Application of 80C196MC waveform generator and intelligent power module in inverter

1 Introduction

Passive inverter technology has been widely used in AC motor speed regulation, uninterruptible power supply, AC-DC-AC frequency conversion circuit, etc. Pulse width modulation technology has made great progress with its obvious advantages in harmonic suppression, dynamic response, frequency and efficiency. Especially after the emergence of mature self-shutdown devices, more and more inverter circuits use pulse width modulation control.

The circuits that use hardware to generate sinusoidal pulse width modulation waveforms are relatively complex and difficult to control accurately; while the circuits that use software to generate sinusoidal pulse width modulation waveforms require a large amount of CPU overhead, thereby reducing the utilization rate of the computer; in addition, the protection and control of high-power power electronic devices are relatively difficult, and the drive circuits are also relatively complex. These factors have hindered the development of inverter technology and reduced the reliability of the device. This article introduces a solution that uses the on-chip waveform generator (WFG) and intelligent power module (IPM) of the 80C196MC microcontroller to implement the inverter circuit.

2 On-chip waveform generator

The on-chip waveform generator WFG (Wave Form Generator) is a unique feature of the Intel 80C196MC/MD microcontroller. It simplifies the control software and external hardware required to generate synchronous pulse width modulation waveforms. Its structure is shown in Figure 1.

The waveform generator in Intel 80C196MC/MD microcontroller has three synchronous PWM modules (only one is shown in Figure 1), each module includes a phase comparison register WG-COMP, a dead time generator and a pair of programmable outputs. The carrier signal can be generated by the combination of the reload register WG-RELOAD, the bidirectional counter WG-COUNT and the comparator 1. In addition to controlling the working mode of WFG, the lower 10 bits of the control register WG-COM can also be used to determine the dead time. The function of the protection register WG-PRO is to disable all 6 outputs of WFG at the same time under software control or external events. The output control register WG-OUT is used to control the function of the output pin. The waveform generator in the 80C196 can generate three independent pairs of PWM waveforms, but they have a common carrier frequency, dead time and operation mode.

Figure 2 takes the center-aligned working mode 0 as an example to illustrate the principle of the waveform generator generating PWM waveforms. At the beginning, the bidirectional counter counts up and the original output is valid. When W-COUNT=WG-COMP, the output becomes invalid. Then the counter continues to count up until the counter count reaches the peak WG-COUNT=WG-RELOAD and a WG interrupt is generated. The system finds the corresponding value from the established sine table and reloads it into the phase comparison register. Later, the counter counts down. During this period, a pair of complementary outputs are invalid. Until WG-COUNT is equal to the value of WG-COMP again, the output becomes valid again. When the counter counts down to 1, it starts counting up again. Repeating this process can generate a pair of complementary SPWM output waveforms on WGx and WGx. [page]

In order to prevent a pair of complementary PWM from being used as the upper and lower arms of the inverter at the same time and generating direct pass, and to ensure that the output of the WFG does not generate overlapping waveforms, a no-signal time generator is set in the WFG. When WG-COUNT=WG-COMP, the phase comparator generates a jump signal. After the jump detector detects this jump, it starts a 10-bit no-signal time counter, whose count value is loaded by the lower 10 bits D9~D0 of the WG-CON special register, and makes the output DT of the counter low level, and then counts by 1 in each state cycle until 0. At this time, the counter stops counting, and DT becomes a high level, thereby generating a dead time to delay the effective opening time of the output. The dead time is mainly determined by the turn-off time of the IGBT in the IPM, and is also related to the delay time of the output isolation device of the single-chip microcomputer. The dead time cannot be too long. Because too long a dead time may cause the WFG to have no PWM output, theoretically, the pulse width should be guaranteed to be no less than 3T-dead.

From the basic principle of the waveform generator WFG of the 80C196MC microcontroller generating PWM waveforms, it can be seen that to generate a sinusoidal pulse width modulation SPWM waveform, the duty cycle of the PWM waveform generated on the WFG must be controlled according to the sine law. Therefore, when the WFG generates an interrupt and reloads the phase register value, the sine function value must be calculated or the sine function table must be checked to obtain the sine value at the corresponding moment.

