Development of a small generator inverter based on Renesas H8/3687

Gasoline generator sets process the original voltage output by the generator through power electronics technology and then output it to the load. Developing a small, stable and reliable generator inverter power supply can save energy and has broad market prospects.

1 Introduction to Renesas H8/3687 MCU
Renesas H8/3687 MCU is a high-precision control industrial motor dedicated processor with high operating speed, powerful processing functions, rich on-chip peripherals, convenient interface and modular design. It is widely used in digital motor control and various power supply equipment in power conversion systems. The specific performance indicators of this MCU are as follows: 1) High-performance static COMS technology, the main frequency can reach 20 MHz, ultra-low power consumption design, and strong anti-interference ability; 2) Built-in 32 K×16 bit ROM program memory; 3) Dynamic PLL, the main frequency can be modified by software programming; 4) 8-channel 10-bit A/D converter, double sampling, maintaining a minimum conversion time of 3.5μs; 5) 2 serial communication interfaces (SCI), I2C bus interface in synchronous clock mode.

2 Structure of generator inverter power system
The overall block diagram of the generator inverter power system is shown in Figure 1. The cylinder of the gasoline engine converts heat energy into mechanical energy through the four processes of intake, compression, expansion and exhaust, and then drives the AC generator through the crankshaft connecting rod mechanism to output AC power with a voltage of 330~470 V and a frequency of 150~330 Hz; it enters the single-phase full-bridge inverter circuit through the rectifier circuit and the large capacitor filter circuit.


The control circuit is based on the Renesas H8/3687 microcontroller, which generates the SPWM signal required for the operation of the single-phase inverter circuit. Through the drive circuit, the single-phase full-bridge inverter main circuit outputs high-frequency pulse width modulated AC power. The AC power is then processed by the output filter to obtain a stable and pure sinusoidal AC power. The output bus voltage is sampled by the Hall current sensor and the detected value is sent to the A/D conversion port of the microcontroller.
2.1 System hardware circuit design
The hardware of the generator inverter power supply mainly consists of a rectifier circuit, a filter circuit, a single-phase inverter circuit and a control circuit. The rectifier circuit is a three-phase bridge uncontrolled rectifier circuit; a large capacitor is used between the rectifier circuit and the inverter circuit to form a surge absorber, which effectively filters out the high-order harmonics generated by the rectifier link and prevents mutual interference between the generator and the load; after the single-phase full-bridge inverter circuit and the LC filter circuit, the required single-phase 220 V/50 Hz sinusoidal AC output is finally obtained. The core of the hardware circuit design of this system is the design of the IGBT drive circuit and protection circuit in the inverter circuit.
2.1.1 IGBT drive circuit design
The IGBT drive circuit must have two functions: 1) to achieve electrical isolation between the control circuit and the driven IGBT gate; 2) to provide a suitable gate drive pulse. This design uses 2ED020I12-F as the IGBT driver. 2ED020I12-F is not only small in size but also fast in speed. The operating frequency can reach up to 60 kHz, and the turn-on and turn-off delays are 120 ns and 94 ns respectively: and a high-end floating bootstrap power supply design is adopted. In the single-phase bridge circuit, only one set of power supply is used, which simplifies the design and saves costs. 2ED020I12-F is used in the half-bridge drive circuit as shown in Figure 2. In the figure, C1 and VDb are the bootstrap capacitor and diode respectively, and C2 is the filter capacitor. It is assumed that C1 has been fully charged during the shutdown period of VQ1. When the high-side input is high, VQ1 is turned on, VQ2 is turned off, and VCC is added between the gate and source of VQ1. As VQ1 is turned on, the source voltage of VQ1 is close to the positive terminal voltage of the DC bus. Since the voltage of C1 cannot change suddenly, the voltage on C1 is raised to maintain the potential difference between the gate and source of VQ1, so that VQ1 remains turned on. When the high-side input is low, VQ1 is turned off, VQ2 is turned on, and C1 is charged through VQ2 to quickly replenish energy for VQ1, and this cycle repeats.


