The relationship between various functions of dcdc power module and temperature

existdcdc power moduleThe main components in the structure are: pulse width modulator (control and transform power), photocoupler (input and output barrier, prevent interference between the front and rear stages, and transmit sampling information to PWM to maintain the stability of the output voltage), VDMOS (power conversion) Components, use its outstanding switching characteristics to improve power conversion) and Schottky diodes (rectification and filtering are the primary components of power output). With the rapid development of electronic technology, the application field of switching power supply is becoming more and more extensive, and the operating environment is getting worse and worse. Statistics show that every time the temperature of electronic components increases by 2℃, the reliability decreases by 10%, and the temperature rise is 50 The life span at ℃ is only 1/6 of that at a temperature rise of 25℃.

In order to find out how the electrical parameters of the power supply module changes with temperature, firstly heat the entire power supply module to test its input current, output current, and output voltage (Vout) electrical parameters. Test conditions: adhere to the input voltage of 28V, the output load of 15Ω, and the output Current 1A; test the change of input current and output voltage with temperature. It is found that the output voltage of the horizontal block has a relatively significant decrease, and the change trend of input current and output current is not very significant.-The change trend is accompanied by an increase in temperature, and the voltage of the power module gradually decreases, and the trend is very significant As can be seen from Figure 1, the heating temperature is 50°C, and Vout is 14.98 V; when the temperature is 142°C, Vout drops to 14.90 V. In addition, because the power of the module is an important goal of its function, when the power is reduced to a certain value, the module will also fail due to excessive heat. For this reason, the change of module power with temperature under the test conditions is calculated. It can be seen from Figure 2 that the power of the module changes more significantly with the increase in temperature. The beginning is relatively slow, and it gradually accelerates with the increase in temperature, showing a glassy Eltzmann's exponential distribution. In the test, it is found that when the temperature rises to 150°C, the output voltage of the module is zero.

In order to find the main components that cause the output voltage of the power supply module to decrease significantly with the increase in temperature, according to the circuit of the module, select the corresponding components to build a circuit. The circuit can complete all the functions of the module after testing. At the same time, because of non-integration, The component can be tested independently, preventing the condition that the integrated component is too small and difficult to test. The important components in the power supply module are heated independently, and the electrical parameters of the power module are tested for changes with temperature, and the circuit Vout is tested for changes.

transformer

The transformer can not only transfer energy, but also has the effect of electrical barrier. The difference in the ratio of turns between the primary side and the secondary side of the transformer can achieve the effect of step-up or step-down. Under the working condition of the module, due to the eddy current effect of the magnetic core, the transformer will generate a lot of heat, which becomes the primary source of heat generation in the module. In the experiment, the primary and secondary inductances of the transformer's primary and secondary coils are tested with temperature. As shown in Figure 3, it can be seen from Figure 3 that as the temperature rises, the inductance of the coil is first added, then decreased slightly, and then slightly Rising, once the ambient temperature was 220°C, the overall trend of the inductance of the transformer's primary side and replica is gradually increasing. When the temperature reaches 220°C and the core temperature reaches the wall point, the inductance of the coil quickly drops to zero. Regarding transformers with different core materials, the Curie point temperature is different. Regarding this type of transformer, it is known that the Curie temperature is close to 220°C. When the transformer temperature is close to the Curie point, the transformer inductance will quickly decrease, which will cause the output voltage to quickly decrease.

The experiment also tested the change of the inductance of other inductance components of the input and output of the circuit with temperature. During the entire heating period, the inductance of other components changes very little with temperature, which can be neglected compared with the change in inductance of the transformer. And during the period when the inductance of the transformer decreases, the change of the inductance of other inductance components is still small.

In order to calibrate the ambient temperature and the temperature of the module due to self-generated heat, select a module, perforate the module shell, and put the temperature sensing wire inside the round hole of the transformer, test the temperature of the transformer, and process the test data to obtain the temperature of the transformer The connection function with ambient temperature: y=1.18x+13. It can be seen that the temperature of the transformer is much higher than the operating temperature of the power module. When the ambient temperature is 150℃, the result of the temperature sensing line test is about 190℃, because the test point of the temperature sensing line is the air inside the round hole of the transformer, not the core temperature of the transformer, so the measurement result of the temperature sensing line is better than that of the actual transformer. The temperature is much lower, so it can be judged that the temperature of the transformer core is close to the Curie point. Therefore, when the ambient temperature of the module exceeds 150°C, the temperature of the transformer in the module will reach the Curie point temperature of the transformer core. The output voltage is almost zero.

