Switching Power Supply Zero-Current Threshold for EMI Control
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Solution Overview
Problem
Switching power supplies face inefficiencies and electromagnetic interference (EMI) due to early zero-crossing triggers in zero-current detection, leading to reverse current discharge and potential latch-up issues, which existing DC trimming methods fail to adequately address.
Innovation Solution
An adaptive zero-current threshold scheme is implemented, using a second circuit to set a threshold for the zero-current crossing event, compensating for propagation delays and input/output voltage dependencies, and incorporating an anti-ringing circuit to reduce EMI.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If DC trimming is used to set the zero-current threshold, then the device complexity is reduced, but the measurement precision and timing accuracy deteriorate due to inability to capture propagation delay
Solution Approach 1:
The patent measures and stores the comparator propagation delay in advance during a calibration phase, then uses this pre-measured delay value to compensate the zero-current threshold setting. This preliminary measurement of the delay allows the system to accurately predict when the comparator will trigger and adjust the threshold accordingly, achieving precise timing without adding complex real-time measurement circuits.
Solution Approach 2:
The patent implements a feedback mechanism where the measured propagation delay is fed back into the threshold setting circuit to adjust the zero-current threshold. The system uses the delay information to dynamically compensate the threshold voltage, ensuring that the HS switch shuts off at the correct moment despite the comparator's inherent delay, thereby maintaining high measurement precision.
2Reliability
If an early zero-crossing trigger is used to account for propagation delay, then the reliability is improved by preventing latch-up, but the use of energy deteriorates due to inefficient body diode conduction
Solution Approach 1:
The patent measures the comparator propagation delay in advance and uses this information to calculate the optimal threshold that will trigger the HS switch shutdown at the precise moment when inductor current reaches zero. This preliminary delay measurement enables the system to avoid both early triggering (which causes energy loss) and late triggering (which causes latch-up), achieving the perfect balance between reliability and efficiency.
Solution Approach 2:
The patent dynamically adjusts the zero-current threshold parameter based on the measured propagation delay. By changing the threshold voltage level according to the specific delay characteristics of the comparator being used, the system optimizes the shutdown timing to coincide exactly with the zero-current point, preventing both energy loss through body diode conduction and latch-up conditions.
3Device complexity
If the zero-crossing detector triggers late, then the device complexity is reduced, but electromagnetic interference increases due to ringing at the switching node
Solution Approach 1:
The patent measures and compensates for the comparator propagation delay before normal operation begins. By pre-characterizing the delay and incorporating it into the threshold calculation, the system ensures that the zero-crossing detector triggers at the correct moment, preventing late triggering that would cause ringing and EMI. This preliminary delay measurement approach maintains simple circuit architecture while eliminating harmful electromagnetic interference.
Data Source
Figure 1A~1C
Figure 2
Figure 3A~3B
AI summary
An apparatus of the subject technology includes a circuit consisting of an inductor and a switch to allow a current to flow through the inductor and charge a capacitor of the circuit. A first circuit is coupled to the circuit and is used to simulate an event. A second circuit sets a threshold for triggering the event, while partially compensating a propagation delay.