Protect Your Board Against Electrical Fast Transients

Key Takeaways

• Transients in high voltage systems or power electronics can occur during a strong discharge event, creating a voltage/current spike that can destroy components.

• Systems that will interact with high voltage/high current sources should be designed to withstand these transients and should include transient suppression mechanisms.

• Simulations are useful for examining the exact transient response seen at an unprotected load and determining the level of protection needed to comply with industry safety standards.

When a relay switches, it will generate an electrical fast transient.

Relays, switches, transformers, and other high power inductive components can exhibit switching action, either manually or by being triggered electrically. However you design your switching system, quickly switching between low and high current delivery to an electrical load will generate an electrically fast transient. These transient voltage spikes are a consequence of fast power delivery and are not trivial. In a minor case, you might see the lights in a room momentarily flicker, while in the worst case, an electrical fast transient will destroy sensitive equipment. Luckily, there are some simple steps designers can take to ensure circuitry is protected and will comply with industry standards on overvoltage/overcurrent immunity.

The Behavior of Electrical Fast Transients

Electrical fast transients are triggered by inductive components that exhibit switching action at high current. A relay is a prime example; when the relay is thrown, high current is delivered to or cut off from some load in a circuit, and the switching action of the current between ON and OFF states generates a voltage spike according to Faraday’s law. This voltage spike can reach kV levels when switching is very fast and/or when the current is very large. In either case, the load component should be protected from the voltage spike and the momentary burst of current it creates.

Electrical fast transient generation can be understood by looking at the below circuit diagram. In this diagram, there is an inductive switch connected to Load 1, and Load 2 is in parallel with the inductive switch and Load 1.

Inductive switch as an RL circuit

Such a topology would be seen on a power bus with two components receiving power in parallel. In this topology, some protection mechanism needs to be added in order to protect Load 1 and Load 2. The exact protection mechanism depends on the value of the current peak and the nature of the load components in the system.

Components for Suppressing Electrical Fast Transients

There are two common components used to suppress electrical fast transients that might arise due to inductive loads:

• Transient voltage suppression (TVS) diodes are commonly used across the load.

• Flyback diodes are used across relay coils in reverse bias.

• Gas discharge tubes can be used with a varistor across a load component, or they can be used for surge suppression on the output of a power source supply or power converter.

The other option is to use isolation, such as with a transformer in a resonant LLC DC power converter, to separate switching elements at high voltage from a low voltage side. However, this is not applicable in every system and may make many designs too bulky to be practical.

Example TVS diodes in SMD packages

Transient Dynamics During Switching

When the inductive switch is activated, the fast-rising current through the inductor creates a large voltage spike pointing around the load. A few things happen at the power supply and in the inductive leg of the circuit which will govern the transient response seen at the load component:

• Power supply’s transient response: As much as we like to think that power supplies are perfect, they do not have an infinitely fast transient response. A power supply has some transient response that is limited by its output impedance. For example, in a switched-mode power supply, the output rise time depends on the output inductance, load impedance, and the resistance of the switching elements in the supply.

• Load impedances: The switching element’s internal/contact resistance, any series resistance (e.g., Load 1 impedance), and the final load impedance create an equivalent RL or RLC circuit. Any series resistance in the inductive leg will slow down the electrical fast transient.

Analyzing Electrical Fast Transients

If instead, the circuit was connected to a DC source and the inductive element was in series with the resistive load, the inductance would slow down the rise of the electrical transient. If the transient response is very fast (low equivalent RL time constant), the large voltage spike can then create overvoltage conditions on the load, which can damage the load component. In determining the level of required protection, one goal in systems design is to simulate the transient response seen at the load component.

Complying with IEC Standards on Electrical Fast Transients

The IEC 61000-4-4 standard defines standards on evaluating immunity to electrical fast transients. Test standards are classified into different levels, with different test procedures defined for each level. Specified areas of testing include test waveforms, calibration procedures, peak test voltage, and waveform repetition frequency. The table below summarizes the four test levels used for electrical fast transient immunity testing.

Other IEC safety standards may be product-specific or industry-specific, such as IEC 62368-1, which recently replaced the IEC 60950 and IEC 60065 safety standards for office and telecom equipment.

No matter how your product and its circuits are constructed, and no matter how the internal circuitry of the load is configured, the entire system can be analyzed as an RLC circuit during the design phase. As long as there are no long transmission lines connecting the load and switching elements to the power source, RLC analysis and simulation tools are applicable. This includes high power circuits, where power MOSFETs may be used for switching. SPICE simulations are the ideal tool for understanding transient dynamics in these systems before running real tests against industry safety standards.

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