Research on low voltage input interleaved parallel dual-switch forward converter

Abstract: Aiming at the DC link of aviation static converter, the staggered parallel double-tube forward converter is studied. The analysis shows that the double-tube forward converter uses two freewheeling diodes to achieve the magnetic reset of the transformer core, which is simple and reliable. After adopting the staggered parallel technology, the input and output current ripple is greatly reduced, the volume of the input and output filters is reduced, and the heat distribution of the converter is more uniform, which improves the performance and reliability of the whole machine. Based on the completion of the aviation DC27V low input voltage, DC190V output, 1kW prototype, the relevant circuit design issues with large input current are discussed and summarized in detail.

Keywords: resonance; DC/DC converter; two-switch forward converter; interleaved parallel connection; low voltage/high current

Analysis and Design of Low Input Voltage Two-module

Interleaved Two-transistor Forward DC/DC Converter

QIN Hai-hong,WANG Hui-zhen,YIN Jian

Abstract:For the purpose of implementation of DC Link of Aeronautical Static Inverter(ASI), research on low input voltage two-module interleaved two-transistor forward DC/DC converter is detailed. In this topology, only two diodes is needed for the demagnetization of transformer core. With interleaving technique, input and output current ripple can be reduced dramatically, and lower size input and output filter can be used. Furthermore, hotspots in the circuit is almost eliminated, which improve circuit performance greatly and make it more reliable. Through a prototype of DC 27V/190V, 1kW DC/DC converter, some useful conclusion and design guideline of the low-voltage high-current input DC/DC converter is presented.

Keywords:DC/DC converter;Two-transistor forward;Interleaving;Low-voltage/high-current

1 Introduction

Aeronautical static inverter (ASI-AeronauticalStatic Inverter) is a static converter that uses power semiconductor devices to convert the aircraft DC27V or 270V power supply voltage into AC115V/400Hz or AC36V/400Hz constant voltage and frequency AC power, and is used as the aircraft secondary power supply. Today's small-capacity static inverters are generally implemented using a two-stage structure as shown in Figure 1: a DC link (front-stage isolated DC/DC part) and a high-frequency inverter link (back-stage DC/AC part). Choosing a reasonable and effective solution to achieve single-stage DC/DC and unipolar DC/AC will be a reliable guarantee for meeting the high indicators of static inverters.

In the DC link of static converters below 1kVA, the forward topology has been widely adopted due to its simple circuit structure, input and output electrical isolation, wide voltage rise/fall range, easy multi-channel output, and suitability for small and medium power power conversion occasions. However, there is an inherent defect in the forward converter: a reset circuit must be added to achieve magnetic reset of the transformer core during the power switch cutoff period to avoid transformer saturation. In recent years, there have been many studies on magnetic reset technology for forward converters. For example: RCD clamping technology, LCD clamping technology and active clamping technology. Although the RCD clamping technology has a simple circuit structure and low cost, part of the excitation energy is consumed in the clamping network. Therefore, it is only suitable for low-power occasions with low efficiency requirements and strict cost requirements. LCD and active clamping technologies overcome the shortcomings of RCD clamping technology, but the circuit structure is relatively complex. The dual-tube forward circuit uses two diodes to provide an excitation current loop, energy feedback power supply, simple circuit structure, reduced loss, the power tube only bears the power supply voltage, and the voltage stress is small. Therefore, after compromise, we use a two-switch forward circuit with a simpler structure and no loss of excitation energy in the clamping network as the DC link of the static converter.

Figure 1 Typical ASI two-level structure

Noting the advantages of interleaving and paralleling technology, we combined the interleaving and paralleling technology in the research of dual-tube forward converters, analyzed the working principle of dual-channel interleaving and paralleling dual-tube forward converters in detail, and completed the production of a 1kW prototype with aviation DC27V input, DC190V output. Through experimental production and analysis, the specific circuit design issues related to high current input in low-voltage input DC/DC converters are summarized.

