Help with amp

lawbadman · 21st February 2004, 07:06 AM

Hi guys, I need some help with a hybrid amplifier I have built. You can see the schematic below.
The problem is that it slowly gets hotter and hotter while playing (regardless of volume), it takes about 10 min for it to reach a temprature were I cant touch the case of the output transistors (IRF540/9540). The transistors do have very good thermal conduction with the heatsink. The bias is only 0.23amps, and I am using 4 ohm speakers. The temprature of the transistors keeps increasing regardless of it playing or not.
I know it seems like I need a bigger heatsink but I doubt that is the problem. A zobel network does not help either. I think I just have a thermal runaway problem, but I could be wrong.

I was wondering if placing the 2sc2240 transistor on the main heatsink would help stabilize the temprature. If not what else can I do?

Thanks
Lawrence

sajti · 21st February 2004, 09:18 AM

Placing the 2SC2240 to the heatsink, probably helps. If no, put it to the case of the output device directly, or increase the source resistors to 0.33R, or 0.47R.

The original design use BUZ900/905 pair which are lateral mosfets, with negative TK, over 100mA. The IRF540/9540 pair is vertical mosfet which has positive TK up to few ampers. This results the thermal runaway, You found.

sajti

Tube_Dude · 21st February 2004, 10:04 AM

Quote:

Originally posted by lawbadman

I was wondering if placing the 2sc2240 transistor on the main heatsink would help stabilize the temprature. If not what else can I do?

Thanks
Lawrence

Hi Lawrence

The answer of Sajti is absolutely correct...and i only want to add a small point..

Check if the off set at the output terminal is the some when the amp become hot...as if you trimm the off set in cold conditions...after warm up maybe it change a litlle ..and this can provoke some increment the output mosfets idle disipation...

lucpes · 21st February 2004, 10:47 AM

I'd throw in a 0.1uf metallized popypropylene cap on each rail as close to the output devices as possible. Also, get the BUZ parts or,

a) Increase the source followers to 0R47 that can but I'm not sure it will help.
b) Replace the output devices with 2sk1058 & 2sj162 (lateral mosfets but cheaper than the BUZ parts) keep the 0R47 source followers if so.
c) add a series coil (10 turns of 1.5mm enameled copper on a crayon//damped with 8R) at the output.

millwood · 21st February 2004, 12:11 PM

using a larger emitter resistor will help but not much. the problem with irf mosfets is that they need a lot of swings at gate to drive. As such, a 0.47ohm resistor isn't going to do a lot of good vs. a 0.22ohm.

using a thermally coupled vbe multiplier is the way to go. better yet, use a "complementory" Vbe multiplier, like the ones in JC3: essentially a npn vbe multiplier for the n-channel and a pnp vbe multiplier for the p-channel output device. and mount them on top of their respective output transistors.

Otherwise, go back to the lateral mosfets.

subwo1 · 21st February 2004, 12:21 PM

Would using a biased mosfet as a bias setter and temperature compensator instead of an amplified diode transistor help?

subwo1 · 21st February 2004, 12:54 PM

I would try using the amplified gate threshold of a mosfet for the bias control and the temperature compensation, possibly either an IRF540 or IRF9540, mounted on the heatsink or on the tab of one of the output mosfets.

djk · 22nd February 2004, 04:49 AM

230mA of idle current is 20W at ±42V

How much sink do you have?

A 2*C sink is about 2" X 3" X 4", that would give you a 40*C rise above ambient.

I have measured a glass front audio rack with just such an amplifier having an internal ambient of 55*C with the room ambient at 25*C.

Under those conditions the outputs will be over 100*C, the boiling point of water.

lawbadman · 22nd February 2004, 06:09 PM

Hi Guys, Thanks for the replies.

I will change the mosfets to the BUZ900P/905P or 2sk1058 & 2sj162. Anyone know were on the internet I can buy the BUZ900P/905P mosfets?

I have a few BUZ901CDP and BUZ906DP double die L mosfets, can they be used?

