how to switch between power supplies using transistor(s)

[chimera_786]

hello EEV forum ! I am back with another question 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

[TerminalJack505]

Have you looked into using OR-ing diodes or an OR-ing controller (aka power multiplexer) IC?

[joersam24]

In your current settings, in what way you can charge the lithium battery?

Have you tried to measure the voltage drop between Collector and Emitter of PNP when you used a lithium battery as a souce? What is the value of your base biasing resistor? Have you tried to lower it?

[joersam24]

You may try to check MAX8677A charger IC from Maxim. Using this, when your external supply is connected, your battery is charged and your load is supplied at the same time. When your external supply is removed the battery will automatically supply your load..

[chimera_786]

In your current settings, in what way you can charge the lithium battery?

Have you tried to measure the voltage drop between Collector and Emitter of PNP when you used a lithium battery as a souce? What is the value of your base biasing resistor? Have you tried to lower it?

Actually, as it turns out, I had biased the pnp incorrectly. After some tweaking, I have gotten it to work. However, voltage at the collector is still low. I need at least 3V at the collector terminal to get my DC DC converter to stay in regulation.

Remember that I am supplying power from a rechargeable battery; so as the current gets drawn, the voltage of the battery drops and when this gets added to the low collector voltage (due to the Vec of the pnp), there is low voltage at the input of the converter.

I might need to change my converter. I am using LT-1302-5V fixed output. I might consider using MC34063. Its cheap and sufficiently efficient and gives better design flexibility.

You may try to check MAX8677A charger IC from Maxim. Using this, when your external supply is connected, your battery is charged and your load is supplied at the same time. When your external supply is removed the battery will automatically supply your load..

In your current settings, in what way you can charge the lithium battery?
Have you tried to measure the voltage drop between Collector and Emitter of PNP when you used a lithium battery as a souce? What is the value of your base biasing resistor? Have you tried to lower it?

I am still tweaking it. the collector voltage drops down to 1.75V when I connect load to the DC converter. Otherwise, with no load on the DC converter, the voltage is around 3.5V. There is also one more thing, the battery size is 750mAH. I know this is small for the load current that it shud be providing and maybe that is why the voltage is sagging; afterall the 750mAH number is just a nominal value.. the actual capacity of the battery is less around 680 to 700mAh. I have a bigger battery on the way, maybe that will help.

The pnp im using is 2n2907 and the biasing resistor are 225ohm for lower one (this resistor provides the ground path for the base current when the battery begins to provide power) and higher one is 22ohms. The 225ohm is from calculation and the 22ohm is use to ensure that base gets pulled high when external source is present. 

I might need to change my converter. I am using LT-1302-5V fixed output. I might consider using MC34063. Its cheap and sufficiently efficient and gives better design flexibility. But a change in the DC converter will only be considered if the bigger battery does not solve the problem (its 1400mAh)

[joersam24]

I think you have a problem in the current rating of PNP transistor you have used.. I have read the datasheet of transistor you used and stated there that its maximum collector current(IC) is 600mA only.. You said that your load needs 5V*600mA=3W. So if the voltage of your battery is 3.6V, your DC-DC Converter needs at least 3W/3.6V=833.33mA of input current.. If this amount of current will pass to your PNP transistor it can destroy it... Try to change your PNP transistor with a high current rating...

[chimera_786]

No, thats not the issue. Besides, I bought it from Fairchild, it has a max of 800mA (pulsed) and a continuous of 600mA. But it still doesnt explain why the low collector voltage. I think its a combo of the battery size, load current and the Vec of the pnp.

Even if I did change the pnp, the low voltage on the collector will still be there. Any ideas?