2 Working Principle

As shown in Figure 2, it is the main circuit and main waveform of the dual-path staggered parallel dual-tube forward DC/DC converter. Q1, Q2, D1, D2 and the secondary side topology constitute one dual-tube forward converter, Q3, Q4, D3, D4 and the secondary side topology constitute another dual-tube forward converter, D5 and D6 are the secondary rectifier diodes of these two converters respectively, and D7 is a freewheeling tube shared by the two paths. Lf and Cf are the output filter inductor and filter capacitor. Coss1~Coss4 are the drain-source junction capacitances of Q1~Q4 respectively, and the transformer primary-secondary turns ratio is n=N1/N2. In one switching cycle Ts, the converter has 6 switching states. Before analysis, the following assumptions are made:

Figure 2 Main circuit and main waveform of dual-path interleaved parallel dual-switch forward DC/DC converter

——All switches and diodes are considered ideal devices;

——Lf is large enough that its current remains basically unchanged during a switching cycle, so that Lf, Cf and the load resistor can be regarded as a constant current source with a current of Io;

——The drain-source capacitance Coss1 of Q1 and Q2 is Coss2, and the drain-source capacitance Coss3 of Q3 and Q4 is Coss4.

FIG3 shows the equivalent circuit of the converter in different states, and its working principle is described as follows.

(a)[t0～t1] (b)[t1～t2] (c)[t2～t3] (d)[t3～t4] (e)[t4～t5] (f)[t5～t6]

Figure 3 Equivalent circuits in various switching states

1) Switching mode 1 [t0～t1] [refer to Figure 3(a)]

Before time t0, the voltages on Q1, Q2, D1, and D2 are all Uin/2, and the voltages on Q3 and Q4 are all Uin. The load current I0 continues to flow through D7, D3 and D4 are turned on, the magnetizing current decreases, and the T2 core magnetic reset. At time t0, Q1 and Q2 are turned on, D1, D2, Q3, and Q4 are still off, D3 and D4 are still turned on, and the T2 excitation current i2M continues to flow through D3 and D4, linearly decreasing and feeding back to the power supply. D7 is turned off, D5 is turned on, and the power supply transfers energy to the secondary side through T1. The T1 magnetizing current i1M rises linearly from zero,

i1M(t)=(Uin/L1M)(t－t0) (1)

i2M(t)=I2M0－(Uin/L2M)(t－t0) (2)

Where: L1M, L2M——corresponding to the primary magnetizing inductance of T1 and T2;

I2M0 is the excitation current value of another path T2 corresponding to the moment when Q1 and Q2 are turned on (t0). Its magnitude is explained as follows: At t1, the excitation current

i2M(t1)=0，t0-1=t1－t0=(2D－1/2)Ts

That is

I2M0=(Uin/L2M)(2D－1/2)Ts

During this period, the voltage on D1, D2, Q3 and Q4 is Uin.

2) Switching mode 2 [t1～t2] [refer to Figure 3(b)]

At time t1, the excitation current i2M (t1) is zero, and D3 and D4 are naturally turned off. At this time, the primary magnetizing inductance L2M of T2, the leakage inductance L2S, the drain-source junction capacitance Coss3 and Coss4 of Q3 and Q4 begin to resonate. i2M flows in the reverse direction to discharge the drain-source junction capacitance of Q3 and Q4. If uds3 (uds4) drops to zero, uds3 (uds4) will be clamped to zero because the body diodes of Q3 and Q4 are turned on. During this period, because Q1 and Q2 are turned on in the other path, the voltage on D7 is clamped to Uin/n, and the secondary voltage of T2 will not exceed Uin/n. Therefore, the secondary side of the single-channel dual-tube forward converter will not be clamped to zero, so a positive voltage appears on the T2 winding (the same name end). Correspondingly, there is

uds3(t)=uds4(t)=Uin·〔1+cosωr(t－t1)〕/2 (3)

i1M(t)=(2D-1/2)Ts+(t-t1) (4)

i2M(t)=－(Uin/Zr)sinωr(t－t1) (5)

Where: ωr=1/;

Zr =;

L2=L2M＋L2S.

During this period, the voltage on D3 and D4 is uD3=uD4=Uin-uds3, uT2PR1M=Uin-2uds3, at time t2

uds3(t2)=uds4(t2)=Uin· (6)

i1M(t2)=I1M(+)=(Uin/L1M)DTs (7)

i1M(t2)=(Vin/Zr)sin(ωrt1-2) (8)

In the formula: t1-2=t2-t1=(1/2-D)Ts.

