Research on a Push-Pull Boost DC/DC Converter

Power electronics technology is a comprehensive discipline that studies the principles and devices of power conversion. It is an electronic technology widely used in the power industry. The research content of power electronics technology is very extensive, including power semiconductor devices, magnetic components, power electronic circuits, integrated control circuits, and power conversion devices composed of the above components and circuits. Among them, power conversion technology is the foundation and core of switching power supplies. Due to the continuous development of production technology, the application of bidirectional DC/DC converters is becoming more and more extensive, mainly in DC uninterruptible power supply systems (DC-UPS), aviation power systems, electric vehicles and other vehicle power systems, DC power amplifiers, and battery energy storage applications. In bidirectional DC/DC converters, boost conversion and buck conversion are two components of bidirectional DC/DC converters. In DC/DC boost circuits, the topologies commonly used are Boost, Buck, Boost and push-pull. When the input voltage is relatively low and the power is not too large, the push-pull structure is generally preferred. This article focuses on a push-pull Boost DC/DC converter, analyzes its working principle, and models and simulates this converter.

1 Working Principle of Push-Pull Boost DC/DC Conversion Circuit

The topology of the push-pull Boost DC/DC converter is shown in Figure 1. The first stage boost circuit can be regarded as a Boost boost circuit, which adjusts the primary input voltage of the transformer by adjusting the duty cycle of the switch tube S1; the second stage boost circuit is a push-pull conversion circuit, which can also be regarded as a combination of two forward converters. The converter is composed of a transformer with a center tap and two switch tubes S2 and S3. The two forward converters have opposite phases during operation and alternately transfer energy to the load in a complete cycle, so it is called push-pull conversion.

Figure 1 Push-pull Boost DC/DC converter

The emitters of the power switches S1, S2, and S3 are directly connected to the negative pole of the power supply. Therefore, the drive circuit of the converter inherits the advantages of the general push-pull conversion circuit: the base drive is very convenient and simple, and can be driven directly without electrical isolation. This topology has the advantages of compact structure, simple drive circuit, and obvious boost effect.

The specific working process of the boost conversion is shown in Figure 2. The driving signal of the high-voltage side switch is blocked. The Boost circuit composed of the power switch tube S1 and the boost inductor L1 initially increases the power supply voltage to a certain voltage value; the duty cycle of the drive signals of S2 and S3 is 50%, and the push-pull conversion circuit formed converts the boosted DC voltage into AC voltage, transmits it to the secondary side through a high-frequency transformer, and further increases the voltage, and uses the anti-parallel diode of the switch tube in the reverse circuit for rectification.

At any one moment, the current flows through only one switching device, which greatly reduces the conduction loss of the converter, while improving the efficiency of the converter and reducing the size of the converter.

The driving signals of the switches S1 , S2 , and S3 , as well as the voltage waveforms borne by the switches and the current waveform in the inductor L1 are shown in FIG. 2 .

Figure 2 Voltage waveforms on the switch tube, current in the inductor, and transformer secondary voltage waveforms during boost conversion

Before the analysis, it is assumed that all switch devices and rectifier diode devices are ideal devices, the transformer is an ideal transformer, and the inductor L1 is large enough to ensure the continuity of the current flowing through it. The capacitor C2 is used to prevent the current from being biased.

The status of each switch is as follows:

(1) t0 to t1 stage

At t0, S1 is turned on, the low-voltage DC voltage is applied to both ends of L1, and the current in the inductor increases linearly. During this period, the power supply charges the inductor and stores energy. In order to ensure the continuity of the current, the inductor L1 must be large enough. During this period, although the switch tube S2 has a trigger signal, the conduction of the switch tube S1 forms a short circuit on the L2 loop, the voltage applied to the primary side of the transformer is zero, and the output voltage of the secondary side of the transformer is also zero.

(2) t1 to t2 stage

At time t1, S1 is turned off, S2 is turned on by the forward voltage, and the current in L1 flows through the transformer via the switch tube S2. At this time, the converter supplies power to the load, and the current in L1 decreases linearly.

