bucknpnswitch-mode-power-supplytransistors
The MC34063 datasheet shows this diagram for a buck configuration with an external (NPN) transistor (in order to be able to handle higher loads than 750mA):
Neither the datasheet nor the application note provide any guidance in choosing the transistor or the bias resistor connected to it. What are the important criteria when choosing a pass transistor, other than current capacity, and how should the bias resistor on the base of the transistor be sized?
Bipolar junction transistors (BJT) have 3 primary modes of operation:
In cases 2 and 3 there is current flowing from the collector to the emitter.
You can saturate the BJT by attaching a smaller load (i.e. higher resistance) than the linear limit allows. Think of a pipe with a valve. If there's something clogging the pipe upstream, at a certain point no matter how much you open the valve you're not going to get any more fluid flowing through the valve.
Yes, the electrons do literally move to generate the current, though this isn't directly used for most electrical engineering analysis.
For most EE analysis we don't consider the motions of charged particles directly, instead relying completely on the abstract concept of current. This is an approximation of reality, but it turns out to be remarkably good in most cases.
Ironically, in my day-to-day work with plasma physics we do consider the motion of charged particles instead of relying on abstract currents because the analysis does differ depending on what species are moving and where/how fast they are moving.
I think you'll probably find your answer in the L293 data sheet - it's the lower-power version of the 298 (without the heatsink). Here's what the 293 output stage looks like: -
The 293 (and I strongly suspect the same for the 298) has a PNP transistor that is much more conveniently driven from a low side logic signal.
I'm fairly sure the two devices work internally the same way - they have pretty-much exactly the same poor output specification. The diagram shown in the 298 spec "hints" at the limitations inherent in this type of H bridge because of the use of NPN transistors at the "top" and darlington transistors at the "bottom". See also this for alternative devices using MOSFETs.
Best Answer
Other than current handling, I would look for a transistor that has a high DC current gain so that it saturates easily, with a low base current, and one that has a low \$V_{CE}\$ (sat). That is to say, when it is fully turned on, the voltage drop across it is low.
I don't know what additional current handling you are looking for, but I have a suggestion. Check out the 2SD882 medium power NPN transistor. I needed a transistor with decent DC gain, current handling and low \$V_{CE}\$ (sat). After quite a search through numerous data sheets, I settled on that one. I managed to get my hands on a bunch of Panasonic made in Japan ones, and the \$H_{FE}\$ of all of them measured over 360. (This is probably no important in this application.)
The PNP complement of this transistor is 2SB772.
Now let's look at the resistors. \$R_{SC}\$ is a current sensing resistor. This will have some tiny value, a fraction of an ohm. The value is important because it determines the trigger threshold for turning on the outboard transistor. The datasheet doesn't give a value for that in your particular circuit, but I think the value from Fig. 11 of 0.33 ohms can be reused for the Fig. 11a circuit.
Then there is the base resistor on the transistor. Its value is does not appear critical. But note that this resistor will function as the emitter feedback resistor for the switch transistor inside (Q2). The feedback which it develops oppposes the turning on of the internal transistors.
There is a reason why Fig. 11a is called "NPN switch" while 11b is "Saturated PNP Switch". The PNP topology does not develop feedback. The switch emitter is simply grounded.
The PNP looks like the superior circuit; I would go for that one.