POWER Frederik Dostal, field applications engineer, Analog Devices
Cascaded and Hybrid Concepts for Voltage Conversion
T
here are different solutions for applications that require conversion from a high input voltage down to a very low output voltage. One interesting example is the conversion from 48V down to 3.3V. Such a specification is not only common in server applications for the information technology market, but in telecommunications as well. If a step-down converter (buck) is used for this single conversion step, as shown in Figure 1, the problem of small duty cycles emerges. The duty cycle is the relationship between the on-time, when the main switch is turned on, and the off-time, when the main switch is turned off. A buck converter has a duty cycle, which is defined by the following formula:
Figure 1. Conversion of a voltage from 48V down to 3.3V in one single conversion step.
With an input voltage of 48V and an output voltage of 3.3V, the duty cycle is approximately 7 percent.
This means that at a switching frequency of 1MHz, or 1000ns per switching period, the Q1 switch is turned on for only 70ns. The Q1 switch is then turned off for 930ns and Q2 is turned on. For such a circuit, a switching regulator must be chosen that allows for a minimum on-time of 70ns or less. If such a component is selected, there is another challenge. Usually, the high-power conversion efficiency of a buck regulator is reduced when operating at short duty cycles. This is because there is only a short time available to store energy in the inductor. The
inductor needs to provide power for a long period during the off time. This typically leads to extremely high peak currents in the circuit. To lower these currents, the inductance of L1 needs to be relatively large. This is because during the on-time, a large voltage difference is applied across L1 in Figure 1. In the example, we see about 44.7V across the inductor during the on-time, 48V on the switch-node side and 3.3V on the output side. The inductor current is calculated by the following formula:
If there is a high voltage across the inductor, the current rises during a fixed period and at a fixed inductance. To reduce inductor peak currents, a higher inductance value needs to be selected. However, a
higher value inductor adds to increased power losses. Under these voltage conditions, an efficient LTM8027 µModule regulator from Analog Devices achieves power efficiency of only 80 percent at 4A output current. Today, a common and more efficient circuit solution to increase the power efficiency is the generation of an intermediate voltage. A cascaded setup with two highly efficient step-down (buck) regulators is shown in Figure 2. In the first step, the voltage of 48V is converted to 12V. This voltage is then converted down to 3.3V in a second conversion step. The LTM8027 µModule regulator has a total conversion efficiency of more than 92 percent when going from 48V down to 12V. The second conversion step from 12V down to 3.3V, performed with a LTM4624, has a conversion efficiency of 90 percent. This yields a total power conversion efficiency of 83 percent; 3 percent higher than the direct conversion in Figure 1.
Figure 2. Voltage conversion from 48V down to 3.3V in two steps, including a 12V intermediate voltage. 38 OCTOBER 2024 | ELECTRONICS FOR ENGINEERS
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