High-efficiency holdup circuit for switch-mode power supply

A method for continuously providing electrical power to a load throughout a source failure not exceeding a predetermined time period, the method comprising:

• unidirectionally conducting, via a source steering diode, DC electrical energy received from the source to a bulk capacitor where the conducted DC electrical energy is stored at a first voltage differential across the bulk capacitor;
• inductively coupling, via a transformer, a source winding, a load winding, and a holdup winding;
• transferring power from the load winding to the load connected thereto;
• providing, via a switching regulator and in response to a signal indicative of the electrical power transferred to the load, switched current drawn from the bulk capacitor and delivered to the source winding of the transformer;
• storing, via a holdup circuit coupled to the holdup winding of the transformer, DC electrical energy on a holdup capacitor, the DC electrical energy stored at a second voltage differential across the holdup capacitor, the second voltage differential greater than the first voltage differential;
• and
• selectively transferring, via a switching circuit and in response to the first voltage differential falling below a predetermined threshold, the DC electrical energy stored on the holdup capacitor to the bulk capacitor, wherein selectively transferring the DC electrical energy stored on the holdup capacitor to the bulk capacitor comprises defining, via a one-shot, a time period during which the DC electrical energy stored on the holdup capacitor is selectively transferred to the bulk capacitor.