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# PNP power rating - the transistor's max power

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#### ringo888

##### Member level 1
PNP power rating

Hi,

If I had a circuit with a PNP transistor and a resistor which will draw 3W on the collector side do I need to ensure that the transistor is capable of taking upto 3W. Its collector current is fine for what i need but i'm not sure if the transistors max power rating needs to be the same as the resistor.

Ringo888

Re: PNP power rating

Power rating of the transistor is independent of the resistor. The power dissipated by the transistor is Pd = Vce x Ic. So, the product of the current flowing through the transistor multiplied by the voltage across its emitter and collector will define the power it dissipates. This is the steady state power. If you´re switching it you´ll have to consider also the power dissipated during transitions. This is valid for bipolars, FETS and any other kind of switchers.

Re: PNP power rating

Power rating of the transistor is independent of the resistor.
I don't agree. If you have a transistor and a load resistor, the maximum transistor power dissipation is V²/(4R). (The resistor rating should be V²/R in contrast)

Re: PNP power rating

V²/4R is the maximum power that can dissipated according to the maximum power theorem. If the design goal is to dissipate maximum power then it´s all right. But in practice that´s not the goal. And again, in a practical circuit, the dissipated power depends on the current and voltage on the transistor at any time.

Re: PNP power rating

FvM said:
Power rating of the transistor is independent of the resistor.
I don't agree. If you have a transistor and a load resistor, the maximum transistor power dissipation is V²/(4R). (The resistor rating should be V²/R in contrast)
I don't understand that math.

I've learned that maximum power dissipation is when the voltage Vce = U_resistor. That means that Pmax = V²/(2R)

Re: PNP power rating

[quote=" Pmax = V²/(2R)[/quote]

In this equation R is the total resistance as seen by the source, which must equal the source output impedance. In your circuit R is only your "resisive" load which, when equal to the transistor´s impedance, generates maximum power. In the discussion above the source output impedance was considered to be zero.

PNP power rating

I was thinking DC only.

Re: PNP power rating

I've learned that maximum power dissipation is when the voltage Vce = U_resistor.
Exactly.

Vce = U_resistor = V/2
P_transistor = P_resistor = (V/2)²/R = V²/(4R) q.e.d

Re: PNP power rating

Agree.

It's just too hard to think while being tired

PNP power rating

Hi ringo888,

I think that you cannot understand what is going on?

2) Load connected in which manner?

3) If its resistive load, what is the value?

4) How much current need to dissipate 3W? at what potential(V)

5) Reffer transistor datasheet for

Operating voltage
gain
current driving capacity.

Note:

otherwise calculate accordingly.

Now you can realize weather your transistor withstand or not for your application.

If you feel its not withstand, you are going to search suitable one with the help of datasheets.

Bye.

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