Hello all!
I designed this amplifier under this schematics - circuit (see Figure 1).
By the time given to work around the amplifier output mosfet IRFP250 burn (churches) !?
Is someone can help and tell what the is problem ?!
Is someone can make a simulation of this amplifier?
What can I do to change the amplifier works without problems?
Thanks and cheers !
I designed this amplifier under this schematics - circuit (see Figure 1).
By the time given to work around the amplifier output mosfet IRFP250 burn (churches) !?
Is someone can help and tell what the is problem ?!
Is someone can make a simulation of this amplifier?
What can I do to change the amplifier works without problems?
Thanks and cheers !
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Do not install the fuses, instead fit 100 ohm 1 watt resistors in their place. These 'safety resistors' will limit current in the event of a fault and prevent the amplifier blowing up. Fuses will not act quick enough if there has been a build error.
Do not connect any input signal nor any load on the output.
Turn the bias preset potentiometer as far anti-clockwise as possible, do not force it. If you wish to set and check this prior to soldering it into the PCB, orient the preset potentiometer so that the middle pin is higher than the outer pins. Now measure the resistance between the middle pin and the rightmost pin, it should be less than 50 ohms. Connect an ammeter in series with the negative rail. Apply power and note the reading on the ammeter, it should be less than 20mA. The 'safety resistors' should also be cool. If either of these is not true, remove power immediately and check the bias preset potentiometer is wound to minimum. If it is then begin to look for solder bridges, broken tracks, dry joints and wrong components, etc.
If all seems OK then proceed to turn the bias preset potentiometer clockwise and you should see the current increase. Set it to 75mA then keep checking the current every minute to make sure it is not running away - it should not reach 100mA within 2 minutes, if it does then turn it down to 75mA again. There will be some movement whilst thermal equilibrium is reached so do not worry too much. Thermal equilibrium should be reached by about 15 minutes, depending on the heatsink. Do not attempt to connect a loudspeaker or play any signals as the 'safety resistors' will prevent the amplifier working in this way.
Once the bias current has been set you can check the output of the amp to make sure that the DC offset is less than 50mV. If it is more than 1V then there is a fault with the amplifier so check for solder bridges, broken tracks, dry joints and wrong components, etc.
Now that everything has been checked you can remove power and replace the 'safety resistors' with the correct rating fuses and re-check the bias procedure above (the bias current will raise a little due to no longer having resistors in the rails) then the amplifier is ready for use. It will not require further adjustment unless a different rail voltage is used or the heatsink is changed.
Do not connect any input signal nor any load on the output.
Turn the bias preset potentiometer as far anti-clockwise as possible, do not force it. If you wish to set and check this prior to soldering it into the PCB, orient the preset potentiometer so that the middle pin is higher than the outer pins. Now measure the resistance between the middle pin and the rightmost pin, it should be less than 50 ohms. Connect an ammeter in series with the negative rail. Apply power and note the reading on the ammeter, it should be less than 20mA. The 'safety resistors' should also be cool. If either of these is not true, remove power immediately and check the bias preset potentiometer is wound to minimum. If it is then begin to look for solder bridges, broken tracks, dry joints and wrong components, etc.
If all seems OK then proceed to turn the bias preset potentiometer clockwise and you should see the current increase. Set it to 75mA then keep checking the current every minute to make sure it is not running away - it should not reach 100mA within 2 minutes, if it does then turn it down to 75mA again. There will be some movement whilst thermal equilibrium is reached so do not worry too much. Thermal equilibrium should be reached by about 15 minutes, depending on the heatsink. Do not attempt to connect a loudspeaker or play any signals as the 'safety resistors' will prevent the amplifier working in this way.
Once the bias current has been set you can check the output of the amp to make sure that the DC offset is less than 50mV. If it is more than 1V then there is a fault with the amplifier so check for solder bridges, broken tracks, dry joints and wrong components, etc.
Now that everything has been checked you can remove power and replace the 'safety resistors' with the correct rating fuses and re-check the bias procedure above (the bias current will raise a little due to no longer having resistors in the rails) then the amplifier is ready for use. It will not require further adjustment unless a different rail voltage is used or the heatsink is changed.
Hi,
I set quiescent current to about 50mA. Amplifier worked well around 2 ... 3 hours, and then they burned the output transistors IRFP250?
It so happened twice and then I gave up ...
Amplifier has a problem i can not find it and fix, because we need help of experienced professionals!?
thanks
I set quiescent current to about 50mA. Amplifier worked well around 2 ... 3 hours, and then they burned the output transistors IRFP250?
It so happened twice and then I gave up ...
Amplifier has a problem i can not find it and fix, because we need help of experienced professionals!?
thanks
Its not the worst design i have seen in fact similar to one of my old designs.
I found thermal runaway could be a problem so the Vbe needs to be on an output transistor or the heatsink at least.
Also I suspect you are gettingt oscillation on the output, put a 100pf from T7 collector to base.
These two fixes worked for me.
I found thermal runaway could be a problem so the Vbe needs to be on an output transistor or the heatsink at least.
Also I suspect you are gettingt oscillation on the output, put a 100pf from T7 collector to base.
These two fixes worked for me.
Do you need all the resistors (R17, R18 and R19) to be the same?
Real opinions, ...
thanks
Correction for error between VAS and output will be driving lower MOSFET 4.5x as hard.
Not so different from single ended with a CCS. I mean, top MOSFET is driven and active,
definitely not a constant current. But not nearly so active as the bottom MOSFET.
If the above behavior is acceptable: No, they don't need to be the same.
And you will run out of headroom on the bottom much sooner if you fix it,
so there is at least one good reason not to fix it.
Not so different from single ended with a CCS. I mean, top MOSFET is driven and active,
definitely not a constant current. But not nearly so active as the bottom MOSFET.
If the above behavior is acceptable: No, they don't need to be the same.
And you will run out of headroom on the bottom much sooner if you fix it,
so there is at least one good reason not to fix it.
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