Based on the LTC7821 design, the DC-DC converter solution can be reduced in size by 50%
Most intermediate bus converters (IBCs) provide isolation from the input to the output through large transformers. They also typically require an inductor for output filtering. Such converters are commonly used in data communications, telecommunications, and medical distributed power architectures. These IBC suppliers are numerous and are typically available in industry standard 1/16, 1/8 and 1/4 brick wall packages. For a typical IBC, the rated input voltage is 48 V or 54 V, the output intermediate voltage range is 5 V to 12 V, and the output power ranges from a few hundred watts to several thousand watts. The intermediate bus voltage is used as an input to the point-of-load regulator, and the point-of-load regulator is used to drive FPGAs, microprocessors, ASICs, I/O, and other low-voltage downstream devices.
However, in many new applications, such as 48 V direct conversion applications, there is no need for isolation in the IBC because the upstream 48 V or 54 V input is already isolated from dangerous mains. In many applications, to use a non-isolated IBC, a hot-swappable front-end device is required. As a result, many new applications integrate non-isolated IBCs at design time, which can significantly reduce the size and cost of the solution while improving conversion efficiency and design flexibility. A typical distributed power supply architecture is shown in Figure 1.
Figure 1. Typical distributed power architecture.
Since some distributed power architectures support non-isolated conversion, we can consider using a single-stage buck converter in this application. The converter has an input voltage range of 36 V to 72 V and an output voltage range of 5 V to 12 V. The LTC3891 from Analog Devices can be used in this application, with an efficiency of around 97% when operating at a lower switching frequency of 150 kHz. When the LTC3891 is operating at a higher frequency, its efficiency will decrease because the MOSFET switching losses will increase when the input voltage is higher at 48 V.
new method
The new innovative controller design approach combines a switched-capacitor converter with a synchronous buck converter. The switched capacitor circuit reduces the input voltage by a factor of 2 and feeds it into a synchronous buck converter. This technique first reduces the input voltage by half and then drops to the target output voltage, supporting a much higher switching frequency, thereby increasing efficiency or significantly reducing the size of the solution. Other advantages include lower switching losses and lower MOSFET voltage stress because the switched capacitor front-end converter has inherent soft switching characteristics that reduce EMI. Figure 2 shows how this combination forms a hybrid buck synchronous controller.
Figure 2. A switched capacitor and a synchronous buck converter combined into an LTC7821 hybrid converter.
New high efficiency converter
The LTC7821 combines a switched-capacitor circuit with a synchronous buck converter to reduce the size of a DC-DC converter solution by up to 50% compared to traditional buck converter alternatives. This performance improvement is due to its ability to increase the switching frequency by a factor of three without compromising efficiency. In other words, the LTC7821-based solution can increase efficiency by 3% when operating at the same frequency. In addition, the device features a soft-switch front-end with low electromagnetic interference (EMI) advantages, making it ideal for power generation, datacom and telecom, and next-generation non-isolated intermediate bus applications in emerging 48 V automotive systems.
Operating from an input voltage range of 10V to 72V (absolute maximum of 80 V), the LTC7821 can generate tens of amps of output current, depending on the choice of external components. The switching frequency of the external MOSFET is fixed and can be set from 200 kHz to 1.5 MHz. In a typical 48 V to 12 V/20 A application, the LTC7821 achieves 97% efficiency at 500kHz switching frequency. The only way to achieve this efficiency in a traditional synchronous buck converter is to reduce the operating frequency to one-third, which requires the use of larger magnetic components and output filtering components. The LTC7821 features a powerful 1 Ω N-channel MOSFET gate driver that maximizes efficiency while driving multiple MOSFETs in parallel for higher power applications. In addition, the device uses a current-mode control architecture, allowing multiple LTC7821s to operate in a parallel, multi-phase configuration to support high-power applications with excellent current sharing control and low output voltage ripple in the absence of hot spots.
