Power Converter Touch Current Reduction via Frequency Limiting
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Solution Overview
Problem
Power converters in burst mode experience rapid voltage decrease of input capacitors when transitioning out of burst mode, leading to excessive touch current, which existing solutions only partially address by adjusting parameters without effectively reducing touch current within safety standards.
Innovation Solution
A control circuit incorporating an auxiliary pin, zero-crossing signal generator, frequency limiting signal generator, and gate signal generator to limit the gate control signal to a predetermined frequency until the power converter leaves burst mode, then switching to a quasi-resonant mode, thereby slowing down the voltage decrease of the input capacitor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the power converter operates in quasi-resonant mode at highest frequency after leaving burst mode, then the power converter achieves normal operation, but the voltage of the input capacitor decreases quickly causing touch current to exceed safety standards
Solution Approach 1:
The control circuit performs preliminary action by detecting when the power converter leaves burst mode and proactively limiting the gate control signal frequency to a predetermined frequency before the rapid voltage decrease occurs. This preventive measure slows down the voltage decrease of the input capacitor, reducing touch current to within safety standards while allowing normal operation to commence.
2Object-affected harmful factors
If the gate control signal frequency is limited to predetermined frequency after burst mode, then the voltage decrease of input capacitor is slowed down reducing touch current, but the power factor of the power converter may be affected
Solution Approach 1:
The control circuit implements dynamic frequency control by transitioning from burst mode to a predetermined frequency limit, and then to quasi-resonant mode based on operating conditions. The frequency limiting is applied dynamically only after leaving burst mode and can be adjusted based on the voltage difference between the input capacitor and input voltage, allowing the system to maintain power factor while controlling touch current.
3Object-affected harmful factors
If parameter adjustments are made to reduce touch current, then some reduction in touch current is achieved, but the solution is incomplete and does not fully meet safety standards
Solution Approach 1:
The control circuit implements feedback by continuously monitoring the voltage of the input capacitor and comparing it with the input voltage. Based on this feedback, the frequency limiting signal generator determines whether to apply frequency limiting to the gate control signal. This closed-loop control ensures that touch current is reduced to within safety standards while maintaining efficient power converter operation.
Data Source
AI summary
A control circuit for reducing touch current of a power converter includes an auxiliary pin, a zero-crossing signal generator, a frequency limiting signal generator, and a gate signal generator. The auxiliary pin is used for receiving a voltage generated by an auxiliary winding of the power converter. The zero-crossing signal generator is used for generating a zero-crossing signal according to the voltage generated by the auxiliary winding and a first reference voltage. The frequency limiting signal generator is used for generating a frequency limiting signal according to a gate control signal, a burst mode signal, and the voltage generated by the auxiliary winding. The frequency limiting signal is used for limiting the gate control signal to a predetermined frequency. The gate signal generator is used for generating the gate control signal to a power switch of the power converter according to the frequency limiting signal and the zero-crossing signal.