Background
Most intermediate bus converters (IBCs) provide isolation from input to output with the use of a bulky power transformer. They also normally require an inductor for output filtering. This type of converter is commonly used in datacom, telecom, and medical distributed power architectures. These IBCs are available from a wide variety of suppliers and are typically housed in industry-standard 1/16th, ⅛th, and ¼th brick footprints. A typical IBC has a nominal input voltage of 48 V or 54 V and produces a lower intermediate voltage between 5 V to 12 V with an output power level from several hundred watts to several kilowatts. The intermediate bus voltage is used as the input to point-of-load regulators that will power FPGAs, microprocessors, ASICs, I/O, and other low voltage downstream devices.
However, in many new applications, such as a 48 V direct conversion application, isolation is not necessary in the IBC since the upstream 48 V or 54 V input is already isolated from the hazardous ac mains. In many applications, a hot swap front-end device is required to use a nonisolated IBC. As a result, nonisolated IBCs are being designed into many new applications, which significantly reduces the solution size and cost while also increasing the operating efficiency and providing design flexibility. A typical distributed power architecture is shown in Figure 1.
Now that nonisolated conversion is allowed in some distributed power architectures, one could consider using the single stage buck converter for this application. It would need to operate over an input voltage range from 36 V to 72 V and produce a 5 V to 12 V output voltage. The LTC3891 from Analog Devices can be used for this approach and provides an efficiency of about 97% when operating at a relatively low 150 kHz switching frequency. Operating the LTC3891 at higher frequencies results in lower efficiency due to the MOSFET switching losses that occur with the relatively high 48 V input voltage.
A New Approach
The new innovative approach to controllers combines a switched capacitor converter with a synchronous buck. The switched capacitor circuit reduces the input voltage by a factor of two and then feeds into the synchronous buck. This technique of reducing the input voltage in half and then bucking down to the desired output voltage results in higher efficiency or a much smaller solution size by operating at a much higher switching frequency. Other benefits include lower switching losses and reduced MOSFET voltage stress due the inherent soft switching characteristic of the switched capacitor front-end converter resulting in lower EMI. Figure 2 shows how this combination has formed the hybrid step-down synchronous controller.
New High Efficiency Converter
The LTC7821 merges a switched capacitor circuit with a synchronous step-down converter, enabling up to a 50% reduction in dc-to-dc converter solution size compared to traditional buck converter alternatives. This improvement is enabled by a 3× higher switching frequency without compromising efficiency. Alternatively, when operating at the same frequency, an LTC7821-based solution can provide up to a 3% higher efficiency. Other benefits include low EMI emissions due to a soft-switched front end ideal for the next generation of nonisolated intermediate bus applications in power distribution, datacom, and telecom, as well as emerging 48 V automotive systems.
The LTC7821 operates over a 10 V to 72 V (80 V abs max) input voltage range and can produce output currents in multiple tens of amps, depending on the choice of external components. External MOSFETs switch at a fixed frequency, programmable from 200 kHz to 1.5 MHz. In a typical 48 V to 12 V/20 A application, an efficiency of 97% is attainable with the LTC7821 switching at 500 kHz. The same efficiency can only be achieved in a traditional synchronous step-down converter by switching at ⅓rd the operating frequency, forcing the use of much larger magnetics and output filter components. The LTC7821’s powerful 1 Ω N-channel MOSFET gate drivers maximize efficiency and can drive multiple MOSFETs in parallel for higher power applications. Due to its current-mode control architecture, multiple LTC7821s can be operated in a parallel, multiphase configuration with excellent current sharing and low output voltage ripple to enable much higher power applications without hot spots.
The LTC7821 implements many protection features for robust performance in a wide range of applications. An LTC7821-based design also eliminates the inrush current typically associated with switched capacitor circuits by prebalancing the capacitors on startup. The LTC7821 also monitors the system voltage, current, and temperature for faults and uses a sense resistor for overcurrent protection. It stops switching and pulls the FAULT pin low when a fault condition occurs. An onboard timer can be set for appropriate restart/retry times. Its EXTVCC pin permits the LTC7821 to be powered from the lower voltage output of the converter or other available sources up to 40 V, reducing power dissipation and improving efficiency. Additional features include ±1% output voltage accuracy over temperature, a clock output for multiphase operation, a power good output signal, short circuit protection, monotonic output voltage startup, optional external reference, undervoltage lockout, and internal charge balance circuitry. Figure 3 shows the schematic of the LTC7821 when converting a 36 V to 72 V input to a 12 V/20 A output.
The efficiency curves shown in Figure 4 represent a comparison of three different converter types for the application that converts a 48 VIN to a 12 VOUT at 20 A as follows:
- A single stage buck running at 125 kHz with a 6 V gate drive (blue curve)
- A single stage buck running at 200 kHz with a 9 V gate drive (red curve)
- The LTC7821 hybrid running at 500 kHz with a 6 V gate drive (green curve)
An LTC7821-based circuit running at up to 3× the operating frequency of the other converters has the same efficiency as the other solutions. This higher operating frequency results in a 56% reduction of the inductor size and up to a 50% reduction to the total solution size.
Capacitor Prebalancing
A switched capacitor converter usually has a very high inrush current when the input voltage is applied or when the converter is enabled, resulting in the possibility of supply damage. The LTC7821 has a proprietary scheme to precharge all switching capacitors before the converter PWM signal is enabled. Therefore, the inrush current during power up is minimized. In addition, the LTC7821 also has a programmable fault protect window to further ensure reliable operation of the power converter. These features result in the output voltage having a smooth soft start just like any other conventional current mode buck converter. See the LTC7821 data sheet for additional details.
Main Control Loop
Once the capacitor balancing phase is completed, normal operation begins. MOSFETs M1 and M3 are turned on when the clock sets the RS latch, and turned off when the main current comparator, ICMP, resets the RS latch. MOSFETs M2 and M4 are then turned on. The peak inductor current at the ICMP that resets the RS latch 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, which is compared to the internal reference voltage by the EA. When the load current increases, it causes a slight decrease in VFB relative to the 0.8 V reference, which in turn causes the ITH voltage to increase until the average inductor current matches the new load current. After MOSFETs M1 and M3 have turned off, MOSFETs M2 and M4 are turned on until the beginning of the next cycle. During the switching of M1/M3 and M2/M4, capacitor CFLY is alternately connected in series with or parallel to CMID. The voltage at MID will be approximately at VIN/2. So, this converter just operates like a conventional current-mode buck converter with a fast and accurate cycle-by-cycle current limit and option for current sharing.
Conclusion
The combination of a switched capacitor circuit to halve the input voltage followed by a synchronous step-down converter (hybrid converter) provides up to a 50% reduction in dc-to-dc converter solution size compared to traditional buck converter alternatives. This improvement is enabled by a 3× higher switching frequency without compromising efficiency. Alternatively, the converter can operate with 3% higher efficiency in a footprint comparable to existing solutions. This new hybrid converter architecture also provides other benefits that include soft switching for reduced EMI and MOSFET stress. Multiple converters can be easily paralleled with active accurate current sharing when high power is needed.
