Using an Adjustable Soft-Start to Combat Highly Capacitive Loads

Cool Power ZVS Family from VicorA customer was looking for a replacement for the power system they had developed for their LED information display, which needed to operate in the extreme temperatures of a desert location.  The previous (non-Vicor) brick-based solution was simply not working in that environment. The customer identified our Cool-Power ZVS Buck Regulators as an ideal solution, given their excellent thermal performance, with very little derating up to high ambient temperatures.  They had been able to use two of these regulators, happily sharing current with just a single wire connection and interleaved by default to reduce output ripple.  They ran cool and had no problem sitting in the oven that simulated the hot desert sun beating down on them all day

However, the customer encountered an issue testing their LED display.  At initial start-up, the power supply would sometimes repeatedly restart. It would always eventually start, but the ‘hiccup’ pulsing behavior was undesirable.

Testing also revealed that once the display was started, if the supply was shut down and then immediately restarted, the issue wasn’t present. This would appear to indicate that the module is shutting down due to an initial overcurrent condition, and then recovering, causing the ’hiccup’ pulses until the rail capacitance is fully charged.

The LED display uses approximately 12 amps in this application, but due to the relatively large amount of bulk capacitance on the LED tiles, the inrush current would be many times that value.

To confirm this we put a current probe on the output to see exactly how large the inrush current into the bulk capacitors is.  If the inrush current is above the Current Limit, the controller inside shuts down the power train and then tries to initiate a soft-start after the Fault Restart Delay time (~30 ms).  As the capacitors are partially charged at every start-up attempt, we see a series of high current pulses until the capacitor voltage increases enough that the inrush falls below the current limit (~18A for this particular part, the PI3424. The current probe confirmed that the inrush was indeed above the current limit.

The solution is to slow down the voltage ramp from the ZVS-Buck Regulator, to limit the capacitor inrush. This is easily accomplished by adding (or increasing the value of) the soft-start capacitor.  With no external capacitance, the soft-start time is typically 2.2 ms.  (See the “Soft-Start Adjust and Tracking” section in the datasheet for details).

An adjustable soft start is useful if we have to deal with things like highly-capacitive loads, as in this example.  This was an LED display panel but applications like Audio Amplifiers, Mixing Desks and so on also tend to have a lot of capacitance on the voltage rails.

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