Charge pump or conventional converter? Hybrid converters are superior with high efficiency and small size
Latest update time：2020-01-06
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Data Center and Telecommunications
Power system design has changed dramatically. Major application manufacturers are replacing complex and expensive isolated 48 V/54 V step-down converters with more efficient non-isolated high-density step-down regulators (Figure 1). No isolation is required in the regulator's bus converter because the upstream 48 V or 54 V input is already isolated from the hazardous AC mains.
图 1. 传统的电信板电源系统架构带有隔离式总线转换器。在48V已经与交流电源隔离的系统中，无需使用隔离式总线转换器。使用非隔离混合式转换器取代隔离式转换器可显著简化设计、降低成本和电路板空间要求
For high input/output voltage applications (48V to 12V), conventional buck converters are not ideal solutions due to the typically larger components required. That is, the buck converter must operate at a low switching frequency (e.g., 100kHz to 200kHz) to achieve high efficiency at high input/output voltages. The power density of the buck converter is limited by the size of the passive components, especially the inductor. The inductor size can be reduced by increasing the switching frequency, but the losses caused by switching will reduce the converter efficiency and cause unacceptable thermal stress.
与基于电感的传统降压型转换器相比，开关式电容转换器(电荷泵)可显著提高效率并缩小解决方案尺寸。在电荷泵中，采用飞跨电容代替电感以存储能量并将其从输入端传递到输出端。电容的能量密度远高于电感，因此与降压型稳压器相比，可将功率密度提高10倍。但是，电荷泵是分数型转换器(它们不能调节输出电压)并且无法扩展以适用于高电流应用。
The hybrid converter based on LTC7821 combines the advantages of traditional buck converters and charge pumps: output voltage regulation, scalability, high efficiency and high density. The hybrid converter regulates the output voltage through closed-loop control, just like a buck converter. Through peak current mode control, the hybrid converter can be easily expanded to higher current levels (for example, from a single-phase design of 48V to 12V/25A to a 4-phase design of 48V to 12V/100A).
All switches in a hybrid converter are subject to only half the input voltage in steady-state operation, so low-rated voltage MOSFETs can be used to achieve high efficiency. The switching losses in a hybrid converter are lower than those in a conventional buck converter, allowing high-frequency switching.
In a typical 48 V to 12 V/25 A application, the LTC7821 achieves over 97% full load efficiency at 500 kHz switching frequency. To achieve the same efficiency using a traditional step-down controller, it must run at one-third the frequency, resulting in a much larger solution size. The higher switching frequency allows the use of smaller inductors, resulting in faster transient response and smaller solution size (Figure 2).
Figure 2. Size comparison of a traditional non-isolated buck converter and a hybrid converter (48 V to 12 V/20 A)
The LTC7821 is a peak current mode hybrid converter controller that provides the functions 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 centers and telecom systems. Key features of the LTC7821 include:
-
Wide V IN Range: 10 V to 72 V (80 V Absolute Maximum)
-
Phase-lockable fixed frequency: 200 kHz to 1.5 MHz
-
Integrated Quad 5 V N-Channel MOSFET Drivers
-
R SENSE or DCR Current Sense
-
Programmable CCM, DCM or Burst Mode® Operation
-
CLKOUT pin for multiphase operation
-
Short circuit protection
-
EXTV CC input for increased efficiency
-
Monotonic output voltage start-up
-
32-pin (5 mm × 5 mm) QFN package
Figure 3 shows a 300 W hybrid converter using the LTC7821 with a switching frequency of 400 kHz. The input voltage range is 40 V to 60 V, the output voltage is 12 V, and the maximum load is 25 A. Twelve 10 µF (1210 size) ceramic capacitors are used for both the flying capacitors C
FLY
and C
MID
. Because the switching frequency is high and the inductor sees only half of V
IN
at the switch node
(small volt-second value), a relatively small size 2 µH inductor (SER2011-202ML, 0.75 in × 0.73 in) can be used. As shown in Figure 4, the solution size is approximately 1.45 in × 0.77 in, and the power density is approximately 640 W/in
3
.
