Emergency Power Supply Schematic Diagram

I am designing an emergency power supply.
GOALS
-The regulated source should provide power to the load when there is power in the mains.
-When a power outage occurs, a backup battery should take over.
-When the mains power is available again, the regulated source should power up the load again and charge the battery at the same time.
-When the battery is full, the charging should stop

This is my current circuit: **broken link removed**
V1: Regulated Buck supply
V2: Battery
R3: LOAD

Switch States: With Mains power, battery is charging
1) Q2 is switched on which in turn switches Q3 on. Battery starts charging.
2) Q4 is switched off since the Regulated supply voltage is greater than the battery voltage.
3) Q1 is switched off since the battery is not yet full.
4) Substate: Battery is full. Zener diode D1 lets current flow to the Gate of Q1 which switches it on.
This switches off Q2 and Q3 since the Gate voltage is insufficient.

Switch States: Without Mains power
1) Insufficient voltage switches Q2 and Q3 off.
2) Q4 switches on since the battery voltage is now greater than the Regulated supply voltage.
3) Battery supplies power to the load.

Any wrong assumptions and mistakes in my design? Any ideas? I am not yet sure if this will work.
All suggestions for improvements are welcome :smile:

whay did you use d3 and d4?

By the way, it looks okay. Good job!

- Q3 will never fully turn on due to insufficient gate voltage, Q3 source voltage is limited near 0 V
- Q4 substrate diode will always conduct for V2 > Vload, Q4 is effectively useless
- threshold of the battery full circuit isn't well defined

rhnrgn said:

whay did you use d3 and d4?

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A higher level of safety? I wanted a unidirectional current. I am not sure if the diodes d3 and d4 can be removed if my desired circuit functionality is "theoretically" correct.

FvM said:

- Q3 will never fully turn on due to insufficient gate voltage, Q3 source voltage is limited near 0 V

Click to expand...

How about this circuit? **broken link removed**
I added a resistor R5

FvM said:

- Q4 substrate diode will always conduct for V2 > Vload, Q4 is effectively useless

Click to expand...

Is the substrate diode same as the body diode? Isn't the body diode in reverse direction of the switch conduction path? Can you enlighten me? Perhaps with internet readings/references?

FvM said:

- threshold of the battery full circuit isn't well defined

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It is assumed that the battery terminal voltage is about 2 volts less than the regulated supply. I can always adjust the regulated supply (say that the needed load voltage is 10 Volts, I will choose a regulated supply of 13-14 Volts and a 12 Volts battery). The zener diode will be chosen in a way that it can detect the full battery threshold.

I am not sure if the diodes d3 and d4 can be removed if my desired circuit functionality is "theoretically" correct.

Click to expand...

At least D3 is necessary to avoid battery discharge into the main source.

How about this circuit? **broken link removed**
I added a resistor R5

Click to expand...

Not essentially better. Did you consider just to omit Q2?

Is the substrate diode same as the body diode? Isn't the body diode in reverse direction of the switch conduction path?

Click to expand...

Yes.

Can you enlighten me? Perhaps with internet readings/references?

Click to expand...

A full MOSFET datasheet has all necessary information.

A lot more could be said about characteristics of a 12V battery, required charge voltage, design of an accurate voltage regulator.

FvM said:

Not essentially better. Did you consider just to omit Q2?

Click to expand...

Oh I see. Q2 was not necessary for the biasing.

All right. This is my new schematic **broken link removed**
I removed Q2.
I removed R2. It has been pointed out to me that R2 will limit the charging current. They also said that I should replace Q3 to a Darlington NPN transistor though I am not sure what the increased Beta will do for me. I only wanted Q3 as a switch. Or would a Darlington NPN serve a double purpose?
I replaced Q1 from a MOSFET to a BJT. So that it is current controlled instead of voltage controlled.

FvM said:

A lot more could be said about characteristics of a 12V battery, required charge voltage, design of an accurate voltage regulator.

Click to expand...

I see. The only thing I know about the battery that will be used is that it is rechargable. More on this later.

Hello guys. This is my newer schematic diagram
**broken link removed**

I decided to use LM338 (LM317 in the figure) to regulate voltage. It is rated 5 amperes
I replaced the FETs with BJTs so that less voltage is needed to switch them on.

R2 and R1 sets the regulator output to
Vreg = 1.25*(1+(R2/R1)) = 14.27 volts

Charging current is limited by R3, R4, and Q1.
Ichg = Vbe,sat/R3 = 0.8/0.25 = 3.2 amperes**
**asuming base current is very small. I got the circuit from https://www.ti.com.cn/cn/lit/ds/symlink/lm138.pdf page 16. I recalculated but I am not sure of the values because Vbe,sat is in the range 0.7 volts to 2 volts depending on the collector current

Charging stops when the battery voltage reaches Vzener + Vbe,sat of D6 and Q2 again assuming that base current is very small. Vzener + Vbe,sat = 13 + 0.8 = 13.8. Q2 shorts R2 which decreases the LM338 output which stops the charging.

Battery supplies power to the load through Q3, when V2 (POWER SOURCE) is open/short. V2 provides power to the load otherwise (it is not shown in the schematic but basically V2 is fed to another LM338).

The extra capacitors and diodes are for improving response time and protection of the LM338 regulator.

Please comment again. Thanks guys!