Zener diodes for combining different power source?

Discussion in 'Power Electronics' started by Farukh Khan, Sep 10, 2019.

1. Farukh Khan

157
2
Jun 12, 2015
Is it possible to combine 3 different independent power sources with different voltage and current regulations together to get an overall high current stable voltage output?

Example:
Power Source 1: [email protected] (Switch Mode - neither the current nor the voltage is constant 5% regulation on both)
Power Source 2: [email protected] (Switch Mode - neither the current nor the voltage is constant 7% regulation on both)
Power Source 3: [email protected] (Switch Mode - neither the current nor the voltage is constant 3% regulation on both)

Combine to get [email protected] at some defined regulation?
Can a zener diode method help to achieve this? If this is not achievable without changing the power source Independence, how can I efficiently switch between the 3 power sources with a priority setup up.

For Example:
Power Source 1: (Priority 1) - So, if this power source is connected it will eliminate both power source 1 and 2.

Power Source 2: (Priority 2) - So, if this power source is connected it will eliminate both power source 1 and 3 until power source 1 comes back online. But it will eliminate power source 3 even if it is online.

Power Source 3: (Priority 3) - So, if this power source is connected it will eliminate both power source 1 and 2 until power source 2 or power source 1 comes back online.

Just a harmonic priority order system for 3 power source selection.

2. Harald KappModeratorModerator

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Nov 17, 2011
Connecting voltage sources in parallel is not a trivial task. Read more e.g. here.
What does the "zener diode method" you envision look like? Why zener diodes, not standard diodes?

3. Farukh Khan

157
2
Jun 12, 2015
@Harald Kapp I am just thinking about clamping the output voltage at constant 5V using the zener. So, that no back current can flow through the power sources can occur due to exactly same clamped 5V voltages.

4. Harald KappModeratorModerator

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Nov 17, 2011
That will not work. Zener diodes have comparatively high tolerances and being parallel to the output will not be able to stop backward current flow.

157
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Jun 12, 2015
6. Harald KappModeratorModerator

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Nov 17, 2011
Read the info I lonkes in post #2.
The short of it: If the power supplies aren't designed to be connected in parallel, doing so requires additional circuitry. This can range from simply a few diodes in series with the outputs (at the cost of additional voltage drop) to sophisticated circuits using transistors and controller ics.

7. Farukh Khan

157
2
Jun 12, 2015
Can I just use 3 power mosfets and pull the gate high or low using a micrcontroller which will be monitoring the voltage at the source terminal before the mosfet? And then use software logic to implement the source priority order function?

What mosfet to use for this application? P channel or N channel? which will provide me the least power loss on this switching circuitry. Please do suggest some part numbers which might be suitable for this application. Max voltage through the mosfet will be 5.7V and max current will be 3A.

8. Harald KappModeratorModerator

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Nov 17, 2011
You could do that, but the measurement would have to be very accurate.
There are special products for that purpose called "Oring controller". For example these (also available from other manufacturers).

9. Harald KappModeratorModerator

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Nov 17, 2011
N channels typically have lower losses (smaller Rdson) for the same price as P channels.
N channels are much harder to control as high side switches (+ side) becasue they need a positive Vgs which means a gate voltage higher than V+.
Apart from tese limitatins any MOSFET with suitable Vds, Ids and Rdson will do, provided you can generate the required control voltaegs for the gate.

10. Farukh Khan

157
2
Jun 12, 2015
I did not understand why I need that precision controllers @Harald Kapp

Consider using an arduino. I can just monitor the voltage before the fet on each of the 3 power source terminals. And if the arduino senses any voltage at power source 1, it will turn the power source 1 mosfet by pulling the gate high (+5V). And if the voltage at the power source 1 drops to 0V then the arduino will simply pull the mosfet gate to low (GND).

But obviously use this method in conjunction with the three power sources, and monitoring each of the power source voltage lines for signal.

Will this method not work?

11. Harald KappModeratorModerator

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Nov 17, 2011
You'll need more than +5 V on the gate to turn an N-MOSFET on in this circuit as Vgs needs to be positive (e.g. ~2 V for a logic level MOSFET) which requires Vgate = 7 V or higher.
which means you would draw power from all three source at the same time. That is where an O-ring controller comes into play.
If you simply want to switch between the power sources, then your circuit will work, provided you can create a gate voltage > 5 V or you use a bit more expensive low Rds P-MOSFETs (which require a negative Vgs to turn on and as source is at +5V a logic low signal can turn these P-MOSFETs on).
There's a catch, however. Consider your statement:
If the power source powers the arduino, then obviously the arduino will be off when the voltage drops to 0 V. Itthen can no longer control the gates of the MOSFETs. So obviously you'll need some kind of buffer storage (a capacitor) to power the arduino during the transistion from one power source to the other power source. But since the voltage drop of the 1st power source may be fast (depending on the load) an accurate measurement of the voltage is required to be able to switch to another power source quickly.

You'll also have to provide a startup circuit that ensures that at least obe power source is active at turn-on. this cannot be the arduino as it has no power initially. So an extra circuit will be required.

As I mentioned: what you wish to achieve is not that simple.

12. Farukh Khan

157
2
Jun 12, 2015
@Harald Kapp Combining those three power sources for total high amp output using Oring method seems very complicated for the project I am trying to implement the 3 power sources. That's why I will stick to the switching or source selection method for now.

So, which P channel mosfet do you recommend for this particular job where the continuous max voltage and current rating is [email protected]?

And is it possible to just ground the gate of this p channel mosfet or just do not apply any voltage to the gate, for turning the mosfet on?

13. Harald KappModeratorModerator

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Nov 17, 2011
Sorry, I do not have the time to look up one. Go to your preferred supplier's website and use the parametric search. You need to look for so called logic level P-MOSFETS with Vgs > -2 V

No. You should never operate a MOSFET with an open (unconnected) gate. Charge can accumulate and the MOSFET may inadvertently turn on. Always drive the gate wit a defined voltage.

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14. Farukh Khan

157
2
Jun 12, 2015
Got it, Thanks for the answer. Will look for parts now and try some prototyping.