Five points elaborate on the thermal solution of switching power supply

  We all know that a large amount of heat will be generated when the switching power supply is working. If the heat cannot be discharged in time and kept at a reasonable level, the normal operation of the switching power supply will be affected, and the switching power supply will be damaged in serious cases. In order to improve the reliability of the switching power supply, I will share with you several specific cooling solutions for the switching power supply today.

  Switching power supplies have been widely used in various types of current electronic equipment, and their unit power density is constantly improving. High power density is defined from 25w/in3 in 1991, 36w/in3 in 1994, 52w/in3 in 1999, and 96w/in3 in 2001. In order to improve the reliability of switching power supply, thermal design is an essential and important part in the design of switching power supply.

  If the temperature rise inside the switching power supply is too high, it will cause the failure of temperature-sensitive semiconductor devices, electrolytic capacitors and other components. When the temperature exceeds a certain value, the failure rate increases exponentially. Statistics show that the reliability of electronic components decreases by 10% for every 2°C increase in temperature; the life expectancy at a temperature rise of 50°C is only 1/6 of that at a temperature rise of 25°C. In addition to electrical stress, temperature is the most important factor affecting the reliability of switching power supplies. High-frequency switching power supplies have high-power heating elements, and temperature is one of the most important factors affecting their reliability.

  A complete switching power supply thermal design includes two aspects: one is how to control the heat generation of the heat source; the other is how to dissipate the heat generated by the heat source, so that the temperature rise of the switching power supply is controlled within the allowable range to ensure reliability of switching power supplies.

 1. The design of controlling the heat generating value

  The main heating components in the switching power supply are semiconductor switching tubes, power diodes, high-frequency transformers, filter inductors, etc. Different components have different methods of controlling heat generation. The power tube is one of the devices with large heat generation in the high frequency switching power supply. Reducing its heat generation can not only improve the reliability of the power tube, but also improve the reliability of the switching power supply and improve the mean time between failures (MTBF). ). The heat generation of the switch tube is caused by the loss, and the loss of the switch tube consists of two parts: the switching process loss and the on-state loss. Therefore, the following measures can be taken to control and reduce the heat.

1. Reduce the on-state loss The on-state loss can be reduced by selecting a switch with low on-state resistance.

2. The switching loss is caused by the size of the gate charge and the switching time. To reduce the switching loss, a device with a faster switching speed and a shorter recovery time can be selected to reduce the switching loss.

3. It is more important to reduce the loss by designing better control methods and buffering technology. For example, the soft switching technology can greatly reduce this loss.

4. Reduce the heat generation of the power diode. Generally, there is no better control technology to reduce the loss of the AC rectifier and snubber diode. The loss can be reduced by selecting a high-quality diode.

5. For the rectification of the secondary side of the transformer, a more efficient synchronous rectification technology can be selected to reduce the loss.

6. For the loss caused by high-frequency magnetic materials, the skin effect should be avoided as much as possible. For the influence caused by the skin effect, the method of winding multiple strands of thin enameled wires in parallel can be used to solve the problem.

2, the thermal design of the switching power supply

In order to dissipate the heat of the heating device as soon as possible, the heat dissipation design of the switching power supply is generally considered from the following aspects: radiator, cooling fan, metal pcb, insulating heat-conducting sheet, etc. In the actual design, it is necessary to comprehensively apply the above methods to the design of the power supply according to the requirements of the customer and the product itself and the best cost-effectiveness ratio.

1. Heat sink design of semiconductor devices

Since the heat generated by semiconductor devices is dominant in switching power supplies, the heat mainly comes from the turn-on, turn-off and conduction losses of semiconductor devices. In terms of circuit topology, the use of zero-switching conversion topology to generate resonance so that the voltage or current in the circuit is turned on or off at zero-crossing can minimize the switching loss, but it cannot completely eliminate the loss of the switch tube, so the use of heat dissipation The device is the common and main method.

