Modern adjustable speed drive circuits use frequency converters to adjust the output current to meet the requirements of three-phase motors. The shape and size of the frequency converter are often limited by the application. In many cases, the circuit board is very close to the motor, and the height of the motor structure is also limited. In addition, the physical properties of the high-power semiconductor devices used and the shape of the selected package also require sufficient space on the circuit board. The voltage and current overlap generated during the operation of power semiconductor switches causes losses and must be eliminated. Although power dissipation can be improved by adding heat sinks, this also limits the layout of semiconductor devices on the circuit board.
Frequency converters are a key technology for achieving EcoDesign energy-saving requirements. Research from the Electric Power Research Institute (EPI) shows that motors using frequency converters are up to 40% more energy-efficient than those without. Whether it's an induction motor, permanent magnet synchronous motor, or brushless DC motor, a frequency converter can generate sinusoidal current for it. Therefore, the switching frequency must be several orders of magnitude higher than the adjustable output frequency of the frequency converter. The pulse-width modulated output voltage is then applied to the inductive load. Thus, the output current is proportional to the average voltage. A higher switching frequency is more beneficial to the frequency converter; resulting in smaller torque ripple, higher dynamic response performance, and lower noise. This requires a high switching rate, which means that the rates of change of di/dt and dv/dt are typically high. Therefore, circuit parasitics become a significant problem that designers must address to meet current and future EMC standards.
Cost is another constraint that must be considered in circuit layout. In many cases, double-sided circuit boards are used. However, different areas on the board often require only one soldering process. Therefore, surface-mount semiconductor devices are becoming an increasingly popular solution for improving cost-effectiveness.
Design considerations
Currently, the development trend of high-power semiconductor devices (such as IGBTs and MOSFETs) is to continuously shrink chip size while improving performance. Reducing chip size can reduce the parasitic capacitance of the device, thereby increasing the switching speed. Therefore, in-depth research on the critical circuits on the circuit board is becoming increasingly important. Figure 1 shows a simplified schematic circuit of two typical switching operation modes of a voltage source inverter (VSI). In high-current applications where the switching frequency is limited, IGBTs are the most popular devices. The figure above shows the commutation from the high-side (HS) freewheeling diode to the low-side IGBT. The current initially flows through the freewheeling path formed by the high-side diode and the corresponding inverting half-bridge IGBT.
Figure 1 Simplified commutation circuit
Once the low-side gate drive circuit turns on the IGBT, a short-circuit current flows through the high-side diode and the low-side IGBT. As a result, the diode current decreases, and the IGBT current increases accordingly (natural commutation: 1-2). During switching, the current of the inductive load can be considered constant. Therefore, stray components are irrelevant to this channel. The switching rate is determined by the conduction of the low-side IGBT and the stray inductance of the half-bridge. To achieve reverse commutation from the low-side IGBT to the high-side freewheeling diode, the voltage drop across the low-side IGBT must be greater than the DC bus voltage to turn on the freewheeling diode. Therefore, the IGBT must be able to withstand both high voltage and high current before commutating with the diode (forced commutation: 2-1).
In Figure 1, the critical current path of the voltage source inverter is marked with a red shading, characterized by a high di/dt rate of change, a feature also reflected in the corresponding gate drive circuit. To ensure the safe operation of the gate drive circuit, stray inductance must be minimized. Especially in the high-voltage side gate drive circuit, there exists a negative voltage caused by the resistive and inductive voltage drop across the low-voltage side diodes and current path, exceeding the minimum allowable voltage of VS, which can lead to abnormal circuit operation.
One solution is to reduce the switching rate by increasing the gate resistance; however, this significantly increases switching losses. In this case, it is necessary to optimize the circuit board layout to fully utilize the overall performance of the voltage source inverter. To decouple the power and signal areas, the grounding of the two areas should be separate. The gate driver should be as close as possible to the IGBT, without any loops or deviations. The signal path between the microcontroller and the gate driver is not critical. Discrete IGBT pin leads should be as short as possible to minimize parasitic capacitance and inductance. The arrangement of the six IGBTs and gate driver packaged together requires careful consideration. In addition, devices on the heatsink need to be equipped with appropriate insulation. In many cases, large heatsinks are required along the edges of the circuit board.
To overcome the above constraints, it is best to use an intelligent power module (IPM, also known as a Smart Power Module (SPM®)). Figure 2 shows a typical fully enclosed module, which contains a complete three-phase voltage source inverter, along with corresponding gate drivers and protection circuitry. Using this module saves up to 50% of board space compared to discrete component solutions. In particular, this module requires very few external components and is designed with EMC requirements in mind. Its peak and average EMC interference levels are significantly lower than traditional designs.
Figure 2 Intelligent Power Module
Similar to the discrete component approach for voltage source inverters, careful consideration must be given to the layout of external components when using intelligent power modules. Figure 3 shows some recommendations for Motion-SPM™ applications. Due to the high switching speed of voltage source inverters, signal ground and power ground must be separate. The two grounds are interconnected at the 15V Vcc capacitor. The path between the Vcc capacitor and the power ground should be narrow to eliminate coupling. To prevent damage from surges, a low-inductance capacitor should be placed between pin P and the power ground. Additionally, because the long leads between the voltage source inverter and the motor can cause high-voltage reflections, some SPM products are equipped with an external gate resistor to regulate the switching speed and minimize reflections.
Figure 3 Layout Suggestions
Figure 4 is an exaggerated diagram illustrating the warping during module installation.
Component installation considerations
Except for TinyDIP/SMD, SPMs all have a certain degree of surface warping. Figure 4 shows an exaggerated illustration of this warping. The module is secured to the heatsink with screws that protrude from the center of the surface. If installed correctly, this convex surface ensures sufficient heat transfer from the module to the heatsink. Uneven tightening of the screws can create stress within the module, leading to module damage or performance degradation. It is recommended to use the screw tightening sequence shown in Figure 4 (pre-tighten in sequence 1-2, then finally tighten in sequence 2-1). Typically, the pre-tightening torque is 25% of the maximum rated tightening torque. Once the heatsink is properly attached to the device, the temperature of the heatsink can be obtained through the SPM's built-in thermistor, simplifying circuit board design.
