MOSFET to uC Guided Learning Q2: – Describe inductive kick (flyback voltage) and mitigation

Category Archives Microcontroller
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This article is part of a guided learning series to explore real-world applications of MOSFETS and microcontrollers.

:pushpin: Canonical Article: How to Interface a Microcontroller with a Relay Using a MOSFET
:blue_book: Learning Companion (Q&A): Explore All Questions

You are reading: Question 2

Describe inductive kick (flyback voltage) including its origin, potential problem, and mitigation.

Answer

Inductive kick otherwise known as flyback voltage originates in the relay coil. A high voltage spike is caused when the current is switched off to an inductor.

Technically, it is a fundamental property of an inductor described by the equation

v = L(di/dt)

Which tells us that the voltage across the inductor is determined by the current’s time rate of change. Opening a switch quickly (e.g. coil MOSFET drive) creates a large di/dt resulting in a large voltage spike.

How to visualize the flyback voltage

Instead of relying on equation we can conduct a simple experiment using a relay, switch, power supply, and a neon lamp such as the A1A (NE-2).

Experiment steps:

We can visualize the flyback voltage by conducting an experiment:

  1. Remove the protection diode from the relay. Refer to this article for more information.

  2. Construct a simple circuit where the switch controls the relay.

  3. Place the neon lamp in parallel with the relay coil.

  4. Observe that the neon lamp will pulse when the relay is turned off.

Note that the A1A neon lamp has a striking voltage of approximately 100 volts. If the lamp lights, we can confidently state high voltage is present.

Tech Tip: The neon lamp can also be used as a polarity detector. Observe that only one electrode will glow.

Why is inductive relay flyback voltage harmful to a MOSFET or transistor?

Turning off a relay causes a high voltage spike. If not handled by a diode or snubber, the high voltage spike will be placed directly across the junction of the drive transistor or MOSFET. In nearly all cases, the unprotected semiconductor will be destroyed.

I learned this lesson the hard way by constructing an alarm system for the OMEGA navigation control room. My circuit worked beautifully but I had forgotten to include the relay protection. My circuit turned on a few times and then failed as the relay drive transistor was destroyed.

How do we protect against a relay’s inductive kick?

In many cases a small signal diode may be placed across the relay. A classic solution is to use the 1N4002. The diode provides a path (commutates) for the high voltage spike.

There are times when the diode is insufficient such as when increased relay turn off speed is required. Refer to this article for a practical example involving a large three-phase motor contactor.

:writing_hand: Article by Aaron Dahlen, LCDR USCG (Ret.), Application Engineer at DigiKey