5
\$\begingroup\$

I am trying to understand in what type of situation I will need an isolated DC-DC converter instead of a non-isolated one. What are the design criteria that can lead one to one way or another?

In my case, I am looking at a Boost Converter design.

\$\endgroup\$
  • \$\begingroup\$ Design criteria. If you need galvanic isolation, then use an isolated power converter. Why might you need galvanic isolation? Noise/transients, safety requirements, floating ground is required. If you don't need galvanic isolation, then use a non-isolated converter. Non-isolated converters tend to be simpler and cheaper than isolated ones. \$\endgroup\$ – Nick Alexeev Feb 21 '14 at 17:02
  • \$\begingroup\$ Please narrow down your question and tell us what you are building and what you're aiming to achieve. (A block diagram wouldn't hurt here.) At the moment, the question is a bit broad. So answers might be more broad than useful. \$\endgroup\$ – Nick Alexeev Feb 21 '14 at 17:03
  • \$\begingroup\$ What they said. And: the output voltage can be "moved" relative to the input voltage as required, as it is independent of the input voltage . Vout+ is relative to Vout- but has no relationship to Vin. This can actually be a problem in some cases as Vout can float due to external influences so that it may aqssume a mean value relative to Vin that causes damage or problems. eg some power supplies have floating outputs BUT regulations require capacitors from Vout_ground to each of two mains input leads. So Vout floats at about Vmains/2 but a high impedance. You can feel a "nip" from such ... \$\endgroup\$ – Russell McMahon Feb 21 '14 at 19:47
  • \$\begingroup\$ ... designs realative to true ground and they canand do kill equipment \$\endgroup\$ – Russell McMahon Feb 21 '14 at 19:48
10
\$\begingroup\$

If you have high currents or frequencies riding around one part of a circuit that you'd like to protect another part of the circuit from, isolation is certainly an option. For example, if you have a microcontroller driving a high-current motor with PWM, you can optoisolate the PWM pin on the microcontroller, then power up the motor circuit with an isolated DC to DC. Doesn't work if you don't optoisolate (or isolate in some other way), obviously.

If you're building a medical device with patient leads, isolation of the patient leads might be critical to meeting safety standards. An isolated DC to DC, along with isolation of all the analog and digital signals that need to connect to the patient would be in order.

Note that an isolated DC to DC becomes a nonisolated DC to DC if you connect the grounds!! Also, if you're putting test points into your board, be sure to include one for every ground, and don't forget that its not hard to connect isolated grounds with your oscilloscope ground clips accidentally!

\$\endgroup\$
4
\$\begingroup\$

The most common use of an isolated DC to DC converter is in virtually every wall-wart. OK you might say "surely it's an isolated AC to DC converter" and I'd reply that the incoming AC is (more often than not) rectified and smoothed before feeding an isolating DC to DC converter. It's a safety thing.

I've also used isolated converters in parallel - both share the same input voltage but the outputs are wired in series to create a split supply.

You can also use one just to create a negative supply - it doesn't matter how you connect positive or negative coming out of the circuit so, connecting positive output to local 0V means you get a negative rail when you didn't have one before.

I've also used them to reduce a 30V rail to 26.7V - a 30V rail was too much for a certain device so I used an isolated 3.3V output device, connected positive output to 30V and this means the negative output is in fact now at 26.7V.

Nearly the same technique can be used when you want to drive a push pull output stage with base/gate voltages that fully turn on the output transistors - the main power is (say) 50V and you create a rail a few volts higher for your driver stage before the main output stage.

There are lots of examples I guess.

\$\endgroup\$

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.