PCB design and layout techniques for optimizing power module performance

- Nov 03, 2018-

Global energy shortages have led governments to push new energy conservation policies.As the energy consumption standard of electronic products becomes more and more strict, it is an eternal challenge for power PCB design engineers to design more efficient and higher performance power supply.Starting from the layout of power PCB, this paper introduces the optimal PCB layout method, instance and technology for optimizing the performance of SIMPLE SWITCHER power module.

When planning the power layout, the physical loop area of the two switch current loops should be considered first.Although these loop areas are largely invisible in the power module, it is still important to understand the current paths of the two loops, as they extend beyond the module.In the loop 1 shown in figure 1, the current self-conducting input bypass capacitor (Cin1) passes through the MOSFET during the continuous conduction time of the high-end MOSFET to the internal inductor and the output bypass capacitor (CO1), and finally returns the input bypass capacitor.

Loop 2 is formed within the closing time of high-end MOSFET and the conduction time of low-end MOSFET.The energy stored in the internal inductor flows through output by-pass capacitors and low end 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 area of high di/dt current.The input bypass capacitor (Cin1) plays a key role in providing high-frequency current to the converter and returning it to its source path.

The output by-pass capacitor (Co1) does not carry a large alternating current but ACTS as a high-frequency filter for switching noise.For these reasons, the input and output capacitors on the module should be placed as close as possible to the respective VIN and VOUT pins.As shown in figure 2, the inductance generated by these connections can be minimized by making the routing between the by-pass capacitor and its respective VIN and VOUT pins shorter and wider.

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

Capacitors with less parasitic inductance are connected to exhibit low resistance to high frequencies, thereby reducing conduction radiation.Ceramic capacitors (X7R or X5R) or other low ESR type capacitors are recommended.The additional input capacitance can only be used if the additional capacitance is placed near the GND and VIN ends.The SIMPLE SWITCHER power module is designed with a unique PCB design that is itself low in radiation and conduction EMI. Following the PCB layout guidelines described in this article will result in higher performance.

Circuit current path planning is often neglected, but it plays a key role in optimizing PCB design.In addition, the ground connection between Cin1 and CO1 should be shortened and widened as much as possible, and the naked pad should be connected directly, which is especially important for the input capacitance (Cin1) ground connection with a large alternating current.

Grounding pins (including bare pads), input and output capacitors, soft start capacitors and feedback resistances in modules should be connected to the loop layer on the PCB.The loop layer can be used as an extremely low return path of inductance current and as a radiator described below.

The feedback resistance should also be placed as close to the module FB(feedback) pin as possible.To minimize the potential noise extraction value on this high impedance node, it is essential to make the routing between the FB pin and the feedback resistance as short as possible.The available compensator or feedforward capacitors should be placed as close to the upper feedback resistance as possible.For examples, see the PCB layout diagram shown in the module data tables.

Cooling PCB design proposal

The compact layout of the modules brings electrical benefits while negatively impacting the cooling PCB design, with the equivalent power dissipating from smaller Spaces.With this in mind, the back of the SIMPLE SWITCHER power pack PCB is designed with a single large bare solder pad and is grounded in an electrical manner.The pad helps provide extremely low thermal impedance from the internal MOSFET(which typically generates most of the heat) to the PCB.

From semiconductor junction to these device packaging thermal impedance (theta JC) is 1.9 ℃ / W.While it's ideal to achieve an industry-leading JC value, a low JC value makes no sense when the thermal impedance of encapsulating the air is too large!Without a low-impedance cooling path that communicates with the surrounding air, the heat will not dissipate on the bare solder pan.So, what determines the value of this?The thermal resistance from the bare solder pad to the air is entirely controlled by the PCB design and the associated radiators.

Now let's quickly learn how to design a simple PCB without radiators. Figure 3 shows the module and PCB as thermal impedance.Compared with the thermal impedance from the junction to the bare plate, due to the relatively high thermal impedance between the junction and the top of the outer package, the heat resistance from the junction to the surrounding air can be estimated for the first time by ignoring the heat path of the bead JA.

The first step in cooling PCB design is to determine the power to be dissipated.The power consumed by the module (PD) can be easily calculated by using the efficiency diagram (contributes) published in the data table.

Then, we use the highest temperature in the PCB design TAmbient and junction temperature rating TJunction temperature (125 ℃), the two constraints to determine the encapsulation on PCB module for thermal resistance.

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

The desired PCB area approximation does not take into account the role of the heat sink, which transfers heat from the top metal layer (encapsulated connected to the PCB) to the bottom metal layer.The bottom layer is used as the second surface layer from which convection can transfer heat from the plate.At least 8 to 10 cooling holes should be used for effective plate area approximation.The thermal resistance of the cooling hole approximates the following equation.

This approximation is applicable to a typical straight through hole with a diameter of 12 mils and a side wall of 0.5 ounces of copper.PCB design as many heat dissipation holes as possible in the entire area below the bare solder pan, and make the heat dissipation holes array with spacing between 1 and 1.5mm.

conclusion

The SIMPLE SWITCHER power module provides an alternative to complex power PCB design and typical PCB layouts associated with dc/dc converters.Although layout problems have been eliminated, some engineering PCB design work needs to be completed to optimize module performance by using good bypass and cooling PCB design.