Basic principles of power switch semiconductor heat sink selection

(1) Basic basis for heat sink selection

The choice of heat sink for power semiconductor devices should be comprehensively considered according to the dissipated power of the device, the junction-to-case thermal resistance of the device, the contact thermal resistance and the temperature of the cooling medium.

(2) Requirements for the fastening force between the device and the heat sink

In order to have good thermal contact between the device and the heat sink after assembly, it must have a suitable installation force or installation torque. And in practical applications, a layer of thermally conductive material is usually added between the device and the heat sink to improve its heat transfer efficiency and reduce the thermal resistance between the two.

(3) Rated cooling conditions of the radiator

Self-cooling radiator: the ambient temperature should preferably not be higher than 40 ℃, the radiator fins should be arranged vertically during installation, and the upper and lower end faces should not be blocked, so that there is a good environment and channel for natural air convection around the radiator.

Air-cooled radiator: The inlet air temperature is controlled below 40°C, and the wind speed at the inlet end is preferably 6 m/s.

Water cooling radiator: the inlet water temperature is not higher than 35 ℃. The water flow rate is determined according to the total heat requirement for heat dissipation and the design temperature difference between the inlet and outlet water.

(4) Comprehensive consideration of the selection of radiators

The selection of radiators should comprehensively consider the range of heat dissipation capacity, cooling method, technical parameters and structural characteristics of the radiator. For a device only from the technical parameters, there may be two or three radiators that can meet the requirements, but it should be combined with cooling and installation. , general interchangeability and economy are comprehensively selected.

2. Fan natural air cooling and forced air cooling

In the actual design process of switching power supply, two forms of natural air cooling and fan forced air cooling are usually used. When installing the natural air-cooled heat sink, the blades of the heat sink should be placed vertically upward. If possible, several ventilation holes can be drilled around the installation position of the heat sink on the PCB to facilitate air convection.

Forced air cooling uses a fan to force air convection. Therefore, in the design of the air duct, the axial direction of the blades of the heat sink should be consistent with the exhaust direction of the fan. In order to have a good ventilation effect, the devices with larger heat dissipation should be closer Exhaust fan, in the case of an exhaust fan, the thermal resistance of the heat sink is shown in the table below:

4. Metal PCB

With the miniaturization of switching power supplies, surface-mount components are widely used in actual products, and it is difficult to install heat sinks on power devices at this time. At present, to overcome this problem, metal PCB is mainly used as the carrier of power devices, mainly including aluminum-based copper clad laminates and iron-based copper clad laminates. There is another copper core PCB. The middle layer of the substrate is a copper plate insulation layer, which adopts a high thermal conductivity epoxy glass fiber cloth bonding sheet or a high thermal conductivity epoxy resin. It can mount smd components on both sides, and high-power smd components can be Solder the smd's own heat sink directly on the metal PCB, and use the metal plate in the metal PCB to dissipate heat.

5. Layout of heating elements

The main heating elements in the switching power supply are high-power semiconductors and their radiators, power conversion transformers, and high-power resistors. The basic requirement for the layout of the heating elements is to arrange them from small to large according to the degree of heat generation. The smaller the calorific value, the higher the wind direction of the switching power supply air duct, the closer the device with the larger calorific value to the exhaust. fan.

      In order to improve production efficiency, multiple power devices are often fixed on the same large heat sink. At this time, the heat sink should be placed as close to the edge of the PCB as possible. However, there should be at least a distance of more than 1cm from the shell or other parts of the switching power supply. If there are several large heat sinks on a circuit board, they should be parallel to each other and parallel to the wind direction of the air duct. In the vertical direction, devices with low heat generation are arranged in the lowest layer and devices with large heat generation are arranged in higher layers. Heat-generating components should be placed as far away as possible from temperature-sensitive components, such as electrolytic capacitors, on the PCB layout.

Sinda Thermal is a professional and experienced heat sink manufacturer, we can provide varieties of heat sinks to the global customers, we can offer the most competitive price and great quality heat sinks, please contact us freely if you have any thermal requirements.