3) Switching mode 3 [t2～t3] [refer to Figure 3(c)]

At t2, Q1 and Q2 are turned off, D1 and D2 are turned on for continuous current flow, and the magnetizing current of T1 decreases linearly from the maximum forward value I1M(+).

i1M(t)=I1M(+)-(Uin/LM)(t-t2) (9)

i1M(t3)=(Uin/LM)(2D－1/2)Ts (10)

D5 is turned off, D7 is turned on, and the load current Io continues to flow through D7. At this time, the primary side of T2 continues to resonate, so the voltage of the T2 winding (the same-name end) is positive, which makes D6 and D7 turn on at the same time, clamping the secondary side of T2 to zero, so that the resonant circuit becomes the resonance of T2 leakage inductance L2S and Q3, Q4 junction capacitance, releasing the leakage inductance energy, making the magnetizing current of T2 zero, uds3, uds4 quickly rise to Uin/2, and then remain at Uin/2 until the next switching state.

4) Switching mode 4 [t3～t4] [refer to Figure 3(d)]

5) Switching mode 5 [t4～t5] [refer to Figure 3(e)]

6) Switching mode 6 [t5～t6] [refer to Figure 3(f)]

At t3, corresponding to the start of the second half cycle, the two dual-tube forward-pulse circuits exchange their working states and repeat the working conditions of the first half cycle. The corresponding related formulas are interchangeable and consistent, so they will not be repeated here. At t6, Q1 and Q2 are turned on again and the next cycle begins.

3 Circuit Characteristics Analysis

From the above switching mode analysis, it can be seen that the dual-path staggered parallel dual-tube forward DC/DC converter works alternately, transmits energy to the secondary side, feeds back energy to the power supply through diodes D1, D2 or D3, D4, realizes the core magnetic reset, and has a simple circuit structure. In addition, the main power tube only bears the power supply voltage during the shutdown period, so low-voltage, high-speed, and low-on-resistance power tubes can be selected, thereby reducing the power tube conduction loss and switching loss.

Moreover, due to the use of the two-way staggered parallel structure, the circuit has the following advantages:

——Under the same switching frequency, the frequency of the voltage on the output filter inductor is doubled, which reduces the volume of the output filter inductor; at the same time, the input current pulsation frequency is doubled, which also reduces the volume of the input filter, thereby further reducing the volume of the entire machine.

——Due to the staggered parallel connection of the two paths, the equivalent duty cycle of the output voltage on the rectifier side is doubled, which brings two advantages: first, when the power tube works at a duty cycle less than 0.5, the duty cycle of the output voltage on the rectifier side can vary between 0 and 1, which improves the response of the circuit and is beneficial to the design of the drive circuit; second, under the condition of the same output voltage, the peak voltage on the rectifier side is reduced by half, and the freewheeling time is reduced, which is beneficial to the selection of freewheeling tubes with low current ratings.

——The parallel structure allows each parallel branch to flow through a smaller power, eliminating the "hot spots" of the converter, making heat distribution uniform and improving reliability.

During the principle analysis and prototype production, we also noticed that the resonance of parasitic parameters will cause a small range of bidirectional magnetization in the transformer. However, since the resonance parameters are small, it has little effect on the selection of transformer core and converter operation, and the maximum duty cycle can still be taken as around 0.5.

4 Experimental results and discussion

Based on the analysis of the working principle of the dual-path staggered parallel dual-tube forward DC/DC converter, a DC 27V/DC 190V, 1kW prototype was developed. The main experimental data of the prototype are:

——Input DC voltage: 20~30V;

——Output DC voltage: 190V;

——Inductor: R2KBDEE40 core;

——Transformer: R2KBDEE42B core;

——Transformer primary-to-secondary turns ratio: 1/10;

——MOSFET: IRF3205;

——Switching frequency: fs=120kHz;

——Magnetic reset diode: IN5822;

——Output rectifier: MUR8100;

——Output freewheeling tube: MUR8100.

