Background information
Most Intermediate Bus Converters (IBC) use a bulky power transformer to provide isolation from input to output. In addition, they generally require an Inductor for output filtering. This type of converter is often used in data communications, telecommunications, and medical distributed power architectures. These IBCs can be provided by many suppliers, and can usually be placed within the industry standard 1/16, 1/8, and 1/4 brick footprint. A typical IBC has a nominal input voltage of 48V or 54V and produces a lower intermediate voltage between 5V and 12V and output power levels from a few hundred W to a few kW. The intermediate bus voltage is used as the input of the point-of-load regulator and will be responsible for powering FPGAs, microprocessors, ASICs, I/Os, and other low-voltage downstream devices.
However, in many new applications called “48V Direct”, there is no need for isolation in the IBC, because the upstream 48V or 54V input is already isolated from the dangerous AC power source. In many applications, hot-swappable front-end equipment requires the use of a non-isolated IBC. Therefore, a built-in non-isolated IBC is designed in many new applications, which significantly reduces the solution size and cost, while also improving work efficiency and providing design flexibility. Figure 1 shows a typical distributed power architecture.
Figure 1: Typical distributed power architecture
Since non-isolated conversion is allowed in some distributed power architectures, single-stage step-down converters can be considered for this application. It will need to operate within a 36V to 72V input voltage range and produce a 5V to 12V output voltage. The LTC3891 provided by Analog Devices can be used in this method, which provides approximately 97% efficiency when operating at a relatively low 150kHz switching frequency. When LTC3891 works at a higher frequency, due to the relatively high 48V input voltage and MOSFET switching losses, the efficiency will drop.
A new method
An innovative method combines a switched capacitor converter with a synchronous buck. The switched capacitor circuit reduces the input voltage by half and feeds it into the synchronous buck converter. This method of halving the input voltage and then stepping down to the desired output voltage can achieve higher efficiency, or by making the device operate at a much higher switching frequency, the solution size can be greatly reduced. Other benefits include lower switching losses and reduced MOSFET voltage stress, which benefit from the inherent soft switching characteristics of switched-capacitor front-end converters, which can achieve lower EMI. Figure 2 shows how this combination constitutes a hybrid step-down synchronous controller.
Figure 2: Switched capacitor + synchronous step-down = LTC7821 hybrid converter
New high-efficiency converter
The LTC7821 combines a switched capacitor circuit with a synchronous step-down converter, which can reduce the solution size of the DC/DC converter by as much as 50% compared to other traditional step-down converter alternatives. This improvement is achieved by increasing the switching frequency by a factor of 3, without sacrificing efficiency. Or, when working at the same frequency, the LTC7821-based solution can provide an efficiency increase of up to 3%. Other advantages include low EMI radiation (due to the use of a soft-switching front end), which is ideal for power distribution, data communications and telecommunications, and new generation non-isolated intermediate bus applications in emerging 48V automotive systems.
The LTC7821 operates in an input voltage range of 10V to 72V (80V absolute maximum) and can generate output currents of tens of amperes, depending on the choice of external components. The external MOSFET performs switching operations at a fixed frequency (settable range from 200kHz to 1.5MHz). In a typical 48V to 12V / 20A conversion application, an efficiency of 97% can be obtained when the switching frequency of the LTC7821 is 500kHz. The traditional synchronous buck converter can only achieve the same efficiency by performing switching operations at 1/3 of the operating frequency, so much larger magnetic components and output filter components have to be used. The LTC7821’s powerful 1Ω N-channel MOSFET gate driver maximizes efficiency and can drive multiple parallel MOSFETs to meet the requirements of higher power applications. Because the device uses a current-mode control architecture, multiple LTC7821s can work in a parallel multi-phase configuration, which uses its excellent current sharing capability and low output voltage ripple to achieve much higher power applications. Generate hot spots.
