Power modules are better than discrete solutions, but choose wisely

To leverage the expertise of power experts and get designs to market quickly, power modules should be used, but they should be chosen wisely. The power module architecture selection can greatly affect the power supply performance.

introduction

Whether choosing a step-down switching regulator at the chip level (with integrated FETs and controllers) or choosing a power module that tends to be easy to use due to integration and a more complete power subsystem, almost all system engineers are under tremendous pressure to integrate more power and more functions into smaller and smaller products, which can have a negative impact on the electrical and thermal characteristics of the system.

System designers must overcome a variety of obstacles, including the increased potential for noise coupling due to extremely small spacing between components, and heat dissipation issues caused by the continued increase in power handling capabilities and smaller footprints.

Fortunately, power module designers can innovate to meet these demanding requirements, such as obtaining the highest performance in the smallest package through various architectural and topological design solutions. However, these innovations put the burden on system designers who need the best power module to carefully select their solutions. The technology used in various power module solutions can greatly affect the total system cost, as well as several key performance parameters such as heat dissipation, transient response, ripple voltage and even ease of use. This is a classic example of "buyer at your own risk".

Modular vs. Discrete Example

There are many reasons for system design engineers to choose power modules instead of designing power converters from the component level, and ease of use and speed to market are relatively the main reasons. By adding only input and output capacitors, these power supply customers can complete their designs relatively easily and quickly, and be confident that their basic performance and space requirements have been met. Power modules are complete power converter systems in a sealed package, including a PWM controller, synchronous switching MOSFETs, inductors, and passive components, see Figure 1.

For example, Intersil’s ISL8203M power module has an extremely low profile package of only 1.83mm, which is about the same height as a 1206 capacitor. Yet it also provides excellent electrical and thermal performance to meet all of the customer’s requirements. Often, this knowledge is sufficient, but the way the module is designed can greatly affect more subtle parameters, features, and functionality.

Figure 1: Highly integrated power modules require only input and output capacitors and sometimes a few external components to meet the needs of system designers

ISL8203M Detailed Explanation

The ISL8203M is a complete DC-DC power module optimized for generating low output voltages from 0.8V to 5V, making it ideal for any low-power, low-voltage application. The supply voltage input range is 2.85V to 6V. The two channels are 180° out of phase to reduce input RMS current and electromagnetic interference. Each channel provides 3A output current. In current sharing mode, the two channels can be combined to provide a single 6A output. In current sharing mode, the phase interleaving of the two channels reduces input and output voltage ripple.

As shown in Figure 2, the ISL8203M is only 1.83mm thick and occupies an area of ​​6.5mm x 9mm. For a given input and output voltage/current range, it has the most compact package outline (see Table 1), with a total package volume of only 106mm3, significantly smaller than all other power module solutions. As shown in Figure 3, despite its very compact package, the ISL8203M still provides very high efficiency.

Figure 2: The ISL8203M power module package measures 6.5mm x 9mm x 1.85mm

Table 1: ISL8203M is the industry’s most compact 6A sealed power module

a) A 3A output at 5Vin input

b) Parallel 6A output at 5Vin input

Figure 3: ISL8203M efficiency at various output voltage and current conditions

Small module package provides excellent heat dissipation performance

As shown in Figure 4, the ISL8203M is packaged in a QFN (quad flat no-lead) copper leadframe package, with the internal components directly soldered to the copper leadframe. Wire bonding can also be performed on the top of the internal components to connect to the leadframe. The mold is then filled to form a complete enclosed package.

This structure allows heat generated by internal components to be dissipated directly through the copper in the lead frame. The thermal conductivity of copper is about 385 W/mK, which is about 1,000 times that of a printed circuit board (PCB), which is about 0.343 W/mK. Therefore, copper-based lead frames are much more efficient at dissipating heat than PCB-based modules. In addition, because the thickness of a copper lead frame can be up to six times the thickness of the 1-ounce copper covering a typical PCB, the module lead frame helps spread heat over a larger area, thereby increasing the effective heat conduction area to the system circuit board.

