High voltage power supply full bridge control strategy-EEWORLD

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Through the understanding of transformer topology, the topology adopted by the high voltage power supply designed this time is determined to be full-bridge conversion, because it is suitable for high power.

The basic principle of the full-bridge circuit structure shown in the figure above is: the DC voltage Uin passes through the full-bridge switching converter composed of VT1, VD1~VT4, and VD4, and obtains the AC square wave voltage uAB at the primary of Tr, which is stepped up or down by the transformer, and then converted into a DC square wave by the output rectifier bridge, and finally passes through the filter composed of inductor Lf and capacitor Cf to obtain a flat DC voltage on Cf.

There are four control strategies for full-bridge DC/DC converters, namely bipolar control, limited bipolar control, asymmetric control and phase-shift control.

Before understanding the above four working methods, first understand two keywords, zvs and zcs.

PWM controls the switch tube to work in hard switching mode, that is, the voltage drop/rise and current rise/fall waveforms overlap during the on/off process. This method is inefficient and has a large loss on the switch tube. Therefore, in order to solve this problem, the so-called zero voltage switching (ZVS)/zero current switching (ZCS) technology has emerged, in which the voltage and current do not overlap, that is, soft switching technology.

The bipolar control method is: the switch tubes VT1 and VT4, VT2 and VT3 are turned on and off at the same time, and their turn-on time does not exceed half a switch cycle, that is, their turn-on angle is less than 180 degrees.

It is a basic control method that adjusts the output voltage by adjusting the pulse width. The switch tube works in a hard switching state, and a buffer circuit must be used to absorb the peak voltage during operation.

The limited bipolar control mode is: In the positive half cycle of the limited bipolar control mode, VT4 is always on, and VT1 is only on for a period of time; in the negative half cycle, VT2 is always on, and VT3 is only on for a period of time. VT1 and VT3 are turned off before VT4 and VT2 respectively. The bridge arm composed of VT1 and VT3 is defined as the leading bridge arm, and the bridge arm composed of VT2 and VT4 is defined as the lagging bridge arm.

This approach achieves zero voltage switching of the leading bridge arm and zero current switching of the lagging bridge arm, thereby reducing power consumption.

The asymmetric control method is: VT1 and VT4 and VT2 and VT3 are turned on and off at the same time. The difference from the bipolar method is that the turning on and off of VT1, VT4 and VT2, VT3 are complementary, that is, when VT1 and VT4 are turned on, VT2 and VT3 are turned off; when VT1 and VT4 are turned off, VT2 and VT3 are turned on. The turning on time of VT1 and VT4 is different from that of VT2 and VT3, so the time of the positive and negative half-cycle of uAB is different.

This circuit has a special advantage while achieving ZVS of the switch tube. There is no dead time between the complementary conduction (but not 180° complementary conduction), and the flat input current is conducive to the design of the input filter.

The phase shift control method is: the two switch tubes of each bridge arm are complementary turned on at 180°, and the conduction of the two bridge arms differs by a phase, which is the so-called phase shift angle. The driving signals of VT1 and VT3 are ahead of VT4 and VT2 respectively. The bridge arm composed of VT1 and VT3 can be defined as the leading bridge arm, and the bridge arm composed of VT2 and VT4 can be defined as the lagging bridge arm.

ZVS is achieved by using the leakage inductance of the transformer and the parasitic capacitance of the switch tube. The output voltage is adjusted by adjusting the phase shift angle. The switching frequency is stable, the switching frequency can be high, and the voltage and current stress of components are small.

The following figure is a waveform diagram of 4 control methods

Advantages and disadvantages of four full-bridge control methods:

Disadvantages of bipolar control: 1. Large spikes. 2. Large losses. 3. The switch tube requires a large safe operating area. 4. The reverse recovery of the diode will also cause high voltage and current spikes and losses. 5. The drive circuit is unstable and generates a lot of noise. 6. The switching loss limits the increase in switching frequency, reduces the dynamic characteristics, and is not conducive to the miniaturization of the filter.

Disadvantages of limited bipolar control: 1. IGBTs generally withstand very high reverse voltages and may experience reverse avalanche breakdown. 2. IGBTs must withstand very high forward voltages. 3. Leakage inductance energy is not fully utilized, so it is consumed in the avalanche breakdown IGBT, resulting in high IGBT power consumption. 4. Because one pair of switch tubes is ZVS, there is still current tailing, which limits the increase in switching frequency.

Disadvantages of asymmetric control: 1. DC blocking capacitors are required. 2. The current stress of the two pairs of switch tubes is different, which is not conducive to the unified selection of switch tubes. 3. Inductors are required, and a DC current always flows in this inductor, and there is loss in the inductor. 4. There is a duty cycle loss phenomenon.

Disadvantages of shift control: 1. Duty cycle loss. 2. Increased current peak of the switch tube. 3. Increased withstand voltage of the secondary rectifier bridge.

From the perspective of miniaturization and lightness of converters, among the four control methods, bipolar control and asymmetric control are not suitable for medium and high power applications. Limited bipolar control and phase shift control have more advantages and are ideal control methods for medium and high power applications.

So the power supply should choose limited bipolar control or phase shift control. Looking at the waveform, I still prefer the phase shift control. Next, we need to understand soft switching, zvs and zcs, and phase shift control.

Reference address：High voltage power supply full bridge control strategy

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