The LTC7821 can perform many protection functions to achieve powerful performance in a wide range of applications. The design based on LTC7821 also eliminates the inrush current usually caused by switched capacitor circuits by pre-balancing the capacitors at startup. In addition, LTC7821 also monitors the system voltage, current and temperature to find faults, and uses a detection resistor to provide overcurrent protection. When a certain fault condition occurs, the device stops switching and pulls the /FAULT pin to a low level. A built-in timer can be set for an appropriate restart/retry time. Its EXTVCC pin allows the LTC7821 to rely on the converter’s lower voltage output or other available power supplies up to 40V, thereby reducing power consumption and improving efficiency. Other features include ±1% output voltage accuracy (over the entire temperature range), a clock output for multi-phase operation, a power-good output signal, short-circuit protection, monotonic output voltage startup, and optional external reference , Undervoltage lockout and internal charge balance circuit. Figure 3 shows the schematic diagram of the circuit when the LTC7821 is used to convert a 36V to 72V input to a 12V/20A output.
Figure 3: LTC7821 application circuit schematic diagram, 36VIN~72VIN to 12V/20A output
The efficiency curve in Figure 4 compares the efficiency levels of three different types of converters for the application of converting 48VIN to 12VOUT/20A output, as follows:
1. Single-stage step-down voltage running at 125kHz, using 6V gate drive voltage (blue curve)
2. Single-stage step-down voltage running at 200kHz, using 9V gate drive voltage (red curve)
3. LTC7821 hybrid step-down operating frequency of 500kHz, using 6V gate drive voltage (green curve)
Figure 4: Efficiency comparison and transformer size reduction
The LTC7821-based circuit can provide the same efficiency as other similar solutions when the operating frequency is three times higher than the operating frequency of other converters. This higher operating frequency has resulted in a 56% reduction in inductor size, while the overall solution size has been reduced by as much as 50%.
Capacitor pre-balance
When the input voltage is applied or the converter is activated, the switched capacitor converter usually has a very large inrush current, which may cause damage to the power supply. The LTC7821 uses a proprietary scheme to pre-balance all switched capacitors before enabling the converter PWM signal. As a result, the surge current during power-up is minimized. In addition, LTC7821 also has a programmable fault protection window to further ensure the reliable operation of the power converter. These characteristics enable the output voltage to achieve a smooth soft start, just like any other traditional current-mode step-down converter. For more details, please refer to the LTC7821 product manual.
Main control loop
Once the capacitor balancing phase is complete, normal operation begins. MOSFETs M1 and M3 are turned on when the RS latch is set by the clock, and turned off when the main current comparator ICMP resets the RS latch. The MOSFETs M2 and M4 are then turned on. The peak inductor current when ICMP 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, and the EA compares the signal with the internal reference voltage. When the load current increases, it causes a slight drop in VFB relative to the 0.8V reference, which in turn causes the ITH voltage to increase until the average inductor current matches the new load current. After the MOSFETs M1 and M3 are turned off, the MOSFETs M2 and M4 are turned on until the beginning of the next cycle. During the switching period of M1/M3 and M2/M4, the capacitor CFLY is alternately connected in series or in parallel with CMID. The voltage on the MID will be approximately VIN/2. Therefore, this converter works like a traditional current-mode converter, and has a fast and accurate cycle-by-cycle current limit function and options for current sharing.
in conclusion
Combining the switched capacitor circuit for halving the input voltage with a synchronous step-down converter (hybrid converter) that follows it, the DC/DC converter solution size can be compared to other traditional step-down converters Converter alternatives have dropped by as much as 50%. This improvement is achieved by increasing the switching frequency by a factor of 3, without sacrificing efficiency. Alternatively, the converter can also achieve a 3% increase in work efficiency while occupying a board area similar to the existing solution. This new hybrid converter architecture also offers other advantages, including soft switching to reduce EMI and MOSFET stress. When high power is required, its active and accurate current sharing capability can be used to easily connect multiple converters in parallel.
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