What are the basic principles of thermal design of power module shared by adapter manufacturers? First look at the following issues:
One: From the perspective of facilitating heat dissipation, the printed version is best installed upright, and the distance between the boards should generally not be less than 20mm;
2: In the horizontal direction, high-power devices are arranged as close to the edge of the printed board as possible to shorten the heat transfer path; in the vertical direction, high-power devices are arranged as close to the top of the printed board as possible to reduce the temperature of these devices on other devices when they are working. Impact;
Three: The devices on the same printed board should be arranged as much as possible according to the amount of heat generation and degree of heat dissipation. Devices with low heat generation or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) At the top of the cooling airflow (at the entrance), devices with high heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed at the most downstream of the cooling airflow;
Four: The temperature-sensitive devices are best placed in the lowest temperature area (such as the bottom of the device). Do not place it directly above the heating device. Multiple devices are preferably staggered on the horizontal plane.
Five: The heat dissipation of the printed board in the device mainly depends on the air flow, so the air flow path should be researched in the design, and the device or printed circuit board should be reasonably configured. When air flows, it tends to flow in a place with low resistance, so when configuring the device on the printed circuit board, avoid leaving a large airspace in a certain area. The configuration of multiple printed circuit boards in the whole machine should also pay attention to the same issues;
When designing the circuit, you must first decide whether the auxiliary power module that supplies the PWM chip is connected to the output ground or the input ground. In most cases, there is DC isolation between the input and output grounds. The output terminal with load is connected to the output ground. The main switching power transistor is connected to the input ground, that is, one end of the DC bus rectified in the grid-voltage power converter (in a battery-powered DC / DC converter, it is an output terminal of the battery).
When adjusting the output voltage, a DC error amplifier connected to the output ground must be used to detect the output voltage, compare it with a reference voltage, and amplify the error voltage. The error voltage is a difference between the reference voltage and a part of the output voltage, and is used to control the pulse width, thereby controlling the on and off of the main power transistor connected to the input ground. Because there is DC isolation between the output ground and the input ground, and the DC voltage levels may differ by dozens or even hundreds of volts, the M pulse of the pin cannot directly drive the transistor through DC coupling. In this way, if the error amplifier and the pulse width modulator are both connected to the input ground (the common connection in the PwM chip), the PwM pulse must pass the pulse transformer to overcome the obstacle of the output ground and input ground isolation.
The function of the auxiliary power supply is to generate an output voltage with the output ground as a reference, a size of 10 ~ 15v, and a power of 1 ~ 3w: and its input power comes from the power supply connected to the input ground. This type of auxiliary module power supply is often used in the case where the prison chip is connected to the input ground. Although the energy required by the chip can be taken from an auxiliary winding of the main transformer after the main power transistor starts to work, if the drive is turned off (due to overvoltage or overcurrent, etc.), the energy transmission will be interrupted and the remote display cannot be powered. At the same time, because the voltage from the auxiliary winding disappears, the voltage on the chip is reduced, which will cause the pulse width to be too large, resulting in the circuit not working properly.
Generally speaking, it is more reliable to supply the PWM chip from the auxiliary power supply than from the auxiliary winding of the main power transformer (bootstrap method). Another method is to use an optocoupler to cross the barrier between the input ground and the output ground, and send the output voltage detection signal back to the input terminal to idle the modulation pulse width to control the on and off of the power transistor. However, if it is not suitable for bootstrap power supply from the auxiliary winding of the main power transformer, an auxiliary power module that shares the ground with the input is still required.
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