Innovative Low Power DC/DC Converters use 3D Power PackagingFollow article
Low power DC/DC converters are vital components in modern electronics, providing efficient power conversion from one voltage rail to another. They can be either non-isolated or alternatively isolated to break ground loops, minimise EMI or provide a safety barrier while up- or down-converting voltage. Despite pressure to increase power density and reduce cost, their fabrication technology however has stubbornly lagged behind other components, at least until recently.
When modular DC/DCs were first developed to replace onboard designs, minimal circuits were used for economy, comprising a few discrete components and typically a toroidal transformer or inductor hand-soldered to a traditional PCB, which was then potted in a plastic box. The DC/DC would often dwarf the other components around, so a through-hole SIP format was developed, minimising board space used and justifying the cost of a module. This complicates assembly, requiring a mix of wave and reflow soldering needed for any adjacent SMT components, so sometimes even hand-soldering of the DC/DC was used for expediency.
SIP DC/DCs are still popular, but an aim has always been for DC/DCs to be in a package that could be stored, handled, placed and reflow-soldered like any other SMT part. A low profile to match other typical SMT components is preferred but not at the expense of footprint, implying ideally a power-dense design using every cubic millimetre of available volume.
The evolving surface-mount DC/DC
As a first attempt, early surface-mount DC/DCs were fabricated from through-hole parts with the leads simply formed into a gull-wing shape. The devices could not withstand typical surface-mount reflow temperatures, however, so were often separately soldered by hand, largely defeating the object of surface mounting. Through-holes were avoided though, with some bottom-side space-saving. As a development, DC/DCs appeared using an internal leadframe with internal components fitted using high-temperature solder. The part was then injection-moulded to produce an IC-like package that could withstand reflow temperatures. With the arrival of lead-free soldering, however, reflow temperatures increased and caused potential reliability problems for DC/DCs fabricated this way. Additionally, a leadframe approach with a mixture of machine placing of discrete components and hand soldering of magnetics seriously limits the complexity of the converter circuit possible, typically even excluding a control IC.
With the pressure for better performance and higher power density, open-frame surface-mount converters have become popular, particularly at higher power and where airflow is available. Very high power density has been achieved in the familiar ‘brick’ formats of open-frame converters using multilayer boards, planar magnetics and exotic control techniques. Being open, it is accepted that internal connection solder joints will reflow along with the motherboard joints but converter components will remain on their pads. Because of the complexity of these products, they are relatively high cost, but this is justified by comparison with a discrete solution at high power, which would occupy hugely more board area and require more layers in the motherboard. However, the technique does not scale down economically to the large market for low power DC/DCs with only convection cooling, so an economic solution was still required.
Isolated and non-isolated converters provide different packaging challenges
Non-isolated switching DC/DC converters are found in almost all circuit designs, often replacing ‘three-terminal’ linear regulators to improve efficiency. Power levels range from milliwatts to tens of watts in the form of ‘Point of Load’ (POL) converters providing power rails for CPUs, GPUS, FPGAs, SoC devices and more.
Packages must be small to have an advantage over a discrete solution and that means high efficiency, yielding high power density. A non-isolated DC/DC design can be highly efficient but there is an inherent trade-off with size – to be able to use a small inductor, the switching frequency must be increased but then dynamic losses in power switches increase proportionally. Modern circuit techniques have overcome the problem to a large extent with fast semiconductors, sometimes wide band-gap types, that anyway have low baseline switching losses. Resonant switching techniques also can eliminate some dynamic losses with ‘zero voltage’ or ‘zero current’ switching. All this is combined with advances in magnetic materials with reduced high-frequency core losses. With reducing inductor size, the component becomes much easier to integrate into an IC-style package.