VDb and C1 are components that need to be strictly selected and designed when designing the drive circuit. They should not be too large to affect the driving performance of narrow pulses, nor too small to affect the driving requirements of wide pulses. The bootstrap diode VDb should be a fast recovery diode with a small reverse leakage current to reduce the loss of charge.
2.1.2 IGBT protection circuit design
2ED020I12-F cannot generate negative bias. The IGBT in the inverter circuit in the off state will be subjected to a sharp rise in CE voltage due to the recovery process of its anti-parallel diode. This phenomenon is called Miller effect. Due to the Miller effect, the IGBT gate drive voltage increases, and even causes the IGWT to be turned on, the upper and lower IGBTs to be directly connected, and the bridge arm to be short-circuited.
In view of the shortcomings of 2ED020I12-F, a negative voltage circuit consisting of a capacitor and a 5 V voltage regulator in parallel is added to the drive circuit of the upper and lower bridge arms. The working principle is that the power supply voltage is 18 V, and the power supply charges the capacitor C6 through the resistor R7, and the voltage across the capacitor C6 is +5 V. When the InL input is high, the OUTL output is a high voltage of 18 V. At this time, the voltage applied to the gate of the lower bridge arm VQ2 is 18 V-5 V=13 V, and the IGBT is normally turned on. When the InL input is low level, the OUTL output is OV, and the voltage on the gate is -5 V, which realizes the generation of negative voltage when turning off; the same applies to the upper bridge arm.
In order to effectively suppress the overvoltage when the IGBT is turned off and reduce the turn-off loss, a turn-off buffer absorption circuit is usually set for the IGBT main circuit. In this design, an RCD type turn-off buffer absorption circuit is used. Capacitors C7 and C8 make the voltage rise slowly when the IGBT is turned off, so it is called a voltage-slowing capacitor. The role of resistors R3 and R4 is to limit the current flowing along the IGBT from the energy stored in capacitors C7 and C8 when the IGBT is turned on. When the IGBT is turned off, the charging current will produce a voltage drop on resistors R3 and R4. The role of the diode VD is to bypass the charging current on the resistor to overcome the overshoot voltage. The requirements for the buffer absorption circuit are: 1) Minimize the wiring inductance L of the main circuit as much as possible; 2) The absorption capacitor should use a low-inductance absorption capacitor, and its lead should be as short as possible, preferably directly connected to the terminal of the IGBT; 3) The absorption diode should use a fast turn-on and fast recovery diode to avoid the generation of turn-on overvoltage and reverse recovery causing a large oscillation overvoltage.
2.2 System software design and algorithm implementation
2.2.1 Basic principles of PWM
As shown in Figure 3, the half-cycle sine wave waveform is divided into n equal parts. The sine wave can be regarded as n pulses of equal width connected to each other with unequal amplitudes. The amplitude of each pulse changes according to the sine law. Replace the n pulse sequence with the same number of rectangular pulses of equal amplitude but unequal width. The area (impulse) of the rectangular pulse and the corresponding sine equal division is equal. This is the pulse width modulation waveform. According to the principle that equal impulses have the same effect, the pulse width modulation waveform and the sine half wave are equivalent. In the sine pulse width modulation waveform, the amplitude of each pulse is equal. As long as the width of each pulse is changed according to the same proportional coefficient, the amplitude of the equivalent output sine wave can be changed. The generator inverter power supply outputs a sine wave AC power of Ursin (wt), and the voltage amplitude is Ur. The output pulse amplitude is Uo, and the online calculation of the IGBT opening time in each carrier cycle is:

2.2.2 System software design
H8/3687 is mainly used for control and data processing, and has the function of generating PWM modulation signals. It drives IGBT through the driving circuit, and the A/D interface collects analog signals of voltage detection and current detection to monitor the operating parameters of key power devices in real time. The program flow of the system is shown in Figure 4. The carrier frequency is 12.5 kHz, then the carrier period T1=50μs, so the number of carriers in one reference wave period is n=12.5 kHz/50Hz=250, and the opening time of IGBT in one carrier can be calculated online by formula (1).



3 Experimental results and analysis
An actual experimental platform with IGBT as the core of the inverter was established. The experimental motor uses HT2700L, 2.2 kW, the IGBT switching frequency is 12.5 kHz, the single-chip microcomputer does not use frequency multiplication, and the base frequency is 20 MHz. Here, an Agilent oscilloscope is used to collect the SPWM signal waveform output by the single-chip microcomputer and the inverter power output waveform, as shown in Figures 5 and 6. The unipolar SPWM pulse signal generated by the Renesas H8/3687 microcontroller is stable and interference-free; the sinusoidal degree of the inverter output voltage waveform is very high and the waveform is good, which means that the total harmonic content of the output voltage is low. The unipolar SPWM inverter control method is effective, and the software and hardware design of the inverter is correct and reasonable.



4 Conclusion
This design uses the advanced Renesas H8/3687 industrial control microcontroller to enhance the stability of the system under harsh conditions; the new driver 2ED020I12-F is used, which not only improves the reliability of the IGWT, but also greatly simplifies the drive circuit design, reduces the product volume, reduces the product weight, and expands the application field of the product.