Audio Amplifier. Which class should I use?

[goldsmith]

Hi shayaan
Let me simplify it ! at first you need a background about mosfet and bjt . at mosfet if you put 15 volts across the GS junction , the DS will become short circuit . but in fact not completely short circuit . the channel will have a little value of resistance . that called Rdson . it is low . ( and sometimes negligible ) .and for bjt : if you put 0.6 volts ( approx ) across the BE junction , the CE will become short circuit . but here you will have base current ( i.e form mosfet there is not any current for gate . because it has pretty high impedance ) . so as you know IC= beta*ib . hence , when vbe is 0.6 volts , ( or a bit more ) but the base current is limited , thus the collector current is limited .
you understand it ?

 

[Shayaan_Mustafa]

Hello sir.

Thanks. I knew all these. But now you told me it in more simplify so I will say yes, I have understood.

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Hello sir.

Thanks. I knew all these. But now you told me it in more simplify so I will say yes, I have understood.

 

[goldsmith]

Humm . shayaan . before continue , let me optimize a thing : one of my best friends noticed me about behavior of bjt . and i think it is better to clarifying it : CE of a bjt won't be short circuit , exactly . it will have a voltage across it self that called VCE sat . ( when we prepared BE voltage and base current .) did you know this ?
Ok now let me continue . so , as you told you are agree with me about on / off region of a bjt or a mosfet . ok ? so , see below , please :

Can you simulate this circuit in pspice and show me the voltage across the resistor and across the DS junction , please ? try to change duty cycle , ( by variation in PW ) . and show me the result . what about power across the resistor ? or power across the mosfet ? can you compare them together ?
Best Wishes
Goldsmith

 

[Shayaan_Mustafa]

Humm . shayaan . before continue , let me optimize a thing : one of my best friends noticed me about behavior of bjt . and i think it is better to clarifying it : CE of a bjt won't be short circuit , exactly . it will have a voltage across it self that called VCE sat . ( when we prepared BE voltage and base current .) did you know this ?

Yes sir I know this.

show me the voltage across the resistor

248.3V across resistor.

across the DS junction

You also asked voltage across DS junction. But sir DS junction will be act as short.

try to change duty cycle , ( by variation in PW )

Sir tried by changing PW, I see the clear result when I decrease PW to 700ns or further low. I have attached thumbnail.

what about power across the resistor

I=4amp to 5amp (approx.) and V=248.3V
P=VI
P=(248.3)(5amp)
P=1241.5W
P=1.2KW

or power across the mosfet

Sir I am confused how to calculate power dissipation across MOSFET..

How much my calculation is right? I have attached thumbnails.

 

[goldsmith]

Hi shayaan
Good night
Of course we can suppose that DS is short circuit ( approximately ) but i want the wave form of the voltage across it ( use differential probe ) ( do you know what is differential probe in Pspice ?
Why 700ns ? of course it is low . increase it until nearby 3.8 usec or decrease it as low as 1usec .

P=1.2KW

Shayaan , can you remember how to calculate the average of a square function ? if yes , you have to show me the average value of power ! if you show me that i will describe my reason by these actions !and about power calculation across the mosfet use a wattage probe on the surface of your mosfet . show me it's average value !
Best Wishes
Goldsmith

 

[Shayaan_Mustafa]

320W (approx.) is dissipated across MOSFET.

And I know how to use differential probes. I have attached result.

Sir I know the formula of average of the square wave but sir I don't know what limits should I use?

 

[goldsmith]

Why your maximum time is 1usec in measurement ? it should be around 20usec hence we can talk about it's power ! increase maximum time of simulation as large as 20usec . and show me again the results .
BTW : why integral ?( limits ?) we have calculated a result for that integral . duty cycle* amplitude of your function

 

[Shayaan_Mustafa]

Why your maximum time is 1usec in measurement ? it should be around 20usec hence we can talk about it's power ! increase maximum time of simulation as large as 20usec . and show me again the results .

