Compact full-bridge DC-DC isolated power supply design example

As the core device of power converter, IGBT, a new type of power electronic device , has its driving and protection circuits that are crucial to the reliable operation of the converter. The integrated driver is an independent driver board with complete functions. It has the advantages of easy installation, efficient driving, and reliable protection. It is currently the best way to drive and protect large and medium power IGBTs. The integrated driver generally includes four functional circuits on the board, including DC-DC isolation power supply , PWM signal isolation, power amplification, and fault protection. The functional circuits cooperate with each other to complete the driving and protection of the IGBT. The input power supply provides power for the primary side functional circuits on the board. The two DC-DC isolation power supply outputs drive the upper and lower half-bridge switch tubes respectively , and provide power for the fault detection and protection circuits on the IGBT side. Therefore, the power supply on the integrated driver board is the premise and basis for the operation of all circuits.

The half-bridge IGBT integrated driver board in this article requires two sets of isolated positive and negative voltage outputs as the IGBT drive and protection circuit power supply. From the driving characteristics of IGBT, it can be seen that its load characteristics are similar to capacitive loads. To achieve reliable and fast opening or closing, the power supply is required to have good pull/sink current capability, that is, good dynamic characteristics. The half-bridge IGBT is composed of upper and lower switch tubes with the same model. The drive voltage and current characteristics of on and off are consistent. As the load of the dual-channel isolated DC-DC power supply, its load characteristics are stable. Therefore, two isolated power supplies can be designed according to the maximum load to be driven, and no feedback control is required. In actual design, it is necessary to calculate whether the power supply power of the driver board is sufficient based on the selected IGBT switch tube parameters and operating frequency. If not, the switch tube needs to be re-selected.

1 IGBT half-bridge integrated driver board power supply design

1.1 IGBT half-bridge integrated driver board power supply characteristics

In the power electronic conversion topology, the topology design based on the half-bridge IGBT as the basic unit is the most widely used, and the effective drive and reliable protection are realized by the half-bridge IGBT integrated driver board. The half-bridge IGBT integrated driver board itself must have two DC-DC isolated power supplies, which require a small PCB area, compact size, high reliability, and the two sets of power supply secondary sides are completely isolated. In the project of high-power half-bridge IGBT integrated drive unit, a power transformer primary side control topology is designed to meet the needs of the drive unit for efficient and reliable isolated power supply, that is, two sets of isolated power transformer primary sides share a set of full-bridge control ideas, which improves the power density and efficiency of the power supply and saves the number of power switches. The full-bridge switch tube is cleverly matched, and no isolation drive is required, which reduces the PCB area occupied on the integrated driver board.

Since the performance parameters of the two IGBT units of the upper and lower half bridges are consistent and the same body is packaged , the load characteristics of the two drives on the half-bridge IGBT integrated driver board are completely consistent. Therefore, in the power supply design of the IGBT half-bridge integrated driver board, the two sets of isolated DC-DC power supply primary sides can completely share a set of control circuits. The IGBT half-bridge integrated driver board is generally embedded in the IGBT power module. It has two requirements for the driver board: first, the half-bridge integrated driver board has very high requirements for the PCB area and volume, and requires the PCB area and volume to be as small as possible; second, because the power required to drive the IGBT is relatively large, the power density and efficiency requirements of the power supply on the board are also high.

1.2 Design of DC-DC power supply with primary side shared full-bridge control

The design uses a full-bridge circuit to control the DC-DC power transformer, and the two transformers share a full-bridge switch on the primary side. In normal mode, two full-bridge conversion topologies require two sets of full-bridge switches, and the pulse drive circuit of the full-bridge switch is also two sets of 8 PWM pulses. The use of a shared full-bridge topology saves the control circuit and full-bridge switch, and simplifies the DC-DC isolated power supply circuit. Since the power supply is used to power the half-bridge IGBT drive circuit, the load is stable and calculable, so the full-bridge DC-DC power supply adopts open-loop control to meet the maximum power requirement. The circuit principle is shown in Figure 1. The power supply consists of 4 parts: 4-way PWM pulse generation circuit, full-bridge drive switch, power transformer and its secondary side rectifier filter circuit. The DC-DC power supply input is a single +15V power supply, and the output is two sets of isolated +15V and -10V dual power supplies. The negative power supply is used to reliably shut down the IGBT.

