hello EEV forum ! I am back with another question
![Tongue :P](https://www.eevblog.com/forum/Smileys/default/tongue.gif)
So here is what I am after:
I want a transistor(s) circuit to switch between to different power supplies. If external power is present (5V), it should use this source to power a DC DC converter (the load is connected to the DC DC converter). In the event that the 5V is not present, it should switch to a lithium battery source(rechargeable). The load connected to the DC DC converter needs 600mA at 5V
Individually, both the DC DC converter when powered by the external source or lithium source works perfectly fine. The load requirements are met.
I have made both the battery charging circuit and the DC DC Converter circuit and was using a pnp transistor as a switch. In my current setting, the load is connector to the collector of the pnp and the emitter is connected to the lithium source. The external source is also connected to the collector of the pnp. The base of the pnp is biased so that when the external source is present, the base goes high and turns off the transistor. This ensures that lithium is no longer the source. When the external source is disconnected, one of the biasing resistors disconnects from the external source and the only resistor then present is the one connected to GROUND of the system and it pulls the base of the pnp low and then base current can travel to ground causing collector current to flow into the DC DC converter and power the load from the lithium source
The issue is that when the source is the lithium battery and load is connected to the DC DC converter, the collector voltage drops to a very low voltage and the DC DC converter struggles and usually drops out of regulation ( I know I can change the design of the converter, but space is limited and i needed one with minimum component count).
I am thinking about using NPN transistor(s). In this way, I will have freedom to choose best fitting transistor (power rating wise) and frankly, I have always run into issues with PNPs.
So any recommendations for my design. Thanks in advance