SnowyDog

Hi all

I would very much welcome some suggestions on how to go about designing a "fallback" power circuit.

I have a load (microprocessor & some logic) which requires +5v at around 160mA. There are two power supply DC voltages available to the circuit, these are +12v and +5v. A common ground is shared by both.

At the moment, the +5v for the load is derived from a linear fixed voltage regulator powered from the +12v line. However, I would like the option of automatically powering the load from either the +5v or +12v supply, in the event that either the +12v or +5v supply voltages fail. Ideally, the +5v to the load would also not be interrupted in the event that one of the supply voltages fails.

I've tried connecting a power diode between the +5v supply and the load so that the load takes power direct from the +5v if the +12v switches off, however the 0.6 volt drop across the diode takes the +5v below an acceptable level. Also, I wondered if the regulator might get damaged by doing this?

So I wondered if there is a better way of adding this automatic fallback without suffering the volt drop using a diode and without risking damage to the voltage regulator. I thought perhaps this could be done using MOSFETs?

Thanks for any suggestions. I'm new to electronics so please be gentle!

ericgibbs

hi,
Get the data sheet for one of these devices , VN05.
It may suit your application, it may require that you use two of these, the +5V and +12V


http://www.datasheet4u.com/share_search.php?sWord=VN05

__________________
Eric
"Good enough is Perfect"

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Pommie

Replace the 5V diode with 100Ω resistor?

Mike.

SnowyDog

Hi and thanks for your replies.

Mike, if I use a 100R resistor instead of the diode, then there is nothing to stop current flowing back from the output of the regulator into the +5v power supply, which I would think is a bad idea? Would you agree?

Pommie

Regulators don't normally object to more voltage on the output than the input. They experience this situation every time the power is switched off. Why don't you explain what you are trying to achieve?

Mike.

on1aag

Hi SnowyDog,

Maybe you are looking for an automatic battery back-up switch ?

http://www.intersil.com/data/fn/fn3183.pdf

on1aag.

SnowyDog

Hi All,

Yes, sorry Mike I realise that I didn't explain myself clearly in my original post.

I also need the microprocessor to know when either one of the supply voltages is missing. A truth table for the desired behaviour is:-

0=voltage missing, 1=voltage present

+5v +12v behaviour
0 0 unit dead (no power)
0 1 unit powers & reports "error +5v missing"
1 0 unit powers & reports "error +12v missing"
1 1 unit powers - normal operation (no error)

The device that you suggested, on1aag, is pretty much exactly what I need. However, it seems that this particular device is end of life (also non-RoHS which is a problem).

However, I'm fairly sure that it must be possible to do the same thing without having to resort to an off-the-shelf device. After all, I'm halfway there - I can easily tell if the +12v is missing (a voltage divider on the +12v input and transistor gives me a signal I can use) but my problem arises if only the +5v goes missing. Unless I put a diode in the incoming 5v rail, the +5v output of the regulator (that is being powered from the +12v) will drive the +5v rail (through the 100R resistor that Mike suggested using) and how can I then actually tell if the 5v input is missing or not?

Hope this makes sense.... thanks!

SnowyDog

on1aag

Hi SnowyDog,

The device has probably been replaced with a ROHS compliant
version, that's probably the reason why it has been discontinued.

http://www.intersil.com/cda/devicein...ICL7673,0.html

on1aag.

SnowyDog

Hi on1aag,

Yes, you're right, thanks for the additional info.

There's still a problem even if I use this device. That is, what happens if the secondary voltage is missing? The device will switch to using the primary voltage and will indicate this, but I won't actually know if the secondary voltage has failed or not.

However, I guess I can do this by monitoring the secondary voltage separately.

Thanks again.

SnowyDog

on1aag

Hi SnowyDog,

You can use the two additional outputs Sbar and Pbar to
indicate which supply is switched on.

on1aag.

SnowyDog

Hi on1aag

Looking at the datasheet, the additional outputs Sbar and Pbar only appear to indicate which supply is providing the output at a given time, rather than the actual presence of Vp and Vs.

What I want to know is whether or not the secondary voltage Vs is actually present, even if the primary voltage Vp is ok. You might wonder why this is important; the reason is that the +12v that provides Vp (via a regulator) and the +5v secondary voltage is used elsewhere in the circuit.

I can easily tell if primary voltage (Vp) has failed, because the Sbar output will indicate this. However, if only the secondary voltage (Vs) fails I won't know - there won't be any change in the Sbar output.

This assumes that I'm reading the DS correctly, but I think that's it?

SnowyDog

on1aag

Hi SnowyDog,

Take a look at this datasheet:

http://www.intersil.com/data/fn/fn3182.pdf

You can monitor two voltages and use the outputs to
drive external mosfets.

on1aag.

SnowyDog

Hi on1aag

I did look at the ICL7665S as a possible option earlier after you suggested the ICL7673 but I discounted it for some reason. Now I've had another look.

Yes, it does what I want. Actually, the application circuit in fig. 15 of the DS looks close to what I'm looking for.

Because the outputs directly reflect the two supply voltages it is a neater solution than the 7673. As you say, I would need to use two MOSFETS, gates controlled by OUT1 and OUT2 to switch the incoming power accordingly, otherwise feedback from the secondary voltage output would affect the primary voltage detect input.

Thanks for your help, I appreciate your time.

SnowyDog