“An interesting example is switching from 48 V to 3.3 V. Such specifications are not only common in server applications in the information technology market, but also in telecommunication applications. If a buck converter (buck) is used for this single conversion step, as shown in Figure 1, the problem of small duty cycle occurs.
For applications that require conversion from high input voltages to very low output voltages, there are different solutions.
An interesting example is switching from 48 V to 3.3 V. Such specifications are not only common in server applications in the information technology market, but also in telecommunication applications.
If a buck converter (buck) is used for this single conversion step, as shown in Figure 1, the problem of small duty cycle occurs.
Figure 1. Voltage reduction from 48 V to 3.3 V in a single conversion step
The duty cycle reflects the relationship between the on time (when the main switch is on) and the off time (when the main switch is off). The duty cycle of a buck converter is defined by:
When the input voltage is 48 V and the output voltage is 3.3 V, the duty cycle is about 7%.
This means that at a switching frequency of 1 MHz (1000 ns per switching period), the on-time of the Q1 switch is only 70 ns. Then, the Q1 switch is turned off for 930 ns and Q2 is turned on. For such a circuit, a switching regulator must be chosen that allows a minimum on-time of 70 ns or less. Choosing such a device presents another challenge.
Typically, the conversion efficiency of a buck regulator decreases when operating at a very small duty cycle. This is because the time available to store energy in the Inductor is very short. Inductors need to provide energy during longer off-times. This usually results in very high peak currents in the circuit. To reduce these currents, the inductance of L1 needs to be relatively large. This is due to the fact that a large voltage difference is applied across L1 in Figure 1 during the on-time.
In this example, the voltage across the inductor during the on-time is approximately 44.7 V, the voltage on one side of the switch node is 48 V, and the voltage at the output is 3.3 V. The inductor current is calculated by the following formula:
If there is a high voltage across the inductor, the current in the inductor will rise for a fixed time with the same inductance. In order to reduce the peak inductor current, it is necessary to choose a higher inductor value. However, higher inductance values increase power losses. Under these voltage conversion conditions, Analog Devices’ high-efficiency LTM8027 µModule® regulator module achieves only 80% conversion efficiency at 4 A output current.
Currently, a very common and more efficient circuit solution to increase conversion efficiency is to utilize an intermediate voltage. Figure 2 shows a cascaded setup using two high-efficiency buck regulators. The first step is to convert the 48 V to 12 V, then in the second conversion step this voltage is converted to 3.3 V. The total conversion efficiency of the LTM8027μModule regulator module exceeds 92% when dropping from 48 V to 12 V. The second conversion step utilizes the LTM4624 to reduce 12 V to 3.3 V with a conversion efficiency of 90%. The total conversion efficiency of this scheme is 83%, which is 3% higher than the direct conversion efficiency in Figure 1.
Figure 2. Voltage from 48 V to 3.3 V in Two Steps, Including a 12 V Intermediate Voltage
This can be quite surprising since all the power on the 3.3 V output needs to go through two separate switching regulator circuits. The circuit shown in Figure 1 is less efficient due to the short duty cycle, which results in a high peak inductor current.
In addition to conversion efficiency, there are many other aspects to consider when comparing single-step buck architectures with intermediate bus architectures.
Another solution to this fundamental problem is the LTC7821, ADI’s new hybrid buck controller, which combines a charge pump with buck regulation. This results in a duty cycle of 2x VIN/VOUT, thus enabling very high step-down ratios at very high conversion efficiencies.
Figure 3 shows the circuit setup for the LTC7821. It is a hybrid synchronous buck controller that combines a charge pump to halve the input voltage and a synchronous buck converter in a buck topology. When using it to convert 48V to 12V at 500 kHz switching frequency, the conversion efficiency exceeds 97%. Other architectures can only achieve such high efficiency at much lower switching frequencies and require larger inductors.
Figure 3. Circuit Design of Hybrid Buck Converter
Four external switching transistors are required. During operation, capacitors C1 and C2 perform a charge pump function. The voltage generated in this way is converted into a precisely regulated output voltage by a synchronous buck function. In order to optimize the EMC characteristics, the charge pump adopts soft switching operation.
The combination of charge pump and buck topology has the following advantages:
The conversion efficiency is very high due to the optimized combination of charge pump and synchronous switching regulator. External MOSFETs M2, M3 and M4 only need to withstand low voltage. The circuit is also compact. The inductor is smaller and less expensive than the single-stage converter approach. For this hybrid controller, the duty cycle of switches M1 and M3 is D = 2×VOUT/VIN. For M2 and M4, the duty cycle is D = (VIN C 2 × VOUT)/VIN.
For charge pumps, many developers assume a power output limit of around 100 mW. Circuits designed with the LTC7821’s hybrid converter switch can deliver up to 25 A of output current. For higher performance, multiple LTC7821 controllers can be connected in a parallel polyphase configuration and frequency synchronized to share the overall load.
Figure 4. Typical conversion efficiency of 48 V to 5 V at 500 kHz switching frequency
Figure 4 shows typical conversion efficiencies for 48 V input voltage and 5 V output voltage at different load currents. At about 6A, the conversion efficiency exceeds 90%. Between 13 A and 24 A, the efficiency is even higher than 94%.
Hybrid buck controllers offer very high conversion efficiencies in a compact form. It offers another interesting solution compared to discrete two-stage switching regulator designs that use intermediate bus voltages, and single-stage converters that operate at very low duty cycles. Some designers prefer cascading architectures, while others prefer hybrid architectures. Using both options, every design should be successful.