FEATURE COVER STORY
Non-isolated IBCs significantly reduce the solution size and cost of power device architechure
Bruce Haug, product marketing manager, Power by Linear Products, Analog Devices, Inc. discusses how the latest 72V hybrid DC/DC reduces intermediate bus converter size by up to 50%
The LTC3891 from Analog Devices can be used for this approach and provides an efficiency of about 97% when operating at a relatively low 150kHz switching frequency. When operating this device at higher frequencies it will result in a lower efficiency due to the MOSFET switching losses that occur with the relative high 48V input voltage.
M
ost intermediate bus converters (IBC) 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 an industry standard 1/16th, 1/8th and ¼ brick footprints. A typical IBC has a nominal input
voltage of 48V or 54V and produces a lower intermediate voltage between 5V to 12V with 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, called “48V Direct”, isolation is not necessary in the IBC since the upstream 48V or 54V input is already isolated from the hazardous AC mains. In many applications, a hot swap front-end device is required to use a non-isolated IBC. As a result, non-isolated IBCs are being
designed into many new applications, which significantly reduce 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 non-isolated 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 36V to 72V and produce a 5V to 12V output voltage.
Figure 1:
Typical distributed power architecture
A NEW APPROACH An innovative approach 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 a 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 to 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.
Figure 2.
Switched capacitor + synchronous buck = LTC7821 hybrid converter
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/DC converter solution size compared to traditional buck converter alternatives. This improvement is enabled by a three times higher switching frequency without compromising efficiency. Alternatively, when operating at the same frequency, a LTC7821-based solution can provide up to a 3% higher efficiency. Other benefits include low EMI
emissions due to a soft-switched frontend ideal for the next generation of non-isolated intermediate bus
12 APRIL 2018 | ELECTRONICS
/ ELECTRONICS
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