# Choosing the right FET to switch between two sources

The circuit below is based on the answer to this question, which i find to be suitable for my application.

3.3VDC is the main supply for the circuit, with 3.6V 1/2AA battery for backup, as defined by the product requirement. The load is basically three switches which should be powered by the main supply until there is an outage, in which case the battery takes over.

In seletcting the Diode, i picked the panasonic DB3X313N with a Vf of 0.55V, this will drope the 3.3V supply to 2.75V which will still work, as the logic input will detect a high.

For the FET, I choose Infineon-BSS84P-DS , with a Vgs(th) of 1-2V.

The load is basically three switches with 10k pull up resistors, so the currrent draw is minimal.

I would like to ask:

1. Will this circuit function as expected, i.e. to supply the load from the 3.3V source and only source from the 3.6V battery when the mains source is unavailable.
2. If so, then is my choice of diode and FET suitable for this circuit, expecially with the FET, as i've not worked with FETs before.

• What was the reason for choosing the FET? What is the maximum input voltage from the Battery – User323693 Aug 10 '16 at 10:50
• @Umar I choose the FET because the battery voltage (which is 3.6V) is higher than the main source voltage which is 3.3V. Without it, the battery will be the one feeding the circuit by default rather than the 3.3V source. – TiOLUWA Aug 10 '16 at 10:53
• Right. the Gate source voltage of the FET at room temperature is -2 V maximum. So, should be fine. The reverse current of chosen diode is in 10 uA range. If the voltage after the diode drop is acceptable, the option is okay. What is the load current expected from the battery? – User323693 Aug 10 '16 at 10:59
• "Will this circuit function as expected?" - what are your expectations? – Andy aka Aug 10 '16 at 11:35
• Forward voltage drop will not be 0.55V. Just look to datasheet. At 25°C and 10mA it will be around 0.25V – Chupacabras Aug 12 '16 at 5:57

## 1 Answer

The device you have chosen is not fully specified for $R_{DS(on)}$ below -4.5V

There is no guarantee that the device will turn on fully with 3.6V of bias, and even if it does then you can expect a relatively high value of $R_{DS}$ of the order of $10 \Omega \ to \ 20 \Omega$ which will give a voltage drop of 100mV to 200mV across the FET.

I would be more inclined to use a part that is fully specified for a lower $V_{GS}$ such as the DMG3415U or perhaps the Si2323CDS, both of which are fully characterised at 2.5V $V_{GS}$

Either of these parts is guaranteed to turn on at the 3.6V bias you have with low on resistance; many others exist and these are simply suggestions.

Basically, look for devices specified for $R_{DS(ON)}$ preferably with $R_{DS}\ \le 100m\Omega$ at $V_{GS} \le$ 2.5V