Confused about cascading and mixing of voltage conversion? A few examples will make it clear
For applications that need to convert from a high input voltage to a very low output voltage, there are different solutions.
An interesting example is the conversion from 48 V to 3.3 V. Such specifications are common not only in server applications in the information technology market, but also in telecommunication applications.
If a step-down converter (buck) is used for this single conversion step, as shown in Figure 1, a problem with small duty cycle arises.
Figure 1. Stepping down the voltage 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 the following formula:
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 cycle), the Q1 switch is on for only 70 ns. The Q1 switch is then off for 930 ns and Q2 is on. For a circuit like this, a switching regulator must be selected that allows a minimum on-time of 70 ns or less. If such a device is selected, another challenge arises.
Typically, the conversion efficiency of a buck regulator decreases when operating at very small duty cycles. This is because the time available to store energy in the inductor is very short. The inductor needs to provide energy during a longer off-time. 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 because 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 the switch node is 48 V, and the output voltage is 3.3 V. The inductor current is calculated using the following equation:
If there is a high voltage across the inductor, the current in the inductor will rise in a fixed time if the inductance remains unchanged. To reduce the inductor peak current, a higher inductance value needs to be selected. However, a higher inductance value increases power losses. Under such voltage conversion conditions, ADI's high-efficiency LTM8027 µModule® regulator module only achieves 80% conversion efficiency at 4 A output current.
Currently, a very common and more efficient circuit solution to improve conversion efficiency is to use an intermediate voltage. Figure 2 shows a cascade setup using two high-efficiency buck regulators. The first step is to convert the 48 V voltage to 12 V, and then convert this voltage to 3.3 V in the second conversion step. When dropping from 48 V to 12 V, the LTM8027 μModule regulator module has a total conversion efficiency of more than 92%. The second conversion step uses the LTM4624 to drop 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. The voltage is reduced from 48 V to 3.3 V in two steps, including a 12 V intermediate voltage.
This may be quite surprising since all the power on the 3.3 V output needs to pass through two separate switching regulator circuits. The circuit shown in Figure 1 is less efficient because of the short duty cycle, which results in higher inductor peak current.
When comparing the single-step buck architecture to the intermediate bus architecture, there are many other aspects to consider besides conversion efficiency.
Another solution to this basic problem is to use the new hybrid buck controller LTC7821 from Analog Devices, which combines a charge pump with buck regulation. This enables 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 of the LTC7821. It is a hybrid synchronous step-down controller that combines a charge pump (to halve the input voltage) with a synchronous buck converter in a buck topology. It achieves over 97% efficiency at 500 kHz switching frequency to convert 48 V to 12 V. Other architectures can only achieve such high efficiency at much lower switching frequencies and require larger inductors.
Figure 3. Circuit design of a 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 to a precisely regulated output voltage by a synchronous buck function. To optimize the EMC characteristics, the charge pump uses 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 the charge pump and the synchronous switching regulator. The external MOSFETs M2, M3, and M4 only have to withstand low voltages. The circuit is also compact. The inductor is smaller and cheaper 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 – 2 × VOUT)/VIN.
For charge pumps, many developers assume a power output limit of approximately 100 mW. Circuits designed with hybrid converter switches using the LTC7821 can deliver output currents up to 25 A. For higher performance, multiple LTC7821 controllers can be connected in a parallel multiphase configuration and frequency synchronized to share the overall load.
Figure 4. Typical conversion efficiency for 48 V to 5 V at 500 kHz switching frequency.
Figure 4 shows the typical conversion efficiency for a 48 V input voltage and a 5 V output voltage at different load currents. At about 6 A, the conversion efficiency is over 90%. Between 13 A and 24 A, the efficiency is even higher than 94%.
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