Understanding Snubber Circuit Types for Capacitor

[Bringitondown]

Hi folks, another Q on snubber circuits. Are they the same as freewheeling diode?

 

[Svein]

Hi folks, another Q on snubber circuits. Are they the same as freewheeling diode?

No. Short and sloppy: A snubber circuit converts flyback voltage spikes into heat, a freewheeling diode converts flyback voltage spikes into voltage.

 

[Bringitondown]

Perfect,thank you Svein

 

[meBigGuy]

a freewheeling diode converts flyback voltage spikes into voltage.

What does that even mean? Is there a typo?

My answer would be:
A freewheeling diode is one form of a snubber circuit. Although, some might consider a snubber to only be RC circuits, many consider snubbers as consisting of RC snubbers and Diode snubbers and other more complex circuits to dissipate inductive energy.

Not that flyback voltage is a "symptom" of the energy stored in an inductor when there is no path (or a changed path) for it to maintain its current flow.

https://en.wikipedia.org/wiki/Snubber
https://en.wikipedia.org/wiki/Flyback_diode

 

[Baluncore]

No. Short and sloppy: A snubber circuit converts flyback voltage spikes into heat, a freewheeling diode converts flyback voltage spikes into voltage.

My answer would be:
A freewheeling diode is one form of a snubber circuit.

You are both wrong. The difference between a freewheel diode and a flyback diode is the way it is used in the circuit.

A freewheel diode conducts in series with the inductor and load to keep the current flowing through the load. It is an "inherent commutation switch" that is applied in switching power supplies to conduct the output current during the period that the active switch is off. A freewheel diode is not a snubber. The inductive energy is transferred efficiently to the load. The circuit inductor is wound to have a minimum internal resistance.

A flyback diode can be connected directly across the inductor to snub the inductive reverse voltage spike at current turn-off. It is used to protect the semiconductor driver and inductor insulation. A flyback diode is a snubber. The inductor is wound to have a resistance that will limit the current while it is turned on. When turned off, the snubbed iductive energy goes to heat the internal resistance of the inductor.

 

[meBigGuy]

I certainly understand *your* distinction. And it seems logical. But the world seems to disagree.
Personally I find your distinction fine, and will probably adopt it for the future.

I was just lazily relying on wikipedia for the definition:
"A flyback diode (sometimes called a snubber diode, freewheeling diode, suppressor diode, suppression diode, clamp diode or catch diode[1]) ..."

If you google freewheeling diode you get lots of snubbers. In fact, mostly snubbers.
http://diotec.com/tl_files/diotec/files/pdf/service/applications/freewheeling-diodes.pdf

There are a few app notes that show freewheeling diodes in bridge motor drivers, which fits your definition.

 

[Svein]

My take on it (and apologies to people who already have introduced parts of the terminology in previous posts):

The purpose of a Snubber Diode is to protect, by dissipating the energy in a coil in a safe way when the current flow is broken.

One example of a snubber circuit.

The purpose of a flyback diode is to create a (high) voltage by storing the excess energy from the coil in a capacitor.

One example of a flyback diodes.

The purpose of a freewheeling diode is to allow current to flow when the switching element is off.

One example of a freewheeling diode.

As others have observed, there is substantial confusion on the internet between these three terms.

 

[Windadct]

So much overlap:

Snubber to me is to usually to dissipate energy itself and usually effective on Voltage ( I have not seen a current snubber - I have not looked - they may exist, and there are other functions) - this can include many different components, MOV, Caps, Resistors, even regular, fast or zener diodes in a basic or relatively complex circuit considering it is just for protection ( not a core function of the system).

Flyback - to typically a DESIRED behavior when you are trying to boost a voltage (yet it may STILL a Free wheeling diode) - Ideally it may not dissipate any energy ( typically this energy loss is NOT desired - but always exists)

A freewheeling diode is applied to a circuit with some (or some expected) inductance, it provides a CURRENT path to prevent significant overvoltage due to V=dI/dt. The current can free-wheel (circulate) back to the inductance.

Still I see these a subtle distinctions in terminology that are often used interchangeably, my German colleagues correct me when I call the Diode mated to the IGBT as a FWD - they call it an Anti-Parallel diode (APD)- this simply describes it's location and not its function since the function ( flyback or FWD (or even rectifier) ) is up to the application..

One of the first / longest used books on this is the GE SCR Manual - out of print, but available on Amazon and it looks like on SCRIBID - behind a paywall.

 

[Baluncore]

A freewheeling diode is applied to a circuit with some (or some expected) inductance, it provides a CURRENT path to prevent significant overvoltage due to V=dI/dt. The current can free-wheel (circulate) back to the inductance.

Where do you get that definition from ?

I agree there is now a whole generation of poorly informed beginners out there. The first surprising thing they learn is that they must put a diode across a DC relay coil to allow the current to keep flowing and so prevent a voltage spike.

A free-wheel ratchet, one way clutch or a diode permits a flow to continue when energy is not being actively inserted into the cycle.

The bicycle free-wheel analogy is being misapplied to inductive flyback snubbers. Beginners wrongly believe the voltage measured across the forward biassed diode is the only voltage across the inductor. The major component of the reverse inductor voltage is actually appearing across the inductor's internal series resistance. They do not see the internally generated voltage across the inductor which is acting to reduce the current at a similar rate to the original turn-on. The same resistance, current and v = L.di/dt works both ways, just the sign is changed.
There is no free-wheeling there, it is maximum brakes for minimum components.

 

[Baluncore]

Attached is part of page 371 from the SCR Manual, 5th Edition, GE, 1972.
D1 is used there to prevent a sudden change in motor torque, the motor current must keep flowing.

It confuses the issue by referring to voltage with the free wheeling diode. That voltage commutates the diode, but the current flows through the motor as the load.