Freewheeling Diode

In an Electrical circuit, a Diode is a semiconductor device that acts like a one-way switch (it allows current to flow in one direction and restricts the current to flow in another direction). Semiconductor is made up of p-type and n-type materials. Semiconductors have conductivity between conductors and insulators. To control the Load voltage, a switching element is used to switch at a particular frequency.

A severe high voltage occurs at the time the switching element is activated due to the stored energy within the magnetic field of the inductive load coming across the switching element terminals in switch OFF condition which may destroy the switching element. To distribute this energy, a Freewheeling diode is used. The Load can be short-circuited at the time the switching element is inactivated. The energy stored in the magnetic field is degenerate like heat.

What is a Freewheeling Diode?

Freewheeling diode also called a flyback diode, kickback diode, clamp diode, commutating diode, snubber diode, or suppression diode. Freewheeling Diode is used to prevent the high voltage over the switch across the inductive load terminals. This diode is used to protect the circuit from unusual damage due to the sudden decrease in the flow of current in the circuit.

Freewheeling diode is used in forward bias only. The function of a freewheeling diode is to fly back the energy stored in the inductor by a short circuit path. This is necessary to prevent the voltage across the diode and switch contacts from rising due to an abrupt decrease in the circuit current.

Based on the applications the types of freewheeling diodes may vary and can be

• Standards silicon diodes
• Schottky diodes or fast recovery diodes (Which are used for switching power converters).

Basics of Flyback Converters

Flyback converters are defined as power converters, that convert AC to DC with galvanic insulation between the inputs and outputs. It stores the energy when the current flows through the circuit and releases the energy when the power is switched off. It uses a mutually coupled inductor and acts as an insulated switching converter for step-down or step-up voltage transformers.

It can control and adjust the multiple output voltages with a broad range of input voltages. The number of components required to design a flyback converter is few when compared to other switching mode power supply circuits. The word flyback is mentioned in the on/off action of the switch used in the design.

Operation of Freewheeling Diodes

The Freewheeling diode circuit is shown below,

Freewheeling diode

The Inductor and resistor are connected across a Freewheeling diode. An ideal flyback diode will have a very large high forward current; which helps control the voltage variations from burning out the diode, the inductor’s power supply is convenient for reverse breakdown voltage and low forward voltage drop. Depending on the instruments being used and the purpose for which the equipment is being utilized, voltage flow can be 10 times that of the power source’s voltage.

The freewheeling diode in the circuit is reverse-biased by the external potential during operating conditions when the switch is closed. Freewheeling diode does not play any crucial role under normal or steady-state conditions. But in the existence of a Freewheeling diode when the switch is opened, the voltage over the inductor forward biases the freewheeling diode.

Need for Freewheeling Diode

In the circuit shown above, External potential is applied when the switch closes, then the diode in the circuit becomes forward biased and current flows through the load Resistance and Inductance.

An inductor is a wire loop that conducts electricity and produces a magnetic field as current passes through it. The inductor carries the energy in the form of an electromagnetic field. The magnetic field created by the current passing through the inductor when the switch is closed then causes the device to fully charge. This resulted in a break in the circuit’s current flow, which ended the previously created magnetic field.

Lenz’s law states that this field creates an opposing current in the circuit, which creates a negative potential across the inductor. This potential is called Flyback voltage. This voltage across the inductor is noticeably higher than the potential that the external source first applied. This results in a rapid current flow through the circuit, which builds up a large reverse voltage across the switch and the diode and may cause damage to other circuit components.

The voltage spike over the inductor is:

[Tex]V = L \frac{di}{dt}[/Tex]

where,

L = inductance

di/dt is the rate of change of current over the inductor.

Then the voltage across the inductor and the flow of current through the circuit carry the relation of direct proportionality. As a result, a freewheeling diode is connected across the inductor to keep away the damage in the circuit.

Importance of Freewheeling Diode in Inductive Load Circuits

In the circuit shown below, the connection of the load Resistor and inductance can be done to the Direct Current source through the switch and diode. When the switch is closed in the circuit for a few seconds, then current flows throughout the load. This current accumulates within the inductor to reach a constant value after some time.

