On the face of it, it all seems quite logical: to obtain a fully functioning switching regulator, all one needs to do is mount a 2x2mm controller IC of the latest generation together with the components recommended by the manufacturer on the printed circuit board. In theory, this should result in a device that offers great efficiency and low idle current and includes all relevant safety and control features – at minimum costs. Unfortunately, practice shows that things are not that easy. As is often the case, the devil is in the detail.
Why modular SMD switching regulators are more efficient
Mar 20, 2019
With modern controller chips, the discrete design of switching regulators should be child's play. Rather than spending money on ready-made modules, manufacturers consider mounting the controller IC together with the recommended external components directly on the SMD board. This approach might work in theory, but practice shows that it is not unproblematic.
Table of contents
Handling dynamic loads
The circuits proposed by chip manufacturers are generally based on the somewhat optimistic assumption that most loads are static. These designs therefore feature only a few additional components. In practice, static loads tend however to be the exception rather than the norm. Load cycles with ratios of 1:1 million are actually quite common – for instance when a microcontroller switches to sleep mode.
What happens in a switching regulator, if the current required by the load drops in an instant from several ampere to a few µA? In such a situation, built-in "intelligence" becomes obsolete, as the laws of physics apply! The energy in the form of inductance generated over a half-wave is transferred to the load over the next half-wave. If the load is suddenly reduced to zero, the energy can only be transferred to the output capacitor.
As the above formula shows, the excess energy results in a rapid voltage increase in the capacitor. , the controller switches the on-time to zero. If at this point the inductor still contains some energy, the output voltage can no longer be properly controlled. In devices designed for low output voltage, it might even double, unless the capacitance is considerably greater than that recommended in the data sheet.
This problem is not easily overcome in discrete solutions. In advanced designs, the output is buffered with 6 parallel capacitors (fig. 1), which is well beyond what chip manufacturers recommend. This configuration comes as standard with all models of the new RPM series of RECOM. Through the parallel arrangement of several small ceramic capacitors, a much larger surface can be achieved than with a single large capacitor. As a result, heat is dissipated much more efficiently from the IC and the inductors to the GND plane. Another advantage of this design is the low ESR.
Fig. 1: Measuring only 1.5cm2, the board of the RPM modules from RECOM features 6 parallel capacitors at the output, mastering also extreme load cycles.
What happens in a switching regulator, if the current required by the load drops in an instant from several ampere to a few µA? In such a situation, built-in "intelligence" becomes obsolete, as the laws of physics apply! The energy in the form of inductance generated over a half-wave is transferred to the load over the next half-wave. If the load is suddenly reduced to zero, the energy can only be transferred to the output capacitor.
As the above formula shows, the excess energy results in a rapid voltage increase in the capacitor. , the controller switches the on-time to zero. If at this point the inductor still contains some energy, the output voltage can no longer be properly controlled. In devices designed for low output voltage, it might even double, unless the capacitance is considerably greater than that recommended in the data sheet.
This problem is not easily overcome in discrete solutions. In advanced designs, the output is buffered with 6 parallel capacitors (fig. 1), which is well beyond what chip manufacturers recommend. This configuration comes as standard with all models of the new RPM series of RECOM. Through the parallel arrangement of several small ceramic capacitors, a much larger surface can be achieved than with a single large capacitor. As a result, heat is dissipated much more efficiently from the IC and the inductors to the GND plane. Another advantage of this design is the low ESR.
Fig. 1: Measuring only 1.5cm2, the board of the RPM modules from RECOM features 6 parallel capacitors at the output, mastering also extreme load cycles.
How to improve EMC
While load cycle issues in discrete designs can be overcome as described above, EMC poses a much greater challenge, as the performance of a filter is not simply determined by the controller IC alone, but also by the overall layout of the printed circuit board. That is why IC manufacturers generally refrain from publishing recommendations. As device designers often know little about the interaction between IC and PCB, they are no really in a position to predict whether their circuits will pass an EMC test.
The issue of EMC becomes even more critical with high switching frequencies, as designers need to reduce the size of inductors. Joseph Fourier showed us than a square wave can be represented as an infinite sum of sinusoidal waves of a higher frequency. The higher the switching frequency, the greater the number of harmonics, and thus the probability of resonance affecting inductors and capacitors "hidden" inside the PCB.
Purchased modules are certified components optimised for EMC. The modules of the RECOM RPM series are for instance equipped with a 4-layer PCB whose bottom layer, together with the metal housing, provides proper shielding on all six sides (fig. 2). As a result, the modules offer excellent EMC ratings. The relevant data sheets contain details regarding simple SMD ferrite beads that allow for reliable Class A or Class B compliance, verified in tests carried out at the RECOM EMC lab (fig. 3). Depending on the quality of the primary power and the distance between the load and the module, there might not even be a need for beads.
Fig. 3: Electromagnetic emissions of an RPM5.0-6.0 equipped with external filter components for Class B as recommended in the data sheet.
