introduction

Some vaccines are highly sensitive to temperature changes and must be stored at ultra-low temperature (ULT) from the start of production until they are administered to patients. For example, some vaccines initially require ultra-low temperature storage, which is kept below -60 °C (-76 °F). Maintaining such temperatures in manufacturing plants and associated warehouses is not difficult. However, maintaining such ultra-low temperatures as the vaccine is transported to the distribution site and then to the recipients is very difficult and costly.

The term “cold chain” refers to the supply chain that transports products or materials that deteriorate or become damaged if the temperature changes beyond a specified range. Many foods, chemicals and pharmaceuticals require tightly controlled “cold chain” logistics. The most difficult part of the deep temperature control supply chain is the “last mile” problem. For vaccines that need to be stored at extremely low temperatures, expensive and technically complex ultra-low temperature freezers are required at or near every vaccination site around the world.

The working principle of ultra-low temperature freezer

A standard short-term storage freezer using a single-stage compressor refrigeration cycle typically only reaches temperatures of -20 °C. The ultra-low temperature (ULT) freezer uses a multi-stage compressor and uses a cascade refrigeration cycle. Most ultra-low temperature freezers use two-stage compressors that can reach temperatures as low as -86 °C.

Figure 1: Schematic diagram of a two-stage cascade refrigeration cycle

A refrigerator/freezer compressor is a device used to compress the refrigerant in the condenser coils as part of the refrigeration process. This compression process increases pressure and generates heat, which is removed from the refrigerant. The pressure in the evaporator coil is much lower, which initiates the evaporation process (which is why the temperature inside the freezer decreases). Evaporation is an endothermic process, so the process absorbs heat. Therefore, the compressor is likened to the “heart” of the refrigeration system.

Refrigeration compressors usually use simple and inexpensive induction motors to do the compression work. In order to provide the corresponding cooling effect at different temperatures, the refrigerant in the ultra-low temperature freezer will be different. But compressors are usually two induction motors based standard equipment.

Bottlenecks faced by ultra-low temperature freezers

There are some difficulties with using these standard induction motor-based compressors in ultra-low temperature freezers. A bigger problem is the inefficiency. Induction motors are fairly inefficient (often around 70% or less), which causes them to generate heat. All the heat generated by the induction motors in the refrigeration system needs to be removed from the system through the refrigeration cycle process. This means that part of the work done by the refrigeration system is to take away the self-heating of the system, not the heat of the refrigerator.

Figure 2: Induction Motor Power Factor and Efficiency

Another problem with induction motors is the power factor. Induction motors are responsive and their poor power factor also varies with load. Under ideal environmental conditions and ideal load conditions, only about 90% of the ideal power factor can be achieved. This means that the apparent power drawn by the induction motor from the line is higher than the actual power applied to the load. In some jurisdictions, including the European Union and Japan (according to IEC61000-3-2), power factor correction circuits are required by law to maintain power transmission balance.

Brushless DC motors could be the solution

Brushless DC (BLDC) motors differ from induction motors in some key principles. One notable point is that brushless DC motors are driven by DC voltage while induction motors are driven by AC voltage. Induction motors have coils for generating a magnetic field, while BLDC motors use permanent magnets. Induction motors are easy to control (as long as they are powered on), while brushless DC motors require more complex Electronic controls. Because of these differences, BLDC motor solutions are more expensive to implement than induction motor solutions.

Figure 3: Induction Motor vs. BLDC Motor

BLDC motors have not been used in compressors for ultra-low temperature freezers due to high cost and complex implementation techniques. But some compressors for new air conditioners have already proven the concept. ROHM Group’s advanced technology and changing market conditions indicate that the application of BLDC compressors in ultra-low temperature freezers may have come to the fore.

Using a brushless DC motor-based compressor in an ultra-low temperature freezer will bring benefits justified by the additional cost. Additionally, with recent technological advancements, the cost and complexity of controlling BLDC motor solutions has improved significantly.

ROHM has an industry-leading line of BLDC motor driver solutions, including DIP modules that can be embedded into designs for all motor control. In addition, ROHM’s BLDC motor control experts are always on hand to provide support in breaking through this bottleneck.

Advantages of designing an ultra-low temperature freezer with a BLDC compressor

The benefits of using BLDC-based compressors in new ultra-low temperature freezer designs are increased power efficiency, reduced self-heating, and power factor correction. BLDC motors are more efficient than induction motors at converting electrical energy into physical work. The heat generated by the BLDC motor is significantly reduced, so less heat enters the refrigeration system, allowing for more efficient cooling. BLDC motors can operate at a power factor of 1 by using power factor correction techniques, thereby reducing apparent power losses.

Eliminate inefficiencies in critical applications

Using an induction motor in a compressor reduces efficiency, and using two induction motors in an ultra-low temperature freezer reduces efficiency exponentially. From a power and heating/cooling cycle point of view, the efficiency is relatively low. Brushless DC motors offer a surprising solution, but at some additional cost and increased technical complexity. But the energy efficiency and compliance of BLDC-based compressors can more than offset this extra cost. ROHM can provide a rich BLDC motor system product portfolio and BLDC motor control expertise to help solve problems of high technical complexity. Replacing an induction motor with a BLDC motor has advantages in terms of efficiency. However, this adds cost and complexity to the design. ROHM has solutions for BLDC motors that make handling complex designs a snap.

Figure 4: ROHM BLDC Motor Solution Reference Design

As shown in Figure 4, the ROHM reference design can help users build ultra-low temperature freezers with excellent energy efficiency.

Whether users choose the ultra-integrated ROHM IGBT IPM series, or wish to obtain an ideal cost-effective product through discrete components, IGBTs, MOSFETs and gate drivers can be used to work together. This allows you to build running systems with low power consumption and small factors.

The ROHM IGBT IPM series has an excellent power management system and is an ideal solution for a highly integrated ultra-low temperature freezer.

The BM6437x series, 600 V IGBT IPM, achieves industry-leading low noise characteristics by utilizing the built-in IGBT characteristics and the soft recovery characteristics of the internal FRD at high power.

ROHM Field Stop Trench IGBT, RGT/RGS series or SiC MOSFET, SCT series are ideal when layout flexibility and thermal performance optimization are the main considerations. These discrete components can be combined with suitable gate driver products.

In terms of power supply, ROHM isolated AC/DC series can provide higher performance and stability.

In order to help users achieve ideal performance while reducing the design workload to a greater extent, ROHM can provide comprehensive application support in the user’s development process based on rich experience.

Summarize

Now, it’s time to seriously consider BLDC compressor designs for ultra-low temperature freezers. The BLDC-based ultra-low temperature freezer is one of the work that needs to be strengthened in preparation for the contingencies of the future, and ROHM is ready to cooperate with design engineers who are willing to face this challenge head-on.