a small circuit around 2 transistors and a capacitor. See drawing. You'll have a signal on the "out" when the AC is on.

Haven't yryied this circuit out yet .... working on protection myself .... so maybe do a bit of sim and trial

[Andy F]

Hi:

For Classd4sure:
I am doing nothing by now, when the amp starts the ne555 holds the SD pin of the driver low for 3 seconds through a small mosfet transistor (2n7002) and from then you can only hear a very small pop on the loudspeaker, nothing to worry about, but when the power supply is switched off the pop become a noticeable and dangerous thump on the loudspeaker. Maybe I need to stop the switching to avoid this during the downward ramp. I dont want to use any relay on the output.

For Baldin:

I checked the overcurrent feature implemented on the driver and it works very good but only for a maximum of 100 watts of power (if you download the IRAUDAMP3 data sheet from IR you can see it, they recommend to use an external overcurrent circuit for more power).

About the IRS20124S you can buy it through Farnell (www.farnell.com)

Your circuit (thanks) could be great if you use a linear supply but I use a SMPS so I dont have an isolated secondary except the output power transformer of the SMPS, I would prefer to implement the shut off circuit on the amplifier pcb board this way do not depends on the supply used (linear or switching).

[classd4sure]

Maybe I need to stop the switching to avoid this during the downward ramp. I dont want to use any relay on the output

Hi,

Yes I think that's the way to go. Either trigger the 555 or pull down the SD pin with another transistor, I think it will make a big difference.

[zilog]

Could you please post your overcurrent protection circuit? I have implemented the circuit from some Iraudamp schematic but it still triggers on close to none output current...

I am also interested in your layout for the overcurrent parts since I suspect I might have a nasty EMI problem in my setup.

/Daniel

[Andy F]

Hi Zilog:

This is the schematic you have requested.
It is not implemented yet on board so I dont have the definitive layout but without it my amp have no Emi issues of relevance.I will test it on a breadboard at first as it simulates good.
You can adjust the sensivity changing R42 and R36 or you can change the sense resistor value.

[Baldin]

Hi Andy

I think you should still be able to use the AC fall out detection circuit with an SMPS. Just connect the diodes to the mains (need diodes to handle the higher voltage).


With regards to the IRS20124S ....... and I thought that was one of the coolest features on a mosfet driver yet ..... 100W hmmm that is really not a lot

.... Farnell ... think they only want to deal with registered firms here in Denmark

Are you using FQP45n15v2 in your circuit or only in the sim? They are pretty slow devices ... but they take a lot of current and have really low Rds
Are you goning for sub-amp or full-range?

[Andy F]

Hi again, Baldin:

I am using it full range.
I did some listenning tests and comparations, it outperforms other amps like B&O 500A, it sounds much better and I have a very clean switching waveform on output with little Hf carrier residue as my main goal was for sound quality. The overcurrent circuit is not ready yet so I rely on the internal protection of the driver, it works very reliable and fast, I tried to trigger the overcurrent prot a lot of times and the amp recovers very well, when the external overcurrent and overvoltage circuits are ready I will check for more power, now my supply has +-45 volts but I want to get 400 or 500 watts so I will try it using +-60 volts or so, now I am waiting for the sense resistors to come before to proceed for more power.
Regarding Mosfets I will try another ones but by now I have very good results with FQP45n15v2 using an average dead time of 25nS on the driver and 33 ohm resistors and SS14 diodes on the Mosfet gates, also I use a snubber (RC) across drain and source (10 ohm and 100pF). My switching frec is about 500kHz.
I dont know yet how well will perform at 400 or 500 watts but
for 150 or 200 watts the sound is so good and clean that it seems incredible, I have a lot of another brands (Class D or analog) to compare as I work for a pro audio company. I really think that one of the reasons of this success is a carefull pcb routing and decoupling, double sided, ground plane, etc... the same schematic can work very diferent on another pcb design.
When I have the shutoff issue solved and the protection ready I will be able to do full power tests but the beginning could not be more promising.

[Baldin]

Hi Andy

Sounds like a very interesting project. What kind of sense resistors are you going to use?
Guess if you are going to go for say 500 W in 8 ohms you'll need to trigger at about 8A, and that will require some 3 W in the resistor. These also needs to be low inductance types.

Actually I was thinking of letting the over current sensor only more or less register mal function or short circuit, and have it trigger at say 15 - 20 A.

And thinking about it you actually need around 14 A in a 0.05 Ohm resistor to trigger the transistor (0.7 V / 0.05 Ohm = 14 A) .... but that requires 14 A * 14 A * 0.05 Ohm = 9.8 W @ DC
........ well only 5 W if you calculate for a sine wave

The snubber you are using 10Ohm/100pF what W rating is the resistor?
Are you also using fast diodes across the mosfets?

Guess I should also get my own amp finished. Needs protection, a new PCB and a few other alterations. I also need to do some more power testing .... I blew my test speaker, which have put the project a bit on hold.

By the way, do you plan to post the whole circuit?