3 Intelligent Power Module

Power electronic devices are an important foundation of power electronic technology. The continuous emergence of various new power electronic devices has promoted the development of power electronic technology. In the late 1980s, composite devices represented by insulated gate bipolar transistors (IGBTs) emerged. IGBT is a composite of power field-controlled transistors (MOSFETs) and power transistors (GTRs). It combines the characteristics of MOSFETs with low drive power and fast switching speeds and the advantages of GTRs with low on-state voltage drop and large carrier capacity. Therefore, its performance is very superior, making it a leading device in modern power electronic technology. However, in actual circuits, the dynamic conditions of high-power and high-frequency switching operations are very harsh. The power circuit, buffer circuit, and gate drive circuit must be designed to withstand the di/dt and dv/dt limit values. If overvoltage occurs, it will occur. At the same time, ground loops and stray capacitances will also cause serious noise problems. Therefore, a reasonable layout is very important for the reliability and working efficiency of IGBTs. Any link that is not designed well will affect the normal operation of the circuit and even damage the device. Mitsubishi Intelligent Power Module (IPM) is a power module that integrates high-speed, low-loss IGBTs and their optimal gate drive and protection circuits. The module uses an advanced online monitoring current sensor IGBT to achieve efficient over-current and short-circuit protection. The over-temperature protection and low-voltage lockout protection of IPM greatly improve the reliability of the system, and the overall module is small in size and compact in structure, which greatly reduces the overall size of the entire device. The following is a combination of the internal structure of PM100CSA120 to explain its principles and protection functions. [page]

The internal structure of PM100CSA120 is shown in Figure 3. Six IGBTs are connected in anti-parallel with freewheeling diodes to form a three-phase full-bridge circuit. Each IGBT has a corresponding isolation drive circuit, so the drive signal needs to be isolated before entering the module, and a separate isolated DC power supply must be provided to each drive module of the upper bridge arm. The lower bridge arm can share a DC power supply because of the common ground. Each drive circuit has a complete protection circuit to prevent device damage. These protections include the following aspects:

(1) Low control voltage lockout

The internal control circuit of IPM requires a 15V DC power supply. When this voltage is lower than a certain voltage value (U1) or higher than a certain voltage value (Uh) for some reason, the drive circuit automatically locks and sends a fault signal Fo to notify the main control circuit.

(2) Temperature protection circuit

There is a temperature sensor inside the IPM that is fixed near the chassis IGBT device. When the module temperature exceeds a certain value To, the drive circuit automatically locks until the module temperature drops below To, and also sends a fault signal Fo.

(3) Overcurrent protection

IPM can use current sensing IGBT to monitor the module online. If the current passing through IPM exceeds the overcurrent threshold value OC and lasts for toff(OC), the protection circuit will lock the gate drive and generate a fault signal. This duration is to avoid protection malfunction caused by instantaneous peaks passing through IPM.

(4) Short circuit protection

If a short circuit occurs, that is, the current exceeds the short-circuit current threshold SC, the protection will immediately operate and send a signal.

The above protection circuit can greatly reduce the possibility of power devices being damaged due to improper operation or control failure, and more importantly, improve the reliability of the system. The optimally designed drive circuit inside the IPM shortens the development cycle of the inverter device, thereby further improving reliability.

4 Circuit Structure

Using IPM and 80C196MC can make the whole circuit concise and clear. Although the internal structure design of IPM is perfect, it is still necessary to provide a reasonable buffer circuit for high-power inverters to eliminate overvoltage and du/dt caused by line inductance. As mentioned above, the PWM control signal must be isolated before entering the IPM mode, and an isolated 15V power supply is required to be provided to the IPM internal drive protection circuit. The output must be filtered after passing through the transformer to obtain a good sine waveform. The voltage sensor and current sensor at the DC input end are used to provide protection signals for the control; the feedback signal provided by the voltage and current transformer at the AC output end is used to automatically adjust the output voltage and frequency of the inverter, and can also be used as a basis for overload protection. In addition, the carrier frequency cannot be too low, because when the frequency is low, the accent pollution is more serious and affects the output waveform; however, the carrier frequency cannot be too high, because the high-frequency switching loss is large, and the larger proportion of dead time will make the output voltage low. This is why the waveform generator in the 80C196MC microcontroller must be interrupted once in each carrier cycle when generating SPWM waves. If the carrier frequency is too high, the program will be interrupted frequently and cannot run normally. Therefore, it is best to choose a range of 10kHz to 15kHz. Except for the slightly larger size of the power frequency transformer, the structure does not require much space. The specific circuit is shown in Figure 4.

5 Conclusion

The use of the 80C196MC's waveform generator WFG can greatly simplify the control software and external hardware used to generate synchronous pulse width modulation waveforms, which is particularly suitable for controlling three-phase AC motors and devices that require multiple PWM outputs. The intelligent power module integrates power devices, drive circuits and protection logic circuits, and has intelligent protection functions, which is particularly suitable for motor control and passive inverters. Practice has proved that the use of these highly integrated special devices can effectively improve the reliability of the system and shorten the development cycle. Modern microelectronics technology and power electronics technology are changing with each passing day, providing a good foundation for the development of new technologies for microcomputers and external transfer chips with increasing integration, greatly shortening the development time and improving the reliability of the system. Similarly, the rapid development of high-power power electronic devices also provides this convenience. IPM integrates power electronics and microelectronics technology and is a very promising power electronic device.