Advanced ‘3D Power Packaging’ Enables Miniaturization of DC/DC Converters

Dec 17, 2021

RECOM components of RxxC05TExxS and RxxCTExxS series on blue background

Recent advancements in "3D power packaging" are driving the continued miniaturization of DC/DC converters while boosting power density. Despite these reductions in size, the performance remains unaffected. The latest models offer extensive control and protection features, with some even providing isolation that meets the highest medical-grade standards.

3D Power Packaging – Thinking in a New Dimension
DC/DCs are now available in SOIC-16 packages
Versions with 2x MOPP medical isolation are also available
The Medical Application for the RECOM R05CT05S DC/DC
Conclusion

DC/DC conversion in modern products focuses primarily on power density – the ability to process a significant amount of power within a limited space while achieving voltage conversion and, often, isolation. This is especially crucial in cutting-edge IoT devices and in medical technology. The continuous improvement in converter efficiency has been a key driver behind miniaturization, allowing products to shift from large, off-board modules to compact, PCB-mounted components. This development positions the conversion and regulation processes right next to the load, ensuring maximum accuracy where it is most needed.

Board-mount products, however, have largely remained in mechanical formats and designs that were first introduced in the last century. Converters are still primarily "mini end-products," featuring an internal PCB, casing, potting, and pins. Expensive hand assembly, particularly for magnetics, remains common. These finished products are often only available in through-hole format, and even when in SMT, they may have strict limitations on the allowable reflow profile. This makes soldering the part to a user’s motherboard alongside other less sensitive components a challenge. As a result, engineers often choose discrete designs that are easier to manage, but these require more board space and bring added costs in design, qualification, purchasing, stocking, handling, assembly, and testing.

A key goal for both converter manufacturers and users has been to develop SMT-format DC/DC converters that are as compact as other modern components. These converters should be cost-effective, require no special heatsinking, and have no strict limitations on reflow temperature or duration. If achieved, designers would no longer consider using a discrete DC/DC design, just as they wouldn't consider implementing an A-D converter with discrete transistors.

3D Power Packaging – Thinking in a New Dimension

In the past, a DC/DC designer might have started with a target footprint and a slightly smaller internal PCB, then filled it with as many discrete components as possible. Today, a more advanced approach is to consider the design in three dimensions from the outset, known as ‘3D Power Packaging^®’ or ‘3DPP^®’ by RECOM. This concept is based on the idea that the DC/DC should be manufactured as a part that is handled and placed like any other. Its internal structure makes full use of the available height, while eliminating unnecessary material.

For example, PCB material often serves only as a space-consuming carrier for copper tracks. Why not have self-supporting connections within the encapsulation instead? An SMT device already features a copper leadframe for external terminations, meaning components can be placed directly on it internally, eliminating the need for a PCB. A prime example is the RECOM RPX-1.0, a switching regulator that provides 1A output with an adjustable 0.8–30V range, all from a wide 4-36V input. Measuring just 3 x 5 x 1.6mm with QFN terminations, the device uses flip-chip technology and advanced thermal management techniques. This design achieves remarkable efficiency with MHz switching, offering an astonishing potential power density of over 1kW/cm³. Figure 1 illustrates the internal construction.

Fig. 1: RECOM RPX-1.0 DC/DC internal construction

Embedded power IC with solder lands — Fig. 2: RECOM RPL-3.0 DC/DC construction with substrate-embedded die

The RECOM range of DC/DC converters utilizes advanced 3DPP® techniques, including the embedding of die in substrates. This approach not only optimizes space for additional components but also enables the use of IC dies with enhanced metallized Re-Distribution Layers (RDL), creating a direct connection to the copper on the PCB layers for solder-free connections. This improves both reliability and thermal performance.

An example of this technique is the RECOM RPL-3.0, a 3A output switching regulator that measures just 3mm square and 1.45mm high. This construction style (Figure 2) is inherently rugged, eliminating the need for over-molding, which saves both cost and space. The inductor surface also serves as the SMT pick-up point. With a high efficiency of over 95%, the thermal performance is outstanding, thanks to the direct connection from the die through the substrate and terminations to the user’s PCB.

The miniaturization and automation of DC/DC construction offer an additional benefit – shorter thermal paths, which improve heatsinking efficiency and lower die temperatures. The electrical connections are also kept short, keeping switching nodes enclosed and loops compact, which reduces EMI. With MHz switching, this further allows for smaller input and output capacitors, contributing to the overall size reduction.