Pulse Width Modem (PWM)

The primary function of PWM is to adjust the duty cycle of the pulse waveform according to the output response, and drive the power equipment, and then obtain a stable DC output voltage.

In this type of power module, the function of PWM-SG3524 is to supply two square wave signals to the transistor and VDMOS, and control the on and off time of VDMOS according to the width of the square wave signal. In this test, the PWM-SG3524 in the working condition of the circuit is heated alone, and the relationship between the output square wave signal and the temperature is tested, and the measured waveform does not change significantly; the input and output current and voltage of the module are performed together with the heating. According to the record, it is found that with the increase of the ambient temperature of the PWM location, the input current and input voltage change very little; the output voltage and output current change also very little. Heating PWM causes the electrical parameter changes to be negligible compared with the overall heating electrical parameters of the module. It is confirmed that PWM-SG3524 has little effect on the temperature characteristics of the module.

VDMOS

VDMOS (Straight Double Diffusion Field Effect Transistor) is used as a switching device in the module circuit. It operates under rational load, accepts high peak voltage and large current, has high switching loss and temperature rise, and its switching frequency can be as high as 130 kHz. Operating at such a high frequency may cause a variety of internal degradation mechanisms, causing the function of the VDMOS to decrease or even fail.

In this experiment, the VDMOS in the module is heated independently, and the electrical parameters of the module are tested. The test shows that when the temperature reaches 180°C, the input current increases significantly with the increase in temperature. The output voltage and output current change little with the increase of temperature. In addition, the output power of the module is calculated to determine whether the module is in normal operation. After calculation, the input current of the module can be quickly increased when the VDMOS is heated to 180°C independently. When the temperature rises to 220°C, the output voltage is almost unchanged, because the module has failed at 150°C, and the independent heating temperature is now as high as 180°C, which is much higher than the temperature at which the entire module fails. Therefore, the temperature characteristics of VDMOS are Affect the cause of the change in output voltage.

Diode (SBD)

The diodes used in the module include Zener diodes and rectifier diodes, among which the rectifier diodes play an important role in the voltage conversion process. At the output end of the transformer, the two rectifier diodes are turned on in different sections, so that the AC pulsating voltage is converted into a DC pulsation. In this experiment, the SBD in the circuit was heated alone, and it was found that as the temperature increased, the output voltage of the module did not change significantly. Therefore, SBD is not the primary factor that causes the module's output voltage to drop under the environment of high temperature operation.

Optocoupler

Optocouplers (hereinafter referred to as optocouplers) use light as the preface to transmit electrical signals. It has an excellent blocking effect on input and output electrical signals. The optocoupler is usually composed of three parts: light emission, light reception and signal expansion. The input electrical signal drives a light emitting diode (LED) to declare light of a certain wavelength, which is received by the photodetector to generate a photocurrent, and then output after further expansion. This completes the conversion of electricity to light to electricity, and then has the effect of blocking input and output. Because the input and output of the optocoupler are separated from each other, and the electrical signal transmission has the characteristics of unidirectionality, it has excellent electrical insulation and anti-interference ability.

In summary, the temperature characteristics of the DC/DC power supply module are as follows: when the temperature is less than 150°C, the output voltage of the module decreases slowly. The reason is the decrease in the optocoupler current transmission ratio; when the temperature is greater than 150°C, the power supply The output voltage of the module quickly decreases, and even the output voltage is almost zero. The reason is that the core temperature of the transformer in the module is close to the Curie point temperature (220°C) at this moment. Caused by the failure of the transformer effect. In this situation, if there is no other damage inside the module, when the heating is stopped, the temperature of the module recovers to room temperature, and the module is powered on again, the output voltage of the module can still recover to a normal value. However, regarding the module tested in this experiment, when the ambient temperature exceeds 150°C or so, because the core temperature of the module transformer reaches the interval point, the core temperature rises. This positive reaction will cause the core temperature to rise rapidly, resulting in The heat is also more, causing damage to other equipment inside the module, which is very simple to form permanent damage to the module.