Conditioning input power supply on PCB

Question

I have had issues in the past with using too close of an input power supply to the max rating of my main power regulator causing failures. For example, I used a 24VDC supply with a regulator with an absolute max rating of 28VDC. When this PCB was used with a PSU with a long cord, there would apparently be an inductive power spike upon turn on, that would destroy the regulator. The PCB had a 0.1uF ceramic cap on the input line parallel with a 47uF electrolytic cap. The chip also had a 0.1uF ceramic on each input pin. So obviously this inductive spike was a long event, as my capacitance should work for short spikes.

I now am working on another project that I don't have any options to go with a higher rated regulator. I have a 15VDC input supply with a chip with a max rating of 24VDC, recommended 20VDC. This system will be using a long (~6') cord.

What are my options to condition the input supply? A bad solution that I thought of, was to buffer with a linear regulator before this chip, that would only drop a small amount of voltage, but would be rated higher..

Do I just need more bulk capacitance? Something like 100uF+? The switching regulator will be going from 15VDC to 17VDC at up to 15A. Switcher: MP3428AGL-Z

EDIT: Some clarification:

• Power cord length is from the PSU -> PCB
• On old PCB, when the PSU was plugged into the PCB and then the PSU was turned on, the regulator would blow. When the PSU was turned on, then plugged into PCB, it would be fine. So the inductive spike was from the initial inrush current.
• New PCB has different PSU, much higher quality, so it shouldn't be an issue, but I want to plan for the worst case, as this is an important design.

Answer 1

There are lots of possibilities, and it really depends on how much energy you expect to come down that cable, which in turn depends on what is in your environment, and also maybe what is driving it. (If the mains supply is just an unregulated transformer + bridge + reservoir cap there could be spikes coming from the mains itself).

One possibility is a transzorb which can simply clamp spikes coming down the line - it's basically like a fast zener (zeners have too soft a curve and are usually too slow for this kind of function).

But the problem is knowing how much energy is in those spikes and how to absorb it. A small amount of series resistance along with some C/L filtering can also reduce the amount of energy hitting your circuit and damp the ringing. (there are various commercially available filter modules as well.)

Is the issue happening when the circuit is plugged? or are you hot plugging the cable and getting spikes as a result of that?

Without knowing more specifics like the amount of DC current you draw and having some rough idea of the source energy it is not really possible to be more specific. This is EMC design, and there tends to be a fair amount of trial and error in getting it right. But this can give you a start point.

Answer 2

I think the best practice is verify the root cause of your failure then correct the cause. Don't guess. Test input start current surge to understand impedance with tower supply using a small sense R or current probe. Low ESR remote load caps with cable inductance can cause startup instability for some regulators on power up.

Usually PTC and Clamps protect for OVP Clamps or NTC inrush limiters for soft cold-start.

If your product is intended to work from these laptop style chargers, test a variety of them for overshoot startup Simulate that load impedance on a variety of long cable laptop supplies for the tendency to overshoot on startup. Some use buck-boost regulators that can reach 45V but regulate to 19.5V.

I used that fact to modify a universal laptop charger into an adjustable V output. It came with different size barrel adapters and a 4 pin header, which I discovered included remote voltage sense to compensate for long cable voltage loss. This meant I could attenuate feedback and raise the voltage. SO I can imagine some impedances that could cause startup overshoot including negative incremental impedances where input current rises if voltage sags during startup.

So the best bet is to verify the potential root cause of failures and then a solution is easy.

• Test startup steps voltage current response with small R sense.
• simulate same input capacitance + load
• then test various external supplies to this remote load to look for overshoot.
  • AC interruption, surge test, DC interruption, AC start-stop cycle test with hot-cold AC start to in case of PTC protection failure
• don't guess. Simply adding more low ESR Caps may compound the problem or make it better when raising the impedance and clamping may be the best overall.