DC/DC power module temperature relationship test analysis

The power supply structure of the DC/DC power supply module includes: pulse width modulator (controls conversion efficiency), optocoupler (isolates input and output to avoid interference between the previous and next stages, and transmits sampling information to PWM to maintain the stability of the output voltage), VDMOS (power conversion component, using its good switching characteristics to improve conversion efficiency) and Schottky diode (rectification and filtering, these are the main components of power output).

We know that the reliability of electronic components decreases by 10% for every 2℃ increase in temperature. When the temperature rises to 50℃, the service life is only 1/6 of that at 25℃. In order to understand the change of the electrical parameters of the power module with temperature, the power module is first heated as a whole to test its input current, output current, and output voltage (Vout) electrical parameters. The test conditions are as follows:

Keep the input voltage at 28V, the output load at 15Ω, and the output current at 1A; test the changes of input current and output voltage with temperature.

It is found that the output voltage of the horizontal block has a significant decrease, and the change trend of the input current and output current is not very obvious. The change trend is that with the increase of temperature, the voltage of the power module gradually decreases, and the trend is very obvious. As can be seen from Figure 1, when the heating temperature is 50°C, Vout is 14.98 V; when the temperature is 142°C, Vout drops to 14.90 V. In addition, because the efficiency of the module is an important indicator of its performance, when the efficiency drops to a certain value, the module will also fail due to excessive heat generation. For this reason, the change of module efficiency with temperature under the test conditions was calculated. From Figure 2, it can be seen that the efficiency of the module changes more obviously with the increase of temperature. It starts slowly and gradually accelerates with the increase of temperature, showing a Boltzmann exponential distribution. In the test, it was found that when the temperature rose to 150°C, the output voltage of the module was zero.

Figure 1: Relationship between power module Vout and temperature T

Figure 2: Relationship between power module efficiency and temperature In order to find the main components that cause the output voltage of the power module to drop significantly as the temperature rises, the corresponding components are selected to build the circuit according to the circuit of the module. The circuit can complete all the functions of the module after testing. At the same time, because it is non-integrated, its components can be tested individually, avoiding the condition that the integrated components are difficult to test due to their small size.

Next, the important components in the power module are heated separately to test how their electrical parameters change with temperature, and at the same time, the changes in the circuit Vout are tested.

Impact of component temperature performance on module temperature characteristics

transformer

The transformer can not only transfer energy, but also play the role of electrical isolation. The difference in the turns ratio between the primary and secondary coils of the transformer can achieve the effect of voltage increase or decrease. When the module is working, due to the eddy current effect of the magnetic core, the transformer will generate a lot of heat, which becomes the main source of heat generation for the module.

In the experiment, the change of the inductance of the primary and secondary coils of the transformer with temperature was first tested, as shown in Figure 3. It can be seen from Figure 3 that as the temperature rises, the inductance of the coil first increases, then decreases slightly, and then increases slightly. Before the ambient temperature reaches 220°C, the overall trend of the primary and secondary inductance of the transformer is gradually increasing. When the temperature reaches 220°C, the core temperature reaches the Curie point, and the inductance of the coil drops rapidly to zero. The Curie point temperature of transformers with different core materials is different. For this type of transformer, it can be seen that the Curie temperature is around 220°C. When the transformer temperature approaches the Curie point, the transformer inductance will decrease rapidly, causing the output voltage to drop rapidly.

Figure 3: Transformer inductance vs. temperature

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

In order to calibrate the ambient temperature and the temperature of the module due to self-heating, a module is selected, the module shell is perforated, and the temperature sensing wire is placed inside the round hole of the transformer to test the temperature of the transformer. By processing the test data, the relationship function between the transformer temperature and the ambient temperature is obtained: 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 wire test is about 190℃. Since the temperature sensing wire test point is the air inside the round hole of the transformer, not the core temperature of the transformer, the measurement result of the temperature sensing wire is much lower than the actual transformer temperature. It can be judged that the core temperature of the transformer will be close to the Curie point. Therefore, when the ambient temperature of the module exceeds 150℃, the temperature of the transformer in the module will reach the Curie point temperature of the transformer core. At this time, the output voltage of the module is almost zero. 

Pulse Width Modem (PWM)

The main function of PWM is to adjust the duty cycle of the pulse waveform according to the output feedback and drive the power devices to obtain a stable DC output voltage.

In this model of power module, the function of PWM-SG3524 is to provide 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 experiment, the PWM-SG3524 in the working state of the circuit was heated separately, and the relationship between the output square wave signal and the temperature was tested. The waveform was measured to have no obvious change; while heating, the input and output current and voltage of the module were recorded, and it was found that as the ambient temperature of the PWM environment increased, the input current and input voltage changed very little; the output voltage and output current also changed very little, and the changes in electrical parameters caused by heating PWM were negligible compared with the overall heating electrical parameters of the module. It proves that PWM-SG3524 has little effect on the temperature characteristics of the module.

