The last blog addressed the AC input stage of the motor drive. This is primarily an AC-DC power conversion stage where the AC input power is converted to a DC level that provides the input to the main three-phase inverter. Several other power domains are required in the drive apart from the high-voltage inverter power domain. Managing these power domains, providing the appropriate ground referencing and isolation, and protecting them from external transients is a critical task of the variable speed drive designer. This blog examines the topic of power management in a variable speed drive and looks at some of the requirements that exist in the different power domains. In the last blog, we introduced a typical architecture diagram, shown here again in Figure 1. This section of the drive under consideration is highlighted.
Figure 1: Detailed Variable Speed Drive Architecture
Firstly, let’s identify the main power supplies within a variable-speed drive. These are:
- Main control power
- Inverter-isolated power supplies (Gate driver and motor phase current sense)
- Multiple low-voltage control power rails
Each of these power supplies carries out a unique function and has certain requirements associated with it. In this blog, we will look at the first two of these power supply sub-systems.
Main Control Power
The main control power supply is usually a 24V DC bus (although it can sometimes be a 12V bus). This power supply has several functions. It acts as the intermediate DC voltage from which the low-voltage rails are distributed. It can also be used to directly supply the power for any cooling fans that may needed within the drive housing or for other higher voltage loads such as motor brakes, as illustrated in Figure 2.
Figure 2: Main Control Supply
This is mostly derived from the main high-voltage DC bus via an isolated DC-DC converter. In some cases, it can be derived directly from one of the AC input phases. In both of these cases, a flyback converter is a popular choice of power topology. Flybacks are not the nicest power converters to work with! They are quite noisy, and it can be challenging to provide tight control of the output voltage. However, they have some major plusses associated with them in their simplicity (one transistor, one magnetic component) and low cost. In the context of variable speed drives, which can be highly competitive in terms of cost, and which are already huge noise generators, the disadvantages of flyback converters are mostly outweighed by their advantages. An example is shown in Figure 3 of a solution from Analog Devices which has the advantage of providing good control of the output voltage without requiring an optocoupler to provide output voltage feedback to the controller, a “no-opto flyback”. In higher power variable speed drives, a flyback may not be suitable and in this case, half-bridge or full-bridge isolated topologies are generally chosen (e.g. LLC converter).
Figure 3: No-Opto Flyback Converter Example
Inverter Isolated Power Supplies
Isolated power supplies are needed for each of the gate driver units, which are responsible for controlling the inverter power switches. Gate drivers and the inverter will be covered in a later blog but for this blog, the requirement is typically three 12-20V supply rails (sometimes with a negative rail also), referenced to the three motor phase switching nodes and 1 low side supply referenced to DC bus negative rail. The “grounds” for these power supplies are not tied to safety earth, or even to a quiet ground reference, and are rapidly moving over hundreds of volts in the case of the gate driver supplies – hence the need for isolation. Overall this usually translates into four isolated DC-DC converters, normally from the intermediate DC bus to the gate driver supply. These isolated voltage rails are designed to provide the switching voltage levels for the power transistors used in the VSD – whether they are power MOSFETs, IGBTs, or SiC/GaN devices. Any current sensors connected in the motor phases, will also generally require isolated supply voltages – these are often derived directly from the gate driver supplies using an LDO (low dropout) regulator as shown in Figure 4.
Figure 4: Inverter-isolated Supplies
Once again, multi-output flybacks can be a solution for these supplies, although these tend to have quite complex and bulky transformer designs due to the need for so many outputs.
The power supplies within a variable-speed drive fulfill different functions that are fundamental to the overall operation of the drive. Isolation for reasons of electrical safety, and galvanic separation of voltage reference levels is an important factor in many of these power supplies. The next blog post will continue this topic and will look at the power supplies needed for the various control and output circuitry, as well as the protection needs of the power supply sub-system.