“System designers and integrators rely on well-designed, drive-by-wire AC/DC power supplies to provide the necessary Voltage rails for their applications, with the elements of accuracy, stability, and fast transient response. Many systems require an AC/DC power supply to provide multiple independent output voltages (rails) simultaneously. These power supplies must also meet a variety of regulatory standards, including electromagnetic interference (EMI), radio frequency interference (RFI), efficiency, and basic user safety. For designers of medical applications, there are additional criteria for allowable leakage current and multiple measures of patient protection (MOPP).
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By Bill Schweber
System designers and integrators rely on well-designed, drive-by-wire AC/DC power supplies to provide the necessary voltage rails for their applications, with the elements of accuracy, stability, and fast transient response. Many systems require an AC/DC power supply to provide multiple independent output voltages (rails) simultaneously. These power supplies must also meet a variety of regulatory standards, including electromagnetic interference (EMI), radio frequency interference (RFI), efficiency, and basic user safety. For designers of medical applications, there are additional criteria for allowable leakage current and multiple measures of patient protection (MOPP).
To meet these application requirements, multi-output power supplies can provide a range of preset output voltages and currents. However, having these preset multi-output AC/DC power supplies complicates bill of materials (BOM) and inventory management as requirements change frequently. If different products require different multi-output power supplies, this limits flexibility, especially for low-volume specialty end products. For designers, a modular AC/DC approach is a better choice in many cases.
This article will explore the features and advantages of this approach in medical, industrial and test applications, especially where unique or customized layouts are required. This article then introduces MEAN WELL Enterprises Co., Ltd.’s high-performance AC/DC modular power supplies and their use.
Power configuration for multiple output requirements
It is perfectly normal for a system to require multiple DC rails for both internal and external loads. For example, processor-centric logic and digital circuits often require low-voltage, single-digit power rails, while loads and their drivers often require higher voltages or different current ratings.
In many cases, the exact values of the required DC output voltage and its maximum rated current are not fixed for two reasons:
・When the same basic design is used for different loads, such as small motors, LED arrays, or medical scanning systems versus larger equipment of the same type, the voltages and currents of some of the power rails need to be changed.
・ Even if a product or product family has a fixed DC output requirement, companies often have multiple related products in their product portfolio, each requiring a different power arrangement.
To meet these different needs, designers have two options:
• Designers can use independent multi-output power supplies and purchase with the required power rail output voltage for each version of the product. Using this non-configurable power source increases inventory management and supply chain issues and creates corresponding inefficiencies in forecasting, ordering, inventory, and lead times.
・ Designers can mix single output AC/DC power supplies (modules) to meet the needs of each product version. This will simplify inventory and supply chain issues to some extent, but may also increase the difficulty of design import and assembly. Because various power supplies may also have different footprint, volume and installation considerations. As a result, complete product components may need to be rearranged for each unique configuration.
While this may seem like a small problem “on paper” (Figure 1), the actual effect may be an unexpected “chain reaction” result.
Figure 1: The difference between a multi-output power supply and multiple single-output power supplies in use may seem small, but the actual impact on the supply chain and production process can be significant. (Image credit: Bill Schweber)
Medical applications add additional requirements
The better choice depends on the situation and the balance between trade-offs, priorities, and design goals. However, for many medical applications, there are more limitations when it comes to instruments that come into contact with the patient and require AC/DC power, and can affect the choice between the two options.
There are corresponding regulatory requirements in this regard, mainly the IEC 60601-1 standard; in addition, there are other standards that govern supplies used in wider applications, such as IEC 62368-1 for information and computer technology (including consumer products) , which completely replaced the existing IEC 60950-1 and IEC 60065 standards in December 2020.
When choosing a power supply, designers need to consider both design requirements and regulatory regulations. For example, leakage current refers to the current flowing to the earth through the protective grounding conductor. In the absence of a good ground and the standard assumes that such a connection may be missing, current flow from the surface of any conductive or non-conductive part to earth can only be life-threatening if there is a conductive path such as a human body.
For medical applications, the maximum allowable leakage current is much smaller than for other general applications. Because this current flows through the human body, especially the chest, even a very low current of less than milliamps can cause cardiac arrest. This current is zero or close to zero under “normal” operating conditions, but in the standard it is assumed that a failure could occur, causing current to flow through the body.
There are two approaches to meeting multiple AC/DC power rail needs, but how does the above situation affect the choice of these two approaches? While the second option may seem attractive in some cases, there is also a subtle but important technical issue to consider due to regulations. Regulatory standards measure the leakage current of the entire end product, not individual components. Therefore, while the leakage current of a multi-output power supply may be lower than the maximum allowable value (Figure 2), the sum of the leakage currents of multiple single-output power supplies may exceed this limit, even if the leakage current of each single-output power supply is low within this limit (Figure 3).
Figure 2: The most common way to provide multiple DC outputs is to use a multiple output AC/DC power supply with a preset output voltage and a specified maximum leakage current. (Image credit: MEAN WELL)
Figure 3: Another option is to provide multiple DC outputs by using multiple single-output AC/DC power supplies, but this increases leakage current and may exceed allowable limits. (Image credit: MEAN WELL)
Second, there is a unique requirement in many healthcare systems that there must be two MOPPs instead of a single MOPP; this is an additional requirement to provide another layer of protection against patient harm in the event of a failure of one MOPP Assure. There are also corresponding requirements for measures of operator protection (MOOP).
