What is the waveform like when the synchronous rectification driver is turned on?

乱世煮酒论天下

What is the waveform like when the synchronous rectification driver is turned on? [Copy link]

Synchronous rectification utilizes the low conduction loss characteristics of MOS tubes or IGBTs. When the applied voltage tends to conduct, that is, when the conduction direction of the applied voltage and the freewheeling diode are consistent, the MOS tube or IGBT is driven to conduct. There are two problems:

1. When used as a rectifier, in the positive half cycle of the sinusoidal input three-phase power supply, the angle is expanded from the peak to both sides, with a maximum of 120 degrees and a minimum of 60 degrees as the rectifier conduction angle. Why should this angle be set instead of the full angle of 180 degrees to open the rectifier?

2. When energy needs to be fed back, the bus voltage should also be detected. When the bus voltage is higher than a certain value, the feedback function is enabled. Then the feedback should be the middle section of the negative half-cycle of the input sine wave as the feedback angle, right?

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1. Conduction angle setting in synchronous rectification

Under the sinusoidal input of three-phase power supply, the reason why synchronous rectification does not adopt full-angle (180 degrees) opening rectification, but chooses a narrow angle range (maximum 120 degrees, minimum 60 degrees) is mainly based on the following considerations:

• Efficiency and loss : In a three-phase bridge rectifier circuit, if each MOSFET or IGBT is turned on at full angle, although complete rectification can be achieved, the conduction loss and switching loss of the switching device will increase. By limiting the conduction angle, these losses can be reduced while ensuring sufficient rectification effect, thereby improving overall efficiency.
• Waveform quality : At a specific conduction angle, by adjusting the duty cycle of the PWM (pulse width modulation) signal, the waveform of the output voltage can be better controlled, the harmonic content can be reduced, and the power quality can be improved.
• Preventing Reverse Current : In some cases, full-angle turn-on may lead to reverse current, which not only wastes energy but may also cause damage to other components in the circuit. By limiting the turn-on angle, this can be avoided.

2. Angle selection for energy feedback

When there is energy in the system that needs to be fed back, it is indeed necessary to detect the bus voltage and start the feedback function when it is higher than a certain value. Regarding the selection of the feedback angle, your mention of "the middle section of the negative half-cycle of the input sine wave as the feedback angle" is a reasonable idea, but the following factors must be considered in the specific implementation:

• Feedback efficiency : When selecting the feedback angle, the feedback energy should be maximized while reducing the loss during the feedback process. This usually requires determining the optimal feedback angle range based on the specific circuit topology and control strategy.
• Grid compatibility : The fed-back electric energy needs to be able to be safely and stably integrated into the grid, so it is necessary to ensure that the quality of the fed-back waveform is good and will not have a negative impact on the grid.
• Protection mechanism : During the feedback process, corresponding protection mechanisms should be set to prevent abnormal situations such as overvoltage and overcurrent.