Hybrid converters outperform traditional isolated bus converters!

Data center and telecom power system designs have changed a lot. Major application manufacturers are replacing complex and expensive isolated 48V/54V step-down converters with more efficient non-isolated high-density step-down regulators (Figure 1). Isolation is not required in the regulator’s bus converter because the upstream 48V or 54V input is already isolated from the hazardous AC power source.

Data center and telecom power system designs have changed a lot. Major application manufacturers are replacing complex and expensive isolated 48V/54V step-down converters with more efficient non-isolated high-density step-down regulators (Figure 1). Isolation is not required in the regulator’s bus converter because the upstream 48V or 54V input is already isolated from the hazardous AC power source.

Hybrid converters outperform traditional isolated bus converters!
Figure 1. Traditional telecom board power system architecture with isolated bus converters. In systems where the 48V is already isolated from the AC mains, there is no need to use an isolated bus converter.Replacing isolated converters with non-isolated hybrid converters significantly simplifies design, reduces cost and board space requirements

For high input/output voltage applications (48V to 12V), traditional buck converters typically require larger components and are not an ideal solution. That is, buck converters must operate at low switching frequencies (eg, 100kHz to 200kHz) in order to achieve high efficiency at high input/output voltages. The power density of a buck converter is limited by the size of passive components, especially the size of the Inductor. Inductor size can be reduced by increasing the switching frequency, but losses due to switching reduce converter efficiency and cause unacceptable thermal stress.

Switched capacitor converters (charge pumps) can significantly improve efficiency and reduce solution size compared to traditional inductor-based buck converters. In a charge pump, a flying capacitor is used instead of an inductor to store energy and transfer it from the input to the output. The energy density of capacitors is much higher than that of inductors, so compared to buck regulators, the power density can be increased by a factor of 10. However, charge pumps are fractional converters (they cannot regulate the output voltage) and cannot scale for high current applications.

The LTC7821-based hybrid converter combines the advantages of a traditional buck converter and a charge pump: output voltage regulation, scalability, high efficiency, and high density. A hybrid converter regulates the output voltage through closed-loop control, just like a buck converter. With peak current mode control, hybrid converters can be easily scaled to higher current levels (eg, from a 48V to 12V/25A single-phase design to a 48V to 12V/100A 4-phase design).

All switches in a hybrid converter withstand half the input voltage in steady-state operation, allowing the use of low voltage rated MOSFETs for high efficiency. Hybrid converters have lower losses due to switching than conventional buck converters, enabling high-frequency switching.

In typical 48V to 12V/25A applications, the LTC7821 can achieve over 97% full load efficiency at 500kHz switching frequency. Achieving the same efficiency with a traditional buck controller would have to run at one-third the frequency, resulting in a much larger solution size. Higher switching frequencies allow the use of smaller inductors, resulting in faster transient response and smaller solution size (Figure 2).

Hybrid converters outperform traditional isolated bus converters!
Figure 2. Size comparison of conventional non-isolated buck and hybrid converters (48 V to 12 V/20 A)

The LTC7821 is a peak current mode hybrid converter controller that provides the functionality required for a complete solution of a non-isolated high efficiency, high density step-down converter suitable for use as an intermediate bus converter in data center and telecom systems . Key features of the LTC7821 include:

• Wide VINRange: 10V to 72 V (80V absolute maximum)
• Phase lockable fixed frequency: 200kHz to 1.5MHz
• Integrated Quad 5V N-Channel MOSFET Driver
• RSENSEor DCR current sense
• Programmable CCM, DCM or Burst Mode® Work
• CLKOUT pin for multiphase operation
• Short circuit protection
• EXTVCCinput for efficiency
• Monotonic output voltage startup
• 32-pin (5mm × 5mm) QFN package
• 48V to 12V/25A hybrid converter
• With 640W/IN3power density

Figure 3 shows a 300 W hybrid converter using the LTC7821 switching at 400 kHz. The input voltage range is 40V to 60V, the output voltage is 12V, and the maximum load is 25A. Flying capacitor CFLY and CMIDAll use twelve 10 µF (1210 size) ceramic capacitors. Because the switching frequency is high and the inductance bears only half the V at the switching nodeIN (small volt-second value), so a relatively small size 2µH inductor (SER2011-202ML, 0.75″ x 0.73″) can be used. As shown in Figure 4, the solution measures approximately 1.45 inches by 0.77 inches and has a power density of approximately 640W/in3.