While there are many ways to implement MOPP in circuits other than the product power subsystem, it is common to implement it through an isolation transformer within the product power subsystem (transformers that comply with medical-specific regulatory standards are considered MOPP). The absence of a ground return on the secondary side of the transformer plus other provisions provides one MOPP, while the isolation created by the primary/secondary side is the second MOPP (Figure 4).
Figure 4: An isolation transformer and pair of primary and secondary windings provide MOPP in AC operating power. (Image credit: MEAN WELL)
In addition, the relevant standards that specify efficiency requirements can also create challenges for designers. Like leakage current, these standards focus on the overall efficiency of the system under specified operating conditions and power levels. Even if individual power supplies in a multi-rail system meet efficiency standards, regulatory approvals are based on total system efficiency, rather than evaluating individual power supplies individually.
Adopt modular power supply method
So far there are two options for multiple power rails: one is to use a multi-output AC/DC power supply with preset fixed outputs, but with limited flexibility; the second is to use a set of different single-output AC/DC power supplies, Can be combined as required.
But there are alternatives: MEAN WELL has developed a modular AC/DC architecture that combines flexibility in output configuration while exceeding all relevant regulatory standards, including medical standards. The MEAN WELL system consists of a modular chassis that includes addable, user-selectable DC output modules (Figure 5).
The chassis is available in two capacity options: the NMP650-CEKK-03 is a four-channel (trough) convection-cooled chassis rated at 650 W; the NMP1K2 is a six-channel forced (fan) cooled chassis rated at 1200 W W. Both chassis feature a 1U slim mechanical design to fit in tight spaces (1U equals 1.75 in/44.45 mm rack height).
Figure 5: The MEAN WELL system consists of a modular chassis with 4 or 6 channels and a series of independent plug-in DC output modules. The cover of the NMP1K2 is shown with the cover removed (above) and not removed (below). (Image credit: MEAN WELL)
The enclosure contains a primary AC line isolation transformer and front-end power conversion/regulation circuitry (Figure 6). For the NMP1K2, its internal temperature detection function automatically adjusts the fan speed to keep the chassis heat below the maximum limit while minimizing energy consumption and noise. The NMP series complies with the medical safety certification requirements of the IEC 60601-1 standard (primary-to-secondary: 2 MOPP; primary-to-ground: 1 MOPP), as well as the Information Technology (IT) industry law specification IEC 62368-1. The series also complies with relevant electromagnetic compatibility (EMC) emissions and immunity requirements, including (but not limited to) those of the EN61000 standard.
Figure 6: The NMP enclosure provides the necessary AC line transformers as well as the first stage of power conversion and control circuitry. (Image credit: MEAN WELL)
The channels (slots) of either chassis are populated with DC output modules with the desired output ratings, such as the NMS-240-5, which is a 5 V (nominal)/36 A unit (Figure 7 and 8). Other models in the single output module series offer 12 V/20 A, 24 V/10 A and 48 V/5 A outputs.
Figure 7: The NMS-240-5 module for NMP650 and NMP1K2 enclosures provides 5 V at up to 36 A (nominal).
Figure 8: The 5 V/36 A NMS-240-5 module slides into the slot in the NMP650 and NMP1K2 chassis. (Image credit: MEAN WELL)
For applications requiring dual DC outputs from one slide-in module, MEAN WELL offers the NMD-240, a 3 V to 30 V/5 A dual output module (Figure 9).
Figure 9: The NMD-240 is a single-slot, dual-output module capable of delivering up to 30 V at up to 5 A on both channels. (Image credit: MEAN WELL)
Additional Features Enhance Versatility
The performance of a power supply is characterized by its main parameter specifications, such as output voltage accuracy, transient and overload response, temperature stability, supply regulation, load regulation, etc. However, there are also features that increase the utility of the power supply to increase user confidence in the product. For the MEAN WELL NMP650 and NMP1K2 chassis and their plug-in modules, these additional features include:
・Protection Functions: Short circuit, overload, overvoltage and overheating protections are built into all output modules, the latter are displayed via TTL level signal output with a maximum source current of 10 mA.
・Auxiliary power output: The NMP650 chassis provides 5 V/1.5 A output, while the NMP1K2 provides 5 V/10 A output, which is suitable for those situations where there is excess power when using full-size modules but need to support multi-function.
・Another function is to solve problems related to multi-output power supplies. In some cases, the user needs a unified “on/off” control of the entire chassis and all outputs, but there are also some tests and even operating situations that require individual control of the outputs so that each can be “on/off” independently output rail. The NMP chassis has a global “on/off” control function, while each DC output module can also be “on/off” individually via remote signals as well as local switches.
• Finally, there is the current and voltage programmability of the module. Using an external 0 – 1 V DC signal, the output voltage of each module is programmable from 50% – 100% of its nominal value, and the output current is programmable from 40% – 100%.
Epilogue
The selection of a multi-output AC/DC power supply involves considerations of performance, functionality, features, procurement, and regulatory standards. MEAN WELL NMP chassis provides a variety of plug-in output card options, which not only provides designers with flexible output configurations, but also can easily and quickly meet the needs of different end users.
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