Bridgeless PFC Converter Zero-Cross Surge Suppression
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Solution Overview
Problem
Bridgeless power factor improvement converters experience surge currents and increased EMI noise due to rapid voltage fluctuations at zero-cross points, which complicate switching control and lead to noise outflow in AC power sources.
Innovation Solution
The implementation of a bridgeless power factor improvement converter with a control circuit that manages dead time periods to gradually shift the voltage of nodes connected between series rectifiers, using parallel capacitors and semiconductor switching elements to discharge capacitors, thereby reducing resonance currents and surge currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high frequency switching is performed immediately after zero-cross point with normal ON time ratio, then power factor improvement is achieved, but surge current is generated and EMI noise increases
Solution Approach 1:
The control circuit performs preliminary action by detecting the zero-cross point of the AC voltage and setting a dead time period immediately after it. During this dead time, the switching elements are kept OFF, preventing the rapid voltage fluctuation that causes surge current. This preliminary preventive measure eliminates the harmful effect before it can occur during the power factor improvement operation.
Solution Approach 2:
The control circuit implements periodic dead time periods at each zero-cross point of the AC voltage cycle. By periodically interrupting the switching operation at these critical moments and then resuming with a gradual ON time ratio increase, the system maintains power factor improvement while avoiding continuous surge current generation and EMI noise.
2Object-generated harmful factors
If soft start control is used to gradually increase ON time ratio after zero-cross point, then surge current is suppressed, but switching control becomes complicated
Solution Approach 1:
The control circuit performs preliminary action by detecting the zero-cross point of the AC voltage and setting a dead time period immediately after it. During this dead time, the switching elements are kept OFF, preventing the rapid voltage fluctuation that causes surge current. This preliminary preventive measure eliminates the harmful effect before it can occur during the power factor improvement operation.
Solution Approach 2:
The control circuit changes the ON time ratio parameter dynamically based on the AC voltage phase. After the dead time period, the ON time ratio is gradually increased from a small value to the normal value over a predetermined period, rather than being set to the normal value immediately. This parameter change approach suppresses surge current while keeping the control logic relatively simple.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach effectively suppresses EMI noise and reduces noise outflow to the AC power source by minimizing voltage fluctuations and surge currents during switching operations.
Implementation Method 1
a capacitor that is connected in parallel to at least one of the series rectifiers; when the capacitor is charged and discharged, the voltage fluctuation of the other terminal of the AC power source is further mitigated
Implementation Method 2
the voltage fluctuation of the other terminal of the AC power source, which is fluctuated by the direct current voltage value of the output voltage every time the input voltage is shifted from negative to positive or from positive to negative, is further mitigated by resonating with each capacitor and an inductor that is connected to one terminal of the AC power source
Implementation Method 3
a first semiconductor switching element that is connected in parallel to one of the series rectifiers; a second semiconductor switching element that is connected in parallel to the other of the series rectifiers; a control circuit that shifts the first subordinate switch to an ON state during a second dead time period
Data Source
AI summary
A bridgeless power factor improvement converter is configured with input terminals for an AC voltage, output terminals from for a DC output voltage, diodes, first through fourth switches, and coils. A control circuit selectively switches the first through fourth switches according to the AC voltage, a first dead time period (the third switch OFF/the fourth switch ON) in which the first and second switches are in a dead time including a zero-cross point from a positive period to a negative period, and a second dead time period (the third switch ON/the fourth switch OFF) in which the first and second switches are in the dead time including the zero-cross point from the negative period to the positive period. The control circuit maintains the third and fourth switches in the OFF state during a period other than the first and second dead periods.