The result is a breed of non-isolated, high-efficiency DC/DC converters which switch in the MHz range and have astonishing power density. For example, the RECOM RPX series provides a 3A output current at voltages from 1V to 18V, or 40.5W maximum from a wide-range 4.5V to 36V input. Size is just 7 x 9 x 4mm in a thermally-enhanced QFN package. A 1A output version is smaller still at a tiny 3mm x 5mm x 1mm. All parts in the RPX range include comprehensive functionality and protection.
The RPX series achieves this power density by using an over-moulded ‘flip-chip on lead frame construction. As an extra benefit, the compact size reduces switching current loops, yielding a low EMI profile.
Figure 1: The RECOM RPX series employing ‘flip-chip on lead frame construction
Another example of advanced packing is in the RECOM RPMB/RPMH series which are rated at up to 45W output in a 25-pad LGA package 12.19mm x 12.19mm x 3.75mm. The parts feature 6-sided metallic screening and a multilayer PCB construction with plugged and blind vias for optimum use of space and best thermal performance. Common output voltages are available and inputs range up to 65V. The parts are particularly efficient, up to 97%, allowing full loading to temperatures up to 100°C, depending on variant.
To utilize space to the maximum, DC/DC converter designers have to think in three dimensions, integrating components above, below and within each other, allowing for interconnects, thermal paths and EMI shielding. This is the ‘3D packaging’ concept championed by RECOM in products such as their RPL series. Here, a power die is actually embedded in the substrate with an additional metallized redistribution layer (RDL) to allow interconnections between the die and PCB internal copper tracks. The substrate top surface now has space to fit a conventional SMT inductor and necessary capacitors while the bottom surface is clear with a 10-pad LGA format for attachment to the application board. The arrangement provides a direct thermal path from the die through the substrate to the customer’s board for excellent thermal performance. As the die is embedded, no over-moulding is necessary and the topside inductor forms the SMT pick-up point. The resulting DC/DC converter has a footprint of just 3mm square and 1.45mm height but can deliver over 15W/3A from its output, adjustable from 0.8V to 5.2V. The input range is 3V to 18V, operating temperature is up to 120°C with derating and full control and protection is provided. EMI is inherently low with the small package size keeping current loops short and the fast switching speed of 2.2MHz allows a wide control loop bandwidth giving sub-100µs load transient response time.
Figure 2: RECOM RPL series with substrate-embedded power IC
As components, materials and efficiencies improve, non-isolated DC/DC converters can continue to miniaturize. However, isolated converters are constrained by insulation and creepage/clearance requirements. When agency safety specifications must be met, these constraints effectively define the converter size, at least in X/Y dimensions. For example, in 250VAC patient-connect medical applications, 8mm creepage is needed for two measures of protection (2 x MOPPs). Given the minimum footprint size this mandates, a low-power DC/DC converter design can use a more traditional over-moulded Leadframe approach with a wire-bonded power die. A standard SOIC-16 footprint is suitable to achieve the creepage, so RECOM has worked to achieve an overall 2 x MOPP/250VAC rating in this package size with their R05CTxx product, which achieves 5kVAC isolation utilizing a very low-profile planar transformer. The result is an overall package height of only 2.65mm, similar to other SMT ICs, for a medically certified, 0.5W DC/DC regulated converter with 5V input and selectable 5V or 3.3V output. Typical applications for the part are for powering isolated interfaces in critical 2 x MOPP medical products.
Figure 3: DC/DC in SOIC-16 package with full 2 x MOPP certification
It can now truly be said that low-power DC/DC converters are ‘IC-like’ in both isolated and non-isolated forms. This is the result of innovative ‘3D packaging’ techniques which promise yet further improvements in power density in the future. RECOM is at the forefront of these developments and offers an evaluation board that features the products mentioned (Figure 4).
Figure 4: RECOM DC/DC evaluation board showcasing 3D packaging technology
RECOM is headquartered in Austria, with multiple manufacturing locations in Europe, Taiwan and China. Comprehensive global logistics and technical support is offered through RECOM distribution partner RS Components.