Figure 3. 48 V to 12 V/25 A hybrid converter using the LTC7821.
Figure 4. A complete bus converter is laid out using both sides of the board, using only 2.7 cm2 of the board's topside.
Because the bottom three switches always see only half the input voltage, 40 V rated FETs can be used. The top switch uses an 80 V rated FET because it sees the input voltage at the beginning of C
FLY
and C
MID
pre-charging during startup (no switching). During steady-state operation, all four switches see only half the input voltage. Therefore, the switching losses of the hybrid converter are much smaller than those of a buck converter where all switches see the full input voltage. Figure 5 shows the efficiency of the design. The peak efficiency is 97.6% and the full load efficiency is 97.2%. Due to its high efficiency (low power loss), the thermal performance is excellent, as shown in the thermal image in Figure 6. At an ambient temperature of 23°C and without forced air cooling, its hot spot temperature is 92°C.
Figure 5. Efficiency at 48 V input, 12 V output, and 400 kHz fSW
Figure 6. Thermal image of the hybrid converter solution in Figure 2.
The LTC7821 uses a unique C
FLY
and C
MID
pre-balancing technique to prevent input surge current during startup. During initial power-up, the voltages across the flying capacitors C
FLY
and C
MID
are measured
. If either of these voltages is not V
IN
/2, the TIMER capacitor is allowed to charge. When the voltage across the TIMER capacitor reaches 0.5 V, the internal current source turns on to bring the C
FLY
Y voltage to V
IN
/2. After the CFLY voltage reaches V
IN
/2, C
MID
is
charged to V
IN
/2. During this time, the TRACK/SS pin is pulled low and all external MOSFETs are turned off. If
the voltage across
C
FLY
and C
MID
reaches V
IN
/2 before the TIMER capacitor voltage reaches 1.2 V, TRACK/SS is released and normal soft-start begins. Figure 7 shows this pre-balancing period, and Figure 8 shows the V
OUT
soft-start
at 48V input and 12V/25A output
.
Figure 7. LTC7821 pre-balancing period at startup avoids high inrush current
Figure 8. LTC7821 startup at 48 V input, 12 V/25 A output (no high inrush current)
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. The PLLIN pin of one LTC7821 and the CLKOUT pin of another LTC7821 are connected together to synchronize the PWM signals.
Figure 9. LTC7821 key signal connections for a 2-phase design.
For designs with more than two phases, daisy-chain the PLLIN pin and the CLKOUT pin. 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, while the odd phases are out of phase with the even phases.
Figure 10 shows a 4-phase 1.2 kW hybrid converter. The power stage per phase is the same as the single-phase design in Figure 3. The input voltage range is 40 V to 60 V, and the output is 12 V with a maximum load of 100 A. Its peak efficiency is 97.5% and 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 200 LFM forced air cooling, its hot spot is 81°C. Inductor DCR sensing is used in this design. As shown in Figure 13, the current sharing among the 4 phases is very balanced.
Figure 10. 4-Phase 1.2 kW Hybrid Converter Using Four LTC7821s
Figure 11. Efficiency of a 4-phase 1.2 kW design
Figure 12. Thermal image of the multiphase converter shown in Figure 9.
Figure 13. Current sharing in the multiphase converter shown in Figure 9.
LTC7821 是一款峰值电流模式的混合式转换器控制器，能够以创新的方式实现数据中心和电信系统的中间总线转换器简化解决方案。混合式转换器中的所有开关都只会接收到一半输入电压，从而显著降低了高输入/输出电压应用中的开关相关损耗。因此，混合式转换器支持的开关频率可高出降压型转换器 2 至 3 倍，且不影响效率。混合式转换器可轻松扩展，以支持更高电流应用。较低的整体成本和易扩展性使混合式转换器比传统的隔离式总线转换器更胜一筹。
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