Author: Liu Kecheng, Institute of Aerospace Science and Technology Group, Wang Weiguo Guo Zuyou Date: 2006- 1- 1
The droop current sharing conversion technology is analyzed theoretically, the circuit model of parallel power supply hot standby switching converter is established, the circuit is analyzed, and the verification results are given.
introduce
One of the development directions of aerospace power supply system is to replace centralized power supply system with distributed power supply system, which has the advantages of simplifying the design of power supply and distribution system and improving the overall reliability of the system. In order to further improve its reliability, DC/DC converters used in distributed power supply and distribution systems generally adopt parallel backup mode to form a reliable parallel system.
At present, the commonly used parallel backup modes of shipborne DC/DC converters in China are cold backup mode (the main part bears all the output power, and the main part needs remote control instruction to switch the main backup) and warm backup mode (the main part bears all the output power, and the backup automatically outputs when the main part fails).
Foreign data show that the service life of electronic components is 1/6 when the working temperature exceeds 50℃, or the failure rate of electronic components increases greatly with the increase of temperature. In order to further improve the service life and reliability of DC/DC converter, the working stress of power devices that mainly affect the service life of DC/DC converter should be reasonably designed and used, and the hot standby mode should be realized in the parallel power supply system (main backup works at the same time, and each bears part of the output power).
In this paper, the parallel current sharing technology of DC/DC converters based on droop method is studied, and a current sharing converter with parallel output of two DC/DC converters based on flyback circuit topology is designed.
Circuit topology and working principle of single-ended flyback circuit
Circuit topology
Figure 1 Flyback Converter
Flyback converter is formed by inserting a transformer into the basic Buck-Boost converter, and the circuit composition is shown in figure 1. In fact, the primary winding of transformer plays the role of energy storage inductance, and it will be mentioned later that the design of primary inductance will affect the working mode of flyback converter.
The first stage of the circuit operation is the energy storage stage, when the switch tube Tr is turned on, and the linear change of the primary winding current Ip follows the formula (1).
( 1)
The second stage of the circuit is the energy transfer stage. When the switch tube Tr is turned off, the primary current is zero, and the secondary rectifier diode D is turned on, resulting in an induced current. According to the principle of constant power, the ampere turns of the secondary winding are equal to those of the primary winding. The secondary winding current follows formula (2).
(2)
Where is the secondary winding voltage and the equivalent inductance of the secondary side of the transformer.
Circuit working mode
(1) Conditions for changing working mode
For the converter shown in figure 1, let the duty cycle of the switch tube be D 1, the duty cycle of the diode be D2, and the duty cycle be Ts. According to the principle that the steady-state inductance current increment is equal, there are:
(3)
In continuous mode, the energy stored in L during D 1 (switch tube turned on and diode turned off) is not completely discharged during D2 (switch tube turned off and diode turned on), so there are:
(4)
In discontinuous mode, the energy stored in L during D 1 (switch tube turned on, diode turned off) has been completely discharged during less than D2 (switch tube turned off, diode turned on), so there are:
(5)
Therefore, the conditions for critical continuity can be deduced as follows:
At the beginning of each cycle, D 1+D2= 1, and IP=0.
Therefore, there are:
(6)
Where Lc is the critical continuous inductance value.
Substitution into formula (3) is:
(7)
The small signal model of flyback converter in CCM mode can be established by using the state space average method, as shown in Figure 2.
Figure 2 Small signal model of flyback converter in CCM mode
It can be deduced that the open-loop output impedance is:
(8)
In ...
It can be seen from Equation (8) that the output impedance of the designed buck-boost converter is only a function of the conduction ratio of the switching tube. The open-loop output impedance of the converter can be controlled by controlling the duty cycle d of the switching tube by PWM.
Sagging current sharing principle
The advantage of parallel use of distributed power supply system is that it can realize modular power supply and standardized system design, realize redundant design and improve system reliability. But at the same time, it is required to take current sharing measures between parallel power supply modules to ensure the uniform distribution of current stress and thermal stress between parallel power supply modules.
Sag method is also called changing output internal resistance method, slope control method, voltage sag method, external characteristic sag method and output characteristic slope control method. The circuit is simple and easy to realize. The accuracy of current sharing is not high, and it is suitable for parallel systems with low voltage regulation rate.