Thanks
Lawrence

lucpes · 22nd February 2004, 06:44 PM

BUZ901CDP and BUZ906DP - Yep, they'll fit & you can increase the rails safely for more power (if you want and the heatsinking is adequate). Keep the .22 followers in this case and bias them to ~100mA per device.

Are the output devices connected with wires to the PCB? You'd want those as short & thick as possible to avoid problems.

21st February 2004, 12:21 PM	#6
subwo1 diyAudio Member Join Date: Jan 2002 Location: North American Continent	Would using a biased mosfet as a bias setter and temperature compensator instead of an amplified diode transistor help? __________________ USMPS http://groups.yahoo.com/group/switchmode/

21st February 2004, 12:54 PM	#7
subwo1 diyAudio Member Join Date: Jan 2002 Location: North American Continent	I would try using the amplified gate threshold of a mosfet for the bias control and the temperature compensation, possibly either an IRF540 or IRF9540, mounted on the heatsink or on the tab of one of the output mosfets. __________________ USMPS http://groups.yahoo.com/group/switchmode/

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21st February 2004, 09:18 AM	#2
sajti diyAudio Member Join Date: Apr 2003 Location: Vác, Hungary	Placing the 2SC2240 to the heatsink, probably helps. If no, put it to the case of the output device directly, or increase the source resistors to 0.33R, or 0.47R. The original design use BUZ900/905 pair which are lateral mosfets, with negative TK, over 100mA. The IRF540/9540 pair is vertical mosfet which has positive TK up to few ampers. This results the thermal runaway, You found. sajti

21st February 2004, 10:47 AM	#4
lucpes diyAudio Member Join Date: Dec 2003 Location: Iasi	I'd throw in a 0.1uf metallized popypropylene cap on each rail as close to the output devices as possible. Also, get the BUZ parts or, a) Increase the source followers to 0R47 that can but I'm not sure it will help. b) Replace the output devices with 2sk1058 & 2sj162 (lateral mosfets but cheaper than the BUZ parts) keep the 0R47 source followers if so. c) add a series coil (10 turns of 1.5mm enameled copper on a crayon//damped with 8R) at the output.

21st February 2004, 12:11 PM	#5
millwood Account Disabled Join Date: Apr 2003 Location: US	using a larger emitter resistor will help but not much. the problem with irf mosfets is that they need a lot of swings at gate to drive. As such, a 0.47ohm resistor isn't going to do a lot of good vs. a 0.22ohm. using a thermally coupled vbe multiplier is the way to go. better yet, use a "complementory" Vbe multiplier, like the ones in JC3: essentially a npn vbe multiplier for the n-channel and a pnp vbe multiplier for the p-channel output device. and mount them on top of their respective output transistors. Otherwise, go back to the lateral mosfets.

22nd February 2004, 04:49 AM	#8
djk diyAudio Member Join Date: Feb 2001 Location: USA	230mA of idle current is 20W at ±42V How much sink do you have? A 2C sink is about 2" X 3" X 4", that would give you a 40C rise above ambient. I have measured a glass front audio rack with just such an amplifier having an internal ambient of 55C with the room ambient at 25C. Under those conditions the outputs will be over 100*C, the boiling point of water.

22nd February 2004, 06:09 PM	#9
lawbadman diyAudio Member Join Date: Mar 2003 Location: St. Catherine, Jamaica	Hi Guys, Thanks for the replies. I will change the mosfets to the BUZ900P/905P or 2sk1058 & 2sj162. Anyone know were on the internet I can buy the BUZ900P/905P mosfets? I have a few BUZ901CDP and BUZ906DP double die L mosfets, can they be used? Thanks Lawrence

22nd February 2004, 06:44 PM	#10
lucpes diyAudio Member Join Date: Dec 2003 Location: Iasi	BUZ901CDP and BUZ906DP - Yep, they'll fit & you can increase the rails safely for more power (if you want and the heatsinking is adequate). Keep the .22 followers in this case and bias them to ~100mA per device. Are the output devices connected with wires to the PCB? You'd want those as short & thick as possible to avoid problems.

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