Fully integrated power modules boast ultra-high power densities

By ALAN ELBANHAWY
Principal Applications Engineer
Exar
www.exar.com

Integrated power modules are becoming widely used in fast-moving electronic appliance, industrial, and high-end markets. Companies, large and small, have adopted this approach because it reduces the development time of power delivery systems by months and almost eliminates prototyping and debugging costs, thus enabling a timely market launch. 

The design challenge  
For the last several years, the electronics industry has been packing more capabilities and features into smaller packages and charging less for them. This industry direction places enormous pressure on component manufacturers to satisfy the moving target of price, size, and performance. Today’s power modules must be small, efficient, and, most importantly, offer competitive total acquisition cost (TAC) when compared to either discrete solutions or other modules available on the market today. 

Reduce components, design time  
Fig. 1  depicts an application example of the XR79115 module. The only external components needed are for setting the output voltage, sequencing, and fault management. Note that no external compensation is required if ceramic capacitors are used for the output filter, significantly shortening the development cycle, because worst-case analysis (WCA) or Monte Carlo Analysis are not required to guarantee a stable control loop under all production scenarios.

Fig. 1: The XR79115 module needs only external components to set the output voltage, sequencing, and fault management.

A host of protection features, including overcurrent, over-temperature, short circuit, and UVLO, ensures safe operation under abnormal operating conditions. Other key features include: 

• Programmable soft start
• Programmable hiccup current limit with thermal compensation
• Precision enable and power-good flag
• Integrated bootstrap diode

Additionally, little to no derating is required for these power modules because their high power conversion efficiency minimizes internal losses. Both packages have good junction-to-case thermal resistance and are optimized for efficient PCB layout. 

The XR79110 and XR79115 modules may be used in multiple applications simply by replacing a few passive components. This saves a company’s development time and R&D expense, and optimizes time-to-market and competitive product positioning.  

Exceptional transient response
Using a proprietary emulated current mode constant-on-time (COT) control scheme, the controller provides extremely fast line and load transient response using ceramic output capacitors. The control loop provides exceptional line and load regulation and maintains an almost constant operating frequency. No loop compensation is required, which simplifies circuit implementation and reduces the overall component count. A selectable power-saving mode allows the user to operate in discontinuous mode (DCM) during light current loads, significantly increasing the converter’s efficiency.

Fig. 2: The XR79115 transient response shows the power module’s fast transient response from DCM to CCM with very little output voltage perturbation.

Optimize thermal, electrical performance
QFN packages offer good thermal and electrical performance at a competitive cost and are easier to debug than LGA and BGA packages when placed on a PCB. The XR79110 and XR79115 QFN packages were designed to maximize performance and minimize the application PCB layout effort. 

Table 1: Comparison of XR79110 and XR79115 Power Modules.

Fig. 3 shows a bottom view of the XR79115 package. Both packages contain several high-current pads that guarantee superior thermal performance while minimizing EMI. Care was taken to provide low thermal resistance from all power-dissipating components to the corresponding bottom pads. This approach ensures low parasitic inductance and resistance for the high-current paths. 

Fig. 3: XR79115 QFN Package Bottom View.

The power train
The XR79110 and XR79115 power train was designed to achieve the highest possible efficiency over a wide range of input and output voltages and at load currents ranging from a few milliamps to full load. Fig. 4 shows the XR79115 module efficiency for 12-V input voltage achieving efficiencies of over 93% at mid-range load currents, and up to 81% at 100 mA. Both modules can be made to operate at switching frequencies from 300 to 800 kHz.

Fig. 4: The XR79115 power train efficiency at V_IN = 12 V and f_s = 600 kHz.

Thermal performance
The high efficiency of these power modules translate into superior thermal performance. Fig. 5 shows an infrared image of a demo board designed for the XR79115 operating at 600-kHz switching frequency, 12-V input voltage and 3.3-V output voltage at 15-A load current for 45-W total output power. The PCB was designed to allow for the lowest thermal resistance from the case of the module 25°C. As can be seen in the figure, the maximum temperature at the hottest point on the module inductor is 76.9°C. This implies that, if the maximum module temperature reaches 120°C, the module will operate at full load current in ambient temperatures up to 68°C without any derating. It is also worth noting that at lower output voltage, the module will dissipate less power and will need almost no derating up to 85°C ambient temperature.

Fig. 5: Infrared image of demo board — V_OUT = 3.3 V, load current = 15 A.

Power system design can be an overwhelming task, especially when there can be more than 15 output rails when all the LDOs and switching regulators in a single system are taken into account. Integrated power modules allow the designer to quickly satisfy the cost, size, and performance demands of complex electronic appliances. Exar’s latest XR79110 and XR79115 power modules offer the advantages of integrated power modules and provide superior thermal performance in a small QFN package. 

Alan Elbanhawy is an Expert Power System Architect at Exar Corporation. He has a B.S. in Electrical Engineering and over 40 years of engineering experience in power supply design and R&D management. Alan holds 18 patents and has applications for four more. He has authored over 60 papers on power delivery that have been translated into five languages for international conferences and technical publications.  Email:  Alan.Elbanhawy@exar.com