[Tex]i = \frac{V}{R} (1-e^{\frac{-R}{L}t})[/Tex]

The constant value of current is,

[Tex]I=\frac{V}{R}[/Tex]

When the switch is opened, there is no current flow in the circuit that moves to degrade throughout the inductive load. This failure current throughout the inductor (L) results in a reverse voltage forming that is equals to L (di/dt) behind the terminals of an inductor.

Inductor circuit with freewheeling diode (In open switch)

The diode will be in forward bias due to this reverse voltage behind the terminal of the inductor. So it acts as a closed switch. Then, the main circuit can be converted to another circuit including the freewheeling diode, resistor and inductor as shown above. In this circuit, the flow of current will decrease exponentially to zero as shown below.

[Tex]i = \frac{V}{R} e^{\frac{-R}{L}t}[/Tex]

Inductor circuit with freewheeling diode (In closed switch)

The diode used in this circuit will consume the stored energy inside the inductor by giving a short circuit path so that high voltage cannot be created. Then the diode and switch are secured from too much voltage. The connection of this freewheeling diode can be done across an inductive load like a relay. Once the drive circuit is set free, the energy that is stored inside the magnetic field has to be left all over.

When this diode is not used or connected, too much voltage will be generated behind the inductor, so there is a possibility for damage to the driving circuit, so a freewheeling diode is used to provide a path to this energy.

Advantages of Freewheeling Diode

• Freewheeling diode increases the load current of various circuits like chopper, inverter and rectifier circuits.
• Freewheeling diode keeps the silicon-controlled rectifiers from damage in various circuits from the maximum reverse voltage that is produced through the Inductive Load.
• The input power factor can be increased for converters or phase-controlled rectifiers.
• It maintains the average output voltage components and ripple components within the output for various circuits.
• This diode can be operated on a broad range of input voltages.
• The Flyback converters use very less components compared to the other kinds of Switched-mode power supply.

Disadvantages of Freewheeling Diode

• Being careful in choosing the freewheeling diode is essential as incorrect diode ratings may lead to its failure and poor protection.
• Energy is dissipated as heat in high-power applications. In that situation, a Freewheeling diode may be provided to raise the temperature and there is a requirement for heat sinks or other cooling methods.
• A few freewheeling diodes may have reverse improvement time after forward bias condition which leads to short period spike.
• There will be energy losses during the freewheeling phase which takes to slight reduction in overall efficiency as energy stored in the inductor is dissolute as heat in the diode.
• The short-circuit collapse of the freewheeling diode may lead to potential harm to the Insulated Gate Bipolar Transistors (IGBTs) and other components, as it makes unrestricted high current to the circuit.

Applications of Freewheeling Diode

• Freewheeling diode is used in the relay drivers, H-bridge motor drivers, and full-wave rectifiers.
• Freewheeling diode is used in power circuits to drive the inductive load.
• It is used to protect the circuits from damage caused by sudden disturbances.
• To control the strong counter Electromotive force ,the freewheeling diode is used with a relay coil.
• It makes sure to get a smooth path for current flow during the disturbances in the circuit and prevents harm to sensitive parts.
• This diode is used to protect the switching devices.

Conclusion

The summary of a freewheeling or flyback diode, its circuit, working, and its applications are discussed above. When designing switching circuits for inductive loads, we need to be responsible for high voltage spikes. Here, switching inductive loads are relays, solenoids, motors and transformers. In the absence of the right circuit protection fixed in the circuit design, the switches would be damaged seriously and may cause circuit failure. To control this, the freewheeling diode is used. Making use of freewheeling diode advantages, used in different applications based on the requirement.

Freewheeling Diode – FAQs

Is Freewheeling diodes used in every inductive circuit?

To protect the components and to prevent voltage spikes freewheeling diodes are used. It is not necessary to use the Freewheeling diodes in every inductive circuit, the circuit may use other components.

What are the Conditions that Freewheeling diodes will fail?

Due to the over voltage, extreme current and voltage rating may cause the freewheeling diode to fail. There should be careful consideration when selecting the diode and installing it.

Freewheeling diode is only used in DC circuits?

To protect the circuit from voltage spikes, freewheeling diode is used in both Alternating current (AC) and Direct Current (DC) circuits with the inductive load.