how to drive half bridge mosfets

Thread Starter

baggio2

Joined Dec 25, 2008
60
thank you so much bro..you are the best...i will try it too tomorrow..it is so late here and have to go to bed...thanks for your helps...if it doesnt work too could you go on helping me?
 

mik3

Joined Feb 4, 2008
4,843
Yes I can help you. Also, note that the part numbers of the transistors used in the circuit were arbitrary chosen.
 

Thread Starter

baggio2

Joined Dec 25, 2008
60
oh man...you are really the best...i can drive mosfets now...thank you so much..you saved my life:D thank you thank you...thank both of you...you are so kind...i know there are still good people like you in the world...thank you again
 

Thread Starter

baggio2

Joined Dec 25, 2008
60
now i have to calculate the output power for 5w...how can i calculate resitors in bjts(50ohm and 1 k)becos my output current is 1.42A..according to formula P=I*I*Rload=1.42*1.42*8=16.1W (hoparlor's resistor is 8 ohm) i need 790mA ouput current...aplifier works but i need 5w ouput power..it is the last step...thanks again..
 

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Audioguru

Joined Dec 20, 2007
11,248
The 50 ohms feeding 1k ohms does not attenuate the signal much. The value of 50 ohms might be much too high to quickly charge and discharge the high gate capacitance of the Mosfets, then they switch slowly and burn out. That is why I did not use the 1k and I used 10 ohms, not 50 ohms.

R81 does nothing except reduce the max output level. Remove it.

The frequency of the oscillator is only about 80Hz. The lousy old LM324 quad opamp can't go much higher.
The values of the lowpass filter reduce all frequencies above only 100Hz.
The inductor is huge.

If the Mosfets swing 21V p-p then the RMS power is 6.9W into 8 ohms. The resistance of the inductor of the lowpass filter will reduce the power a little or a lot.

EDIT: Math errors corrected
 
Last edited:

mik3

Joined Feb 4, 2008
4,843
The 1K resistor is used to limit the current through the base and its value depends on the gain of the transistors. A 1K will ensure that there is enough current through the base to drive the transistor into saturation.

The 50 ohm resistor prevents high currents to be developed through the BJT when the switching frequency is very high due to the capacitance of the MOS gate. To an approximation, just calculate it to limit the current below the transistors max limit if the MOS was a short circuit.
 
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