Auxiliary power supplies

[NiHaoMike]

Let's suppose you need another power supply in your design that only needs to source a few mA or less. For example, you might need +-12V in a design that is mostly 3.3V to power analog circuits or RS-232 transceivers. Perhaps go over designs like charge pumps that use existing circuits in your design or low cost/small size separate power supplies.

In a network router I built, I needed +-12V for RS-232. The input power supply is a regulated 12V, so that is already taken care of. To derive the -12V, I made a simple charge pump out of two diodes, two capacitors, and a resistor, deriving the drive voltage from one of the existing buck converters.

[Zero999]

There area all sorts of options, another one is using a mains adaptor with an AC output so negative voltages can be made using a couple of diodes and capacitors.

Here's a multistage boost converter schematic I found using Google, you could probably use an octal inverter such as the 74HC240.

http://www.solarbotics.net/library/circuits/misc_pump.html

[NiHaoMike]

Here's a multistage boost converter schematic I found using Google, you could probably use an octal inverter such as the 74HC240.

http://www.solarbotics.net/library/circuits/misc_pump.html

That would definitely cost more and use more board space than making a charge pump that derives the drive voltage from an existing switching power supply.

The interesting part is that a really old ADSL modem I took apart used an AC power supply so it could use a charge pump to derive a negative voltage. I'm surprised they didn't source the charge pump from one of the two buck converters in order to reduce the size of the charge pump capacitors, along with using a more common DC power supply and getting rid of the huge filter capacitors.

[Zero999]

I misread your post, so you want to derive -12V from +12V not 3.3V.

It looks like you've answered your own question.

[alm]

You could also use an RS-232 transceiver with built-in charge pumps (eg. the popular MAX(3)232 and siblings), but I'm sure this was considered and rejected.

[NiHaoMike]

Yes, that particular problem is solved.
Let's suppose that in another application, you have a similar requirement (need +12V or -12V but only have 3.3V or 5V), but there are no existing switching power supplies (or they are shut down when the auxiliary power is needed). Then go over the different options.

A similar problem shows up in the standby power supplies for consumer electronics, but in the other direction. An old TV I took apart used a shunt regulator to supply the (nonisolated) standby 12V supply. Use a X2 capacitor (no isolation) or a pair of Y2 capacitors (isolation, low current) instead of a resistor and it would be a very efficient way to derive standby power.

You could also use an RS-232 transceiver with built-in charge pumps (eg. the popular MAX(3)232 and siblings), but I'm sure this was considered and rejected.

The board (just a modified ATX Pentium 3 motherboard) already had RS-232 transceivers onboard. I used a 12V regulated adapter since normal ATX power supplies are very inefficient at such low loads (733MHz rated CPU underclocked to 550MHz with just 1.25V core!) and in fact have regulation problems. For testing with a standard ATX supply, I had to connect a car light bulb to provide enough load.

[alm]

The board (just a modified ATX Pentium 3 motherboard) already had RS-232 transceivers onboard. I used a 12V regulated adapter since normal ATX power supplies are very inefficient at such low loads (733MHz rated CPU underclocked to 550MHz with just 1.25V core!) and in fact have regulation problems. For testing with a standard ATX supply, I had to connect a car light bulb to provide enough load.

Ah, is was thinking about something like an embedded ARM solution. Yep, for off the shelf hardware, you're stuck with supplying the required power, although one simple solution might have been to mount an USB-to-serial converter internally . These all have internal charge pumps. Although these are not available to the boot loader when using the standard BIOS. Not sure about PCI cards, I don't think PCI supplies anything except 5V/3.3V, so these probably also use charge pumps.