Power Management: Meeting Complex DC-DC Power Conversion Requirements

introduction

In industrial applications such as test and measurement equipment or embedded computing, the system architecture of embedded DC-DC converters can be quite complex, with requirements on many different aspects such as output voltage and current , ripple, EMI, and power-up sequencing. This article focuses on the impact of converter power stage selection in DC-DC applications.

Requirements for Advanced Embedded DC-DC Converters

Many industrial systems, such as test and measurement equipment, require embedded DC-DC converters because of the increasing computing power required for these applications. This computing power is provided by DSPs, FPGAs, digital ASICs, and microcontrollers, which are constantly improving thanks to shrinking process geometries. On the other hand, this also brings three major requirements: first, the power supply voltage is getting lower and lower (of course, there is also the permissible voltage ripple and load variation); second, the power supply current is getting higher and higher; third, these ICs usually need to provide separate voltages for the core and I/O structures in an accurate sequence to avoid latch-up.

Embedded DC-DC converters must have excellent efficiency. The small space available for such converters makes thermal design particularly challenging, as embedded converters rely primarily on the copper area around the components on the PCB to improve the thermal impedance of the system. Since power dissipation is proportional to the square of the current, this situation worsens as the load current increases. This requires power switches with low on-resistance RDSON and low switching losses. However, a certain trade-off must be made, as the lower the on-resistance RDSON of the device, the higher the parasitic capacitance and therefore the switching losses, and ultimately the higher the power dissipation. Another major requirement for embedded DC-DC converters is that EMI must be low. The noise generated by these converters can interfere with the surrounding circuits and must be as small as possible. However, switching large currents at high speeds (to reduce switching losses) (if required by the load) inevitably generates a lot of switching noise, including conducted noise and radiated noise (mainly magnetic fields). Therefore, special attention must be paid to the optimization of power stage component selection and layout, especially in terms of driver connections. In addition, the PWM control topology has a certain impact.

For example, a digital IC using 0.09μm technology may require a supply voltage of 1.2V ± 40mV. According to the data sheet of this DSP, its supply current can be as high as 952mA. Another example is a large-size FPGA manufactured in 65nm process, which requires an idle supply current of 4.2A at 1.0V +/-50mV supply voltage and 85℃. In active mode, the current can increase to 18A depending on the specific configuration, because the dynamic requirements are very high at high switching frequencies.

It is quite common for these applications to include multiple different ICs, for example, a smaller microcontroller (when the supply voltage is higher) to handle all the interface and host functions, with a larger DSP or dedicated hardware performing the compute intensive functions. Many times, high-performance A/D converters with another set of voltage requirements are also used specifically to improve noise performance and really take advantage of the resolution and bandwidth of these converters. These trends have led to complex power management systems with many interdependencies.

Modular control lifting system design

One application recommendation is to place the DC-DC converter as close to the load as possible. This minimizes EMI, reduces the board space occupied by wide, high-current traces, and improves the converter's dynamic characteristics. This has led to the emergence of "distributed" power management systems in which all converters are ideally connected to each other. An example of a controller that can work in conjunction with other converters in a network is the FD2004, as shown in the block diagram of Figure 1.

FD2004 is a member of Digital-DC product family, integrating digital loop control and highly integrated power management functions. This controller and its peers can be connected to the host controller and other DC-DC converters through SMBus (System Management Bus), easily implementing many different functions, such as in-system configuration of converters, power-on sequence, margining, fault protection and system monitoring. All these functions help to shorten time to market and, more importantly, improve system reliability.

The FD2004 can work with an external gate driver (such as the FD1505) and discrete MOSFETs , or with single- package Dr MOS products that integrate the driver and MOSFET. It can also be programmed by resistors in stand-alone applications —specifically, the maximum output voltage is set by resistors, and the maximum voltage set by software commands must not exceed more than 10% to protect the load. In applications requiring higher currents, such as multiphase converters, the selected architecture allows current sharing of up to 8 phases, and phase shedding can be implemented at low output powers to maintain high efficiency. The controller is based on a digital control loop with adaptive performance algorithms and loop compensation, supporting switching frequencies up to 1.4MHz. Clock synchronization can help improve EMI performance. The FD2006 is also a good choice for applications that require both an integrated driver and discrete external MOSFETs.

Integrated DC-DC converters are recommended for system voltages with lower output currents, where PCB area and ease of use are the most important considerations. Digital converters, such as the FD2106 (6A max), have communication capabilities like other products in the Digital-DC family and can be used with discrete MOSFETs or converters based on DrMOS that can provide higher currents. For stand-alone applications, integrated converters (such as Fairchild Semiconductor 's FAN2106) can also be used because they do not need to be connected to other converters in the system.