In summary, the module's thermal performance is better than discrete solutions where components are soldered directly to the PCB system board.

Figure 4: ISL8203M internal structure

It is worth noting that the packaging material in this structure has a similar heat spreading effect as the copper lead frame. Although the thermal conductivity of the packaging material is lower, the heat can still be conducted horizontally through the packaging material and then spread to the copper lead frame. The packaging material also increases the effective heat conduction area from the internal power components, thereby reducing the thermal resistance from the internal components to the environment. This is another comparative advantage of the power module - it can provide higher power handling capabilities in a smaller package size relative to discrete solutions.

Thermal performance of the ISL8203M on a standard four-layer evaluation board with 2 ounce copper on the top and bottom layers and 1 ounce copper in the middle layer is shown in the detailed structure diagram in Figure 5. Under the worst case operating conditions of 5Vin - 3.3Vout/6A, no fan, and an ambient temperature of 25oC, the maximum temperature of the module is only 66.8oC.

Figure 5: In the worst case, converting 5Vin input to 3.3Vout output at 6A current (no fan, ambient temperature 25oC), the maximum operating temperature of the ISL8203M is only 66.8oC

Current mode power modules provide better performance for transient conditions

Two types of control schemes are commonly used in module applications: current mode and voltage mode. As shown in Figure 6, to ensure fast transient response under various load conditions, the ISL8203M uses a current mode control scheme to regulate the output voltage. The current sensing signal of this scheme is obtained from the on-resistance (Rdson) of the upper FET of the synchronous buck converter. The signal is then fed into the current amplifier, whose output undergoes slope compensation before being compared with the output error amplifier to produce the current pulse width modulation (PWM) signal. Through the driver, the PWM signal can control the synchronous buck converter to achieve the required voltage regulation. The error amplifier needs to be compensated to increase the loop gain and phase margin for better performance and stability.

The structure of voltage mode control is simpler than that of current mode control. It replaces the dashed block area in Figure 6 (b) with a sawtooth ramp at a fixed frequency as shown in Figure 6 (a). The sawtooth ramp, rather than the current sensing signal of the current mode design, is then compared with the output of the error amplifier to generate the required PWM signal.

Voltage mode control is also easy to understand. As shown in Figure 7, its open-loop system is a second-order system with the inductor and output capacitor forming a double pole. Obviously, its normalized phase Tv(s) curve (shown in Figure 7 (b)) quickly drops 180o at the 20kHz resonant frequency of the double pole. The system relies on compensation components to improve the phase margin to achieve stability. Otherwise, as shown in Figure 7 (b), it only has 10o phase margin at a crossover frequency of 50kHz. A large phase margin (usually higher than 40o) is a necessary condition for loop stability.

If the voltage mode control system in 6(b) is used and modified to the current loop shown in Figure 6(a), it becomes a current mode control system. The system open loop Bode plot is shown in Figure 7 for Tc(s). The system is similar to a single-order system at low frequency, so the phase increases significantly from 20kHz to 500KHz, as shown in Figure 7(b). Even without compensation components, this is still a stable system. If a simple second type of compensation is added to improve the low frequency gain and increase the crossover frequency to about 50KHz, the current mode control phase margin is still about 80o, which is sufficient to ensure stability. Therefore, for current mode control, the compensation is simpler than voltage mode, and it can cover a wider range of output capacitor applications due to the increase in open loop phase.

(a) Simple schematic diagram of ISL8203 current mode control

(b) Typical voltage mode control schematic

Figure 6: Schematic diagram of current and voltage mode control

(a) Open-loop gain in voltage and current modes

(b) Open-loop phase shift in voltage and current mode

Figure 7: Open-loop Bode plots for voltage and current mode control

For power module applications, the compensation circuit is fixed inside the package, so if the output capacitance changes with different customer applications, the double pole in voltage mode control will change significantly. Fixed compensation may not be enough to cover a wide range of output capacitance changes because its open loop phase becomes too low once the LC resonant frequency is exceeded. To avoid this problem, voltage mode modules must reduce the loop bandwidth (crossover frequency) to ensure that the phase margin is sufficient to ensure system stability under various load conditions (compared to current mode). The cost of reducing this bandwidth is a deterioration in transient response performance.