The working principle of two sets of DC-DC isolated power supplies sharing a full-bridge switch is: the diagonal switch tubes are turned on at the same time, and the other diagonal switch tubes are turned off. At this time, the primary sides of the two sets of magnetic cores are excited in positive and negative phases at the same time, and the secondary sides are coupled, and then full-wave rectification and filtering are performed to obtain a stable power supply. The full-bridge switch operating frequency is designed to be 360kHz, and full-wave rectification is used at the same time, so the secondary side does not need large filtering and energy storage components, which is conducive to the miniaturization of DC-DC power supplies.

The parameters of the full-bridge DC-DC power supply are: input +15V; output +15V, -10V; output power 6W; operating frequency 360kHz. The dynamic characteristics under rated load are required to meet: +15V fluctuation < +1V; -10V fluctuation < -2V; the operating frequency meets the 5% deviation tolerance. The operating frequency is determined by the Schmitt trigger CD40106 parameters and RC values. The specific parameters are: R = 2.2kΩ; C = 748pF; VDD = 15V; VT+ = 8.8V; VT- = 5.8V. According to formula (1), the oscillation frequency is calculated to be 748.792kHz. Because the multivibrator output in the design charges and discharges two RCs, the charging capacitor capacity is doubled, so the oscillation frequency is 1/2 of the above calculated frequency, that is, 374.396kHz.

1.2.1 Primary side sharing Full bridge Controlled 4-way PWM signal generation

The traditional full-bridge DC-DC topology consists of 4 identical switches and requires 2 mutually inverted PWM control signals. Each PWM signal drives the 2 diagonal switches. The 2 PWM signals require dead zones to avoid full-bridge direct pass. The upper arm drive of the full-bridge topology must be isolated, otherwise the correct drive cannot be completed. The isolation circuit is generally implemented with optocouplers or magnetic devices, which are complex and large in size. The design uses 2 power transformer primary windings to share a full-bridge switch. Since the system is a +15V single power input, the full-bridge switch is implemented with 2 S14532ADYs containing P MOS and NMOS. At this time, the PWM drive pulse does not need to be isolated, that is, the upper and lower arm drive pulses of the full-bridge do not need to be isolated. The logic gate of the oscillation circuit is used for driving, which simplifies the control circuit. At the same time, the full-bridge switch is a small-volume SO-8 package , which realizes the minimum PCB design. According to this principle, the design of the full-bridge switch requires 4 PWM pulse drives, which are divided into 2 groups. Each group is mutually reversed to drive the diagonal PMOS and NMOS switches. There is a dead zone between the 2 groups. The specific 4-way drive pulse timing requirements are shown in Figure 2. G11, G2, G22, and G1 are 4-way PWM drives. T1 and T11 are two DC-DC power transformers. Only the primary winding is drawn here. C is a DC isolation capacitor , which can effectively prevent the transformer core from saturation. It can be seen that the diagonal switches are turned on at the same time, and the two diagonal switches are switched alternately. The two transformer cores work in the I and III working quadrants, and the bidirectional excitation is conducive to achieving high power density.

With the above design, the 4-way PWM timing must be generated strictly as shown in Figure 2. The general PWM drive generation method uses MCU, DSP or dedicated IC, which is difficult to achieve low cost and compact design. In this paper, the general multivibrator circuit is improved by adding two diodes , resistors and capacitors respectively, so that the 4-way PWM drive signal that meets the above requirements can be output, which simplifies the power supply design and improves reliability.