Fig. 3 Circuit model of switching power supply
Fig. 4 Output curve of switching power supply
The output characteristic curve of a single switching power supply shown in Figure 3 is shown in Figure 4. The relationship between the output voltage Vo and the load current Io is as follows:
(9)
Fig. 5 Circuit model of two parallel switching power supplies
When two switching power supplies are connected in parallel as shown in Figure 5, the load current of each switching power supply is:
( 10)
( 1 1)
In ...
Fig. 6 External characteristic slope of switching power supply after parallel connection
It is obvious from fig. 6 that a power supply with a small external characteristic slope (i.e., a small output impedance) has a larger increase in the distributed current than a power supply with a large external characteristic slope.
The main means to realize current sharing by droop method is to adjust the external characteristic slope of each converter through current feedback, so that the output impedance of parallel converters is close to the same, thus realizing output current sharing.
As mentioned above, the output impedance of the flyback circuit is a function of the duty cycle of the switch tube. Therefore, the duty ratio of the switch tube should be controlled by the current detection signal, and the current sharing mode should be realized by the flyback circuit, or the current detection signal should participate in PWM control.
In this paper, two parallel DC/DC converters with output of 12V are designed by droop method. The structure is shown in Figure 7, and the technical requirements are as follows.
Fig. 7 DC-DC design principle block diagram of droop current sharing method.
Input voltage:17v ~ 32vdc;
Output voltage:12 VDC;
Maximum output power: 30W;;
Working frequency: 200kHz.
Voltage regulation rate: less than 3%;
Load adjustment rate: less than 3%;
Efficiency: greater than 70%;
Ripple: at 70mV.
Design result
● Load adjustment rate
The output mode of the flyback converter studied in this paper is offline design, and the voltage sampling signal is not directly sampled from the output, but adopts magnetic isolation sampling technology. This design can realize off-line output without starting isolation circuit and isolation drive circuit, and the circuit is simple, but the disadvantage is that the load adjustment rate is not very high. Theoretically, it is difficult to achieve a load adjustment rate of 5%. According to related literature, this design can achieve load regulation rate (output 12V, current changes from 0. 1 ~ 0.3A). After taking some effective measures, when the load current changes in the range of 0. 1 ~ 1.3A, the load adjustment rate can be achieved. ..
1. Inductive coupling
Because the voltage sampling signal is obtained by coupling the voltage sampling signal winding of the transformer with the output voltage change signal, the signal coupling directly affects the output voltage load regulation rate. After repeated experiments, I got two practical experiences:
1. The transformer is wound in a "sandwich" winding mode, that is, the primary winding is wound halfway, then the secondary winding is wound around the remaining turns of the primary winding, and the secondary winding is wrapped inside, so that the leakage inductance is minimum, as shown in Figure 8.
Fig. 8 Winding method of transformer
2. The output winding and the voltage sampling winding are wound in parallel to achieve the best coupling effect.
2. Working mode
It is found that the different working modes of the circuit also have great influence on the load regulation rate. When the primary inductance of the circuit is large and it works in continuous mode (CCM), the waveform slope of current signal (peak inductance current) caused by load change is relatively flat (the change rate is small), which affects the load regulation rate of output voltage. When the circuit works in discontinuous mode (DCM), it will affect the efficiency.
Therefore, after repeated experiments, the primary inductance of the circuit design is moderate (the primary turns of the transformer are adjusted to 6 turns), and the circuit works in critical continuous mode. Therefore, the output voltage load regulation rate has been improved to some extent.
3. Voltage sampling signal
It is also found in the experiment that reducing the output impedance of the voltage sampling winding is equivalent to amplifying the voltage sampling signal to a certain extent, which can improve the output voltage load regulation rate to a certain extent, as shown in Figure 9.
Figure 9 Reducing the output impedance of the voltage sampling winding can improve the output voltage load regulation rate.
conclusion
According to the relevant research and discussion in this paper, and the solutions to practical problems encountered in design, the designed single-ended flyback hot standby current-sharing switching power supply has good performance, and the output parameters are shown in table 1.
Table 1
The current sharing results of two parallel DC-DC converters are shown in figure 10.
Figure 10 Current sharing results of two parallel DC-DC converters
From the results, since the output impedance of DC/DC 1 is less than that of DC/DC2, the output current of DC/DC 1 is always greater than that of DC/DC2, and the unbalance of the output current is about 12.78%.
The output impedance of DC/DC 1 can be adjusted by series resistance, which can further reduce the unbalance, but this will reduce the output efficiency and increase the output load regulation rate.
Judging from the design results, the hot standby of DC/DC output has been basically realized, and the overall efficiency and various indicators have met the design requirements.
refer to
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