Considerations for choosing the right flyback diode and rating

Flyback diode insights

• A flyback diode protects the circuit from overvoltage if the contactor is switched off from a controller and can damage semiconductors and other mechanical devices in the process.
• Picking the right flyback diode will extend the life span of the system and improve its overall reliability.

Flyback diodes are a simple, essential protective component that impacts the overall life of a semiconductor and other mechanical switches. They are commonly found in the terminals of 24 volts direct current (Vdc) industrial contactors or relays.

Sometimes the supplier will have a suggested product, but this is not always true. Some exceptions include legacy products or applications in unique locations. A situation may occur where users need to select the right diode for an application. When this is the case, three major considerations need to be front of mind.

1. Overvoltage protection

A flyback diode protects the circuit from overvoltage if the contactor is switched off from a controller. Without it, the magnetic field around the inductor collapses and produces harmful voltage levels. Aside from damage, higher voltage generates noise that confuses the controllers, which can trigger false trips.

The basic configuration of a flyback diode for industrial applications is to use a classic rectifier in reverse polarity across each contactor terminal.

2. Four flyback diode considerations

Flyback diodes relieve the magnetic circuit of its energy in a reverse flow system. When inserted into a circuit, the diode creates a reverse current path to flow through it .

While the application is simple, there are four parameters to consider when choosing a diode that helps mitigate damage to the diode, controller and contactor:

1. Selecting a proper reverse voltage (V) that is suitable for the application.

2. Selecting a current (I) that can support the application.

3. Selecting a repetitive energy (J) dissipation for the application.

4. Ensuring that the diode is as close to the target as possible.

Several factors must be considered before committing to the purchase of a diode. These include reverse voltage rating, current and energy, which will prevent long-term damage to the contactor.

Figure 1: Flyback electrical circuit. Courtesy: Hedgehog Technologies

3. Reverse voltage rating

When determining reverse polarity rating for a 24 V system, it should be 10 times the nominal voltage of the system when selecting a diode, though it is advisable to go higher if possible. Reverse polarity voltage adds stress to the diodes reducing its reliability and lifetime.

Reducing the voltage stress ratio is critical to the health of the system. The ratio is applied voltage versus rated voltage. Selecting 10 times the nominal voltage is useful because the system voltage can spike from external influences such as noise, reflections and other circuit interferences.

Consider a 120/208 volts alternating current (Vac) circuit that operates at higher voltage levels. The nearby 24 Vdc-powered line becomes corrupted by the radiated noise from the higher voltage circuit causing damage to the diodes. 

Figure 2: Flyback device types. Courtesy: Hedgehog Technologies

Three steps for choosing the correct flyback diode current rating

Choosing the right current setting is just as important as selecting the voltage rating. Overvoltage can destroy the internal P-N junction in the diode while overcurrent can result in the diode overheating.

Step 1: Determine the average current rating and multiply by 10

The average current listed by the contactor manufacturer is a reliable source for choosing a rectifier diode. When a listing is not available, the safest parameter is to use the coil resistance and apply Ohm’s law (V=IR) or power ratings and use the power equation (P=IV) to determine the average current. Then multiply the result by 10.

If the solenoid nominal current’s consumption rate is known, a flyback diode is only required to carry 10 times the nominal solenoid for a limited time during the turn-off period. The reason for this is that the flyback diode current decays similar to the RL circuit current. Diodes also have a short-term surge current that is much greater than the average current, which offers additional protection.

Step 2: Using approximated repetitive I^2 t values

A powerful analysis tool is using the approximate rectifier diode repetitive I^2 t values. It can be used with either repetitive switching or a flyback diode without a sufficient average current rating.

The diode datasheets provide an approximate number of equivalent amplitude half-cycle current pulses (N) versus peak current in chart form. The half-cycle pulses are based on a 60 Hz wave and last about 10 milliseconds (ms).

If the system is constantly switching, this may help to understand what the peak current should be adjusted to. Apply the I^2 t using the value “t” as 10 ms and “I” as the peak current. Ensure this number is well below the max A^2s value.

Figure 3: Factory commissioning team. Courtesy: Hedgehog Technologies

Step 3: Flyback diode placement

Flyback voltage can reach levels of up to 10 times in a high impedance circuit. To minimize the potential for a voltage spike, place the diode in the terminals of the contactor. There are suppliers that offer adaptors for adding flyback diodes that can be clipped to relay or contactor terminals. While users might want to place flyback diodes in a control panel when a terminal block is easily accessible, this should only be done if the contactor is located inside the same panel.

Four alternative flyback diode control options

There are several options that can be considered as alternatives to a basic rectifier diode and each are useful in specific applications.

• Metal oxide varistors are used in ac circuits operating as logarithmic functioning devices. They are useful in protecting them from severe spikes that occur in events such as lightning strikes.

• Transient-voltage-suppression diodes are fast-acting diodes that can be bi-directional or mono-directional to protect electronics from transient spikes. While they can be used in flyback applications, they cost more than a basic rectifier diode and may not be required in a typical contactor situation.

• Zener diodes are reverse triggered for a set voltage. Though they are not intended for surge protection, they can work if a suitable part is selected.

• Schottky diodes carry a low forward voltage drop thus increasing the opening time of the contactor. This diode is not recommended as the reverse voltage tolerance is lower than typical rectifier diodes.

Flyback and current suppression with contactors are essential components to most industrial designs. Making the right decision when selecting a circuit will extend the life span of the system and improve its overall reliability.

Consider alternate designs such if the application requires it. If the system is sensitive and demands high reliability, take a moment to test performance with each option and compare the voltage, current and energy using a datasheet.

Hedgehog Technologies is a CFE Media and Technology content partner.

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