The controller and converter chain of the digital power management system can be controlled through a graphical user interface, which makes it easy to modify all parameters and monitor system performance. The software runs on a PC and is connected to the controller via a USB interface. When the parameters are all good, they are stored in the non-volatile memory of the controller, so that the PC is no longer required to run the system.

The DC-DC power stage shown in Figure 2 can be designed in different ways to achieve the best electrical and thermal performance at the same time.

1. Discrete solutions with drivers and MOSFETs are still widely used. To meet all design requirements, Fairchild Semiconductor now offers products in small-size thermally enhanced MLP (QFN) packages to achieve high system performance. MOSFETs are the first to use MLP packages (see Figure 3). Its Power56 and Power33 product families use the latest PowerTrench technology to provide ultra-low RDSon and low Qg at the same time, making them suitable for high switching frequency applications. Bonding technology reduces the inductance of the package and improves the limited ID of the package, making it suitable for high current applications. Its low-end FET product portfolio uses SyncFET integrated with Schottky diodes to achieve high switching performance while reducing heat dissipation.

The FDMS9600S integrates a high-side FET and a low-side SyncFET in an asymmetrical Power56 package, further improving thermal performance and enabling compact PCB designs with a small footprint (Figure 4).

2. The above discrete solutions with driver and MOSFET are also available in MCM (Multi- Chip Module) with 8x8mm or 6x6mm MLP packages. These Dr MOS (DriverMOS) product lines include 8x8mm products FDMD87xx and 6x6mm products FDMF67xx to meet different design requirements. Evaluation boards can help designers familiarize themselves with the application and test the performance for comparison with discrete solutions (Figure 5).

A discrete solution with Power56 MOSFETs and SO-8 drivers has a board footprint of around 120mm2, while an MCM requires only 64mm2 or 36mm2. The individual components in the latter module are carefully selected and fully optimized to provide higher performance and better thermal performance than discrete solutions (Figures 6 and 7). Driven by the computer industry, this solution can handle currents up to 30A and is optimized for switching frequencies up to 1MHz. Even in high-current designs, even with thermal design rules in mind, a heat sink is not required because the air flow in the system is sufficient to dissipate heat for the chip.

3. Finally, fully integrated switches will make power stage design faster and easier. In addition to the FD2106 in the Digital-DC family, the FAN210x TinyBuck family also provides fully integrated synchronous buck functions for 3A FAN2103 and 6A FAN2106 applications (Figure 8).

The entire IC is packaged in an MLP package measuring only 5x6mm, which helps in compactness of the design while achieving optimal thermal performance and high efficiency.

At first glance, higher integration may seem to result in higher bill of materials (BOM) costs, but considering all the advantages, such as space savings, better thermal performance, fewer passive components, etc., it actually reduces the final system cost. Such a fully integrated solution also supports high system reliability, because fewer components mean a lower risk of failure, and considering thermal design rules, lower system temperatures are also very important.

Thermal design is a very important part of the design task. With today's MOSFETs , Dr MOS or gate drivers, you can generally get a pretty good junction-to-case thermal impedance, but the thermal impedance from case to ambient depends on the design and is usually much higher. In most systems, if only the PCB is used, the thermal impedance (case to ambient) is around 40K/W, and the best designs can reach 25K/W, which is still much higher than the junction-to-case thermal impedance, which is typically 2K/W for MOSFETs. Therefore, the thermal design of the PCB is very important, because both thermal impedances are in series and affect the maximum temperature of the PCB, which is usually the limiting factor (if the junction-to-case thermal impedance is low, the junction temperature cannot be much higher than the PCB).

For higher currents , a multi-phase discrete solution (e.g. 2-3 DrMOS devices) is preferred to spread the heat over a larger surface. Another trade-off is switching frequency – lower switching frequency helps reduce switching losses and ultimately lower temperatures if it is not predetermined by EMI requirements or space constraints (higher switching frequency reduces the size of passive components).

As for layout, more metal obviously helps. A thicker top layer helps keep temperatures down, but may not be appropriate for the rest of the PCB because of the added cost and the impossibility of finer spacing required for other components. Larger copper areas are helpful but consume PCB space. These should be covered with solder whenever possible, as metal surfaces dissipate heat better than painted surfaces. In multi-layer PCBs, inner layers are sometimes used to assist with heat dissipation. Thermal vias (filled with solder) are sometimes used to spread heat to the other side of the PCB (Figure 9).

For forced air convection systems, care should be taken when placing components to avoid placing the converter in the "wind shadow" of other larger components. It is recommended to place the controller upstream of the MOSFET, which will not increase power consumption much and will operate more reliably at a lower case temperature.

summary

Modern embedded DC-DC converters benefit from many different technical solutions that can improve system performance and reliability and reduce costs. The interdependencies on the control side between stand-alone converters or interconnected digital converters, and on the power stage side between integrated or discrete solutions show that DC-DC converters operating in the network can be optimized and minimized .