DIY production of wireless power supply for laptop

Problems faced by wireless power supply for laptops

The wired power supply of laptops is usually around 20V/3A. For common switching power supplies, with some losses, the reserve power of this power supply is required to be above 70W, which is a bottleneck value. When the power of this wired power supply is close to the limit value, it is undoubtedly a great technical challenge to use wireless power supply to achieve the same power supply and charging functions as wired power supply, which will face the following technical problems:

1. Power Issues

2. Efficiency Issues

3. Eddy current problem

4. EMC issues

5. Structural issues

The main physical difference between wireless power supply and wired power supply is that wireless power supply has no physical connection with the electrical appliance. If the transformer coupling of wired power supply is regarded as a closed system, then wireless power supply must be an open system, that is, the energy is not coupled through the transformer core, but transmitted from the transmitter to the receiver after a certain distance in the form of electromagnetic wave radiation. Therefore, the loss is much greater than that of wired power supply, and the efficiency is much lower. What is more troublesome is that the eddy current problem, EMI and EMC problems that follow are very prominent. In such a powerful magnetic field, eddy currents may cause serious heating and even damage to the components inside the laptop, especially the chips (including CPU); at the same time, due to the huge electromagnetic radiation, it may cause serious interference and even threats to the laptop itself, the power network and the surrounding environment. In addition, the laptop is a highly integrated portable product with extremely limited internal space. Therefore, the wireless power supply must be light in weight and small in size to be embedded in the laptop.

Figure 1

Introduction of Dot Magnetic Resonance Chip

Fortunately, the chips VOX330MP05S and VOX20K3A make wireless power supply for laptops possible. This is a pair of dedicated high-power transmitting and high-power receiving ICs based on electromagnetic resonance, with high transmitting and receiving efficiency. Its appearance is shown in Figure 1.

VOX330MP05S is a 3-pin package (equivalent to TO-220) electromagnetic resonance dedicated transmitter chip with a transmission capacity of up to 100W, and a volume of only 22mm×12mm×9.5mm. Its first pin is the power supply end, the working voltage is 9~12V, the third pin is the ground, and the second pin is the output end. The IC has a built-in oscillator, voltage comparator, power limiter, driving circuit and power output tube. Its output end can withstand high-voltage pulses above 1000V, which is very suitable for high-voltage drive applications. Figure 2 is an example of using 220V~ to power the electromagnetic resonance circuit after rectification.

In the figure, C4, T1 and C3 form a filter network, which is connected in series between the power grid and the transmitting circuit. It is used to absorb the harmonics in the transmitting circuit and feed them back to the power grid, and also prevent the surge voltage on the power grid from affecting the transmitting circuit. The four diodes and C2 are rectification and filtering circuits, which directly rectify the 220V~ mains power to obtain a DC voltage of about 300V. This voltage is added to the output end of VOX330MP05S through the parallel resonant circuit composed of L1 and C1. The DC in the figure is a 12V/100mA power conversion module, which provides working voltage for IC1.

VOX20K3A is a five-pin thick film package circuit with a size of 33mm×30mm×6.3mm. It integrates the phase detection, voltage detection, current detection, power correction and other functions required for electromagnetic resonance. The working principle diagram of its application is shown in Figure 3.

VOX20K3A needs to provide a 5V working voltage and a working current of about 30mA, which can be provided by LM78L05. The silicon voltage regulator on pin 2 determines the output voltage of the entire power supply. The relationship is: Vout = DW + 1.2V. Therefore, different voltage regulators will get different output voltages, but the voltage regulator must be selected between 9 and 24V. A1 and A2 are the compensation input and output terminals respectively, C1 is the input capacitor, L1 and L2 are two receiving coils in series, which can also be replaced by one coil, VD1 and VD2 are rectifiers, fast tubes or Schottky tubes are selected, and the current is greater than 3A. L3, L4 and C4~C9 are filter circuits used to reduce ripple and stabilize voltage.

