Power Oring Using Mosfet

1. Power Oring Using Mosfet

Here's the Circuit, as shown in fig1, There's Supply from adapter(9.5V DC), there's also a supply from the battery(8.4V Max). Diodes used(D1,D2) are shottky diodes, with a typical drop of (0.3V-0.5V). The problem i was facing was even though battery voltage at given point of time is less than Adapter Voltage(ADPT), At TP3, I was still getting battery Voltage.
The issue was resolved when Implemented the same circuit with a small change(Adding a resistor RL1), as shown in fig2.

Queries:

1. I would want to know what was the issue with the circuit in fig1 and how did it get resolved in fig2?

2.Is there a batter solution to the given problem ?

Part Numbers Used:
D1,D2 : SK85
U4: Pmk35EP,518
LI-Ion Battery (7.4V, 8.4V Max)

•

2. Re: Power Oring Using Mosfet

Hi,

1. I would want to know what was the issue with the circuit in fig1 and how did it get resolved in fig2?
Without the resistor the node is called "floating". (No predictable current flow through either diode)
The voltage of floating nodes is undetermined.
When you measure the voltage of a floating node you get unpredictable values.

As soon as you connect the resistor you get predictable current flow through (at least one of) the diodes. Thus you get predictable voltage of the node.

2.Is there a batter solution to the given problem ?
If you write "better" then you also need to write "in regards if what".
"Better" in your case can have a lot of meanings:
* lower cost
* low part count
* least board space
* lower power consumption
* less voltage drop
* higher current capability
* fast response
* better availability
* ....

Klaus

•

3. Re: Power Oring Using Mosfet

The report makes no sense. According to given schematic, the MOSFET is switched off when V(ADPT) - V(out) > Vth. Assuming a threshold of 1 V, this happens already with V(ADPT) > V(bat) - 1.

The purpose of RL is probably different, avoid inadvertent charging of V(ADPT) by diode leakage.

•

4. Re: Power Oring Using Mosfet

Better means in terms of better circuit design and stability, also would be better if its a low-cost Solution

5. Re: Power Oring Using Mosfet

Hi,

Originally Posted by zuirgham
Better means in terms of better circuit design and stability, also would be better if its a low-cost Solution
Depending on current draw, a simple solution is to replace all of the PMOS + Schottkys + etc. section/circuit with a DC barrel jack, pcb mount or panel mount, the type that have three pins: one for gnd, and two for V+ (insert connector = disconnect battery from circuit, remove connector = connect battery to circuit).

If a barrel connector is the wrong shape, there must be other types that fulfil the same connect/disconnect function, e.g. some audio jack connectors do this, too.

•

6. Re: Power Oring Using Mosfet

There is also a charging circuit present. That will not work as the battery gets disconnected when adapter is connected.

7. Re: Power Oring Using Mosfet

There is also a charging circuit present. That will not work as the battery gets disconnected when adapter is connected.
Why are you telling this now? There's no information about a charging circuit, it's operation principle and how it's connected in your posts up to now.

You should either tell the full complete story or omit the point.

1 members found this post helpful.

--[[ ]]--