The LTC7821 implements multiple protection features to maintain robust performance across a wide range of applications. The LTC7821-based design also pre-balances the capacitor at startup to eliminate the inrush current that is common in switched-capacitor circuits.
The LTC7821 also monitors system voltage, current, and temperature faults and uses sense resistors for overcurrent protection. When a fault occurs, it stops switching and pulls the FAULT pin low. In addition, the onboard timer can be used to set the appropriate restart/retry time. The LTC7821's EXTVCC pin can be connected to the converter's lower voltage output or other available power supply (up to 40 V) to reduce power consumption and increase efficiency. Other features include: ±1% output voltage accuracy over temperature range; clock output for multiphase operation; power good output indication; short circuit protection; monotonic output voltage; optional external reference; undervoltage lockout; And an internal charge balancing circuit. Figure 3 shows the schematic of the LTC7821 when converting a 36 V to 72 V input to a 12 V/20 A output.
Figure 3. LTC7821 schematic (36VIN to 72VIN/12V/20 A output).
The efficiency curve shown in Figure 4 is a comparison of the performance of three different converters in the same application. The purpose of this application is to convert 48VIN to 12VOUT/20 A, as follows:
Single stage step-down with 125 kHz operation with 6 V gate drive voltage (blue curve)
Single stage step-down with 200 kHz operating frequency with 9 V gate drive voltage (red curve)
LTC7821 Hybrid Buck Synchronous Controller Operating at 500 kHz with 6 V Gate Drive Voltage (Green Curve)
Figure 4. Efficiency comparison and transformer size reduction.
The LTC7821-based circuit operates at the same frequency as three times the other converters, and its efficiency is the same as other solutions. At this higher operating frequency, the inductor size can be reduced by 56% and the overall solution size can be reduced by up to 50%.
Capacitor pre-balancing
Switched-to-capacitor converters typically experience high inrush currents when applying an input voltage or when the converter is enabled, which can damage the power supply. The LTC7821 integrates a proprietary mechanism to precharge all switched capacitors before the converter PWM signal is enabled. This minimizes the inrush current during power-up. In addition, the LTC7821 has a programmable fault protection window to further ensure reliable operation of the power converter. These features allow the output voltage to achieve a smooth soft start, just like any other conventional current mode buck converter. Please refer to the LTC7821 data sheet for details.
Main control loop
As soon as the capacitor balancing phase is over, normal operation begins immediately. M1 and M3 of the MOSFET are turned on when the clock sets the RS latch, and are turned off when the main current comparator ICMP resets the RS latch reset. Then, M2 and M4 of the MOSFET are turned on. The peak inductor current at the ICMP responsible for resetting the RS is controlled by the voltage on the ITH pin, which is the output of the error amplifier EA. The VFB pin receives the voltage feedback signal and the EA compares this signal to the internal reference. As the load current increases, the result is a slight decrease in VFB relative to the 0.8 V reference voltage, which in turn causes the ITH voltage to increase until the average current of the inductor matches the new load current. After M1 and M3 of the MOSFET are turned off, M2 and M4 of the MOSFET are turned on until the next cycle begins. During M1/M3 and M2/M4 switching, the capacitor CFLY will alternate in series or parallel with the CMID. The voltage at the MID is approximately equal to VIN/2. It can be seen that this converter works in the same way as a conventional current mode buck converter, except that the cycle-by-cycle current limit is faster, more accurate, and supports current sharing options.
in conclusion
Install a synchronous buck converter (hybrid converter) after a switched-capacitor circuit that halve the input voltage, which is the highest DC-DC converter solution compared to conventional buck converter alternatives The size is reduced by 50%. This performance improvement is due to its ability to increase the switching frequency by a factor of three without compromising efficiency. It is also possible to increase the efficiency of the converter by 3%, which is comparable in size to existing solutions. This new hybrid converter architecture has other advantages, including soft switching characteristics that help reduce EMI and MOSFET stress. When high power is required, multiple converters can be easily connected in parallel for active and accurate current sharing.
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