Space dictates a different design for DC-DC converters

Limited space requires DC-DC converters designed with thermal performance in mind, advises Timur Uludag of Würth Elektronik.An example of a typical industrial environment with multiple electronic applications and rail voltages is a warehouse.

Industrial warehouses can accommodate complex conveyor systems with many sensors, actuators, and control units. Each of these applications has its own requirements in terms of power supply voltage, current, and installation. For example, a surveillance camera with a microcontroller unit and an image sensor requires a voltage of 3.3 v and a current in the ampere range. Cameras are installed in different locations, which are usually not directly connected to the DC bus and have very limited installation space. Industrial-grade inductance sensors are usually connected directly to the DC bus.

The next step is to evaluate the fundamentals of the switching behavior of DC-DC power modules to address design challenges.

Switch behavior

Battery-powered devices do not always operate at full load. For example, a measurement application shows a high current demand during a measurement and a low current demand between measurement instances. These two load scenarios are common. Under light load, applications work in idle or standby mode, reducing energy consumption. Under heavy load, the application works under nominal conditions, which results in nominal energy consumption. The power management system must be flexible enough to provide optimal performance and efficiency under each load. Most industrial power supplies have a forced PWM mode. For this type of application, this is desirable because the power supply operates under heavy load conditions for most of its life. However, applications such as sensors have different load conditions. In this case, light load is the main running condition. Therefore, the switch behavior must be modified to perform optimally under this load. Pulse frequency modulation (PFM) mode provides higher efficiency values as the load current decreases. This supports longer battery life in battery-powered devices.

Working in PFM mode, the inductor current will burst switch. During each burst, the load and output capacitors are energized. During idle time (the time between two bursts) , the high-and low-end switches are turned off. This allows the output capacitor to provide its own load current. The energy expenditure between the two bursts drops sharply until the feedback system triggers the next burst. On the other hand, the efficiency of PFM mode is obviously higher than that of conventional PWM mode because of less switching loss. The idle time is inversely proportional to the load current, that is, the greater the load current, the shorter the time between burst actions. When the idle time approaches zero, the module switches back to PWM mode from PFM, returning a constant switching behavior at the default switching frequency of 4 Mhz. The peak inductance current in PFM mode is higher than that in PWM mode, which allows the same amount of energy to be provided to the load in a fixed period of time, while reducing the loss in the converter. In the idle time of burst mode, compared with PWM mode, the module does not produce any loss.

If the output voltage is above a certain value, both switches remain closed. The output capacitor is the only source of power for the load, which means that the output voltage decreases over time. When the output voltage drops to a certain level, the next burst cycle begins. The resulting value of the ripple depends on the threshold set inside the controller IC. The output voltage ripple in PFM mode can be reduced by simply increasing the output capacitance.