PCB News - PCB layout technology for power module performance

PCB layout technology to optimize the performance of power modules
The global energy shortage problem has caused governments all over the world to vigorously implement a new energy saving policy. The energy consumption standards of electronic products are becoming stricter and stricter. For power supply design engineers, how to design a higher efficiency and higher performance power supply is an eternal challenge. Starting from the layout of the power supply PCB board, this article introduces the best PCB layout methods, examples and techniques for optimizing the performance of the SIMPLE SWITCHER power supply module.

When planning the power supply layout, the first thing to consider is the physical loop area of the two switched current loops. Although these loop areas are basically invisible in the power module, it is still important to understand the respective current paths of the two loops because they will extend beyond the module. In loop 1, the current self-conducting input bypass capacitor (Cin1) passes through the MOSFET during the continuous on-time of the high-side MOSFET, reaches the internal inductor and output bypass capacitor (CO1), and finally returns to the input bypass capacitor.

Loop 2 is formed during the off-time of the internal high-side MOSFET and the on-time of the low-side MOSFET. The energy stored in the internal inductor flows through the output bypass capacitor and low-side MOSFET, and finally returns to GND. The area where the two loops do not overlap each other (including the boundary between the loops) is the high di/dt current area. The input bypass capacitor (Cin1) plays a key role in providing high-frequency current to the converter and returning the high-frequency current to its source path.

The output bypass capacitor (Co1) does not bring large AC current, but it acts as a high-frequency filter for switching noise. In view of the above reasons, the input and output capacitors on the module should be placed as close as possible to their respective VIN and VOUT pins. If the traces between the bypass capacitors and their respective VIN and VOUT pins are shortened and widened as much as possible, the inductance generated by these connections can be minimized.

Minimizing the inductance in the PCB layout has the following two major benefits. First, improve component performance by promoting energy transfer between Cin1 and CO1. This will ensure that the module has a good high-frequency bypass and minimize the inductive voltage peaks generated by high di/dt currents. At the same time, the device noise and voltage stress can be minimized to ensure its normal operation. Second, minimize EMI.

Connecting a capacitor with less parasitic inductance will exhibit low impedance characteristics to high frequencies, thereby reducing conducted radiation. It is recommended to use ceramic capacitors (X7R or X5R) or other low ESR capacitors. Only when the extra capacitance is placed close to the GND and VIN terminals, the additional input capacitance can be effective. The SIMPLE SWITCHER power module is uniquely designed to have low radiation and conducted EMI. Follow the PCB layout guidelines introduced in this article to achieve higher performance.

The path planning of the loop current is often overlooked, but it plays a key role in optimizing the power supply design. In addition, the grounding traces between Cin1 and CO1 should be shortened and widened as much as possible, and directly connected to the exposed pad. This is particularly important for the ground connection of the input capacitor (Cin1) with a large AC current.

The grounded pins (including the exposed pad), input and output capacitors, soft-start capacitors, and feedback resistors in the module should all be connected to the circuit layer on the PCB. This loop layer can be used as a return path with extremely low inductor current and as a heat sink that will be discussed below.

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 on this high-impedance node, it is important to keep the trace between the FB pin and the middle tap of the feedback resistor as short as possible. Available compensation components or feedforward capacitors should be placed as close as possible to the upper feedback resistor. For an example, please refer to the PCB layout diagram given in the relevant module data sheet.

Thermal design recommendations

The compact layout of the module brings benefits in the electrical field, but also has a negative impact on the heat dissipation design. The equivalent power is dissipated from a smaller space. Considering this problem, a single large exposed pad is designed on the back of the SIMPLE SWITCHER power module package, which is electrically grounded. This pad helps to provide extremely low thermal resistance from the internal MOSFET (which usually generates 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°C/W. Although it is ideal to reach the industry-leading θJC value, when the thermal resistance (θCA) from the package to the air is too large, a low θJC value is meaningless! If a low-impedance heat dissipation path is not provided to the surrounding air, the heat will *cannot be dissipated on the exposed pad. So, what exactly determines the value of θCA? The thermal resistance from the exposed pad to the air is completely controlled by the PCB design and the related heat sink.

Now let's quickly understand how to perform a simple PCB heat dissipation design without a heat sink. Figure 3 shows the module and the PCB as a thermal impedance. Compared with the thermal resistance from the junction to the die pad, since the thermal resistance between the junction and the top of the outer package is relatively high, we can ignore the thermal resistance (θJT) from the junction to the surrounding air for the first time θJA Heat dissipation path.

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

Then, we use the two temperature constraints of the design's maximum temperature TAmbient and the rated junction temperature TJunctiON (125°C) to determine the required thermal resistance of the module packaged on the PCB.

Finally, we use the most simplified approximation of the convective heat transfer on the PCB surface (both the top and bottom layers with undamaged one-ounce copper heat sinks and countless heat dissipation holes) to determine the board area required for heat dissipation.

The approximate PCB area required does not take into account the role of heat dissipation holes, which transfer heat from the top metal layer (the package is connected to the PCB) to the bottom metal layer. The bottom layer serves as the second surface layer, from which convection can transfer the heat from the board. In order for the approximate board area to be effective, at least 8 to 10 heat dissipation holes must be used. The thermal resistance of the heat dissipation hole is approximate to the value of the following equation.

This approximation applies to a typical through hole with a diameter of 12 mils and a copper sidewall of 0.5 ounces. Design as many heat dissipation holes as possible in the entire area under the exposed pad, and make these heat dissipation holes form an array with a pitch of 1 to 1.5 mm.

in conclusion

The SIMPLE SWITCHER power module provides an alternative to complex power supply designs and typical PCB layouts related to DC/DC converters. Although the layout problem has been eliminated, some engineering design work still needs to be completed in order to optimize the performance of the module with a good bypass and heat dissipation design.