The development of circuits continues to use diodes, so do you know the rectification of diodes? In recent years, with the rapid development of electronic technology, the working voltage of the circuit is getting lower and lower, and the current is getting bigger and bigger. Low-voltage operation is beneficial to reduce the overall power consumption of the circuit, but also brings new challenges to the power supply design.
The loss of the switching power supply is mainly composed of three parts: the loss of the power switch tube, the loss of the high-frequency transformer, and the loss of the rectifier tube at the output end. In the case of low voltage and high current output, the conduction voltage drop of the rectifier diode is relatively high, and the loss of the rectifier tube at the output end is particularly prominent. Fast recovery diode (FRD) or ultrafast recovery diode (SRD) can reach 1.0~1.2V, even if a low-drop Schottky diode (SBD) is used, a voltage drop of about 0.6V will be generated, which leads to an increase in rectification loss. large, the power efficiency is reduced.
However, if a supply voltage of 3.3V or even 1.8V or 1.5V is used, the current consumed can reach 20A. At this time, the rectification loss of the ultra-fast recovery diode has approached or even exceeded 50% of the output power of the power supply. Even if a Schottky diode is used, the loss on the rectifier tube will reach (18% to 40%) PO, accounting for more than 60% of the total power loss. Therefore, the traditional diode rectifier circuit can no longer meet the needs of realizing low-voltage, high-current switching power supplies with high efficiency and small size, and has become a bottleneck restricting the improvement of efficiency of DC/DC converters.
Introduction to Synchronous Rectification Technology
In the field of power conversion, isolated converters with low output DC voltage all use MOSFETs as rectifier devices. Due to the lower conduction losses on these devices, they are increasingly used to improve efficiency; for this circuit to function properly, a synchronous rectifier (SR) must be controlled, which is a basic requirement. The synchronous rectifier is used to replace the diode, so an appropriate method must be selected to drive the synchronous rectifier according to the working law of the diode. The drive signal must be formed with a PWM control signal, and the PWM control signal determines the different states of the switching circuit.
Features of Synchronous Rectifier Devices
Synchronous rectification technology is to use low on-resistance power MOS tube to replace the fast recovery diode of switching converter to play the role of rectifier tube, so as to achieve the purpose of reducing rectification loss and improving efficiency. Usually, the main switch tube of the converter also uses a power MOS tube, but there are still some differences between the two.
The power MOS tube is actually a bidirectional conductive device, which leads to some other differences due to different working principles. For example, the MOS tube as the main switch is usually hard-switched, so the switching speed is required to be fast to reduce the switching loss; and as the synchronous MOS tube for rectification/freewheeling, the MOS tube is required to have low on-resistance, body diode reverse Therefore, although both are MOS transistors, their operating characteristics and loss mechanisms are different, and their performance parameters are also different. Understand this One point, it is beneficial for how to choose the MOS tube correctly.
The basic circuit structure of synchronous rectification
Synchronous rectification is a new technology that uses special power MOSFETs with extremely low on-state resistance to replace rectifier diodes to reduce rectification losses. It can greatly improve the efficiency of the DC/DC converter and there is no dead-time voltage caused by the Schottky barrier voltage. Power MOSFET is a voltage-controlled device, and its volt-ampere characteristics are linear when it is turned on. When using a power MOSFET as a rectifier, the gate voltage must be synchronized with the phase of the rectified voltage to complete the rectification function, so it is called synchronous rectification.