Sir you mean by this PW?

BTW : why integral ?( limits ?) we have calculated a result for that integral . duty cycle* amplitude of your function

Yes sir, we have evaluated integral. Now we have formula,

Average=amplitude * D.C.

Sir I have given amplitude with 1.2KW
But sir duty cycle??

 

[goldsmith]

Hi shayaan .
No of course i'm not referring to the PW . i referred to Edit simulation time ! ok ?
And about Duty cycle .
I think i told you at past . but no problem . again : Ton/T T is 1/period time . ok ?
Best Wishes :wink:
Goldsmith

 

[Shayaan_Mustafa]

No of course i'm not referring to the PW . i referred to Edit simulation time ! ok ?

Sir I don't know how to edit simulation time. how much time should I set, I don't know.

I think i told you at past . but no problem . again : Ton/T T is 1/period time . ok ?

Sir still confusing.

Average=amplitude * D.C.
Average=1.2K * 1/period

Like this?

 

[goldsmith]

Sorry if my descriptions weren't as clear as enough .
see below , please . i told you what step needed ! increase it as large as 20 usec :



And about duty cycle , my mean was not that ! let me give an example : consider that we have a square wave , with amplitude = 50 volts and frequency = 250KHZ and D.C=0.5 ( it means ton =2usec ) . t=4usec . and ton =2 usec .
D.C= ton/T ======> 2/4=0.5 . ok ? so can you find your own ?

Don't lose your hope ! ( i believe that you will learn these things very soon and then you will laugh to these simple problems ! :smile
Good luck
Goldsmith

 

[Shayaan_Mustafa]

Thanks for reply sir.

Now I have understood but with little confusion.

1) You told me to increase step as large as 20usec. But how would I know how much I should increase step? Because every time you don't have me to tell me.

2) I know the formula for D.C. But how to know the value of ton and T? as you have used ton=2 and T=4. What should I use? Is it given to me anywhere?

 

[goldsmith]

1) You told me to increase step as large as 20usec. But how would I know how much I should increase step? Because every time you don't have me to tell me.

there isn't any special methodology ! simply , i told you to do that because thus we can see some cycles of this square wave ! is it unclear ?

2) I know the formula for D.C. But how to know the value of ton and T? as you have used ton=2 and T=4. What should I use? Is it given to me anywhere?

You can measure it from your waveform ! but don't worry about it ! you will select it at future ! don't forget that i still didn't tell you what is my purpose by these things ! i bet after some day you will be curious and wonderful about what i have tried to do with your mind !

 

[Shayaan_Mustafa]

there isn't any special methodology ! simply , i told you to do that because thus we can see some cycles of this square wave ! is it unclear ?

Yes sir, because by increasing step I can't see any cycle further. here is attachment with 20usec step.

You can measure it from your waveform ! but don't worry about it ! you will select it at future ! don't forget that i still didn't tell you what is my purpose by these things ! i bet after some day you will be curious and wonderful about what i have tried to do with your mind !

So by this time can I use D.C.=0.5?

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here is attachment

 

[goldsmith]

It doesn't good . what kind of mosfet you have used for it ?

 

[Shayaan_Mustafa]

Sorry sir... I have managed pulse width..
Now I have result... attached.

 

[goldsmith]

Hi shayaan
Well , now it is good .calculate average power of RL and average loss power across the DS and show me the result of your calculations .

 

[Shayaan_Mustafa]

Hello sir..

Average for RL's power:

Average=amplitude*D.C.
Average=1.2K*0.5
Average=600W

Average of Ploss across DS:

Average=amplitude*D.C.
Average=320*0.5
Average=160W

Here is my calculation.

 

[goldsmith]

Well done , shayaan ! . now can you calculate the efficiency ? Recall : efficiency= (pout/pout+ploss )*100

 

[Shayaan_Mustafa]

Yes sir I can,

efficiency = pout/(pout+ploss) *100
efficiency = 600/(600+160)*100
efficiency = 78.94%