PCB Assembly Services - Screaming Circuits: Flyback Diodes - a Question


Flyback Diodes - a Question

Parallel caps A while back, I posted about putting caps in parallel. Sometimes it makes sense to do that either to reduce the effective series resistance (ESR) or to better respond to different frequencies of ripple, spikes or distortion.

But here is my question - Take a look at the schematic below. This is mostly for you motor control and power component folks, but anyone can take a stab at it.

MOSFETs typically have their own flyback diode built in. But it may not be fast enough or good enough in some way or other, so it's common practice to use external flyback diodes in parallel with the internal one. In this schematic, each leg of the H-bridge has three MOSFETs in parallel. It also has D7 and D8 as flyback diodes for the bottom legs. For the moment, ignore the fact that the top legs don't have any external flyback.

Barrier diode

Here's the question: Is is equally effective to have a single big flyback for the three parallel transistors as it is to have an individual flyback for each MOSFET?

Duane Benson
Is snow cold? Perhaps, but it's all relative.

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So if the chosen MOSFET internal diode has a Vf of, say 1.3V, then a single big external flyback diode with a fast turn-on and a Vf of 600mV, should do well here.

It would depend on the manufacturing spread of the forward voltage, but you can see how this could work. Initially, only one diode will be conducting. This will be the one with the lowest Vf. If the current is large enough, the Vf on the first diode will rise to a level above the Vf of the next diode. That second diode will begin to conduct a small part of the current. If there's enough current to go around, or if the Vf are well-matched, the third diode will turn on. In any case, the vast majority of the current will be hogged by the diode with the low Vf.

If this circuit is designed this way to overcome the max forward current limits of the part, then it is an error. The current sharing won't be equitable.

Usually when theory and practice differ it's to our disadvantage. I enjoy the occasional pop when that discrepancy actually helps us mere mortals.

In theory, current-sharing is not supposed to work between LEDs. In practice it is widely used.

The good engineering practice allows for small discrepancies between devices, thus one big diode is preferred.

In addition, flyback voltage is generated at single point (at the motor), not at each individual mosfet.

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