Author: Texas Instruments Akshay Mehta
This article will introduce some ways that power modules help improve DAQ performance compared to discrete power solutions.
Power Architecture of DAQ
In DAQ, it is not uncommon to see parallel power rails and different load current (and ripple) requirements across multiple subsystems. Figure 1 shows the power architecture of a DAQ system and how power modules generate the required power rails for various subsystems.
Figure 1: DAQ Power Architecture Using Power Modules
Using power modules helps to improve overall performance, efficiency, and reliability. Power modules also have the following advantages:
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Output current in the same package provides design flexibility and scalability through optimized cost.
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Methods to improve light load efficiency through automatic Pulse Frequency Modulation (PFM) mode.
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Excellent transient response during load regulation.
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Compact solutions through integrated, innovative packaging and assembly.
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Improved power module performance through selective passive component choices.
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Can operate over a wide temperature range.
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Transition from traditional substations to smart substations.
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Improved power module performance through selective passive component choices.
In addition to package selection and layouts designed to mitigate EMI and output ripple, the choice of passive components is equally important. Non-original components may perform well during prototyping but show signs of stress and lead to field damage or failure.
Inductors are one of the key components in DC/DC buck converter design. Selecting the right inductor takes time and know-how, including understanding the subtle parameters of inductor cores and their impact on power performance and lifespan. A common issue with inductors is failure during High-Temperature Storage (HTS) testing, indicating that the inductor can withstand high temperatures for extended periods.
During HTS testing, inductors are placed near the DC/DC converter, restricting airflow. The coating and/or adhesive of the iron powder begins to break down over time and under high-temperature conditions, leading to increased core losses and reduced power efficiency. This issue is most pronounced at higher input voltages and higher switching frequencies. Figure 2 compares the efficiency degradation of inductors at multiple input voltages before and after HTS stress testing.
Upon inspection, inductors often appear to be undamaged. The L and DCR values of the inductors may not change. However, exposure to high temperatures increases AC impedance, as shown in Figure 2.
Figure 2: Efficiency Degradation and Changes in AC Impedance of Inductors Before and After HTS Testing
Meanwhile, Texas Instruments’ power modules integrate inductors. These inductors exhibit excellent performance over time and with increasing temperatures. Figure 3 shows the HTS test results of various inductors after exposure to high temperatures. Our power modules use inductors that show little or no change in Q and core resistance after HTS testing.
Figure 3: Texas Instruments Power Module Inductor HTS Performance
Additionally, our power modules undergo working voltage testing to ensure no insulation breakdown occurs.
Efficiency with automatic PFM and load transient response (total load and light load)
Power modules provide MODE/SYNC options to switch to automatic energy-saving mode during light loads. During normal operation, power modules use Pulse Width Modulation (PWM) to regulate output.
When the load current is very low, the control logic switches to PFM operation and diode emulation. In this mode, the high-side Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) turns on one or more pulses to provide energy to the load. The on-time of the high-side MOSFET depends on the input voltage level and the pre-programmed internal current level (IPEAK-MIN). The higher the input voltage, the shorter the on-time. The duration of the off-time also depends on the load current level. Lighter loads result in longer off-times.
This operating mode achieves excellent conversion efficiency at very light loads. When using automatic PFM mode, the output voltage at no load is approximately 1% higher than in Forced Pulse Width Modulation (FPWM) operation. Figure 4 shows the efficiency curves for PFM and FPWM modes.
Load transient response is a measure of how well the power supply responds to sudden changes in current demand or tracks load impedance changes. Load transient response is an increasingly important performance parameter, especially for microprocessors or Field-Programmable Gate Arrays (FPGAs) that feature low core voltages, high current consumption, and fast load switching. Figure 4 shows the load transient response of the power module.
Figure 4: Power Module Efficiency and Load Transient Response
By keeping the equivalent series resistance sufficiently low, transient response can be improved by adjusting output capacitance. Increasing input capacitance can enhance the response to longer and/or deeper transient steps. Thanks to the reduction of current per phase, increasing the phase of the converter can also improve transient response by increasing the effective switching frequency and allowing for smaller output inductors and capacitors.
Reducing Solution Size
We have developed innovative compact packaging technologies for power modules.
Such packaging technologies include the Quad Flat No-lead (QFN) package shown in Figure 5, featuring a single copper lead frame. Integrated circuits (ICs) with bypass components are mounted on this lead frame. By mounting inductors on top of the IC and passive components, switching nodes become compact and shorter, reducing EMI. Examples include Texas Instruments’ LMZM33603 and LMZM33602, both rated for 36V input and capable of providing up to 3A of load current.
Our MicroSiP™ or QFN packaging technology can be used for power rails requiring lower power. This packaging technology uses bare DC/DC regulator chips embedded in a thin printed circuit board substrate. Copper traces connect the chip to the substrate without using bonding wires, as shown in Figure 5. Examples include Texas Instruments’ LMZM23601 and LMZM23600, which integrate input bypass capacitors and inductors to provide better EMI performance.
Figure 5: Power Module
Can Operate Over a Wide Temperature Range
One advantage of power modules is that they can operate under high-temperature conditions. With optimized design, packaging, layout, and suitable component selection, power modules can provide 50% load current even at high temperatures of 100°C, as shown in Figure 6.
Figure 6: Ambient Temperature vs. Output Current
Generating Negative Power Using Power Modules
In DAQ, the ADC used for sampling AC analog inputs specifies an input range of ±10.24 V. The AC current or voltage output of the sensor is scaled to the ADC input range using a gain amplifier, and the operational amplifier used for scaling gain is powered by ±12 V DC power. Various methods can be used to generate the required bipolar DC power. One method is to generate the negative power by using power modules in a reverse buck-boost configuration.
In a standard buck configuration, the positive output (VOUT) connects to an internal inductor, and the loop connects to system ground. In the reverse buck-boost configuration, system ground connects to VOUT, and the loop is now the negative output. This topology allows for the generation of reverse output voltage, as shown in Figure 7.
Figure 7: Transitioning from Buck to Reverse Boost
Conclusion
In addition to providing the above detailed advantages, power modules in DAQ applications also improve system performance and reliability, reduce design work, and help designers optimize circuit board space. Texas Instruments offers a series of pin-compatible power modules with different load currents and programmable output voltages to provide scalability for DAQ designs.
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