Circulating Current and Oscillation Control of DC Converters in Parallel Operation

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Circulating Current and Oscillation Control of DC Converters in Parallel Operation [Copy link]

The circulating current and oscillation generated by the parallel system of switching power supplies will produce high voltage impact on electronic components, reduce the power factor, and cause inhibition between the modules in parallel. Therefore, the technical research on the parallel system of switching power supplies has received extensive attention. The causes and processes of the circulating current and oscillation generated by the parallel system of DC converters are analyzed and verified by experimental results. Finally, several effective methods for solving the circulating current and oscillation problems of parallel systems are summarized.

Keywords: DC converter; parallel connection; circulating current; oscillation

　

0 Introduction

The parallel connection of multiple switching power supply modules is a key technology to solve the problem of high-power power supply system. Its advantages are that it can be flexibly combined into power supply systems of various power levels, improve the reliability of the system, obtain fault-tolerant redundant power through N +1 redundancy, realize hot replacement, and facilitate maintenance.

The rectifier diodes originally used in switching power supplies have been mostly replaced by MOSFETs due to their low efficiency. This has also created some problems while using high-efficiency MOSFETs.

The MOSFET in the synchronous rectifier is equivalent to a bidirectional switch, which can not only pass forward current but also allow reverse current to pass.

In a synchronous rectifier, the circuit that controls the MOSFET is cross-coupled with the circuit that turns on the MOSFET. The topology of such a circuit is very similar to that of a transistor multivibrator, that is, the circuit itself can oscillate.

Therefore, when the modules are connected in parallel, due to the difference in output voltages of each module, a circulating current will be generated between the module with high output voltage and the module with low output voltage, forming an oscillation. In this way, the module with low output voltage not only does not provide current to the outside, but also absorbs the current of the module with high output voltage; the module with high output voltage not only provides load current, but also provides current to other modules. Therefore, the module with high output voltage will be impacted by large current; oscillation will produce large voltage impact; several modules interfere with each other, and the module with high output voltage will suppress the module with low output voltage.

This paper mainly focuses on the parallel system of DC switching power supplies. By theoretically analyzing the structure that may produce circulating current, it clarifies the causes and processes of circulating current and oscillation, and summarizes several effective control methods to solve the circulating current.

1 Analysis of circulating current generated by parallel system

FIG1 is a schematic diagram of a parallel system of two forward DC/DC power modules using self-driven synchronous rectification.

Figure 1 Two forward power modules in parallel

In module 1 of Figure 1, S1 _is a synchronous rectifier, S2 _is a freewheeling tube, L1 is a filter inductor, _C2 is _a filter capacitor, and R is the load of the parallel system. _{S3 is a MOSFET} switch that controls the conduction of the primary coil of the transformer. C1 and D4 _form the freewheeling loop of the primary coil of the transformer.

Since S1 _replaces the original diode, the branch that was originally only unidirectional allows reverse current to pass. In a parallel system, when there is a difference between the two modules, there will be a difference in the output voltage, which is the main reason for the circulation in the rectifier circuit.

The input voltages of the two modules are the same, and the control methods are the same. When the reference voltage of one of the modules is higher, assuming here that the reference voltage of module 2 is higher, the conduction angle of S7 will be greater than that of S3 _{, making} the output voltage of module 2 higher.

At this time, from the output end, the two modules can be equivalent to the structure of an ideal voltage source and a resistor in series, as shown in Figure 2.

Figure 2 Parallel equivalent circuit

It can be clearly seen from FIG. 2 that when V _out2 > V _out1 , a loop is very likely to be formed and a circulating current will be generated.

2 Theoretical analysis of self-oscillation[1]

Due to the existence of the circulating current phenomenon, the parallel-operating power supply system shown in FIG. 1 may generate a self-oscillation phenomenon.

According to the switch status, it can be divided into 4 time periods.

1) State 1: S3 _is off, S1 _is off, and S2 _is on.

_{At this time ,} the equivalent circuit of module 1 is shown in Figure 3. In the figure, Lm is the excitation inductance of the transformer, _{Cp is} the capacitance value of the transformer primary side equivalent to the secondary side, S1 _, S2 _and S3 _are equivalent to capacitors CS1 , CS2 and CS3 respectively when _{they are turned off} , and V2 _is the output voltage.

C _p = n ₂ C _S3（1）

Where: n is the transformer ratio.

Figure 3 Equivalent circuit 1

At this time, v _S2 = 0, and the voltage across S _{1 is}

v _S1 =－L _m（2）

i _{L m} =( n ² C _S3 + C _S1 ) (3)

- L1 = V2 ( _{4 )}

Since S1 _changes from on to off, the initial value of v _{S1 is zero, so we can get}

v _S1 ( t ) = i _{L m0} L _m β sin( βt ) (5)

i _{L m} ( t ) = - v _S1 d t + i _{L m0} (6)

i _{L 1} ( t ) = - t + i _{L 10} (7)

In the formula: i _{L 10} and i _{L m0} are the initial values of i _{L 1} and i _{L m} .

β = (8)

When v _S1 decreases to zero, state 2 is entered.

2) State 2: S3 _is off, S1 _is on, and S2 _is off. The equivalent circuit diagram at this time is shown in Figure 4.

Figure 4 Equivalent circuit 2

At this time, v _S1 = 0. And

v _S2 = L _m（9）

i _{L m} =－i L _{1－ (} n 2 ^C S3 ₊ C S2 ₎ ( 10)

v _S2－L ₁ = V ₂（11）

Since the initial value of v _S2 is zero, we can get

v _S2 ( t ) = （12）

i _{L m} ( t ) = - v _S2 d t + i _{L m0} (13)

i _{L 1} ( t ) = v _S2 d t t + i _{L 10} (14)

Where:

A2 = ( ₁₅ )

α = （16）

θ ₂ =arctan （17）

Where: i _{L m0} and i _{L 10} are the initial values of i _{L 1} and i _{L m in the second stage;}

T _s is the unit time.

