Single-phase inverter power supply design

1 Introduction

The inverter power supply that converts 27V DC to 115V AC, 400Hz is widely used in the army and ships.

There is a great demand for it. To address this situation, we have developed a 800VA single-phase static inverter power supply, which uses a DC 27V input and can output a 115V, 400Hz sine wave voltage. And by properly connecting three identical power supplies, under the control of the peripheral circuit, it can be used as a three-phase inverter power supply.

At present, new technologies are constantly emerging, and there are many ways to form DC/AC inversion. However, considering the specific conditions of use, cost and reliability, this power supply adopts a relatively typical two-stage conversion method, that is, the first stage uses DC/DC conversion to convert 27V into a DC high voltage of about ±130V, and the second stage uses DC/AC conversion to convert the DC high voltage into AC output, and the ±130V high voltage DC is adjusted by feedback to ensure a stable AC 115V output. In this way, the circuit debugging and production process are simplified, the quality is also easy to control, and it is convenient for industrialization.

2 Main circuit design

2.1 Using DC/DC converter to achieve voltage regulation

The converter adopts a push-pull working mode, which has the advantages of high efficiency and reliable operation. As shown in Figure 1, the function of the converter is to convert low-voltage DC into high-voltage DC. The primary of the main transformer T1 is connected in a push-pull form. Because the secondary voltage is higher, it is rectified in a full-bridge manner. The switch tubes S1 and S2 are connected in parallel with 4 IRF3710s, which effectively reduces the conduction loss. The symbiotic diode of the power MOSFET can also be used as an AC path when the switch tube is turned off to suppress the turn-off overvoltage at both ends of the switch tube. R2, C3, R3, and C4 are resistance-capacitance absorption circuits, which can further reduce the peak voltage when the MOSFET is turned off. The principle of selecting the absorption resistor is that the voltage on the capacitor can still be discharged completely at the minimum conduction time, and the absorption capacitor is as large as possible within the power consumption allowable range of the absorption resistor. After experiments, the absorption resistor of this circuit is 5Ω, 5W, and the absorption capacitor is 0.1μF, 250VDC.

The main transformer T1 uses TDK's PQ50/50 core. After calculation (see the formula in Reference 1), the primary of this transformer is 2 turns and the secondary is 30 turns. Because the primary current is large, a thin copper sheet with a thickness of 0.5 mm is used for winding. At the same time, the primary and secondary are wound alternately to minimize leakage inductance, skin and proximity effects.

The filter inductors L1 and L1' are wound together on the same CD-shaped iron core, with an inductance of 1.0mH. In terms of connection, L1 and L1' are in the form of series inductors, which can increase the inductance and ensure that the output voltage of ±130V with good dynamic and static characteristics to the ground is ensured. L2 and L2' are a group of auxiliary filter inductors.

In actual circuit debugging, it should be noted that the voltage waveform of the power tube of the push-pull converter is significantly different when the first stage is connected to a resistive load and when the second stage DC/AC load is connected. In the second case, the peak voltage when the power tube is turned off is smaller.

2.2 Using DC/AC inverter to output sine wave

Because this power supply outputs a fixed frequency and voltage 115V, 400Hz power supply, from the perspective of system reliability and practicality, a square wave conversion and resonance filtering method are used to output a sine wave voltage. The main circuit is shown in Figure 2.

S3 and S4 use IRFP460, and their driving circuit uses SKHI21. The circuit is simple and reliable. For detailed information on SKHI21, see reference 2. The inductance of L3 is 6.8mH, and it is wound with a CD12.5×25×60 iron core plus an air gap, with a wire diameter of 1.65mm; in order to improve the utilization efficiency of the iron core, the two windings share one iron core and are connected in series to form an inductor. C7~C10 are 5.0μF, 400VDC MKC capacitors; C14~C16 are 5.0μF, 250VDC MKP capacitors; C11~C13 are 1μF, 400VDC MKC capacitors. L6 and C11~C13 form a series resonant circuit, which is mainly used to filter out the third harmonic. L6 is equal to 4.3mH. The filter circuit is simulated and calculated by circuit simulation software EWB5.1D, and the parameters of each component are optimized. Under certain load conditions, the harmonic distortion can be controlled at about 4%.

3 Control Circuit Design

The control circuit consists of two parts, one part is used to provide the driving signal and current, voltage feedback control and protection of the push-pull boost circuit, as shown in Figure 3; the other part provides driving and protection signals for the inverter circuit, as shown in Figure 4. In Figure 3, the PWM driving and protection circuit is composed of TL494 as the core, and the conversion frequency is 50kHz. The 115V output voltage is sent to P8 after isolation and voltage reduction, and then sent to the error amplifier A1 of TL494 after taking the effective value through AD536 , which is used as the closed-loop voltage control of the system. The current of the push-pull inverter circuit is sampled by the current transformer and sent to the control board by P7 for signal processing and then sent to the error amplifier A2 of TL494 as the current loop control of the system. The driving signal output by TL494 is output to the driver board through P6 for amplification and then sent to the main circuit. The system circuit also has perfect protection measures, including:

(1) 115V output current overcurrent protection. The output signal of the current transformer enters the comparator U2 through P1, and after judgment and comparison, it is sent to the slow start blocking terminal (DT) of TL494.

(2) Thermal protection. The system is equipped with temperature relays on the main power devices. When the temperature is too high, the corresponding P4 ends are short-circuited, so that the emitter of T12 outputs a high level and blocks the PWM pulse output.

(3) DC/AC inverter main circuit overcurrent protection and DC overvoltage protection. Both of them are sent by corresponding sensors to the control board P5 and P21 for processing, and then the PWM pulse output is blocked.

The circuit shown in Figure 4 is mainly used to generate the driving signal required for DC/AC inversion. The driving signal can be generated by the signal circuit of this system or supplied by an external circuit to realize the parallel output of the system. Its conversion is automatically performed through the U12 (MAX4544) multi-way switch. When there is an external signal, after being processed by the circuit composed of U11, U11B-7 outputs a high level to U12-7, thereby causing the electronic switch to operate and realize the switching of the driving signal from the internal circuit to the external circuit. The driving signal is amplified by U3A and shaped by U10E and then sent to the rising edge delay circuit respectively. The delay circuit is mainly composed of R50, C3, and D15 to complete the function of adding the "dead zone" time to the rising edge of the driving signal. In this way, the "straight-through" phenomenon of the two switch tubes in the same arm is avoided. Some other circuits are mainly used for protection to improve the reliability of the system.

4 Conclusion

This power supply has high reliability, low output waveform distortion, and no spike interference generated by the SPWM modulation circuit. It has been put into production and use.