Examining High-Power Buck Switching Regulator Modules
Designers seeking a power supply providing a very high operating current should consider this alternative.
This article is published by EE Power as part of an exclusive digital content partnership with Bodo’s Power Systems.
Designers may need a power supply providing a very high operating current on certain occasions, but such circuits can be bulky and require a considerable amount of board space. Heat dissipation and sufficient safety measures are also important to think about.
NC2700MA, NC2701MA, and NC2702MA (NC270xMA) series switching regulators (Figure 1) represent a possible solution with a small module and easy setup despite a few settings and extra components. The modules are intended to be used in applications like point-of-load converters, communication base stations, network servers, and other industrial and medical devices.
Figure 1. NC270xMA series switching regulators. Image used courtesy of Bodo’s Power Systems [PDF]
Three versions offer maximum output currents of 20 A, 10 A, and 6 A, respectively, accepting a wide input voltage from 4 up to 28 V, and the output voltage can be externally set from 0.7 to 5.3 V. (Figure 2). For convenience, all versions share the same pin layout configuration, making it easier to swap devices if, during the design stage, you need a higher current capacity.
Figure 2. NC270xMA efficiency curves. Image used courtesy of Bodo’s Power Systems [PDF]
Package and PCB Space
The modules have a core regulator controller integrated along with other key peripheral components such as the inductor, high-side and low-side MOSFETs, and a bypass capacitor. This results in a smaller footprint on the circuit board and simplifies the power supply circuitry design efforts as there is no need to choose and evaluate the integrated components, saving time, effort, and cost. The input and output capacitors and other peripheral components are simply placed adjacent to the module. The total occupied PCB space of the complete circuit measures only 19 x 21 mm versus 23 x 28 mm when using a similar solution with discrete components, saving 38% of PCB space (Figure 3).
Figure 3. PCB area comparison. Image used courtesy of Bodo’s Power Systems [PDF]
Given the high output currents delivered by the modules, efficient heat dissipation is crucial. Therefore, the NC270xMA series has a special package (Figure 4) with dissipation pads optimized for high heat dissipation performance.
Figure 4. Package details and heat dissipation performance. Image used courtesy of Bodo’s Power Systems [PDF]
On the QFN0910-65-MA, all pins are exposed on the four sides of the package, making it easier to perform measurements and probing inspections for evaluation compared to LGA or BGA packages, which have only access to the pins from the bottom. With this approach, designers do not need additional tools or special test pads to measure, simplifying the evaluation and testing process.
Protection and Safety
The NC270xMA series has built-in safety features to protect the system and ensure reliable operation. A thermal shutdown circuit prevents overheating and protects the components from damage. Over- and under-voltage detection circuits measure if the output voltage goes above or below a specified threshold window. The regulator will respond accordingly to protect the system from potential damage or unstable operation. The UVLO feature ensures that the regulator operates within a specified voltage range. If the input voltage falls below the UVLO threshold, the regulator turns the output off to prevent improper operation. As for the output current limitation, a selection can be made between a hiccup or latched current limit circuit to protect against excessive output currents. In the case of a Hiccup current limit, the regulator will automatically try to restart periodically after an overcurrent condition is detected, while a Latched current limit will require manual intervention to reset the circuit. The current limit function restricts the peak current through the high-side MOSFET to prevent an excessive current flow. It is measured by the voltage difference over an additional sense resistor (Rsense) connected between the SENSE pin and the VOUT_S pin. The threshold is set to the desired level by using external components and when exceeding the limit, the high-side MOSFET is turned off. One should take into account the accuracy and value of the sense resistor and PCB wiring to achieve an accurate current limit detection and to minimize power loss by high output currents, fine-tuning is possible by adding resistors RS1 and RS2 to the current sense circuit. It is also possible to control the current limit value using the DC resistance (DCR) of the internal inductor as well but is not recommended due to the significant reduction in accuracy.
This output provides a logic signal when the switching regulator is operating within the specified parameters, and the output voltage is stable. The power-good output signal is only valid when all the next circuits and events are in the normal state, which is: chip-enabled and powered up after a soft-start, and the thermal shutdown, OVD, UVD, UVLO, and Hiccup or Latched current limit circuits are not active.
The NC270x series has a soft-start function that controls the timing for the output voltage to increase gradually from zero to the set voltage value after the chip is turned on. The soft-start time is internally set to 500 µs by default but can be extended by connecting an additional external capacitor (CSS). Alternatively, you can also control the soft-start sequence by connecting an external voltage to the CSS/TRK pin. This voltage should then increase from 0 to 0.64 V, and the output voltage will follow the same ramp-up curve to the set output voltage. Moreover, both up- and down-tracking is possible.
Output The oscillation frequency of the circuit can be controlled by connecting a resistor (Rrt) between the RT pin and GND. The range of the frequency control is specified from 250 kHz to 1 MHz (Rrt = 32 kΩ to 135 kΩ respectively). In case the Rt resistor is disconnected, shorted, or beyond its normal setting range, the switching regulator will stop operating to protect the IC, also the CLKOUT pin is pulled to Gnd level. After the cause of the abnormal state is removed, the IC will reboot with a soft-start sequence. The CLKOUT pin provides a clock signal synchronized with the internal switching frequency.
Mode Pin/Frequency Synchronization
The NC270xMA series can synchronize with an external clock signal connected to the MODE pin using a Phase-Locked Loop (PLL), making it possible to operate in a forced PWM mode using the external clock as a reference, even during the soft-start procedure. To achieve synchronization, the external clock must have high/low periods of at least 100 ns or more, and the external clock must be within 0.5 to 1.5 times the set frequency. Provided that specific conditions are avoided, such as maximum duty or duty-over and a small difference between input and output voltage.
This article originally appeared in Bodo’s Power Systems [PDF] magazine.