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Q&A

What should be considered when picking a flyback diode?

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Which are the parameters I should looking for when picking a flyback diode to be placed across a generic coil, such as a relay? That is: a coil with plain on/off functionality, for example a 24VDC relay coil with 700mW max coil power.

I assume that these are the important ones:

  • Power dissipation. According to this - together with the total resistance across the coil.
  • Reverse voltage. Ensuring that that it can handle the input voltage to the coil.
  • Repetitive peak forward current. I assume this is what's relevant the for EMF current, rather than continuous forward current.

Q1: Is the above correct? Anything missing?

Q2: What about using TVS diodes as flyback? They are a bit more expensive, but multi-purpose since they could also help with other EMI not related to the coil EMF. Any technical drawbacks?

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3 comments

I have used the TVS and schottky in series (pointing opposite way), for higher turn-off voltage in the coil. No obvious problems after it was appropriately sized. The catch is that the power dissipation in the TVS is higher in proportion to its voltage drop. With rapid cycling, the package size that worked for the schottky can turn into a smoky surprise. Pete W‭ 3 months ago

Not sure if this way of faster turn-off would be important for a relay, maybe better achieved with a SSR? I found it useful for a solenoid that produces some mechanical action. The advantage of a TVS (avalanche diode) vs a zener, I'm not sure. In principle it is an opportunity to reuse a part that is likely already part of your world if you are doing, e.g. 24V systems, but the power dissipation needs may be higher (depends). Pete W‭ 3 months ago

@Pete W‭ Yeah indeed, I'll already have some 33V TVS elsewhere in the BOM. As for replacing the relay, indeed some manner of SSR-like solution is preferable (faster, cheaper, more reliable). There's various "smart high side drivers" that are very good for this and they can also drive analog signals. Though if you use such for driving a solenoid, you are going to need a flyback diode for the solenoid anyway. Lundin‭ 3 months ago

2 answers

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a coil with plain on/off functionality, for example a 24VDC relay coil with 700mW max coil power.

That means a coil current of 0.7 watts / 24 volts = ~30 mA.

A relay might have a coil inductance of anything from 1 henry upwards but this can be estimated by the relay activation time in the data sheet. It's approximate but you could assume L/R equals the activation time. Let's say it's 20 ms.

What does R equal? It equals voltage squared divided by power (700 mW): -

$$R = \dfrac{V^2}{P}$$

So, with 700 mW and 24 volts, the resistance is 823 Ω. Hence inductance equals ~16.5 henries based on L/R = 20 ms.


Energy stored is half of $0.03^2 \times 16.5$ = 7.425 mJ.

So, if the relay was toggled on and off continuously (20 ms to activate and 20 ms to deactivate) that is a frequency of 25 Hz and the worst case power dissipated in everything due to magnetic energy stored is 186 mW.


It's looking fairly trivial for virtually any diode that has a reverse voltage rating of 30 volts or above because most of that power would be eaten by the relay's internal resistance of 823 Ω.

But, if in doubt, you could easily simulate this circuit.


Q2: What about using TVS diodes as flyback?

When the relay is deactivated, the diode becomes forward biased so a TVS is ineffective in the normal position for a flyback diode. If, however, you wish to reverse the direction of the TVS in order to dissipate the power more quickly in order to turn the relay off more quickly, then you will still need a diode in addition to the TVS to avoid the TVS conducting when the relay is activated.

This time, the power dissipated will be higher than the previously calculated 186 mW because, in the worst case scenario, the relay could be energized and rapidly deactivated nearly twice as fast. So assume 372 mW (worst case) and assume also that most of this will be "spent" in the TVS (if you want to be cautious).

I'd still use a simulator because the time taken to set it up and get numbers is about the same length of time I took to write this answer.

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3 comments

Thanks for your reply. Regarding the TVS it would be 30V-something zener breakdown voltage, mounted with the anode towards ground. The use-case I had in mind is something like an industrial/automotive application with a fairly long cable (3-5m) between the driver circuit on the PCB and the actual coil, which in turn may be subject to all manner of conducted or radiated EMI. The TVS would then prevent that EMI from hitting the driver circuitry while the same time acting as flyback. Lundin‭ 3 months ago

If you want me to comment on your proposal, a schematic will be needed @Lundin. Andy aka‭ 3 months ago

Yeah the TVS part should perhaps have been posted as a separate question. Lundin‭ 3 months ago

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Catching a flyback pulse is actually a rather easy application for a diode. Many of the diode parameters don't matter much. The basics are:

  1. Reverse voltage. This is simply the maximum voltage the coil will be driven with, which is 24 V in your example. There is nothing magic about the flyback catching role here.

  2. Forward current capability. This is just the maximum current there will ever be in the coil when it is suddenly turned off. Your relay coil dissipates 700 mW with 24 V across it. (700 mW)/(24 V) = 30 mA. The current can't somehow magically be higher than that.

Note that for a relay that isn't regularly pulsed, you can use the maximum pulse current spec for the diode instead of the continuous current spec. Every time the relay is switched off, there is a finite amount of energy that will be dissipated by the diode. At most, the diode will dissipate only the energy stored in the coil. In practice, the coil resistance actually dissipates most of that energy.

One spec you didn't mention that sometimes does matter for flyback-catching applications is reverse recovery time. That is important when the coil may be switched on again before the current from the previous pulse has fully died down yet. This is common in PWM applications. In such cases, during the time the diode is still on but the new pulse has already started, the diode becomes a short across the coil. This can damage the diode, damage the switch, and/or draw too much supply current. At the low voltages in your example, a Schottky diode would be an obvious solution to this.

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