use of power dissipation in transistor and using higher current rated transistor

Question

I am using using a PNP transistor in one of my application to switch on and off the boost convertor. My requirement for collector current is 500mA.

My question is does it matter if I use 500mA/600mA/800mA/1A/1.5A collector current rated transistor ? Because using one with slightly higher value will give some tolerance so I am thinking to go for 1A. My concern is mainly voltage drop across transistor(emitter to collector).I am operating transistor at 3.3 volt so I can not afford high voltage drop across transistor. So does it even matter ?

NOTE: assume enough base current is provided according to gain.

Another thing I don't understand in datasheet while choosing transistor is power dissipation. How one can filter that parameter for application. For example if I have 2 transistor of same rating with only difference is one have power dissipation of 250mW and other 500mW.

Any explanations or pointers will do. I am just trying to understand different parameters of transistor to choose most optimum one. Thank you.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

There's no harm in using a transistor with a higher current rating, and allowing a good safety margin is wise to prevent the circuit overheating. Running a 500mA rated transistor at 500mA is pushing it a bit.

The voltage drop (and hence the power dissipated in the transistor) won't change significantly, provided you ensure that the new transistor is still saturated. Higher current transistors may have a lower gain than small ones, so check the data sheet and see if R1 needs adjusting.

Answer 2

If your concern is the voltage drop across the transistor, then you rather use a MOSFET which will have very little drop compared to a BJT.

You do not need lots of tolerance using a MOSFET as long as your current is below its rated current. If you have capacitors in your Boost converter, consider the rush current.