Advantages and applications of asymmetric half-bridge isolation drivers-EEWORLD

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Today's technology is developing rapidly, and semiconductor device technology is also developing rapidly. The emergence of various fully controlled devices has accelerated the development of switching power supply technology, and asymmetric half-bridge converter technology has gradually surfaced. This technology has a simple structure and uses only a small number of components, which can be said to have various advantages. This article introduces several commonly used asymmetric half-bridge MOSFET drive circuits and analyzes the advantages and applicable occasions of each circuit.

Introduction and analysis of several asymmetric half-bridge drive circuits

Non-isolated asymmetrical half-bridge driver circuit

Figure 1 is a commonly used low-power drive circuit, which is simple, reliable, and low-cost, and is suitable for low-power switching devices that do not require isolation. One path is directly connected to the lower tube, and the other path is reversed by the inverter to drive the upper tube. RP1 and RP2 are used to adjust the dead time.

Forward asymmetric half-bridge isolation drive circuit

A forward asymmetric half-bridge isolation drive circuit is shown in FIG2 .

Taking the forward circuit as an example, the pulse signal is coupled through a high-frequency pulse transformer to drive the power MOSFET. When the secondary pulse voltage is positive, the MOSFET is turned on. During this period, VT3 is turned off, and the discharge circuit formed by it does not work. When the secondary pulse voltage is zero, VT3 is turned on, quickly discharging the MOSFET gate charge and accelerating the cutoff of the MOSFET. R7 is used to suppress the peak of the drive pulse, and R9, VD3, R11, VD5, and R13 can accelerate the drive and prevent the drive pulse from oscillating. Together with the pulse transformer winding connected to it, it forms a demagnetization circuit.

This circuit achieves isolation and can output a better driving waveform. However, it also has some shortcomings:

1. The structure is complex and requires dual power supply (±12V);

2. There are many components, especially two isolation transformers, which not only take up a large space, but also increase the circuit cost; dedicated chip drive circuit

ST's L6384 is a dedicated asymmetric half-bridge driver chip, and its schematic diagram and peripheral circuit are shown in Figure 3. A single pulse is input from pin 1 (IN), and complementary pulses are output from pins 5 (HVG) and 7 (LVG). Pin 3 (DT/ST) is connected to an external resistor and capacitor to control the dead time of the two outputs. When the level of pin 3 is lower than 0.5V, the chip stops working. Dedicated chips have the characteristics of simple peripheral circuits and small space occupation, but due to their high cost, they are not suitable for low-cost design products.

A new type of asymmetric half-bridge isolation drive circuit

According to the above driving circuits, a new type of asymmetric half-bridge isolation driving circuit is proposed to solve the problems of complex structure, large space occupation and limitation of driving circuit application of traditional isolation driving circuit. It is suitable for chips with single pulse output, has the characteristics of simple and reliable structure, small space occupation, and realizes electrical isolation, so it can be used in medium and high power occasions.

The driving circuit is shown in Figure 4. The operating frequency is determined by the characteristics of the magnetic core. Generally, high-frequency magnetic cores are used, and the operating frequency can reach 100kHZ. The push-pull power amplifier circuit is composed of VT1 and VT2 on the primary side. When the pulse output is high level, VT1 is turned on to provide MOS tube driving power; when the pulse output is low level, VT2 is turned on, and the energy storage on the capacitor provides a reverse pulse. The two waveforms output by the secondary side of the transformer are converted into complementary pulse signals after the conditioning circuit, thereby driving the MOSFET. When the driving pulse is positive, the MOSFET is turned on. During this period, VT1 and VT2 are turned off, and the discharge circuit formed by them does not work. When the secondary pulse voltage is zero, VT1 and VT2 are turned on, and the gate charge of the MOSFET is quickly discharged to accelerate the cut-off of the MOSFET. The voltage regulator tubes VD1 and VD2 clip the pulse waveform in the positive direction.

Under SABER simulation, the driving waveforms of the secondary side N2, N3 and the upper and lower tubes of the transformer are shown in Figure 5(a) and (b) respectively.

This circuit has the following advantages:

1. The circuit structure is relatively simple and reliable, and has electrical isolation. When the duty cycle is fixed, through reasonable parameter design, this drive circuit has a faster switching speed.

2. This circuit only needs one power supply, that is, single power supply operation. Experiment and conclusion

This paper designs an asymmetric half-bridge converter prototype: the operating frequency is 98kHz, the input voltage is 400VDC, and the output voltage is 30VDC. The driving waveform Ug1 is measured when the duty cycle is 0.47, and Ug1 is shown in Figure (6).

After verification in this article, this new type of asymmetric half-bridge isolation drive circuit structure is not only simple, but also perfectly realizes the complementary drive with the MOS tube, and this drive has good stability, enough to become a high-performance drive circuit. I hope everyone can fully understand this new drive circuit and make full use of it.

Reference address：Advantages and applications of asymmetric half-bridge isolation drivers

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