Figure 2 Transmitter circuit

Figure 3 Receiving circuit

Figure 4 Relationship between the base and the laptop

Figure 5 Hierarchical relationship between transmission and reception

Planning of wireless power supply for laptops

Wireless power can eliminate the trouble of frequent plugging and unplugging of power cords, and can also charge laptop batteries at any time, bringing convenience to users. However, to use wireless power, the above-mentioned problems must be solved.

1. Spatial structure issues

Laptops generate a certain amount of heat during use, especially in the summer, when the heat is more obvious. Many users equip them with a base to help dissipate heat.

The wireless power supply is divided into two parts. The transmitting part can be installed on the base, and the receiving part is placed inside the laptop. There is a ventilation layer in the middle to facilitate air convection. The structural distribution is shown in Figure 4.

The internal structure of a laptop is very full and there is no space to place the wireless power receiving circuit. Therefore, if it is to be practical, the internal structure of the laptop must be adjusted through engineering design. However, as an experiment, the following adjustments can be made: remove the optical drive of the laptop and place the wireless power receiving part in the position of the optical drive.

2. Eddy current solution

When a large area of ​​metal (a good conductor of electricity and heat) is placed in an electromagnetic wave, eddy currents will be generated and heat will be generated. Induction cookers are made based on this principle. This situation is absolutely not allowed in laptops. The high-density circuits in laptops, especially the CPU, have many internal loops. If they are placed in an electromagnetic field, it is equivalent to a large area of ​​metal, which is bound to generate serious heat! How to solve it?

To eliminate eddy currents, the key is to prevent high-density devices such as CPUs from working in electromagnetic fields. Therefore, an isolation layer must be added above the receiving coil to prevent electromagnetic waves from propagating further upward. The isolation layer can be made of anti-wave materials and is required to have a certain thickness, as shown in Figure 5. The protective layer can be a metal sheet to absorb the remaining electromagnetic waves after passing through the isolation layer. Of course, it will generate heat, so the quality of the isolation layer not only affects the heat generation, but also seriously affects the overall efficiency of the power supply. The protective layer also serves as a heat sink, and all heating components are attached to it with heat dissipation adhesive.

3. EMI and EMC solutions

There are two main aspects of EMI, namely conducted interference and radiated interference. C4, T1 and C3 in Figure 2 can effectively eliminate the former. As for the latter, an isolation layer and a protective layer can also be added under the transmitting coil to minimize the external radiation of electromagnetic waves. Of course, the quality of the receiving circuit, especially the quality of the material, is the key to reducing electromagnetic radiation and improving efficiency, and the quality of the PCB and the quality of the coil play an important role in reducing secondary resonance and resonant radiation.

Figure 6 Launch board (viewed from above) Figure 7 Launch board (viewed from below)

Production of wireless power supply for laptop

1. Production of the transmitter and base

The transmitter mainly consists of two parts: the transmitting circuit board and the transmitting coil board.

The transmitting coil board has 4 layers, from top to bottom: surface insulation protection layer, coil layer, isolation layer and bottom metal protection layer, each layer is firmly attached with adhesive. The size of the transmitting coil board is about 10cm square, but the metal isolation layer at the bottom must be longer, about 13cm, and the extra area is mainly used to install the transmitting chip VOX330MP05S and serve as a heat sink for the chip.

The DC module on the transmitting circuit board uses a small AC/DC with AC220V input and 12V/200mA output. In addition to providing 20-30mA working current for the transmitting chip, this power supply is also the power supply for the external fan. The transmitting chip must be installed horizontally so that it can be fully attached to the heat sink, that is, the bottom metal protective layer, as shown in Figures 6 and 7.

Figure 8 Launch base (viewed from above) Figure 9 Launch base (viewed from above)

Take a plastic board, the size of a laptop computer, and fix the launch board with screws at the position corresponding to the optical drive. Dig a hole near the launch board to install the fan. Of course, you also need to install 4 rubber pads. At the same time, don't forget to fix the input end of the power cord to prevent it from falling off or breaking during use and causing accidents, as shown in Figures 8 and 9.