VDMOS

VDMOS (vertical double diffused field effect transistor) is used as a switching device in the module circuit. It works under inductive loads, withstands high peak voltages and large currents, has high switching losses and temperature rises, and its switching frequency can be as high as 130 kHz. Working at such a high frequency may cause multiple internal degradation mechanisms, resulting in performance degradation or even failure of VDMOS.

In this experiment, the VDMOS in the module is heated separately to test the changes in the module's electrical parameters. The test shows that when the temperature reaches 180°C, the input current increases significantly with the increase in temperature. However, the output voltage and output current change slightly with the increase in temperature. In addition, the output efficiency of the module is calculated to determine whether the module is in normal working condition. Through calculation, it can be found that when the VDMOS is heated to 180°C alone, the input current of the module increases rapidly. When the temperature rises to 220°C, the output voltage hardly changes. Since the module has failed at 150°C, and the temperature of the separate heating is as high as 180°C at this time, which is much higher than the temperature at which the module fails as a whole, the temperature characteristics of VDMOS are not the reason for the change in output voltage. Diode (SBD)

The diodes used in the module include voltage regulator diodes and rectifier diodes, among which the rectifier diode plays an important role in the voltage conversion process. At the output end of the transformer, two rectifier diodes are turned on at different times to convert the AC pulsating voltage into DC pulsating voltage. In this experiment, the SBD in the circuit is heated separately, and it is found that the output voltage of the module does not change significantly with the increase of temperature. Therefore, when the module is working in a high temperature environment, the SBD is not the main factor causing the module output voltage to drop.

Optocoupler

Photoelectric coupler (hereinafter referred to as optocoupler) uses light as a medium to transmit electrical signals. It has a good isolation effect on input and output electrical signals. Optocouplers are generally composed of three parts: light emission, light reception and signal amplification. The input electrical signal drives the light emitting diode (LED) to emit light of a certain wavelength, which is received by the photodetector to generate photocurrent, and then output after further amplification. This completes the conversion of electricity, light and electricity, thereby playing the role of input and output isolation. Since the input and output of the optocoupler are isolated from each other, and the electrical signal transmission has the characteristics of unidirectionality, it has good electrical insulation and anti-interference capabilities.

In the module, the optocoupler is an important component for isolating input and output, and transmits the current signal output by the output comparison amplifier to the 9th pin of PWM. The 9th pin is the compensation end of PWM, which is connected to the reverse input end of the comparator to control the width of the output pulse of the 11th and 14th pins of PWM, thereby adjusting the output voltage of the module to keep it stable.

In this experiment, we first tested the change of the proportional coefficient of the input current to the output current of the optocoupler NEC2705 used in the module with temperature. The current applied to the input end was 11 mA. The results showed that at 25°C, the current transfer ratio of the optocoupler was close to 1:1. However, as the temperature increased, the input current remained unchanged and the current at the output end gradually decreased. For every 10°C increase, the current transfer ratio of the optocoupler decreased by 4%. The results are shown in Figure 4.

Figure 4: Optocoupler current transfer ratio vs. temperature 

Figure 5: Output voltage vs. optocoupler temperature

Then heat the optocoupler of the module in the working state separately (the optocoupler of the module is large, and it can be heated separately after removing the welding wire), and measure the output voltage of the module, as shown in Figure 5. It is found that as the temperature rises, the module voltage gradually decreases, and it is basically consistent with the trend of the output voltage measured when the module is heated as a whole, which decreases with the increase of temperature. Through analysis, it can be seen that as the ambient temperature increases, the power consumption of each component of the power module increases, which will cause the output voltage of the module to decrease. At this time, the feedback circuit connected by the optocoupler should be used to increase the pulse width of the PWM output and increase the voltage at the output end. However, due to the decrease in the transmission efficiency of the optocoupler, the result of the negative feedback cannot be fully transmitted to the PWM. The PWM output pulse width is narrower than the actual one, that is, the voltage adjustment ability is reduced, and the output voltage decreases with the increase of ambient temperature.

In summary, the module temperature characteristics are as follows: when the temperature is less than 150℃, the module output voltage slowly decreases, which is caused by the decrease of the optocoupler current transfer ratio; when the temperature is greater than 150℃, the output voltage of the power module drops rapidly, and even the output voltage is almost zero, because the core temperature of the transformer in the module is close to the Curie point temperature (220℃). The transformer fails. In this case, if there is no other damage inside the module, when the heating is stopped, the module temperature returns to room temperature, and the module is powered on again, the module output voltage can still return to normal. However, for the module tested in this experiment, when the ambient temperature exceeds about 150℃, the core temperature of the module transformer reaches the distance point, causing the core temperature to rise. This positive feedback will cause the core temperature to rise rapidly, and more heat will be generated, causing damage to other components inside the module, which can easily cause permanent damage to the module.