(3) t2 to t3 stage

At time t2, S1 is turned on again, and the working process is the same as that of stage t0 to t1.

(4) t3 to t4 stage

At time t3, S1 is turned off, S3 is turned on by the forward voltage, and the current in L1 flows through the transformer via the switch tube S3. At this time, the transformer supplies power to the load, and the current in L1 decreases linearly.

The following conclusions are drawn through analysis: This circuit uses a combination of two boost circuits, the Boost circuit and the push-pull boost circuit, to boost the input voltage, greatly improving the overall boost efficiency. However, its main disadvantage is that the main part of the circuit still uses a hard switching circuit, which causes relatively large switching losses and limits the efficiency of the converter. Therefore, it is necessary to improve the conversion circuit, and the series resonant soft switching technology [4, 5] can be introduced into the push-pull Boost converter.

2 Modeling and Simulation

In order to verify the above analysis, the PSPICE circuit simulation software is used to model and simulate this push-pull Boost DC/DC conversion circuit and observe its simulation waveform.

(1) Figure 3 shows the simulation diagram of the main circuit of the boost converter circuit. The main simulation parameters are as follows:

Input DC voltage: Uin = 28 VDC; Output DC voltage: Uo= 270 VDC; Transformer primary and secondary turns ratio: n = 5; Boost inductor: L4= 200 μH; Output filter capacitor: C1 = 200 μF; Switching tube:

IRF460; Power diode: MUR460.

(2) Setting the drive signal of the power switch tube

First, draw the circuit schematic diagram shown in Figure 3 in Pspice Schematic, select transient analysis, and call the PspiceA/D program to simulate the circuit under the action of a given input excitation signal.

Figure 3 Main circuit simulation diagram of boost converter circuit

The driving signals of the three switch tubes are set as shown in Table 1.

The driving signal of this simulated switch tube adopts the pulse signal excitation source VPULSE, which mainly has 7 parameter settings.

The switching frequency of the boost switch is twice that of the push-pull switch, and the switching period of the push-pull switch is 25 μs.

Table 1 Switch tube drive pulse signal setting table

(3) Simulation results and analysis

Figure 4 shows the driving waveforms of the boost switch and the push-pull switch in the boost conversion circuit. S1 is the boost switch, and S2 and S3 are push-pull power switches. In the figure, S2 and S3 are the driving waveforms of the push-pull switch, with a duty cycle of 50%, which are two square waves 180° apart.

Figure 4 Driving waveforms of boost switch S1 and push-pull switches S2 and S3

Figure 5 shows the drive waveform of the converter boost switch tube and the current waveform in its boost inductor. As can be seen from the figure, when the boost switch tube S1 is turned on, the DC voltage Uin on the low voltage side is applied to both ends of the boost inductor L5, so the current in the inductor rises linearly. At this time, the DC voltage source charges the inductor to store energy. At this time, although the push-pull switch tube S2 is driven to turn on, the conduction of S1 forms a short circuit on the loop of S2, and the voltage applied to the primary side of the transformer is zero. When the switch tube S1 is turned off, the current in the boost inductor L5 will flow through the switch tube S2 through the transformer to supply power to the load. At this time, the current in L5 decreases linearly and circulates in sequence.

Figure 5 The driving waveform of the switch tube S1 and the current waveform in the boost inductor

Figure 6 shows the voltage waveform between the drain and source of the boost switch tube S1 and the push-pull switch tube S2. It can be seen from the figure that there is a small amount of oscillation between the drain and source voltage of the switch tube. This is due to the voltage peak caused by the leakage inductance in the transformer. This voltage peak is directly added to the two ends of the turned-off switch tube.

Figure 6 Voltage waveform between drain and source of S1 and S2

3 Conclusion

Through the above simulation analysis, this new type of boost method that combines Boost boost and push-pull boost has greatly improved the boost efficiency, but the disadvantage is that it still uses hard switching, which makes the converter large in size and has certain switching losses. The next step of research is to introduce soft switching technology on this basis.