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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.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ First off, the difference between your control signal and the "cell" voltage (which is probably a low value and not necessarily the higher value) is enough to keep a PNP always ON. So that already may be a problem you'll need to consider. Beyond that, you do have to deal with dissipation. So we need a lot more information about your situation than you have provided here, I think. What exactly is the "cell" there? What's the peak current for the boost converter -- and in fact what exactly is the boost converter? What's the I/O control signal's current compliance? Etc. \$\endgroup\$ – jonk May 12 at 8:06
  • \$\begingroup\$ by cell I mean li ion cell(3.3 to 4.2v range) which is also a source for MCU(atmega328p), So when I write pin high it will be same as cell voltage which is given to emitter of PNP hence transistor will turn off. I am using this configuration currently (used pull up resistor too on signal pin, figured out not needed). Boost converter is to boost the 3.3v to 12v to drive RF transmitter. I have measured peak current of around 500-530mA, and transistor I'm using have rated 600mA.Up to this point everything is working. \$\endgroup\$ – Dharmik Prajapati May 12 at 9:04
  • \$\begingroup\$ My question is what if I replace this transistor with the one with 1.5 A current rating anything changes? \$\endgroup\$ – Dharmik Prajapati May 12 at 9:05
  • \$\begingroup\$ Al higher current rating or wattage rating will not hurt if the other parameters are OK, it is actually better. \$\endgroup\$ – Gil May 14 at 2:45
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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.

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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.

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  • \$\begingroup\$ mosfets that switches at input logic 3.3v are quite rare and comparatively costly and not locally available. So ill like to stick to transistor. Even if I found one I like to know how it is affecting. It is more like learning stuff then completing task. \$\endgroup\$ – Dharmik Prajapati May 12 at 9:12

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