1
\$\begingroup\$

In general, there are many ways for DC-DC conversion: SMPS, charge pump, linear regulators, zenner diodes, even resistive dividers. But lets say I want to create 2 voltage rails, +Vcc and -Vcc.

I can use an explicit negative voltage converter like an inverting charge pump or a buck-boost inverting topology converter, or even build a dual PSU that produces 2 rails centered around earth.

Can I achieve the same by just have 2 DC-DC converters, calling the middle node GND, and calling the low voltage -Vcc and the high voltage +Vcc? Are there any downsides to this method rather than using an already established GND and pulling a rail down with reference to that, aside from potential shorting risks to earth?

\$\endgroup\$
3
\$\begingroup\$
  1. At least one of the converters must be galvanically isolated (input to output). There cannot be a common between input and output. Using two isolated converters can help you deal with noise and ground loops in an analog design.

  2. There is a potential issue with most single output converters not expecting a load that goes beyond their negative or positive rail. You can prevent any issues by putting a reverse-biased Schottky diode across each output.

  3. There is an advantage over using an inverting buck converter for the negative rail in that the startup surge of an inverting buck can be very high, enough to potentially pull down the input source and prevent proper starting.

\$\endgroup\$
  • \$\begingroup\$ Regarding statement 1., specifically that "There cannot be a common between input and output", doesn't the buck-boost topology violate that? I know it stands out from a buck or boost because during the charging up cycle, it is the inductor that's in parallel with the load instead of the input voltage. But the inductor doesn't have to isolate input from output for the negative voltage to be generated. \$\endgroup\$ – Michael E Aug 9 '18 at 7:29
  • \$\begingroup\$ @MichaelE Yes, I was referring to DC-DC converters specifically, which usually don't have buck-boost topology. There may be something out there that does, but they usually are either isolated or have the negative rail (or "Ground") common (possibly on different pins, but connected internally). \$\endgroup\$ – Spehro Pefhany Aug 9 '18 at 11:41
2
\$\begingroup\$

In general, there are many ways for DC-DC conversion: SMPS, charge pump, linear regulators, zenner diodes, even resistive dividers. But lets say I want to create 2 voltage rails, +Vcc and -Vcc.

Most of us would exclude linear regulators, Zener diodes and resistive dividers from a list of DC-DC converters and call them regulators as they burn-off excess voltage rather than convert it.

I can use an explicit negative voltage converter like an inverting charge pump or a buck-boost inverting topology converter, or even build a dual PSU that produces 2 rails centered around earth.

You don't have to use earth. You can call any point on your circuit GROUND and use that as your reference for everything else. A battery-powered, hand-held device, for example may need a dual supply but can't have an earth connection.

Can I achieve the same by just have 2 DC-DC converters, calling the middle node GND, and calling the low voltage -Vcc and the high voltage +Vcc?

Yes. You might be able to avoid one of them if your battery voltage or primary power-supply suits, say, the positive rail. Then you just need one DC-DC converter to generate the negative rail.

Are there any downsides to this method rather than using an already established GND and pulling a rail down with reference to that, aside from potential shorting risks to earth?

No, if the outputs are isolated from each other you are free to connect any one point on one to one point on the other. That's standard practice and good engineering.

\$\endgroup\$
  • \$\begingroup\$ Right, I was using converter more to say that these methods would produce a voltage output different from their input. Thank you for the detailed response! \$\endgroup\$ – Michael E Aug 9 '18 at 0:12
0
\$\begingroup\$

If you use an isolated DC-DC converter, you can just connect its positive output terminal to Ground, and call the negative output terminal "-xV".

This WILL NOT work with a non-isolated DC-DC converter!!

\$\endgroup\$
  • \$\begingroup\$ By isolated you mean that the rail that you're inverting was generated via a transformer from mains? \$\endgroup\$ – Michael E Aug 8 '18 at 23:59
  • \$\begingroup\$ No - on some DC-DC converters, the output terminals are galvanically isolated from the input terminals - this type can be used to make a negative supply from a positive supply. If the converter description does not explictly say "isolated", the input ground will be connected directly to the output ground, so that type cannot be used to make a negative supply from a positive input. \$\endgroup\$ – Peter Bennett Aug 9 '18 at 1:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.