Design of current-sharing converter based on parallel output of DC/DC converters

Figure 6 External characteristic slope of the switching power supply after parallel connection

It is obvious from FIG6 that the increase in the distribution current of a power supply with a small external characteristic slope (ie, a small output impedance) is greater than that of a power supply with a large external characteristic slope.

The main means of achieving current sharing in the Droop method is to use current feedback to adjust the external characteristic slope of each converter so that the output impedance of the parallel converters is close to the same, thereby achieving output current sharing.

As mentioned above, the output impedance of the flyback circuit is a function of the switch tube's conduction duty cycle. Therefore, the way to use the flyback circuit to implement the Droop method current sharing should be to control the switch tube's conduction duty cycle through the current detection signal, or in other words, the current detection signal should participate in PWM control.

This paper uses the Droop method to design two 12V output parallel DC/DC converters. The structure is shown in Figure 7 and the technical indicators are as follows.

Figure 7 Droop method current sharing DC-DC design principle block diagram

Input voltage: 17V～32VDC;

Output voltage: 12VDC;

Maximum output power: 30W;

Operating frequency: 200kHz.

Voltage regulation: less than ±3%;

Load regulation: less than ±3%;

Efficiency: greater than 70%;

Ripple: less than 70mV.

Design Results

● Load Regulation

The output mode of the flyback converter studied in this paper is an offline design, and the voltage sampling signal is not directly sampled from the output end, but a magnetic isolation sampling technology is used. This design can achieve offline output without the help of a startup isolation circuit and an isolation drive circuit. The circuit is simple, but the disadvantage is that the load adjustment rate cannot be very high. In theory, it is difficult to achieve a load adjustment rate of ±5%. Relevant literature introduces that this design (output 12V, current changes from 0.1 to 0.3A) can achieve a load adjustment rate of ±3%. This design has taken some effective measures to achieve a load adjustment rate of ±3% when the load current changes from 0.1 to 1.3A.

Transformer coupling

Since the voltage sampling signal is obtained by coupling the output voltage change signal through the transformer voltage sampling signal winding, the quality of signal coupling directly affects the quality of the output voltage load regulation rate. After repeated experiments, two practical experiences were obtained:

The transformer is wound in a “sandwich” manner, that is, the primary winding is wound halfway, then the secondary winding is wound, and then the remaining turns of the primary winding are wound, wrapping the secondary winding inside, so that the leakage inductance is minimized, as shown in Figure 8.

Figure 8 Transformer winding method

The output winding and the voltage sampling winding are wound in parallel to achieve the best coupling effect.

Working Mode

Through experiments, it is found that the different circuit working modes have a great impact on the load regulation rate. When the circuit is designed with a large primary inductance and works in continuous mode (CCM), the current signal (peak inductor current) caused by load changes has a relatively flat waveform slope (small rate of change), which affects the output voltage load regulation rate; when the circuit works in discontinuous mode (DCM), it affects the efficiency.

Therefore, after repeated experiments, the circuit was designed with moderate primary inductance (the number of primary turns of the transformer was adjusted to 6 turns), and the circuit operated in critical continuous mode, which resulted in a certain improvement in the output voltage load regulation rate.

Voltage sampling signal

It is also found in the experiment that reducing the output impedance of the voltage sampling winding is equivalent to having a certain amplification effect on the voltage sampling signal, 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

in conclusion

According to the relevant research and discussion in this article, and combined with the solution of practical problems encountered in the design, the designed single-ended flyback hot backup current sharing switching power supply has better performance. 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/DC1 is smaller than the output impedance of DC/DC2, the output current of DC/DC1 is always greater than the output current of DC/DC2 as a result of steady-state adjustment, and the imbalance of the output current is about 12.78%.

The output impedance of DC/DC1 can be adjusted by adding a series resistor to further reduce the imbalance, but this will reduce the output efficiency and increase the output load regulation rate.

Judging from the design results, hot backup DC/DC output has been basically achieved, and the overall efficiency and various indicators have met the design requirements relatively well.