Holdup time extension for power supply unit (psu)

A power supply unit comprising:

• an AC or DC input;
• a supply line and a ground line connected to the AC or DC input to an input voltage;
• a first holdup capacitor coupled to the ground line;
• a first switch selectively coupling the first holdup capacitor to the supply line;
• a second holdup capacitor coupled to the supply line;
• a second switch selectively coupling the first holdup capacitor to the second holdup capacitor;
• a third switch selectively coupling the second holdup capacitor to the ground line;
• control circuitry configured to control the first, second, and third switches, wherein the control circuitry is configured to detect that a voltage across the third capacitor has dropped to a threshold voltage and, in response, turn off the first and third switches and turn on the second switch.

A method comprising:

• detecting a voltage at an input of a DC-to-DC converter of a power supply unit has fallen to a threshold voltage, and in response:
• open a first switch disposed between a first holdup capacitor and a supply line, the first holdup capacitor being coupled to a ground line;
• open a third switch disposed between a second holdup capacitor and the ground line, the second holdup capacitor being coupled to the supply line;
• close a second switch disposed between the first and second holdup capacitors to connect the first and second holdup capacitors in series.