1.2.2 Selection and design of DC-DC power transformer

The system power supply adopts full-bridge drive, and the magnetic core works in quadrants I and III. The drive must be able to prevent the magnetic core from saturating, and at the same time require high efficiency and small size. Based on the above considerations, the toroidal magnetic core T10×6×5 is selected, and the material is PC40. The toroidal magnetic core has low leakage and high efficiency. The specific parameters are: μi=2400, Ae=9.8mm2, Aw=28.2mm2, J=2A/mm2. The system working state is: ηB=90%, Km=0.1, fs=366kHz, Bm=2000GS, according to P0=Ae×Aw×2×fs×Bm×J×ηB×Km×10-6. It is obtained that P0=9.8×10-2×28.2x10-2×2×366×103×2000x2×0.9×0.1×10-6=7.3W. Theoretical calculation shows that the selected magnetic core meets the design power requirements.

The transformer turns are designed based on equations (2) and (3), where μi is the minimum input voltage , △Vce is the full-bridge circuit switch voltage drop at rated current , Dmax=0.48; μo is the output voltage rating; △Vd is the full-wave rectifier diode voltage drop at rated output current. Theoretically calculated primary and secondary turns are: primary Np=4.6 turns, secondary Ns1=5.8 turns, and Ns2=3.9 turns.

Np=[(μi-△Vce)×Dmax]／(2△B×Ae×fs)(2)

Np=[(μo-△Vd)×(1-Dmax)]／(2△B×Ae×fs)(3)

The actual debugging results are: primary side p=6 turns, secondary side Ns1=8 turns, Ns2=5 turns.

1.3 Simulation of 4-channel complementary PWM signals with dead zone

Two DC-DC power supplies Transformer primary side shared Full-bridge Topology, full-bridge Circuit The 4-channel PWM signals are generated by adding several Passive devices to the multivibrator circuit , and the two sets of drive signals generated have dead zones, which can effectively prevent the full-bridge switch device from passing through. The working principle of the circuit is: improve the output of the general multivibrator to make its charge and discharge Capacitor Capacity different, generate 2 waveforms with slightly different charge and discharge curves, this difference will generate a dead zone between the two sets of PWM waves, and then pass through the same phase device and inverter respectively, to generate 4 PWM pulses that meet the drive requirements.

The Saber simulation schematic diagram and simulation results of the 4-way PWM generation circuit are shown in Figure 3(a) and Figure 3(b). It can be seen from the simulation results that the 4-way PWM pulse can meet the control requirements of the shared full-bridge topology.

2 Experimental Results

Figure 4(a) shows the waveform of the primary and secondary windings of the actual full-bridge DC-DC power transformer with carriers, where CH1 is the voltage across the primary coil and CH2 is the positive voltage of the secondary coil. Due to the dispersion of the device, the actual tested DC-DC power supply operating frequency is 366kHz, and the frequency deviation is 3.8%, which meets the design requirements. Figure 4(b) shows the dynamic loading output waveform, where CH1 is the output positive voltage and CH2 is the output negative voltage. During the test, the load is 35Ω/10W. It can be seen that the output positive voltage is relatively stable when the rated load is suddenly added or removed, and the fluctuation is <1V, which meets the design requirements; the negative voltage fluctuates slightly. Considering that the negative voltage of the IGBT is used to maintain the off state, the negative voltage can be between -5 and -15V, so it meets the requirements of the half-bridge integrated drive power supply.

3 Conclusion

Aiming at the design requirements of green energy and combining the specific use conditions of the integrated driver board, the efficient and reliable design of the DC-DC isolated power supply is realized, and it is easy to integrate with the IGBT module and easy to install. The circuit uses a DC-DC isolated power supply with two sets of magnetic core primary windings sharing a high-frequency full-bridge switch; it generates 4 full-bridge pulse signals that do not require isolation, and realizes the compact design of a high-power density on-board power supply. The simulation and experimental results show that the power supply circuit is simple, efficient, and reliable, and achieves the expected purpose.