To show this important difference in transient performance, a competitor’s 4A power module using a voltage mode control scheme was selected for comparison with the ISL8203M. The resulting loop Bode plots for the two power modules are shown in Figure 8.

If we choose the same output capacitor for the test, the phase margin is about 60o, and the loop bandwidth of a 3A output ISL8203M is significantly higher than that of the voltage mode module, resulting in much better transient performance of the ISL8203M, as shown in Figure 9. Under the same test conditions, the ISL8203M has a 240mV output voltage peak-to-peak variation and a recovery time of only 25µS, while the voltage mode module has a 275mV peak-to-peak variation and a long recovery time of 70µS.

(a) A 3A output of ISL8203M

(b) A competitor’s voltage-mode module

Figure 8: Closed loop Bode plots of current mode and voltage mode control for module applications (5Vin - 1Vout/3A, using the same COUT = 2x10µF ceramic + 47µF tantalum)

(a) A 3A output of ISL8203M

(b) A competitor’s voltage mode controlled power module

Figure 9: Output load transient response with same output capacitance (5Vin - 1Vout 0 - 3A, COUT = 2x10µF ceramic + 47µF tantalum; 1A/µs load current step slew rate)

Parallel operation provides low output ripple

Finally, the ISL8203M can operate with dual 3A outputs or a single 6A output. When operating at 6A, the two 3A outputs can be connected in parallel, as shown in Figure 10. Since the two outputs are staggered 180o in phase, the input and output ripples can be significantly reduced. As shown in Figure 11, the parallel output ripple is only 11mV, while the competitor's single-phase module ripple is as high as 36mV under the same conditions. More importantly, for a given output ripple, the ISL8203M requires less than half the amount of output capacitors required by the single-phase module, providing significant cost savings.

Figure 10: The ISL8203M can be quickly and easily programmed to operate in parallel.

(a) Ripple of ISL8203M at two parallel outputs and 4A

(b) Ripple of a competitor’s 4A single-channel output module

Figure 11: Output ripple performance with the same output capacitance (5Vin - 1Vout 4A, COUT = 2x4.7µF ceramic + 68µF tantalum; 1A/µs load current step slew rate)

Conclusion

The ISL8203M is housed in a compact package, yet still meets the customer's electrical and thermal performance requirements. The module's standard evaluation requires no heat sink or fan, delivering 20W of total power to the load with a maximum temperature of only 66.8o C.

The current mode control scheme allows the ISL8203M to achieve better transient response performance, with excellent voltage peak-to-peak swing variation and only one-third the recovery time of competing power modules. The special parallel mode of the ISL8203M also enables it to provide 6A output current with very low output ripple, and the two output phases are staggered by 180o. For a given ripple limit, this feature is also accompanied by significant component cost savings.

With all these outstanding performance characteristics, the ISL8203M is an excellent candidate for any low-power, low-voltage application that requires high density and excellent performance, such as test and measurement, communication infrastructure, and industrial control systems.

To overcome the challenges of designing power subsystems for these systems, many design engineers are using power modules instead of traditional discrete point-of-load power supplies, driven by factors such as time to market, size constraints, reliability and design capabilities. For more information on Intersil's ISL203M power module, visit www.intersil.com/products/ISL8203M.

About the Author:

Jian Yin is the Application Engineering Manager for Industrial and Infrastructure Products at Intersil. He is responsible for analog and digital power module design and development, as well as all customer application support related to power modules. Mr. Yin is the recipient of eight U.S. patents (including patents under examination) and has published more than 50 magazine articles and technical papers. Prior to joining Intersil, Mr. Yin was a senior engineer at Monolithic Power System and a module design engineer at Linear Technology Corporation, designing and launching more than nine power module products. Mr. Yin holds a Ph.D. in Electrical Engineering from Virginia Polytechnic Institute and State University.