DC/DCs are now available in SOIC-16 packages

Another example of 3DPP® is the development of an isolated product that closely resembles an IC. The new RECOM RxxCTExxS series comes in a molded SOIC-16 (wide) package with a compact 10.3mm x 7.7mm footprint and a height of just 2.65mm, making it ideal for space-constrained applications. Unlike the previous examples, this product (Figure 3) features galvanic isolation. The gull-wing terminations extend from an internal leadframe, which includes a control IC and proprietary planar transformer, both wire-bonded to it.

Achieving robust isolation in DC/DC converters has long been a challenge, especially as their size decreases. Safety approvals from agencies also require minimum insulation levels, as well as specific creepage and clearance distances, both internally and externally across the pins. The RxxCTExxS series has the advantage of a gull-wing arrangement, which allows for more than 8mm of external clearance. This new cost-effective series includes models that provide 3kVDC/1 minute isolation with IEC/UL/EN 62368-1 agency-rated ‘Basic’ isolation, making them suitable for use in product safety isolation systems.

The product designers had to adopt a radical new approach to packaging in order to achieve the desired functionality and high isolation ratings. A traditional toroidal transformer couldn't meet the insulation requirements, and a discrete E-core with a bobbin or similar design would have been too large. As a result, the switching frequency was increased to 30MHz, allowing for fewer winding turns and a smaller planar transformer core. Solid interwinding insulation was incorporated to meet the isolation standards.

Two cost-effective versions are available, offering a regulated 5V output at either 0.5W or 1W, with a 5V input. These versions meet the market demand for DC/DC converters, providing 5V ground isolation, voltage inversion, and isolated power for data interfaces (Figure 4).

Electronic circuit schematic with RECOM RxxCTExxS DC/DC Converter and RS485 Interface IC

Fig. 4: The RECOM RxxCTExxS DC/DC powering an isolated data interface

The markets addressed are diverse, including IoT, industrial, medical, smart metering applications, as well as any area requiring reliable, robust components with wide temperature ratings ranging from -40°C to 125°C ambient, with derating. Despite their compact size, these components feature a low isolation capacitance of around 7pF, which is crucial for medical applications and for powering high-side gate drives for wide-bandgap switches with high dV/dt waveforms. Additionally, their EMI emissions performance is exceptional.

Both the 0.5W and 1W versions have no minimum load requirements, making them ideal for applications with light-load standby modes. Unlike many competing parts, they offer full protection against short circuits, overcurrent, and overtemperature.

Versions with 2 x MOPP medical isolation are also available

The RxxCTExxS DC/DC converters from RECOM are part of their Econoline series, designed for cost-effective applications without compromising performance. Higher-specification models are also available for applications requiring medical-grade isolation while remaining cost-effective. The R05CT05S, housed in a compact SOIC-16 package, is a 0.5W-rated converter with a 5V nominal input. It offers selectable outputs of 3.3V or 5V, as well as 3.7V or 5.4V to provide headroom for low-dropout regulators (LDOs).

A key feature for medical applications is its 2x MOPP / 250VAC continuous rating, compliant with IEC/EN 60601-1, along with a 5kVAC test voltage. Its ultra-low 3.5pF coupling capacitance ensures minimal leakage current in 250VAC/50Hz applications. For non-medical environments, its ratings are even more robust, providing reinforced isolation at 800VAC working voltage, in accordance with IEC/EN/UL 62368-1. The converter operates at temperatures up to 140°C (with derating) and includes enable, sync, and trim functions, along with built-in undervoltage lockout, short circuit, overcurrent, and overtemperature protection.

The Medical Application for the RECOM R05CT05S DC/DC

It is worth examining how DC/DC converters, such as the R05CT05S, are used in medical applications and the benefits they offer. Electronic devices are increasingly utilized in both medical and home healthcare settings, where product designers must adhere to strict electrical safety standards. Key regulations include IEC 60601-1:2005, along with its collateral documents and national adaptations, such as EN 60601-1:2006 in Europe and ANSI/AAMI ES-60601-1 in the U.S. These standards focus on protection against electric shock, whether from high voltages or AC mains leakage current. While AC/DC converters are commonly associated with these regulations, DC/DC converters also play a crucial role in the safety isolation system. When properly specified, they can even eliminate the need for the highest-grade medical AC/DC converters, simplifying compliance and design.

First, we will review the terminology and medical safety requirements. The standards refer to "measures of protection" (MOPs) in both operator and patient environments. An MOP can take various forms. For example, a securely grounded metallic case with a mains fuse constitutes one MOP. A prescribed creepage or clearance distance may count as one or two MOPs, depending on the actual separation. Similarly, solid insulation can provide one or two MOPs. The required distance for MOPs depends on several factors, including system voltage, overvoltage category, environmental pollution degree, and operating altitude. Additionally, protection requirements for patients (MOPPs) are more stringent than those for operators (MOOPs).

