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The output generated will show the complete schematic, bill of materials (external component listings), and typical estimated performance data like efficiency and duty-cycle variation at different operating points. WEBENCH software can generate additional information such as the complete list of suggested components and basic performance estimates, as shown in Figure 10.
Test and measurement of your power converter
When troubleshooting (or simply verifying the proper operation of) a power-supply circuit, you should follow some basic practices to ensure accurate measurements. Because DC/DC switch-mode converters involve fast switching devices (power FETs used in the regulation circuits), as well as large current and voltage transients, improper technique can easily lead to misleading measurements. For example, the input power supply for a circuit being tested on a lab bench might be located a foot or two away from the test board. If current is flowing through a few feet of cable, there can be 10 s or even 100 s of millivolts difference from one end of the cable to the other. This amount of voltage drop can have a noticeable effect on the (perceived) efficiency of the circuit. In general, use larger wires (good conductors with low loss) and shorter lengths of cable wherever possible. When measuring fast signals such as the gate drive or switching nodes in a DC/DC converter, even the conventional 2- to 3-inch-long ground lead on an oscilloscope probe can lead to misleading indications of output ripple and/or transient response ringing. The articles in References 6 and 7 explain some of the procedures associated with testing a power circuit in greater detail.
Power losses, thermal management and basic thermal calculations
While an ideal power conversion circuit is always 100 percent efficient, any real system will have some amount of loss (inefficiency). “Loss” in this case means that some portion of the energy taken from the input is not delivered to the output – and thus translates to heat generated within the power-supply components. Some typical sources of power loss are:
When power loss occurs, it results in heat generation within the semiconductors and passive components. Depending on the physical design of a system (IC packaging, PCB layout and trace widths, airflow, etc.), a given amount of electrical power loss (Watts dissipated) will translate to a specific amount of heat generated (temperature increase above ambient).
For reliable operation as well as prevention of any potential user safety issues, semiconductor operating temperatures need to stay below the recommended maximum values (e.g. 125ºC). Although in most cases, you will not be able to directly measure the actual semiconductor (die) temperature (because the IC is encapsulated within an external package), there are methods of estimating internal die temperature based on (measurable and/or specified) external temperatures on the PCB surface or external IC package locations.
Figure 11 shows a photograph of a typical mid-power DC/DC converter board next to the thermal image of the circuit during operation. For an ambient temperature of 20ºC, you can see that specific components (power FETs, power inductor) may actually be as much as 50ºC hotter due to operating power losses. This implies that to prevent internal component temperatures from exceeding a specified maximum such as 125ºC, the overall circuit needs to be in an ambient environment of less than 70ºC for reliable operation. See References 8 and 9 for further explanation of the basic thermal calculations required to ensure safe and reliable power-circuit operation. Table 2 lists part numbers for commonly used power management devices.
Table 2.
References
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