Based on the PCB layout of power supply, this paper introduces the best PCB layout method, examples and techniques to optimize the performance of simple switcher power module.

When planning the power supply layout, the first consideration is the physical loop area of the two switching current loops. Although these loop regions are largely invisible in the power module, it is important to understand the respective current paths of the two loops because they extend beyond the module. In loop 1 shown in Figure 1, the current self-conducting input bypass capacitor (Cin1) passes through the MOSFET to the internal inductor and output bypass capacitor (CO1) during the continuous conduction time of the high-end MOSFET, and finally returns to the input bypass capacitor.

Schematic diagram of loop in the power module www.elecfans.com

Figure 1 Schematic diagram of loop in power module

Loop 2 is formed during the turn-off time of the internal high-end MOSFEts and the turn-on time of the low-end MOSFEts. The energy stored in the internal inductor flows through the output bypass capacitor and low end MOSFEts before returning to GND(see Figure 1). The region where two loops do not overlap each other (including the boundary between loops) is the region with high DI/DT current. The input bypass capacitor (Cin1) plays a key role in supplying the high frequency current to the converter and returning the high frequency current to its source path.

The output bypass capacitor (Co1) does not carry much AC current, but acts as a high-frequency filter for switching noise. For the above reasons, input and output capacitors should be placed as close as possible to their respective VIN and VOUT pins on the module. As shown in Figure 2, the inductance generated by these connections can be minimized by making the wiring between the bypass capacitors and their respective VIN and VOUT pins as short and wide as possible.

Figure 2 SIMPLE SWITCHER loop

Minimizing inductance in a PCB layout has two major benefits. First, improve component performance by promoting energy transfer between Cin1 and CO1. This ensures that the module has a good hf bypass, minimising inductive voltage peaks due to high DI/DT current. It also minimizes device noise and voltage stress to ensure normal operation. Second, minimize EMI.

Capacitors connected with less parasitic inductance exhibit low impedance characteristics to high frequencies, thus reducing conducted radiation. Ceramic capacitors (X7R or X5R) or other low ESR type capacitors are recommended. Additional input capacitors can only come into play if additional capacitors are placed near the GND and VIN ends. The Power module of the SIMPLE SWITCHER is uniquely designed to have low radiation and conducted EMI. However, follow the PCB layout guidelines described in this article to achieve higher performance.

Circuit current path planning is often neglected, but it plays a key role in optimizing power supply design. In addition, ground wires to Cin1 and CO1 should be shortened and widened as much as possible, and bare pads should be directly connected, which is especially important for input capacitor (Cin1) ground connections with large AC currents.

Grounded pins (including bare pads), input and output capacitors, soft-start capacitors, and feedback resistors in the module should all be connected to the loop layer on the PCB. This loop layer can be used as a return path with extremely low inductance current and as a heat dissipation device discussed below.

FIG. 3 Schematic diagram of module and PCB as thermal impedance

The feedback resistor should also be placed as close as possible to the FB(feedback) pin of the module. To minimize the potential noise extraction value at this high impedance node, it is critical to keep the line between the FB pin and the feedback resistor’s middle tap as short as possible. Available compensation components or feedforward capacitors should be placed as close to the upper feedback resistor as possible. For an example, see the PCB layout diagram in the relevant module data table.

For AN example layout of LMZ14203, see the application guide document AN-2024 provided at www.naTIonal.com.

Heat Dissipation Design Suggestions

The compact layout of the modules, while providing electrical benefits, has a negative impact on the heat dissipation design, where equivalent power is dissipated from smaller Spaces. To address this problem, a single large bare pad is designed on the back of the Power module package of the SIMPLE SWITCHER and is electrically grounded. The pad helps to provide extremely low thermal impedance from the internal MOSFEts, which typically generate most of the heat, to the PCB.

The thermal impedance (θJC) from the semiconductor junction to the outer package of these devices is 1.9℃/W. While achieving an industry-leading θJC value is ideal, a low θJC value makes no sense when the thermal impedance (θCA) of the outer package to the air is too great! If no low-impedance heat dissipation path is provided to the surrounding air, the heat will accumulate on the bare pad and cannot be dissipated. So what determines θCA? The thermal resistance from bare pad to air is completely controlled by the PCB design and associated heat sink.

Now for a quick look at how to design a simple PCB without fins, figure 3 illustrates the module and PCB as thermal impedance. Because the thermal impedance between the junction and the top of the outer package is relatively high compared to the thermal impedance from the junction to the bare pad, we can ignore the θJA heat dissipation path during the first estimate of the thermal resistance from the junction to the surrounding air (θJT).

The first step in heat dissipation design is to determine the amount of power to be dissipated. The power consumed by the module (PD) can be easily calculated using the efficiency graph (η) published in the data table.

We then use the temperature constraints of the maximum temperature in the design, TAmbient, and the rated junction temperature, TJuncTIon(125 ° C), to determine the thermal resistance required for the packaged modules on the PCB.

Finally, we used a simplified approximation of the maximum convective heat transfer on the PCB surface (with undamaged 1-ounce copper fins and numerous heat sink holes on both the top and bottom floors) to determine the plate area required for heat dissipation.

The required PCB area approximation does not take into account the role played by heat dissipation holes that transfer heat from the top metal layer (the package is connected to the PCB) to the bottom metal layer. The bottom layer serves as a second surface layer through which convection can transfer heat from the plate. At least 8 to 10 cooling holes should be used for the board area approximation to be valid. The thermal resistance of the heat sink is approximated by the following equation.

This approximation applies to a typical through-hole of 12 mils diameter with 0.5 oz copper sidewall. As many heat sink holes as possible should be designed in the whole area below the bare pad, and these heat sink holes should form an array with a spacing of 1 to 1.5mm.

conclusion

The SIMPLE SWITCHER power module provides an alternative to complex power supply designs and typical PCB layouts associated with DC/DC converters. While layout challenges have been eliminated, some engineering work still needs to be done to optimize module performance with good bypass and heat dissipation design.