Common Faults and Solutions for DC-DC Power Modules
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1. Input voltage is too high
The input parameter of the power module is abnormal - the input voltage is too high. This abnormality can cause the system to not work properly, or even burn the circuit. So what are the common causes of excessive input voltage?
l The output end is suspended or has no load;
l The output load is too light, lighter than 10% of the rated load;
l The input voltage is too high or there is interference voltage.
For this type of problem, you can adjust the load at the output end or adjust the input voltage range, as shown below:
l Ensure that the output end is not less than 10% of the rated load. If there is no-load phenomenon in the actual circuit operation, connect a dummy load of 10% of the rated power to the output end;
l Change the input voltage to a reasonable range. If interference voltage exists, consider connecting a TVS tube or a voltage regulator tube at the input end.
2. Output voltage is too low
Abnormal output parameters of the power module - the output voltage is too low. This may cause the overall system to not work properly. For example, in a microcontroller system, a sudden increase in load will pull down the microcontroller supply voltage and easily cause a reset. And if the power supply works at a low input voltage for a long time, the life of the circuit will be greatly reduced. Therefore, the problem of low output voltage cannot be ignored. So what are the reasons for the low output voltage? As shown in Figure 1 below.
l The input voltage is low or the power is insufficient;
l The output line is too long or too thin, resulting in excessive line loss;
l The voltage drop of the anti-reverse diode at the input end is too large;
l The input filter inductance is too large.
Figure 1 Reasons for low output voltage
For this type of problem, it can be improved by adjusting the power supply or replacing the corresponding peripheral circuit, as shown below:
l Increase the voltage or use a higher power input power supply;
l Adjust the wiring, increase the wire cross-sectional area or shorten the wire length to reduce the internal resistance;
l Use a diode with a small conduction voltage drop;
l Reduce the filter inductance value or lower the internal resistance of the inductor.
3. Output noise is too large
Abnormal output parameters of power modules - excessive output ripple noise. As we all know, noise is a key indicator to measure the quality of power modules. In the application circuit, the design layout of the module will also affect the output noise. So what are the reasons for excessive output ripple noise?
l The power module is too close to the noise-sensitive components of the main circuit;
l The power input terminal of the noise-sensitive components of the main circuit is not connected to a decoupling capacitor;
l Differential frequency interference occurs between power modules with single output in a multi-channel system;
l Improper grounding treatment.
The ZDS2024 oscilloscope tests a power module with a large noise interference problem, as shown in Figure 2:
Figure 2 Power supply ripple waveform
For this type of problem, you can improve it by isolating the module from the noisy device or using decoupling capacitors in the main circuit, as follows:
l Keep the power module as far away from the noise-sensitive components of the main circuit as possible or isolate the module from the noise-sensitive components of the main circuit;
l Connect a 0.1μF decoupling capacitor to the power input terminal of the noise-sensitive components of the main circuit (such as A/D, D/A or MCU, etc.);
l Use a multi-channel output power module instead of multiple single-channel output modules to eliminate differential frequency interference;
l Use remote single-point grounding to reduce the ground wire loop area.
4. Poor power withstand voltage
Abnormal performance parameters of power modules - poor withstand voltage of power modules. Usually, the withstand voltage of isolated power modules is as high as several thousand volts, but it may fail to reach this indicator during application or testing. So what factors will greatly reduce its withstand voltage capability?
l The voltage withstand tester has overshoot when it is turned on;
l The isolation voltage value of the selected module is not enough;
l Reflow soldering and hot air guns are used many times during maintenance.
The method of testing the isolation voltage of the power module using a withstand voltage tester is shown in Figure 3:
Figure 3: Hi-pot test diagram
For this type of problem, it can be improved through standardized testing and standardized use, as shown below:
l The voltage is gradually increased during the withstand voltage test;
l Select a power module with a higher withstand voltage;
l When welding the power module, select the appropriate temperature to avoid repeated welding and damage to the power module.
