Synchronous Rectification

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This year's smart car competition and electronic competition both have a set of wireless charging car model operation questions. How to effectively receive wireless power is an important design content to improve performance. In rectifying the high-frequency signal of the wireless power receiving coil, some teams chose the synchronous rectification circuit.

Synchronous rectifier, also known as active rectifier, is mainly used to improve the efficiency of diode rectifier circuit. The diode in the rectifier circuit is replaced by an active switching device, such as a triode, MOS tube, etc., which works synchronously in the on and off state under the control signal of the same frequency as the alternating voltage, thereby forming a rectification effect.

Since their switching is synchronized with the signal being rectified, such rectifiers are called synchronous rectifiers, or active rectifiers.

The reason for using synchronous rectifiers is that when ordinary diodes are rectified, there will be a stable voltage drop across the diodes in the on state.

Although the on-state voltage of a silicon diode is usually 0.6V, when used in a rectifier circuit and operating at a rated high current, the on-state voltage drop can be as high as about 1V.

The use of Schottky diodes can reduce the forward voltage drop, but the forward voltage drop is still a problem, especially when the circuit efficiency is very high. In this case, synchronous detection can provide a more efficient rectification effect than Schottky rectifier diodes.

In a typical diode rectifier circuit, an ordinary diode automatically turns on when forward biased and turns off when reverse biased. An active switching device (such as a triode, MOS tube, etc.) can be used to achieve the same switching effect. The advantage of these active devices is that their on-state voltage and on-state resistance are much lower than those of diodes.

Synchronous rectification gets its name from the fact that the active device switches on and off on time, that is, in sync with the waveform being rectified.

Power MOSFE is an ideal device for synchronous rectification. They have very low on-resistance (RDS), which may be only tens of mΩ or even less, so the voltage drop of the device is much lower than the voltage drop across the diode. If you want to further reduce the voltage drop loss across the MOS power tube, you can use several MOS tubes in parallel.

The difficulty encountered in using synchronous rectifiers is that they require a relatively complex synchronous control circuit, which usually includes a voltage detection circuit, a signal conditioning circuit, a MOS tube drive circuit, etc.

The most critical factor in designing the control circuit is to ensure that the opposite devices in the rectifier circuit cannot be turned on at the same time, otherwise it will cause a partial circuit break, thus burning out the rectifier circuit. Therefore, the control circuit needs to ensure that the switching devices are not turned on at the same time during the switching process.

There are usually two solutions to achieve synchronous rectification:

The first solution is to have an AC signal generate the original circuit to give a synchronous signal. For example, in a DC-DC conversion circuit, the primary oscillation circuit gives a synchronous rectification signal while performing alternating inversion on the input DC power supply. This solution usually requires power isolation devices, such as optocouplers, pulse isolation transformers, etc., which often occupy circuit board area and also limit the increase in synchronization frequency.

The second solution is to generate a synchronization signal by independently detecting the voltage signal at both ends of the rectifier switch device. The voltage polarity at both ends of the rectifier switch device will change when it is turned on and off. By detecting the voltage difference at both ends of the switch device, it is determined whether the switch device is turned on, thereby providing a correct drive signal to be applied to the control end of the switch device.

The circuit below is a simple self-driven synchronous rectification circuit. Now the control switch MOS tube is completed by a dedicated integrated circuit, and the circuit efficiency will be higher.

The conditions for the control circuit to turn on and off the switch device are different. When the switch device is in the off state, the control circuit detects whether the voltage across the device is reversed in the forward direction, thereby determining to turn on the switch device. When the switch device is turned on, the control circuit detects whether the current flowing through the switch device is reduced to zero, thereby quickly turning off the switch device.

In the technical report of the Energy Saving Group of Shanghai Maritime University, a synchronous rectification was used, using TI's UCC24612-1 series synchronous rectification controller to perform synchronous rectification on the wireless received power.

The technical report of the Energy Conservation Group of Shanghai Maritime University did not provide a comparison of the efficiency of the above solution compared with the ordinary Schottky voltage doubler rectifier diode. This was shown by the students who participated in this year's Electronic Design Competition: UCC24612 synchronous rectifier controller and field effect transistor replace the traditional rectifier diode, which can eliminate the loss caused by the forward conduction voltage drop of the diode, making the efficiency of wireless charging up to 70% or more. The use of synchronous rectification can increase the efficiency of wireless charging by about 10%.

The technical report also mentions the ideal diode circuit: using ANALOG DEVICES's LTC4352 ideal diode controller to replace the traditional diode to isolate wireless charging and supercapacitor control of the main power supply, in order to reduce the loss of diode conduction.

The circuit widely uses switch DCDC change circuit to complete various low-voltage power conversions in the circuit, greatly improving the efficiency of electric energy utilization. Two 5V voltage regulator circuits are used to power the laser radar and main equipment respectively, and Infineon's IFX91041-5.0 step-down DC-DC converter is used. The chip has a wide voltage (4.75V to 45V) input and can adapt to most lithium batteries on the market. The maximum current output is 1.8A, supports soft start, and can be started by the microcontroller control circuit.

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