Hybrid converters outperform traditional isolated bus converters!
Figure 3. 48V to 12V/25A Hybrid Converter Using the LTC7821
Hybrid converters outperform traditional isolated bus converters!
Hybrid converters outperform traditional isolated bus converters!
Figure 4. A complete bus converter is laid out using the front and back of the board, using only 2.7 cm2 of the front of the board

Because the three switches on the back always receive only half the input voltage, a 40V rated FET can be used. The top switch uses an 80V rated FET because during startup CFLY and CMIDWhen precharge begins (without switching), it receives the input voltage. During steady-state operation, all four switches receive only half of the input voltage. Therefore, the switching losses of a hybrid converter are much smaller than that of a buck converter in which all switches receive the full input voltage. Figure 5 shows the design efficiency. Peak efficiency is 97.6% and full load efficiency is 97.2%. Due to its high efficiency (low power loss), thermal performance is excellent, as shown in the thermal image in Figure 6. With an ambient temperature of 23°C and no forced air cooling, its hot spot temperature is 92°C.

Hybrid converters outperform traditional isolated bus converters!
Figure 5. Efficiency at 48V Input, 12V Output, and 400 kHz fSW
Hybrid converters outperform traditional isolated bus converters!
Hybrid converters outperform traditional isolated bus converters!
Figure 6. Thermal image of the hybrid converter solution in Figure 2

The LTC7821 uses a unique CFLY and CMID Pre-balance technology to prevent input inrush current during startup. During initial power-up, measure the flying capacitance CFLY and CMIDvoltage across both ends. If any of these voltages are not VIN /2, the TIMER capacitor is allowed to charge. When the voltage on the TIMER capacitor reaches 0.5V, the internal current source turns on so that CFLY Y voltage reaches VIN /2. in CFLY voltage reaches VIN After /2, put CMID Charge to VIN /2. During this time, the TRACK/SS pin is pulled low and all external MOSFETs are turned off. If before the TIMER capacitor voltage reaches 1.2 V, CFLY and CMID The voltage across the terminals has reached VIN/2, TRACK/SS is released, and normal soft start starts. Figure 7 shows this pre-balance period and Figure 8 shows V at 48V input, 12V/25A outputOUT soft start.

Hybrid converters outperform traditional isolated bus converters!
Figure 7. LTC7821 Startup Pre-Balance Cycle Avoids High Inrush Current

Hybrid converters outperform traditional isolated bus converters!
Figure 8. LTC7821 Startup at 48V Input, 12V/25A Output (No High Inrush Current)

1.2 kW Multiphase Hybrid Converter

The LTC7821 is easily scalable, making it ideal for high current applications such as those in telecom and data centers. Figure 9 shows the key signal connections for a 2-phase hybrid converter using multiple LTC7821s. Connect the PLLIN pin of one LTC7821 to the CLKOUT pin of the other LTC7821 to synchronize the PWM signals.

Hybrid converters outperform traditional isolated bus converters!
Figure 9. LTC7821 Key Signal Connections for 2-Phase Design

For designs with more than two phases, daisy-chain the PLLIN and CLKOUT pins. Since the clock output on the CLKOUT pin is 180° out of phase with the LTC7821’s master clock, the even phases are in phase with each other, and the odd and even phases are out of phase with each other.

Figure 10 shows a 4-phase 1.2kW hybrid converter. The per-phase power stage is the same as the single-phase design in Figure 3. The input voltage range is 40V to 60V, the output is 12V, and the maximum load is 100A. Its peak efficiency is 97.5%, and its full load efficiency is 97.1%, as shown in Figure 11. Its thermal performance is shown in Figure 12. With an ambient temperature of 23°C and 200LFM forced air cooling, its hot spot is 81°C. The design uses inductive DCR detection. As shown in Figure 13, the current sharing among the 4 phases is well balanced.

Hybrid converters outperform traditional isolated bus converters!
Figure 10. 4-Phase 1.2kW Hybrid Converter Using Four LTC7821s

Hybrid converters outperform traditional isolated bus converters!
Figure 11. Efficiency of 4-Phase 1.2kW Design

Hybrid converters outperform traditional isolated bus converters!
Figure 12. Thermal image of the multiphase converter shown in Figure 9

Hybrid converters outperform traditional isolated bus converters!
Figure 13. Current Sharing for the Multiphase Converter of Figure 9

in conclusion

The LTC7821 is a peak current mode hybrid converter controller that enables innovative solutions for intermediate bus converter simplification in data center and telecom systems. All switches in a hybrid converter receive only half the input voltage, significantly reducing switching-related losses in high input/output voltage applications. As a result, hybrid converters can support switching frequencies 2 to 3 times higher than buck converters without compromising efficiency. Hybrid converters are easily scalable to support higher current applications. Lower overall cost and easy scalability make hybrid converters superior to traditional isolated bus converters.

The Links:   NL6448BC20-18D CPT30040