The explosive growth in mobile electronic devices such as smartphones and tablets creates an increasing need in the art for compact and efficient switching power converters so that users may recharge these devices. A flyback switching power converter is typically provided with a mobile device as its transformer provides safe isolation from AC household current. This isolation introduces a problem in that the power switching occurs at the primary side of the transformer but the load is on the secondary side. The power switching modulation for a flyback converter requires knowledge of the output voltage on the secondary side of the transformer. Such feedback can be obtained through opto-isolators bridging from the secondary side to the primary side but this adds to cost and control complexity. Thus, primary-only feedback techniques have been developed that use the reflected voltage on the primary side of the transformer in each switching cycle.
In a switching cycle for a flyback converter, the secondary current (the current in the secondary winding of the transformer) pulses high after the primary-side power switch is cycled off. The secondary current then ramps down to zero as power is delivered to the load. The delay between the power switch off time and the secondary current ramping to zero is denoted as the transformer reset time (Trst). The reflected voltage on the primary winding at the transformer reset time is proportional to the output voltage because there is no diode drop voltage on the secondary side as the secondary current has ceased flowing. The reflected voltage at the transformer reset time is thus directly proportional to the output voltage based upon the turn ratio in the transformer and other factors. Primary-only feedback techniques sample this reflected voltage through an auxiliary winding to efficiently modulate the power switching and thus modulate the output voltage.
Although primary-only feedback techniques reduce complexity and cost, the associated transformer is relatively heavy compared to other board-mounted components such as integrated circuits. In particular, the transformer is commonly interconnected to its circuit board through the use of solder. Modern recycling standards typically require the use of lead-free solder, which is relatively brittle and thus prone to cracking. The resulting failure of the solder interconnect may occur with regard to the coupling to either the primary or second windings. Such failures will render the resulting flyback unusable but the output voltage will never be driven too high as a result. In contrast, if the auxiliary winding's interconnects fail, a reflected voltage will still appear across the auxiliary winding due to trace inductive, resistive, and capacitive (LRC) effects despite the open circuit fault. The power controller will thus react to this reflected voltage and continue to cycle the primary winding's power switch. As a result, the output voltage may be driven to dangerously-high levels due to the interconnect fault for the auxiliary winding, which results in damage to the associated load. But conventional power controllers have no way of determining that the auxiliary winding interconnects have failed.
Accordingly, there is a need in the art for improved fault detection for primary-only-feedback-regulated flyback converters.