2. Preparation of the receiver

The receiver also consists of two parts: a receiving circuit board and a receiving coil board.

The size of the receiving circuit board is about 10cm×3cm. There are several components on the board that must be mounted on the radiator, namely, rectifier tubes VD1, VD2 and receiving chip VOX20K3A, as shown in Figures 10 and 11.

Figure 10: Receiving circuit board (viewed from below) Figure 11: Receiving circuit board (viewed from above)

The structure of the receiving coil board is the same as that of the transmitting coil board, and is also divided into 4 layers, but the arrangement direction is just the opposite. From top to bottom, they are: top metal protection layer, isolation layer, coil layer and surface insulation protection layer. The metal protection layer is also used as a heat sink, and several heat-sensitive components are closely attached to it, as shown in Figure 12.

Figure 12 Receiver plate (top view)

3. Laptop computer transformation

The following two modifications are required inside the laptop:

First, open the laptop case and remove the screws that fix the optical drive. This way, the optical drive can be removed at any time to leave space for the receiving module. When you want to use the optical drive, just reinsert it.

The sizes of the module and the optical drive are shown in Figure 13.

In addition, there is a layer of electroplating on the inner side of the laptop shell to prevent external electromagnetic interference. Electromagnetic waves cannot (or are not easy to) penetrate. The electroplating layer at the corresponding position of the optical drive must be scraped off to meet the requirements of wireless power supply. Scraping off the electroplating layer at this position will not affect the use of the laptop, because the optical drive itself is a metal structure. When the optical drive is plugged back in, it will make up for the original electroplating layer.

The above treatment can preserve the original appearance and performance of the laptop to the greatest extent possible, and will not cause it to be damaged by this experiment.

Finally, insert the receiving module into the laptop, plug the power output end into the laptop, and place the laptop on the transmitting base, aligning the transmitting module and the receiving module up and down, keeping a distance of about 2cm (Note: if the distance is too large, the radiation increases and the efficiency decreases; if the distance is too small, there is no gap between the laptop and the base, which is not conducive to heat dissipation) and then power on and work, see Figure 14.

Figure 13 Comparison between module and optical drive Figure 14 Laptop computer with wireless power supply

Summarize

Laptop wireless power is actually a comprehensive solution that integrates power, efficiency, eddy current, EMC and structural issues. These issues are mutually influential and mutually constrained, and it is meaningless to consider any one of them alone. For example, the treatment of eddy current is actually a process of reducing radiation and improving efficiency. With less secondary radiation, the eddy current is smaller, and the efficiency is correspondingly improved; and the structural problem is a primary issue, because the structure directly affects the practicality of the wireless power supply. If the structure is unreasonable, it may cause interference, heating, or even burning of the internal components of the laptop.

After the above treatment, the laptop wireless power supply and receiving module can be directly inserted into the laptop (unplug the optical drive first), realizing the functions of wireless power supply and wireless charging. The test results are shown in the attached table.

Schedule

Item Parameter Remarks

Antenna thickness 5mm including isolation layer and protective layer

PCB thickness 15mm including heat sink thickness 1mm

Size 135mm×100mm Transmitter and receiver are the same

Transmitter voltage 220V~

Transmitting no-load current 0.04A~ No receiving part

Transmitting no-load current 0.05A~ There is a receiving part but it does not power the laptop

Load current 0.28～0.39A~ Laptop computer works normally and charges

Receive output voltage 20V DC ±0.5V

Receive output current: 3.0A DC when charging, 1.8A DC when not charging

Ripple voltage 50～100μV DC

Transmitter module temperature 42℃ Temperature 28℃/ 1 hour/air convection

Receiver module temperature 58℃ Air temperature 28℃/1 hour/no ventilation in laptop