“Most intermediate bus converters (IBCs) achieve isolation from input to output using large transformers. They also typically require an Inductor for output filtering. These converters are commonly used in datacom, telecom, and medical distributed power architectures. There are numerous suppliers of these IBCs, typically in industry standard 1/16, 1/8 and 1/4 brick wall packages.
Author: Bruce Haug
Most intermediate bus converters (IBCs) achieve isolation from input to output using large transformers. They also typically require an inductor for output filtering. These converters are commonly used in datacom, telecom, and medical distributed power architectures.There are numerous suppliers of these IBCs, often using industry standards1/16,1/8and1/4Brick wall package. For a typical IBC, its rated input voltage is 48 V or 54 V, the output intermediate voltage range is 5 V to 12 V, and the output power varies from several hundred watts to several kilowatts. The intermediate bus voltage is used as the input to the point-of-load regulator, which is used to drive FPGAs, microprocessors, ASICs, I/Os, and other low-voltage downstream devices.
However, in many newer applications, such as 48 V direct conversion applications, isolation is not necessary in the IBC because the upstream 48 V or 54 V input is already isolated from the hazardous mains. In many applications, the use of non-isolated IBCs requires a hot-swappable front-end device. As a result, many new applications are designed with non-isolated IBCs integrated, which can significantly reduce solution size and cost while increasing 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 a single-stage buck converter for 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 for this, with efficiencies around 97% when operating at a lower switching frequency of 150 kHz. When the LTC3891 operates at higher frequencies, its efficiency drops because the MOSFET switching losses increase when the input voltage is a higher 48 V.
New innovative controller design method combines a switched capacitor converter with a synchronous buck converter. A switched capacitor circuit reduces the input voltage by a factor of 2 and feeds it into a synchronous buck converter. This technique reduces the input voltage by half and then down to the target output voltage, enabling a much higher switching frequency, thereby increasing efficiency or significantly reducing the size of the solution. Other advantages include lower switching losses, lower MOSFET voltage stress, and reduced EMI due to the inherent soft switching characteristics of switched capacitor front-end converters. Figure 2 shows how this combination forms a hybrid buck synchronous controller.
Figure 2. A switched capacitor and a synchronous buck converter combine to form 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 DC-DC converter solutions by up to 50% compared to traditional buck converter alternatives. This performance improvement is due to its ability to triple the switching frequency without compromising efficiency. In other words, an LTC7821-based solution can be 3% more efficient when operating at the same frequency. In addition, the device features a soft-switching front end with low electromagnetic interference (EMI) benefits, making it ideal for next-generation non-isolated intermediate bus applications in power distribution, datacom and telecommunications, and emerging 48 V automotive systems.
Operating over an input voltage range of 10V to 72V (80 V absolute maximum), the LTC7821 can generate output currents in the tens of amps, depending on the choice of external components. The switching frequency of the external MOSFET is fixed and can be programmed 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 by one-third, which necessitates the use of larger magnetics and output filtering components. The LTC7821 features powerful 1 Ω N-channel MOSFET gate drivers that maximize efficiency while driving multiple MOSFETs in parallel for higher power applications. In addition, the device’s current-mode control architecture allows 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 without hot spots.
The LTC7821 implements several protection functions to maintain robust performance in a wide variety of applications. Designs based on the LTC7821 also pre-balance the capacitors at startup, eliminating the inrush currents that often occur 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. Additionally, an onboard timer can be used to set an appropriate restart/retry time. EXTV of LTC7821CCThe pins can be connected to the lower voltage output of the converter or other available power supply (up to 40 V) for power, reducing power consumption and improving efficiency. Other features include: ±1% output voltage accuracy over temperature; clock output for multiphase operation; power good output indication; short circuit protection; output voltage monotonic startup; optional external reference; undervoltage lockout; and internal charge balancing circuitry. Figure 3 is a schematic of the LTC7821 converting a 36 V to 72 V input to a 12 V/20 A output.
Figure 3. LTC7821 Schematic (36VINto 72VIN/12V/20A output).
The efficiency curve shown in Figure 4 is a comparison of the performance of three different types of converters in the same application. The function of this application is to convert the 48VINConvert to 12VOUT/20 A, as follows:
• Single-stage buck operating at 125 kHz with 6 V gate drive (blue curve)
• Single-stage buck operating at 200 kHz with 9 V gate drive (red curve)
• LTC7821 hybrid buck synchronous controller operating at 500 kHz with 6 V gate drive (green curve)
Figure 4. Efficiency comparison versus transformer size reduction.
The LTC7821-based circuit operates at up to three times the frequency of other converters with the same efficiency as other solutions. At this higher operating frequency, the inductor size can be reduced by 56%, and the size of the entire solution can be reduced by up to 50%.
Switched-capacitor converters typically experience high inrush currents that can damage the power supply when the input voltage is applied or when the converter is enabled. The LTC7821 integrates a proprietary mechanism to precharge all switched capacitors before the converter PWM signal is enabled. This minimizes 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 enable a smooth soft-start of the output voltage, just like any other conventional current-mode buck converter. Refer to the LTC7821 data sheet for details.
main control loop
As soon as the capacitor balancing phase ends, normal operation begins immediately. MOSFETs M1 and M3 turn on when the clock sets the RS latch and turn off when the main current comparator ICMP resets the RS latch. Then, M2 and M4 of the MOSFET are turned on. The inductor peak current at the ICMP responsible for resetting RS is controlled by the voltage on the ITH pin, which is the output of the error amplifier EA. VFBThe pin receives a voltage feedback signal, which the EA compares to an internal voltage reference. As the load current increases, the result is a VFBA slight dip relative to the 0.8 V reference, again resulting in ITHThe voltage is increased until the average current of the inductor matches the new load current. After the M1 and M3 of the MOSFET are turned off, the M2 and M4 of the MOSFET are turned on until the next cycle starts. During the switching process of M1/M3 and M2/M4, the capacitor CFLYwill alternate with CMIDseries or parallel. The voltage at 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 limiting is faster, more accurate, and supports the current sharing option.
A synchronous buck converter (hybrid converter) followed by a switched capacitor circuit for halving the input voltage can increase the maximum DC-DC converter solution performance compared to traditional buck converter alternatives 50% reduction in size. This performance improvement is due to its ability to triple the switching frequency without compromising efficiency. It is also possible to increase the converter’s operating efficiency by 3%, at which point its size is comparable 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 precision current sharing.
Bruce Haug graduated from San Jose State University in 1980 with a bachelor’s degree in electrical engineering. He joined Linear Technology (now part of Analog Devices) in April 2006 as a Product Marketing Engineer. Previously, Bruce held positions at Cherokee International, Digital Power and Ford Aerospace. He is also keen to get involved in sports.
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