A Novel Multi Output Forward and Reverse Converter Using Post Stage Adjustment Technology

The working principle of a novel multi-channel output forward flyback converter using post stage adjustment technology is analyzed in detail. The converter can use the forward part of the circuit to output high power and the flyback part to output low power. Therefore, the circuit combines the high efficiency of the forward converter with the low cost of the flyback converter. At the same time, the post stage adjustment technology also ensures the accurate output of each winding. The characteristics of the converter are verified by a 250W prototype.

Key words: multi output; Post adjustment technology; Forward and reverse converter; DC / DC

0     introduction

At present, many electronic devices not only require that the power supply can provide two or more mutually isolated power supplies with high adjustment rate, but also require that the efficiency of the power supply can be continuously improved and the power density can be continuously increased, so as to reduce the volume of the circuit. The combination of these requirements puts forward higher requirements for power supply design.

Generally speaking, any topology isolated by transformer and capable of providing multiple windings on the secondary side can be a candidate for multi-channel output converter. At present, from the perspective of low cost and relatively high efficiency, multi-channel output circuits mostly use multi winding forward converter or flyback converter.

Flyback converter with multi-channel winding output is the simplest and easiest to realize in multi-channel output circuit. It has many advantages, such as simple structure, low cost, easy design and so on. However, because the overall efficiency of this kind of circuit is not high, it is rarely used in occasions above 150W.

The characteristics of forward converter are just opposite to that of flyback converter. The converter has the advantage of high overall efficiency, so it has been widely used in high-power occasions. However, due to the addition of a freewheeling diode and filter inductor on the secondary side structure of the circuit, the cost will be increased accordingly [1].

In the actual circuit design, we will encounter the following situation, that is, the output power is not evenly distributed on each output in the multi-channel output power supply. That is, the output power of one and two channels is particularly large, accounting for more than 80% of the overall output power, while the other channels are particularly small. If the total power of the whole circuit exceeds 150W. From the perspective of efficiency, the forward converter should be selected in the circuit topology. However, if each circuit uses a forward circuit, the cost of several low-power circuits is too high and unnecessary.

In view of this situation, a new multi-channel output forward and backward converter is proposed in this paper. The converter can use the forward part of the circuit to output high power and the flyback part to output low power. It is a compromise between forward converter and flyback converter, which fully combines the advantages of high efficiency of forward converter and low cost of flyback converter. Therefore, the circuit is especially suitable for those occasions where the output power of each channel in the multi-channel output circuit varies greatly. At the same time, in order to make each circuit have accurate output, the post stage adjustment technology is also used in the output circuit of the auxiliary circuit to accurately control the output voltage.

one     working principle

Figure 1 shows the principle block diagram of the multi-channel output forward and reverse converter.

Figure 1     Multi output forward and reverse converter using post stage adjustment technology

The circuit uses the output filter inductance in the forward converter as the flyback transformer of other auxiliary output circuits. In the time period (1-D) ts, the distribution of energy above the secondary inductor between each output is essentially the same as that of the flyback conversion circuit (where D is the duty cycle of the main S1 and TS is the switching cycle).

Like the ordinary flyback converter, without additional post stage adjustment measures for the circuit, the adjustment rate of the output of other auxiliary circuits without feedback in this converter will be relatively poor, usually fluctuating about 10%. If each circuit is required to have accurate output, the most common method is to add a linear voltage stabilizing chip (such as 7805) to the auxiliary circuit output. However, this will bring huge power loss. Therefore, this method is only suitable for low output power. In the case of medium and high power, the common method is magnetic switch (magamp). However, in the case of high frequency, the post stage adjustment technology (sspr) has more advantages [2]. Therefore, this paper uses this technology to obtain accurate output voltage on other auxiliary output windings.

For the flyback converter, when the main pipe is turned off, the energy is transmitted from the primary inductor to the load side in the form of current source. More importantly, there is no output inductance in the flyback converter, and the output voltage of each circuit will be determined by the turn ratio of the transformer. That is, the output voltage of the main circuit and the output voltage of the auxiliary circuit in the flyback circuit are determined by equation (1).

Uo1/N1=Uo2/N2（1）

In this operating mode, if the circuit is not adjusted, the voltage blocking characteristic of sspr will not work in this place. Therefore, a "time-sharing reuse" mode is proposed in reference [3]. In this mode, when the main pipe is turned off, the energy will be transmitted to different output branches in different time periods. In order to realize the working state of "time-sharing multiplexing", equation (2) must be satisfied.

Uo1/N1>Uo2/N2（2）

Figure 2 shows several main working waveforms of the circuit. Figure 3 shows the equivalent circuit diagram of each stage of the converter using sspr.

Figure 2     The main working waveforms of sspr circuit are used

(ugs1 is the gate signal of main switch S1. Ugs2 is the gate signal of auxiliary switch S2)

(a) Phase 1   DTs

(b) Phase 2   D1Ts

(c) Phase 3   D2Ts

Figure 3     Equivalent circuit diagram of each stage

The specific process of the work is described below.

The main circuit output uo1 of the circuit controls the on time of the main pipe through the feedback controlled PWM, which determines the energy transmitted to the secondary side of the transformer. The function of sspr is to transfer these energy to different output branches respectively in the time period of (1-D) ts, so as to distribute this part of energy between the two outputs uo1 and UO2. "Time sharing multiplexing" can be realized by adjusting the blocking time of sspr.

Phase 1     Before time T1, in the time period DTS, as shown in Fig. 3 (a), the main pipe S1 is turned on, part of the energy on the DC bus is stored in the output inductance L1 through the forward transformer, and the other part is transmitted to the main output uo1.

UL1=Us1=Uin/n－Uo1（3）

Where: n is the transformation ratio of forward transformer t.

Phase 2     After time T1, in the time period d1ts (D1 is the duty cycle of diode D2 on), as shown in Fig. 3 (b), the main pipe S1 and diode D1 are turned off, and the sspr blocks the auxiliary circuit output. Therefore, only diode D2 on uo1 branch is turned on at this time. The voltage US1 on the inductor L1 is clamped to the main output voltage uo1.

UL1=－Us1=－Uo1（4）

Phase 3     In the time period d2ts (D2 is the duty cycle of S2), as shown in Fig. 3 (c), S2 on the secondary side of the transformer has been triggered and D3 is also turned on. It can be seen from equation (2) that the voltage US1 on L1 will be clamped at n1uo2 / N2