Main Purpose of using Freewheeling or Flyback Diode

Freewheeling Diode



Freewheeling Diode is nothing but a normal PN Junction Diode. Freewheeling Diode used in the Power Circuits which drives the Inductive Load. The other names of Freewheeling Diode are Flyback Diode, Clamp Diode, Snubber Diode, Suppressor Diode etc.

You may notice in Rectifier Circuits, Inverter Circuits, Chopper Circuits Freewheeling Diode used. There is a question arise why we use Freewheeling diode?

At first, let's know the purposes of using Freewheeling Diode then we will discuss in detail.

Always remember that we need to use Freewheeling Diode in the circuit which drives inductive loads. For a resistive load, we have not to need to use the Freewheeling diode.





Purpose of using Freewheeling Diode:



(1) The Freewheeling Diode improves the waveform of the load current of Rectifier circuitsinverter circuits, and chopper circuits by making it continuous.

(2) The Freewheeling or Flyback Diode protect the SCRs from damage in the above mention circuits from the excessive reverse voltage creating by the Inductive Load.




(3) The Flyback Diode improves the Input Power Factor of Phase controlled Rectifiers or converters.

(4) The flyback diode sustains the average output voltage of the above mention circuits.

(5) It also helps to reduce Ripple components in the output of the above mention circuits.


These are the Advantages of using Freewheeling Diode in rectifier circuits, converter circuits, chopper circuits.





Now let's discuss how the Freewheeling or Flyback diode provides the above Advantages.


click on the image to enlarge
single phase half wave controlled rectifier using SCR


 Here we have taken a circuit of single phase half wave controlled rectifier using SCR. An R-L load is connected with the circuit.


single phase half wave controlled rectifier using SCR



According to the above circuit diagram, during the positive half cycle when we trigger the SCR then the SCR acts as a closed switch and it conducts current. So the current flow through the SCR and the load which shown in the below figure. During this time the Inductor storing electrical energy in form of the magnetic field.



single phase half wave controlled rectifier using SCR




Now when the SCR  goes to the OFF condition or it acts as open switch there is no current flow through the SCR and load. At this time the Inductor creates a high reverse voltage and want to discharge the energy. If the produced reverse voltage is greater than the reverse breakdown voltage of the SCR then the SCR will be damaged.


click on the image to enlarge

Freewheeling Diode in single phase half wave controlled rectifier using SCR





But if we keep connected a freewheeling diode across the load then at the OFF condition of the SCR, the inductor will discharge its stored energy through the Diode and the reverse voltage produced by the inductor will be zero. Thus the Freewheeling Diode prevents the SCR from Damage. You can understand this phenomenon seeing the below waveforms.


There is another advantage is, in case of the previous condition(freewheeling is not connected) there is no current flowing through the load when the SCR is OFF but after connecting the freewheeling diode the current will be continuous flow through the load even when the SCR is OFF until the inductor is fully discharged. Thus the Freewheeling Diode improves the waveforms of the load current.






See the below Waveforms carefully to understand the advantages of Freewheeling Diode in a Rectifier Circuit,


click on the image to enlarge

advantages of Freewheeling Diode



Why it is called Freewheeling Diode?



We already know that the freewheeling diode is nothing but a PN Junction diode but why it is called freewheeling diode?

It is called freewheeling diode because it causes to flow the free current which stored in the inductor as the cyclical way which is shown in the above figure.

In this case, the current which flowing, does not draw from the supply or any other source, it is flowing due to the stored energy of the inductor.

The interesting thing is that the current which flows through the inductor is produced by the inductor itself.

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Main Purpose of using Freewheeling or Flyback Diode Main Purpose of using Freewheeling or Flyback Diode Reviewed by ETechnoG on 21:06 Rating: 5
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