Finding the right balance

Jon Cronk, Exar


In search of a winning combination in power modules

Equipment designs increasingly utilize advanced MCU, DSP and FPGA devices to deliver the functionality and performance demanded of them. With this sophistication comes the challenge of providing the multiple power rails these devices require. The use of carefully chosen power modules can significantly ease this task and allow system designers to get to market on time and without compromise.

A quick examination of almost any piece of industrial and telecommunications equipment will clearly show that it requires a large number of individual DC bus voltages operating at different currents to provide power to all the sub-circuits in the design. Each DC bus voltage is typically supplied by a DC-DC switching regulator or an LDO that has to fit in ever smaller PCB footprints and yet meet tighter and more demanding performance specifications relating to efficiency, input and output voltage regulation, and operating temperature, to mention a few.

These specifications are necessary to ensure the equipment can operate for longer, run cooler and achieve higher reliability. In order to design these converters in-house, electronic equipment companies must employ highly qualified and experienced power engineers and, depending on the number of converters needed, allow sufficient time in the project schedule for the design and prototyping of the power system, including full test and verification. This is a process that is expensive and time consuming, impacting both the project budget and time-to-market in a very competitive market where time and cost are rare commodities.

A “Wish List” for the ultimate power system design

From a cursory analysis of subsystem power requirements, it is a short step to identifying a wish list for an ideal power delivery solution, which must:

  1.         Meet the most stringent specifications of the subsystem, e.g. voltage tolerance.

2.         Operate with the highest possible efficiency - this allows battery-powered devices to last longer and reduces power dissipation so that devices can run cooler and be more reliable. Every 10˚ rise in operating temperature halves the Mean Time Between Failure (MTBF).

3.         Provide excellent transient response for FPGA and CPU operation to avoid spurious operation caused by false clocking or incorrect power sequencing.

4.         Offer a programmable output voltage and switching frequency, and provide protection against over current and over voltage conditions.

5.         Ideally require no external compensation - this can otherwise be a time consuming task to ensure design stability over worst-case conditions of input/output voltages and load currents.

   6.       Provide a solution that can fit in tight spaces without compromise.

7.       Achieve “plug-and-play” simplicity, requiring no system troubleshooting once the PCB is properly designed.

  8.       Achieve all of the above objectives at a low cost in medium to large volumes.

The significance of these points can be appreciated by considering a modest subsystem that needs to provide five rails with output voltages ranging from 0.6V to 3.3V and operate from an input voltage of 5V 20V. A buck (step-down) DC-DC converter might typically be used to generate each of these rails, as illustrated by the simplified functional schematic in Figure 1.


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Figure 1: Simplified functional circuit schematic of a COT buck (step-down) switching regulator

But, implemented as a discrete solution, such a design can easily take from two to four months, with a good part of the time spent ensuring that the stability of the control loops and the resulting transient response of the outputs can be unconditionally guaranteed over worst case temperature conditions and across the 6-sigma distribution of discrete component parameter values. This length of time, in some cases, can make or break the chance of a new product making it to market ahead of the competition.

Greater integration helps but is trumped by a module

Today many semiconductor companies provide DC-DC converter solutions that integrate many of the control elements of a switching regulator, such as the compensation amplifier and the ramp generator and comparator, which form the pulse-width modulator that controls the power switch. However, this still leaves the MOSFET, LC output filter and most of the passive compensation feedback components off-chip. These components need to carefully chosen and their operation modeled, which even using the design tools many vendors offer is still time-consuming.

Far better, especially for system designers who are not power experts, is to use power modules that integrate the PWM controller, MOSFET power switches and inductor in a single small form-factor package. These modules are typically designed, optimized and tested by a multi-disciplinary team of engineers who are each experts in their own field. Consequently, this provides the application design engineer with a device that delivers superior performance, high reliability and the benefit of immediate availability, cutting down the time to market by several months.

Good examples of these devices are the latest members of Exar’s expanding family of modules; the XR79103/06 and XR79203/06 deliver 3A and 6A outputs at voltages down to 0.6V from input supplies of up to 22V and 40V respectively.

What do these modules offer?

Implemented as synchronous step-down buck converters, these power modules employ a proprietary emulated current-mode Constant On-Time (COT) control loop. They require no external loop compensation and are unconditionally stable using ceramic output capacitors and operate at near-constant switching frequency, requiring very few external housekeeping components. The wide input voltage range allows the XR79103/06 to operate from industry standard 5V, 12V and 19.6V rails while the XR79203/06 can cope with standard 24V and 18-36V DC rails, and also rectified 18VAC and 24VAC rails.

Consequently these modules not only fulfill the earlier wish list, which was key to their design brief and hence the requirements of modern electronic equipment, but do so at a competitive price.

Modules do not have to be a compromise

The controllers used in Exar’s power modules offer the same performance as their IC counterparts and provide exceptional line and load regulation. Furthermore, to achieve maximum efficiency at both ends of the output current range, operation can be switched from the regular constant current mode (CCM) to discontinuous conduction mode (DCM) to save power at light load currents.

These modules also provide all the usual protection features, such as over current, over temperature, short circuit and under voltage lockout, to ensure safe operation under abnormal operating conditions. Additional features include: programmable soft-start, a programmable hiccup current limit with thermal compensation, flags for Power-Good and Precision Enable, and an integrated bootstrap diode.

Where the module approach really wins out is in integrating the control IC, with its rich feature set, together with the power train components, which have been selected for optimal switching performance, in one very small, cost-effective package. The QFN packages used for these modules (see Figure 2) include large pads that connect to key nodes in the power train, ensuring good Ohmic contact and minimizing EMI. These pads also provide excellent heatsinking for the module, which result in the package’s low junction-to-case thermal resistance and superior thermal performance.

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Figure 2. The QFN package used provides pads that ensure excellent thermal conductivity

Combined with the regulator’s intrinsic high power conversion efficiency that minimizes internal losses, these modules can deliver full power with little or no de-rating required – see Figure 3. This solid performance specification allows the modules to be easily used in multiple applications simply by replacing a few passive components. This saves development time and R&D expense and results in a faster time-to-market and a more competitive product positioning (see Figure 4).

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Figure 3. Thermal derating curve for Exar’s XR79106 power module at VIN = 12V

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Figure 4. Power modules require just a few external housekeeping components

Keeping up

Today’s fast paced innovative electronic design environment places enormous pressure on the practicing engineer to complete his/her designs in record times without any sacrifice in performance or cost. Coming to their rescue is a family of fully integrated power modules that are conceived, designed and produced to help turn the development of a power delivery subsystem into an easy five step process:

(1)       Know your requirement

(2)       Select the appropriate module

(3)       Calculate the values of the housekeeping components – see figure 4 below

(4)       Layout your PCB

(5)       Test and verify your prototype … then move on to your next achievement!

The first three tasks should take less than 30 minutes for guaranteed performance and reliability.

Modules can still be highly efficient

Modules should not compromise power converter performance and much less their efficiency, which needs to be as high as possible to reduce power losses and issues with heat dissipation. The efficiency plot of the XR79206 in Figure 5 shows it achieving almost 95% efficiency at a 700KHz switching frequency. Figure 6 further illustrates how the module switches mode briefly from DCM to CCM operation to execute a fast transient response before returning back to DCM to save power.

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Figure 5. The XR79206 achieves nearly 95% efficiency at 700kHz switching frequency and VIN = 24V

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Figure 6. The XR79206 executes a load step from 0.05A to 3A by seamlessly changing from DCM to CCM