ALLPCB
Overview
Moving the inverter from the control cabinet to the motor creates new inverter design requirements, chief among them thermal management. Standard cabinet-style inverters operate in a relatively benign environment without special IP protection. By contrast, embedded drives install the inverter inside the motor housing, which requires both motor and inverter to be waterproof and dustproof, and this affects the system's thermal performance.
Thermal Constraints in IP65 Systems
In an IP65-protected system, all dissipated power must be removed through the system surface (primarily through the heatsink). Therefore the inverter design used in embedded systems must minimize losses and use effective thermal interfaces to connect all power-dissipating components to the heatsink. Keeping the overall temperature low and avoiding internal hotspots is essential for system reliability.
Size and Partitioning
Embedded inverters must be compact to fit the target environment and enable compact motor designs and high installation density in applications such as water treatment, heat pumps, and ventilation systems. Reduced system surface area increases the importance of optimized thermal management. Reducing overall system volume also requires intelligent system partitioning to minimize interconnects between the power stage, energy storage, and inverter control board.
Necessary Functions
All applications require a rectifier stage, passive or active power factor correction (PFC), and the inverter stage for the motor, as shown in Figure 1. Additional PFC circuitry is often needed to meet efficiency regulations introduced around 2012 and to achieve higher system efficiency.
Figure 1. CI + PFC topology. Image provided by Bodo Power Systems [PDF]
Role of Power Modules
A major challenge is relocating components that have creepage and clearance requirements and therefore occupy large PCB area. These components include CI+PFC semiconductors, bootstrap circuits, shunts, buffer capacitors, and similar parts. Power modules address this by encapsulating such components. Power modules are typically filled with electrically insulating silicone with a dielectric strength of several kilovolts per millimeter. Encapsulating components in silicone enables denser packaging.
Fine-Pitch and Thick-Film Solutions
One constraint is the distance between two rails. Using standard Al2O3 DCB substrates typically requires distances greater than 0.5 mm. While this is acceptable for power-semiconductor bare dies, it becomes challenging for discrete resistors, capacitors, diodes, and especially for ICs. In these cases, Vincotech's thick-film technology offers a promising solution that enables fine pitch below 0.5 mm.
Vincotech offers two types of modules for embedded-drive applications: DCB modules that contain only power semiconductors, and thick-film modules that also incorporate active and passive components. Both are based on alumina, but they differ mainly in substrate thickness and whether copper planes are present on both sides. Because Al2O3 is brittle, the thick-film substrate is about twice the thickness of a DCB substrate (roughly 1 mm). Thick-film modules also have higher thermal resistance, since the absence of copper on both faces limits lateral heat spreading.
Thick-film modules are manufactured by printing multiple layers on an alumina substrate and firing them at about 850 °C. A variety of pastes are available for different functions, for example low-ohm conductors for high-power rails and resistor pastes that span from low shunt values up to megaohm values for various resistor functions.
Laser trimming of resistors improves accuracy, and adding a glass passivation layer enhances reliability.
Figure 3. Comparison of a DCB-based module (left) and a thick-film module (right). Image provided by Bodo Power Systems [PDF]
Integrated Circuitry in Thick-Film Modules
The diagram below shows components integrated into a thick-film module. Unlike standard power modules, the gate-driver IC implementation can include diodes, resistors, and capacitors—for example, gate resistors and gate-emitter capacitors. Other Vincotech thick-film products offer separate turn-on and turn-off gate resistances and an optional gate-emitter resistor to discharge the gate when the gate supply is absent.
Figure 4. Internal circuit of a thick-film power module. Image provided by Bodo Power Systems [PDF]
Figure 5. EMI measurements without Cge (left) and with integrated Cge (right). Image provided by Bodo Power Systems [PDF]
PFC and EMI Considerations
The module's circuitry can also include a PFC stage using fast 650 V IGBTs or faster silicon MOSFETs with fast silicon diodes, or SiC diodes when higher switching frequency and efficiency are required.
A ceramic capacitor between DC+ and DC- closes the high-frequency loop in the power circuit. A capacitor between gate and emitter further improves EMI performance, as illustrated in Figure 5. The placement of these capacitors has a significant impact on measured EMI and loop behavior.