5. The power module is difficult to start
The first is the case with less destructive force - the power module has difficulty starting or even fails to start. When using the power module, you may find that the voltage at the output end of the power module is normal, but there is no output at the output end, and the power module is not damaged. What is the reason? The specific reasons are as follows:
l The external capacitor is too large;
l The capacitive load is too large;
l The load current is too large;
l The input power is insufficient.
For this type of problem, it can be improved by adjusting the output capacitance and load or adjusting the input power, as shown below:
l The external capacitor is too large. It takes a long time to charge the power module when it starts, making it difficult to start. You need to select a suitable capacitive load.
When the capacitive load is too large, a suitable inductor must be connected in series first;
l When the output load is too heavy, the startup time will be prolonged, so choose a suitable load;
l Use a higher power input power source.
6. The module is seriously overheated
Compared with the difficulty in starting, the more serious abnormal situation of use is that the power module heats up seriously when in use. The root cause of this phenomenon is that the power module has energy loss in the voltage conversion process, which generates heat energy and causes the module to heat up, reducing the power conversion efficiency. This will affect the normal operation of the power module and may affect the performance of other surrounding devices. This situation needs to be checked immediately. So under what circumstances will the power module heat up seriously? The specific reasons are as follows:
l A linear power module is used;
l Load overcurrent;
l The load is too small: if the load power is less than 10% of the module power output power, it may cause the module to heat up (the efficiency is too low);
l The ambient temperature is too high or the heat dissipation is poor.
The heating power module under the observation of the thermal imager is shown in Figure 4:
Figure 4 Thermal image of power module
For this type of problem, it can be improved by optimizing the external environment or adjusting the load, as shown below:
l When using a linear power supply, a heat sink should be added;
l Increase the load of the power module to ensure that it is not less than 10% of the rated load;
l Reduce the ambient temperature and maintain good heat dissipation.
7. The module power supply is damaged quickly
It goes without saying that there is a more serious abnormal situation than the power module heating, that is, the power module is directly damaged. So the power module is damaged not long after use, and it is broken again a few days after replacement. What is the reason? First of all, we need to rule out the use of inferior power supply. Then what other factors may cause this problem? The specific reasons are shown in Figure 5 below:
l The output load is too light, which reduces its reliability;
l The output capacitance is too large, causing damage to the module when it starts;
l The input voltage is too high for a long time, causing the switch tube at the input end of the module to be damaged.
Figure 5 Power module damage
This type of problem is also caused by load mismatch, which can be improved by changing the output load, capacitor or changing the appropriate input voltage, as follows:
l Ensure that the output end is not less than 10% of the rated load. If there is no-load phenomenon in the actual circuit operation, connect a dummy load of 10% of the rated power to the output end;
l Select capacitors that meet the specifications in the power module technical manual;
l Select the appropriate input voltage.
8. The power module burns out quickly after powering on
Compared with the previous case of power module damage, the more terrible situation is that not only the power supply is damaged, but also the entire circuit is burned. The specific phenomenon is that the power module burns and smokes as soon as it is powered on, and the capacitor at the input end explodes, as shown in Figure 6. This type of problem is the most serious and needs to be avoided as much as possible in the early design. So if this situation has already occurred, what exactly caused it? The details are as follows:
Figure 6 Power module burnt out
l The input voltage polarity is reversed;
l The input voltage is much higher than the nominal voltage;
l The polarity capacitor at the output end is connected in reverse;
l The output circuit is prone to short circuit or the external load has a large current at the moment of power-on.
This type of problem is the most serious fault, and the circuit needs to be rechecked for corresponding optimization or voltage adjustment, as shown below:
l Before wiring, please check or add reverse connection protection circuit;
l Select the appropriate input voltage;
l Check the polarity of the capacitor before powering on to ensure it is correct;
l Add short-circuit protection to the output end of the power module.
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