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Power system design engineers always want to achieve higher power density on a smaller circuit board area, and for data center servers and LTE base stations that need to support large current loads from FPGAs, ASICs, and microprocessors that consume more and more power This is especially true. In order to achieve higher output currents, the use of multi-phase systems is increasing. In order to achieve higher current levels on a smaller circuit board area, system design engineers began to abandon discrete power solutions and choose power modules. This is because power modules provide a popular choice for reducing the complexity of power supply design and solving PCB layout issues related to DC/DC converters.
This article discusses a multi-layer PCB layout method that uses a through-hole layout to maximize the heat dissipation performance of a dual-phase power module. The power module can be configured as two-channel 20A single-phase output or single-channel 40A dual-phase output. The example PCB design with through holes is used to dissipate heat from the power supply module to achieve higher power density, so that it can work without a heat sink or fan.
So how can this power module achieve such a high power density? The power module shown in the circuit diagram in Figure 1 provides an extremely low thermal resistance θ of only 8.5°C/W, due to the use of copper as the substrate. In order to dissipate heat for the power module, the power module is mounted on a high-efficiency thermally conductive circuit board with direct mounting characteristics.
The multi-layer circuit board has a top wiring layer (on which the power supply template is installed) and two buried copper planes connected to the top layer with through holes. This structure has a very high thermal conductivity (low thermal resistance), which makes the heat dissipation of the power module easy.
To determine the thermal resistance of the copper layer on the top of the PCB, we take the thickness of the copper layer (t) and divide it by the product of the thermal conductivity and the cross-sectional area. For the convenience of calculation, we use 1 square inch as the cross-sectional area, at this time A=B=1 inch. The thickness of the copper layer is 2.8 mils (0.0028 inches). This is the thickness of 2 ounces of copper deposited on a 1 square inch circuit board area. The coefficient k is the W/(in-°C) coefficient of copper, and its value is equal to 9. Therefore, for this 1 square inch of 2.8 mil copper heat flow, the thermal resistance is 0.0028/9=0.0003°C/W.
From these figures, we know that the thermal resistance of the 33.4 mil (t5) layer is the highest. All the numbers in Figure 4 show the total thermal resistance of the four-layer 1-square-inch circuit board from the top to the bottom. What if we add a through-hole connection from the top of the PCB to the bottom of the PCB? Let's analyze the situation of adding this through-hole connection.
The hole size of the through holes used in the circuit board is about 12 mils (0.012 inches). When making the through hole, a hole with a diameter of 0.014 inches is drilled first, and then copper is plated. This will add about 1 mil (0.001 inch) of copper to the inside of the hole. The circuit board also uses the ENIG plating process. This adds about 200 microinches of nickel and about 5 microinches of gold to the outer surface of the copper. We ignore these materials in our calculations and only use copper to determine the thermal resistance of the via.
Using this formula for 12 mil (diameter) holes, we have r0=6 mils (0.006 inches), r1=7 mils (0.007 inches) and K=9 (copper plating).
The variable l is the length of the via (from the top copper layer to the bottom copper layer). There is no solder mask on the circuit board where the power module is soldered, but for other areas, PCB design engineers may require a solder mask on top of each through hole, otherwise the area above the through hole will be vacant. Since the via only connects to the outer copper layer, its length is 63.4 mils (0.0634 inches). The thermal resistance of the total via length itself is 167°C/W.
Note that when the heat flows down through the via and reaches another layer, especially another copper layer, it will diffuse laterally to that material layer. Adding more and more vias will eventually reduce the effect, because the heat spreading laterally from one via to the nearby material will eventually meet the heat from the other direction (from another via). The size of the ISL8240MEVAL4Z evaluation board is 3 inches x 4 inches. The top and bottom layers of the circuit board contain 2 ounces of copper, and the two inner layers each contain 2 ounces of copper. To make these copper layers work, the circuit board has 917 12-mil diameter through holes, all of which help spread heat from the power module to the copper layer below.
concluding remarks
In order to adapt to the increase in the number of voltage rails and higher-performance microprocessors and FPGAs, advanced power management solutions such as ISL8240M power modules help improve efficiency by providing greater power density and lower power consumption. The optimal realization of through holes in the PCB design of power modules has become an increasingly important factor in achieving higher power density.
