# Relevance of Galvanic Isolation of Boost Converters

Flyback Boost converters use a transformer and this setup is still referred to as galvanic isolation.

In most use cases, the output voltage is usually orders of magnitude greater than the input, and still at a significant power rating.

Considering that the higher output voltage means that whatever circuitry connected to the output lends to a higher risk of shock (even at low currents).

How does the use of the term galvanic still apply?

(I understand using the term galvanic isolation for step down circuits such as phone chargers; but cannot wrap my head around it for boost converters esp. high power).

• The values of input and output voltages of a flyback converter has nothing to do with galvanic isolation. The isolation is just a property of the circuit when a transformer is used. Likewise the ratio of the transformer windings (which set the voltages) has nothing to do with the isolation. Sep 4, 2017 at 11:31
• I'm sure you don't understand the term "galvanic isolation" at all. The fact that you think that voltage has anything to do with proves it. Sep 4, 2017 at 13:51

Galvanic isolation means that no (significant) current can flow between two parts of the circuit (here, the input side and the output side). It doesn't mean the output voltage of a circuit has to be lower than the input voltage.

Galvanic isolation is very important in mains power supplies, because a failure to galvanically isolate the two sides may put the low-voltage output side of the power supply (and whatever it's powering) at mains potential.

This galvanic isolation is the subject of safety regulations, and can be a real concern with low-quality power supplies. See for example this teardown of a fake Apple charger, or this teardown of a USB charger (that is actually putting mains on the USB ports!)

### Galvanic isolation in step-up converters

In a step-up converter (and indeed, anywhere else), galvanic isolation still means the same thing: no (significant) current can flow between two parts of a circuit.

One way this can be relevant is if the input side of your circuit is (or can become) earth-referenced. With galvanic isolation, there is only a voltage across the two output terminals, but the output terminals are floating with respect to earth ground. A shock risk should only exist when someone touches both terminals.

However, if the galvanic isolation fails, one of the output terminals may become earth-referenced, putting the other terminal at a high voltage relative to earth ground, creating a shock risk when just the one terminal is touched!

• Usually you don't get shocked by connecting ground to two parts of your body. Sep 4, 2017 at 14:46
• @ThePhoton You do if there's a voltage source (such as the transformer in the picture) in the connection path ;) Sep 4, 2017 at 15:47
• What The Photon is saying is that you have drawn your gree/yellow wire incorrectly (and I agree). The pictures are cool but please fix up the 2nd one to be more meaningful. Sep 4, 2017 at 18:24
• @Andyaka Better? :P Sep 4, 2017 at 21:01
• @JulianF.Weinert The Y1 capacitor accross the transformer is necessary for EMI suppression; without it significant noise from the high frequency switching may leak out of the power supply. It is a compromise, because it indeed compromises galvanic isolation to a degree. However, normally the current this capacitor passes is severely limited to a ballpark 100-200µA. I'd say this mostly preserves galvanic isolation, but not 100%. This turns the neat yes/no question into scale question, leaving others to decide where to draw the line. Alas, such are the complexities of real life :) Apr 5, 2019 at 7:06

Galvanic insulation refers to the fact that the two circuits (primary and secondary, or input and output) do not share a conductive path, so that virtually the impedance between any two points (each belonging to one of the two circuits) is infinite.

In reality it is not: stray capacitance is always there, maybe a few pF or a fraction of pF.... This is an important parameter to check, especially for electrical safety and common mode noise coupling.

Objective : so, what's for?

• Electrical safety, as said in the previous answer, reducing the leakage current from a circuit with dangerous voltage to safe level

• Common mode noise suppression : think of optocoupled amplifiers such as Avago ones, able to suppress very large common mode voltages (in normal OpAmp CMRR is large but large common mode voltages are not tolerated)

• Freedom of series connection at output: for power supplies output circuits may be stacked in series for multi-tap outputs and higher output voltage, up to the rating of the insulation (with due margins). That does not mean that touching two wires of the output does not bring any shock: a 100W 48+48 V arrangement may kill you.