The issue of EMC becomes even more critical with high switching frequencies, as designers need to reduce the size of inductors. Joseph Fourier showed us than a square wave can be represented as an infinite sum of sinusoidal waves of a higher frequency. The higher the switching frequency, the greater the number of harmonics, and thus the probability of resonance affecting inductors and capacitors "hidden" inside the PCB.
Purchased modules are certified components optimised for EMC. The modules of the RECOM RPM series are for instance equipped with a 4-layer PCB whose bottom layer, together with the metal housing, provides proper shielding on all six sides (fig. 2). As a result, the modules offer excellent EMC ratings. The relevant data sheets contain details regarding simple SMD ferrite beads that allow for reliable Class A or Class B compliance, verified in tests carried out at the RECOM EMC lab (fig. 3). Depending on the quality of the primary power and the distance between the load and the module, there might not even be a need for beads.
Fig. 3: Electromagnetic emissions of an RPM5.0-6.0 equipped with external filter components for Class B as recommended in the data sheet.
Good heat management requires 4-layer board
Having successfully overcome the issues described so far, designers of discrete solutions now need to consider the problem of heat build-up. The compact design of modern controller ICs makes proper heat dissipation difficult. Heat dissipation is however crucial for a long service life and a reliable ambient temperature rating.
Again, 4-layer boards are the most suitable solution, as the GND plane acts as a heat sink. For devices where two layers would be sufficient to carry all components, it is generally more economical to use off-the-shelf modules. The RPM series from RECOM for instance comes with optimised heat management.
At the RECOM R&D lab in Gmunden, engineers worked many months on a solution that combines best electronic design with best thermal design. As a result, the 12x12mm boards of the RPM modules now include many advanced thermal features, including various vias designed as heat pipes. While this technology is not exactly cheap, it ensures that the heat of the BGA ICs and the passive components is dissipated in a most homogeneous manner to the metal housing and the GND plane.
With this innovative approach, RECOM has once more set a new global standard, as its RPM modules work at ambient temperatures of up to 105°C without any derating, cooled exclusively through the housing and the GND plane. The most powerful RPM module provides up to 6A, and its power density of >50W/cm3 is around 50% higher than that of similar modules from other suppliers.
Again, 4-layer boards are the most suitable solution, as the GND plane acts as a heat sink. For devices where two layers would be sufficient to carry all components, it is generally more economical to use off-the-shelf modules. The RPM series from RECOM for instance comes with optimised heat management.
At the RECOM R&D lab in Gmunden, engineers worked many months on a solution that combines best electronic design with best thermal design. As a result, the 12x12mm boards of the RPM modules now include many advanced thermal features, including various vias designed as heat pipes. While this technology is not exactly cheap, it ensures that the heat of the BGA ICs and the passive components is dissipated in a most homogeneous manner to the metal housing and the GND plane.
With this innovative approach, RECOM has once more set a new global standard, as its RPM modules work at ambient temperatures of up to 105°C without any derating, cooled exclusively through the housing and the GND plane. The most powerful RPM module provides up to 6A, and its power density of >50W/cm3 is around 50% higher than that of similar modules from other suppliers.
Other advanced technical features
The RPM series includes non-insulated SMD switching regulator modules designed according to the latest technical standards. At the moment, they are available with 3.3V and 5V outputs and currents of 1, 2, 3 or 6A, and can be combined with external resistors to achieve any output voltage between 0.9V and 6.0V. The ultra-low profile modules offer excellent efficiency ratings of between 97 and 99%, depending on the model. They are designed for high efficiency, especially in the 5 to 20% load range (fig. 4).
Equally high are the maximum permissible ambient temperatures, e.g. +107°C for the 1A model without external cooling. The switching regulator modules include a number of additional features such as soft start, sequencing and output voltage tracking. The modules of the RPM series are manufactured in Europe in a fully automated plant and are available through the normal distribution channels. The price of the most powerful module is around 4 Euro, depending on the order quantity.
Equally high are the maximum permissible ambient temperatures, e.g. +107°C for the 1A model without external cooling. The switching regulator modules include a number of additional features such as soft start, sequencing and output voltage tracking. The modules of the RPM series are manufactured in Europe in a fully automated plant and are available through the normal distribution channels. The price of the most powerful module is around 4 Euro, depending on the order quantity.
Evaluation boards for accelerated prototyping
By opting for modular DC/DC converters rather than discrete designs, manufacturers are able to start developing their prototypes without delay. Not so long ago, modules came with little feet, so that mounting was never a problem. This has however changed, and the latest RPM series now features 25 pads measuring only about 1mm2 each. To facilitate prototyping, RECOM has developed special evaluation boards that allow for the proper integration of the switching regulator and all its functions, as well as external filter components, without soldering.
Conclusion
Although highly integrated controller ICs make it relatively easy to produce non-insulated switching regulators, the use of ready-made modules is often the more viable option. On the one hand, modules speed up prototyping. On the other, they reduce the risk of EMC test failure. In addition, they appear in BOMs as single items, replacing numerous discrete components from a range of suppliers with different shipping terms. Last but not least, they do away with the not always easy task of positioning controller chips measuring no more than 2mm2 on a PCB, which becomes an even greater challenge if the chips are to be installed beside much larger components.
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