Easily solve common application problems of DC-DC power modules

Micro powerDC-DC power moduleWith multiple advantages such as high integration, high reliability, and simplified design, it is favored by many electronic designers. Although the power module has simple application circuit and simple operation, it often encounters some common problems in application. In view of this article, a detailed analysis of the common application problems of the power supply module and how to troubleshoot is carried out, hoping to help the designer in the selection of the power supply module.

Common problem 1: The output ripple noise is too large

Reason 1: When the module is in use, the load is a dynamic load, which makes the peak-to-peak output voltage of the module larger, but note that this is not ripple noise.

When the load current undergoes periodic sudden changes, the peak-to-peak value of the output voltage of the module will become larger. This is a transient quantity, but it is sometimes mistaken for ripple noise. Therefore, when using a power module to supply power to multiple circuit units, for circuits with periodic load changes, the front stage needs to add π-type filtering to reduce the interference of transient changes in this part of the circuit to other circuits.

For example, circuit B in the figure below causes the input voltage to fluctuate due to the change in load size. In order to reduce the interference of circuit B to circuit A, it is recommended to add π-type filtering at the input of circuit B.

Reason 2: Oscilloscope ground wire problem

When testing the ripple noise of the power supply output, the ground clamp of the oscilloscope, the ground wire, and the output pin of the module form a loop, which is similar to an antenna receiver and introduces other noise. If the test environment has large interference, this noise will also be introduced by the oscilloscope, which will affect the results of the ripple noise test.

And usually the ground of the oscilloscope probe we buy is connected to the ground wire inside the oscilloscope. In this case, the anti-interference ability of power frequency interference is weak and it is easy to introduce interference noise. Therefore, it is best to ensure that the oscilloscope probe is floating (isolate the power ground of the oscilloscope or directly use a battery-powered oscilloscope) to reduce the interference introduced. If the power supply of the measurement object is also floating, this is better, so that it will not cause changes in circuit characteristics and increase the output noise of the module.

Common problem 2: After the module is started, the output voltage is low

Reason 1: There is an anti-reverse circuit at the input end

Example: The ZY_FKES-3W module in Figure 2 is a constant-voltage input non-regulated output module, and its output voltage will vary with the input voltage and load size. In the circuit design, an anti-reverse diode is added on the input side, so this will cause the voltage to the input of the module to decrease, and thus the output voltage becomes smaller. Because when we design and use anti-reverse diodes, we must consider the forward voltage drop of the diode.

Reason 2: The impedance of the output wire is too large or the connection of the voltmeter is not standardized

When using the power supply module, we are testing the output voltage and often try to directly test the voltage at the input end of the powered circuit. However, because the impedance between the output of the module and the input of the powered circuit is too large, the measured value will be lower than the actual value. Therefore, when testing the output voltage of the power supply module, the voltage between the output pins of the module should be measured, not the voltage at the input end of the powered circuit.

Reason 3: When using the module, the required power of the power module is not accurately estimated, which makes the module in an overload start or overload working state.

For example, the ZY_FKES-3W power supply module with constant voltage input and unregulated output has a rated power of 3W, that is, the power range used by the module cannot exceed 3W. The output voltage of ZY0505FKES-3W module under 5V input and 400mA output is about 4.98V; at 600mA output, the output voltage is about 4.88V. If it is overloaded, the output voltage will be lower. As the load increases, the output voltage will decrease. This is the characteristic of the constant voltage input and unregulated output module.

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