3) State 3 S3 _is on, S1 _is on, and S2 _is off. The equivalent circuit diagram at this time is shown in Figure 5.

Figure 5 Equivalent circuit 3

V ₁ /n is the voltage of the secondary winding of the transformer, and at this time, i _{L 1} and i _{L m} both increase linearly.

i _{L 1} ( t ) = t + i _{L 10} (18)

i _{L m} ( t ) = t + i _{L m0} (19)

4) State 4 _S3 is off, S1 _is on, and S2 _is off. The equivalent circuit diagram at this time is the same as that of state 2, and the time function expressions of all quantities are also the same, except that the initial values are different.

3 Simulation and Experimental Results

In order to verify the analysis results of the above-mentioned circulating current and oscillation phenomena, the two self-driven power module systems shown in Figure 1 were simulated using PSpice, and an experimental module was made.

The main parameters of the simulation and experimental system are: input voltage 60V, output voltage 5V, switching frequency 200kHz. The output voltage of module 2 when running alone is slightly higher than the output voltage of module 1.

Figure 6 and Figure 7 are simulation results and experimental results respectively. Where V1 is the voltage between the source and drain of the rectifier tube S1 in module 1; _{V3 is} the voltage between the source and drain of _{the switch tube S3 .}

Figure 6 Simulation waveform

Figure 7 Experimental waveform

The simulation and experimental results show that due to the existence of circulating current, self-oscillation occurs in the parallel system.

4 Several measures to solve circulation and oscillation problems

If circulating current and oscillation occur in the power modules running in parallel, it will affect the normal operation of the system. Appropriate measures must be taken to avoid the occurrence of circulating current and oscillation. The following measures can be taken.

4.1 Resistor method

Adding a resistor to the loop that generates the circulating current is equivalent to increasing the resistance of the entire circulating current loop, which can reduce the circulating current. However, the added resistor in the output loop of the switching power supply will inevitably reduce the output voltage and current. This method can only be used when the output requirements of the switching power supply are not high.

4.2 Use detection methods to control and eliminate

A current detector is added to each switching power supply module. When the current of a module changes abnormally, the detected signal is sent to the controller. The controller restores the module to normal operation through the control circuit to prevent the occurrence of circulating current.

This method can be combined with current sharing control to evenly distribute the current among the modules while preventing the generation of circulating current.

4.3 Changing the driver of the rectifier MOSFET

4.3.1 Improved self-driving[5]

FIG8 shows a self-driven synchronous rectification module.

Figure 8 Self-driven power module

When multiple modules are connected in parallel, if a module stops outputting voltage for some reason, the output voltage V _out still exists because other modules are still working and the output terminal of the module is connected to other modules. Although the module is not working at this time, due to structural reasons, the voltage between the source and drain of S1 and S2 is _V ds ₌ V _out , and _the voltage between the gate and drain is V _gs = V _out , so S1 _and S2 _are both turned on, thus short-circuiting V _out and inevitably causing circulating current.

The improved self-driving module is shown in FIG9 .

Figure 9 Improved self-driven power module

S5 and S6 _{are P -} channel MOSFETs. When the module is working normally, S5 _and S6 _only play the role of driving voltage buffering and do not affect the driving voltage waveform of S1 _and S2 _. When the module is not working, although Vout still exists, due to the blocking of S5 _{and S6} , _the voltage _Vout cannot be added to the gates of S1 and S2, and due to the resistors _R5 and _{R6 , static} electricity will not accumulate on the gates. At this time, the pin voltages of S1 _and S2 are _Vds = _{Vout and} Vgs = 0. Therefore , neither S1 _nor S2 _will be turned on. This effectively improves the self-driving structure _.

4.3.2 Change self-drive to external drive

The rectifier MOSFET is not driven by itself, but by another driver. The single rectifier MOSFET structure is modified in this way as shown in Figure 10. The gate of the rectifier MOSFET S1 _is connected to the PWM control circuit, changing the original cross-coupled structure and avoiding the generation of circulating current and oscillation.

Figure 10 Other drive scheme 1

The self-driving structure with two rectifier MOSFETs is changed to an external drive structure as shown in Figure 11. The driving signals of the rectifier MOSFETs S1 _and S2 _are provided by the PWM control circuit, which also changes the original cross structure and effectively avoids the generation of circulating current and oscillation.

Figure 11 Other drive scheme 2

5 Conclusion

Based on the analysis of the causes of circulating current and oscillation in parallel DC converter systems, several effective methods for solving the problems are proposed.

　

About the Author

Li Quan (1980-), male, master's degree student, majored in electrical engineering at the Department of Electrical Engineering of Tsinghua University, and graduated from the Department of Electrical Engineering of Tsinghua University. He is currently conducting scientific research cooperation in the Department of Electrical Information Engineering of Nagoya Institute of Technology in Japan. His research direction is power electronics technology and power conversion.

Liu Xiucheng (1959-), male, associate professor, majored in electrical engineering, Department of Electrical Engineering, Tsinghua University, graduated with a bachelor's and master's degree from the Department of Electrical Engineering, Tsinghua University. His main research directions are circuit systems and electromagnetic engineering, power electronics technology and superconducting application technology.

　

gtekled

It can be flexibly combined into power supply systems of various power levels, improve the reliability of the system, obtain fault-tolerant redundant power through N+1 redundancy, realize hot replacement, and facilitate maintenance, etc.