VNH5019A Flyback issue on custom PCB

Question

I have a custom high power PCB I recently designed and am currently testing. It is designed to use 2 linear actuators link. I am using a VNH6019A H-bridge to control them. They operate simply as -12v goes up, 12v goes down (wrt each terminal).

My schematic for the bridge is:

I am using the suggested capacitor size for the driver:

"Note: The value of the blocking capacitor (C) depends on the application conditions and defines voltage and current ripple onto supply line at PWM operation. Stored energy of the motor inductance may flyback into the blocking capacitor, if the bridge driver goes into 3-state. This causes a hazardous overvoltage if the capacitor is not big enough. As basic orientation, 500 μF per 10 A load current is recommended."

I am using one 12V power supply, which is used for the main power to the actuators, as well as using an "all in one" switching 5v regulated power supply for my logic level components.

I am using an intelligent LCD display by 4DSystems to control the board, so it has internal decoupling and is powered from the 5v line.

Power regulator schematic:

LCD connector schematic:

Since my actuators can draw a maximum of 10A, I am using a 500uF electrolytic capacitor.

When I hook up one (or two) actuators, they work fine with no noticeable issues. However, if I use one (same issue with two) actuators, and hook up a scope to the 5v output of my regulator when the actuator stops moving, I get a nasty voltage spike on the 5v line up to 16v.

I have sufficient decoupling, as there is a small (<20mA) sag when the actuators turn on. Since it is when the actuators stop, it must(?) be flyback from the inductive load.

My questions:

1. Why is this occurring when I am using significantly higher flyback caps than required (my single actuator with no load draws only ~800mA).
2. Since there is such a large spike, why is my LCD or anything else powered by the 5v line getting damaged?
3. What is the solution?

Thanks

EDIT: I assume the easiest is just to throw a flyback diode across the actuator terminals. Assuming that works, I would still like to know number 1 and 2

EDIT2: Nevermind, I can't do that from an h-bridge, since the polarity switches. Back to square one.

Answer 1

I'm not experienced with using scopes. I have a single probe connected to channel one. This probe has a ground alligator clip. This I clipped to a ground standoff on the pcb.

That forms a small loop antenna suitable for picking up magnetic field impulses and it is highly likely that what you are seeing is a phantom. The currents in your circuit are around 10 amps (and with quite fast edges) and this type of scope probing problem is commonplace to find on switching converters. You do need to use a ground plane and proper probing on a circuit like this or you'll be fighting an uphill battle.

Here's something you can try. Connect the alligator clip to the scope probe end to form a short. Now connect the scope probe (with intact alligator clip) to the grounding post on your circuit. What would you expect to see and what do you actually see?