Electrical device schematic with safety features — Fig. 5: One medical power supply scenario allowing the highest level of patient connection with economical parts

In the case of AC-powered equipment, there must be a minimum of 2 x MOOPs or 2 x MOPPs isolation from the AC line to the output, depending on whether the equipment is designed for use in an ‘operator’ or ‘patient’ environment, respectively. However, patient-connect outputs must also be isolated from ground by a minimum of 1 x MOPP, accounting for the possibility that the patient could become electrically ‘live’ from other faulty equipment, allowing lethal current to flow through the patient to the earth of the non-faulty equipment.

Many AC/DCs do not provide this level of isolation from output to ground, or at best only offer 1 x MOOP. An additional isolated DC/DC converter providing power to the patient-connect circuitry can help in this situation. If it has suitable medical-grade isolation, it can enable an AC/DC with only 1 x MOOP output isolation to ground to be used in a patient-connect application (Figure 5).

Compared to a single AC/DC with 1 x MOPP to ground, the combination of a more common AC/DC with 1 x MOOP plus a cost-effective medical DC/DC converter can be a more economical solution. Although the DC/DC normally only has low-voltage inputs and outputs, in this application, its safety rating in terms of MOPs must account for the ‘system’ voltage, typically 250VAC. Be cautious, as some competitor parts advertised as ‘2 x MOPP’ apply only to low system voltages, perhaps 30VAC, which offers little value in this application.

Another scenario is illustrated in Figure 5, where unspecified signal input/output connections (SIPP and/or SOPP), such as communication ports to equipment, are either AC- or battery-powered. There must be a 2 x MOPP barrier between the patient connections and the signals to prevent an external fault from making the signals ‘live.’ An extra DC/DC converter can add the required isolation. Figure 5 shows a Class I AC/DC power supply, rated for 2 x MOOP medical applications, with unspecified system signal inputs. An additional 2 x MOPP-rated DC/DC converter is included, providing full isolation to the signal inputs and allowing a lower-cost ‘operator-rated’ AC/DC supply to be used in a patient-connect situation. The AC/DC supply alone is likely to have excessive leakage current for patient-connect applications, but the DC/DC converter, with its low isolation capacitance, reduces leakage to an acceptable level. The DC/DC converter typically only needs to provide minimal power for a signal-isolated patient-connect interface, making it relatively small and low-cost.

Battery-powered equipment falls into the same category, requiring 2x MOPP isolation between patient-connected outputs and any unspecified equipment that could be connected—such as a battery charger or a printer through a USB cable. Again, a 2x MOPP DC/DC converter to power isolated patient-connected interface circuitry is a solution.

Conclusion

DC/DC converters with high, medical-grade certified isolation play a key role in achieving the required overall isolation ratings for the most sensitive applications, including worst-case cardiac floating (CF) patient connections. With careful application, systems can be designed using models like the RECOM R05CT05S to minimize costs without compromising safety. The advanced thermal, circuit, and 3D power packaging® techniques used in this DC/DC converter are typical of those found in the latest RECOM products, supporting the company’s drive toward smaller, more compact, and cost-effective DC/DC and AC/DC converters across various applications, including medical.

3D Power Packaging^®

Applications

	Series
1	RPL-3.0 Focus	Wide input range (4 - 18V) Low profile 1.45mm	Small footprint 3x3mm Adjustable output 0.8 to 5.2V	Datasheet	New Order now
2	RPL-3.0-EVM-1 Focus	Evaluation platform for RPL-3.0 Buck Regulator Module Thermal design considerations included	EMI Class A filter Easy evaluation of output voltage selection, control, power good and sensing functions	Datasheet	New Order now
3	RPX-1.0 Focus	Buck regulator power module with integrated shielded inductor 36VDC input voltage, 1A output current	SCP, OCP, OTP, and UVLO protection 3.0 x 5.0mm low profile QFN package	Datasheet	New Order now
4	RxxCTExxS Focus	Compact 10.35 x 7.5mm SMD package Low profile (2.5mm)	3kVDC/1min isolation Low EMI emissions	Datasheet	New Order now
5	RPX-1.0-EVM-1	Evaluation platform for RPX-1.0 buck regulator module Thermal design considerations included	EMI class B filter Easy evaluation of output voltage selection, control, and sensing functions	Datasheet	New Order now