Temperature is one of the key factors limiting the power and performance of devices. Any power chip application engineer should pay attention to the thermal resistance problem when designing. The decrease in θJA is particularly important when θJC is already relatively low, and the thermal resistance θJA is highly dependent on the PCB design. In the following, TSSOP28 is taken as an example to discuss the influence of several factors on heat dissipation through several experiments. In the following figure, A is an ideal board with few cables. B is A peripheral board with heat dissipation pad in the middle and upper and lower. C is to expand the heat dissipation area.
On the premise that the holes are the same, compare 5 boards of the same size, ABCD is a 4-layer board, and the comparison result is shown in the figure above.
There are several conclusions
The 4-layer plate has a significant heat dissipation effect than the 2-layer plate.
Although the Dog bond does not help heat dissipation in absolute terms, it has a lot of effects in relative proportion with the increase of power consumption. It is worth emphasizing that the pins on the left and right sides of the chip in the top layer cannot dissipate heat, so try not to wire up and down, and increase the area of Dog bond as much as possible.
The heat dissipation of the top and bottom welding resistance layer has a role;
Copper area is helpful for heat dissipation;
In addition to the above factors, the size of the holes, the number of holes and the structure of the holes also affect the heat dissipation. The following figure shows the influence curves of the holes of 2-layer and 4-layer plates on the heat dissipation.
As can be seen from the figure, there is a balance between the number of holes, and too many holes may not improve the heat transfer. This also depends on package size. Take the 0.33mm(13mil) hole as an example
In addition, heat dissipation is DIE conduction to the periphery. The following figure is the thermal radiation diagram of a 2-layer plate for reference thermal conductivity design.
Schematic design for heat dissipation help:
In addition to PCB and heat sink design, there is also to pay attention to the circuit design. The sound box with battery is an example, the two chips with the biggest caloric output are DC-DC boost and power amplifier. Special market demand for more and more power, such as power amplifier output 2X25W (THD+N=1%) demand, the need for battery boost chip to 16V or so.
When the battery voltage is boosted to 16V and the current is continuously increased, the chip efficiency will gradually decrease with the rise of heating, and the greater the pressure difference, the lower the chip efficiency will be, and the lower the efficiency, the heat will continue to increase.
The same principle of the power amplifier is the same, the output of the PWM waveform of the high and low level is PVDD and GND, PVDD the more efficient the lower, the more easy to hot power amplifier.
So if the PVDD voltage becomes low, it can not only solve the efficiency problem of DCDC, but also solve the efficiency problem of power amplifier, killing two birds with one stone. To achieve this function, we can use the CLASS H function of ACM3108. The principle of this function is as follows:
According to the characteristics of music signal, ACM3108 provides the control signal control chip FB, so as to control the boost PVDD
With the change of PVDD, the efficiency of boost and power amplifier are both improved. When playing music, the overall calorific value is greatly reduced, and the playing time is also improved by about 40%.