Figure 4 shows the waveforms of the gate-source voltage ugs and drain-source voltage uds of the switch tube MOSFET at full load, which is basically the same as the theoretical analysis. Figure 5 shows the voltage waveforms of the secondary rectifier diode D5 and the freewheeling diode D7. It can be seen that when the freewheeling tube is turned off, the reverse recovery causes voltage oscillation. Figure 6 shows the relationship between the converter efficiency and output current when the rated input voltage is DC 27V.

Figure 7 shows the secondary rectifier circuit. The staggered parallel circuit structure doubles the equivalent duty cycle of the secondary output voltage UA. Although it can reduce the volume of the output filter inductor, it doubles the switching frequency of the freewheeling tube D7, which is in a higher frequency switching process. Due to the reverse recovery of D7, a loop current will be formed in D5, D7 and the secondary side of T1 (D6, D7 and the secondary side of T2), causing greater losses. If the di/dt is too high in the t1-t2 section (as shown in Figure 8), it will not only cause ringing and generate serious electromagnetic interference, but also may damage the diode or other semiconductor devices in the circuit due to the high transient spike voltage. Therefore, D7 should use a fast recovery diode with a short recovery time of t0-t1 and a long time of t1-t2, that is, a large flexibility coefficient.

At the same time, the secondary leakage inductance of the transformer should be minimized as much as possible, and the area enclosed by D5, D7, and T1 secondary winding (D6, D7, and T2 secondary winding) should be minimized to reduce line parasitic inductance.

Figure 4 Ch2-ugsCh1-uds

Figure 5 Ch1—uD7 Ch2—uD5

Figure 6 Efficiency changes with load at rated input voltage

Figure 7 Secondary circuit

Figure 8 Diode reverse recovery

5 Summary of low voltage/high current input circuit design

This paper studies the DC link of the aviation static converter and the low-voltage input dual-path interleaved parallel dual-transistor forward converter. Due to the large input current, it brings many related design problems, which must be paid enough attention in the design and production. Combined with the prototype development, this paper gives some suggestions on the specific circuit design of the low-voltage/high-current input converter.

1) The primary current of this type of converter is large, and even a small resistance will cause considerable loss. Therefore, the components of the main circuit as shown in Figure 9 should be laid out as compactly as possible, and the winding resistance of the transformer should be reduced as much as possible. A large input area can be laid to reduce the resistance of the input wire, and a low-loss core material with high Bs and low Br can be selected.

2) Due to the large primary current, in order to reduce the conduction loss of the power device, the power tube should use a power MOSFET device with a low on-resistance, or use multiple MOSFETs in parallel. However, it must be noted that the power device working in the hard switching state has a relatively high switching loss when working at high frequency. Therefore, when selecting devices, the conduction loss of the MOS device and the capacitive related loss (switching loss, driving loss) must be compromised. If necessary, soft switching technology can be considered.

3) The main power MOSFET works in the hard-off state, and the di/dt at the time of shutdown is very large. Due to the inevitable parasitic inductance in the circuit, a large voltage spike will be excited between the drain and source of the MOSFET, causing circuit oscillation and even damage to components. In order to reduce the spike, in addition to using the method in 1) as much as possible, the following points must be noted:

——As shown in Figure 9, capacitors with good high-frequency performance are connected in parallel at points a and b close to the pins of the power device to eliminate the influence of some parasitic parameters;

Figure 9 Primary circuit diagram

——The transformer adopts the process of staggered winding of primary and secondary sides to minimize leakage inductance;

——Appropriately slow down the power tube shutdown speed, but this will also increase the power tube shutdown loss, so a compromise should be considered in practical applications;

——Select a fast recovery diode with a faster turn-on speed as the freewheeling diode for the primary excitation current.

Low voltage/high current input DC/DC converters have very high requirements for the main circuit design. The quality of the design directly affects the height of the spikes borne by the power tube, circuit losses, heat generation, etc., which in turn affects the reliability, efficiency, volume and cost of the entire machine. It must be fully and reasonably considered in actual circuit production.

6 Conclusion

Aiming at the DC link of aviation static inverter, the dual-path interleaved parallel dual-tube forward converter with low voltage input is studied, and the experimental results of 1kWDC/DC converter prototype with DC27V low voltage input, DC190V output are given. Combined with the research on the low voltage input converter, several design summaries of low voltage/high current input DC/DC converter are given, which has certain guiding role in engineering practice.