Intermediate bus converters (IBC) are designed for use in implementing the intermediate bus architecture (IBA) that helps in saving the space of the PCB. IBA is a power distribution scheme that gives more options to the designers to reduce the size of the board and use low-cost point of load (POL) converters. In this scheme, all individual converters do not have to convert between the main voltages supplied to the end voltages of the load.
For example, suppose there is a main power supply of 48 V, and there are several loads that work between 3 V to 6 V. According to the traditional approach that involves single-step conversion, all individual loads will have converters that will have to convert from 48 V to the lower voltages required by the loads. With the IBA scheme, a two-step conversion will take place. The first step will be to convert the 48 V to 6 V. Then, the individual POL converters will receive 6 V to convert them to the desired load voltages. In this way, all converters will not have to convert from 48 V to 6 V to 3 V. They will only convert between 6 V to 3 V, according to the load power requirement.
How the IBA approach saves PCB space
The conversion from higher input voltages requires a large voltage process to ensure that there is a reasonable margin between the device's operating range and the breakdown. This will ensure that voltage spikes generated during commutation of the power switches will not fail the device. There will be a need for more space between the drain, source and gate of the transistors, so it will make the device size large.
In comparison to this, the IBA approach lowers the high input voltage to an intermediate lower voltage. Then the low voltage converters are required to step down the voltages further, according to the power requirements of each load. These voltage converters do not need internal circuitry to handle the high input voltages, thus they are available in small sizes at a reasonable price.
Apart from this, an inductor is used to minimize the ripples in the output. When stepping down from high DC voltages to low, there are two possible ways to minimize the ripples in the output: a higher switching frequency or higher inductance. A high switching frequency leads to more power losses and hence decreases the efficiency. Therefore, the feasible option is to go for the high inductance. A high inductance leads toward the large size of the inductor because there will be a need for more winding around the magnetic core of the inductor. This problem could be solved with the help of IBC. A designer can step down the high DC voltages to low in the first step. Hence, there will be no need to increase the size of the inductors at each POL.
Comparison IBA vs single-step power distribution
Two-step conversion has some advantages over single-step conversion. The IBA approach is less expensive because of the low-cost inductors and low-cost POL converters. On the other side, the single-step conversion approach is expensive due to the large inductors and higher voltage process technology. Another advantage of using IBA is the smaller size of the PCB. The disadvantage of using IBA is lower efficiency as compared to the single-step conversion approach.
How to improve efficiency using the IBA approach
The efficiency of the IBA approach depends upon IBC and it can be improved if a high efficient intermediate converter is used. It is also possible to design the IBC on low current rails that will reduce wattage lost.
The IBA approach reduces the efficiency of the system due to the two-step conversion, but there are many applications where it is more important to save space than to consider reduced efficiency.
Keynotes for a design engineer while selecting IBC
The design engineer needs to select an IBC that will not compromise system efficiency below to a certain level. The selected IBC should optimize system cost and stability. There are three basic types of IBC that a designer could choose based on what suits his or her design most: these are fixed ratio, regulated and semi-regulated IBC. A fixed ratio IBC can offer high efficiency (nearly 98%) which means there are only 2% losses during conversion. The output of these IBCs is a fixed fraction of the input voltages. These IBCs are capable of handling only a short range of input voltages with no regulation. Regulated IBC are capable of handling a wide range of input voltages with regulation. The drawback of these IBCs is a lower efficiency. Semi-regulated IBCs are also capable of handling a wide range of inputs, but they provide regulation up to a specific range of input voltages. They have high efficiency as compared to the regulated IBCs.
It is recommended that a design engineer compares the competitive benefits and tradeoffs of selecting one type of IBC over the other.
When to consider the IBA power distribution scheme
The IBA scheme has lower efficiency compared to the single-step conversion approach. If there are fewer than three individual loads, then it is recommended to use a single-step conversion approach. A design engineer can still go for IBA if there is a need for a small size and less cost design. While there is a relaxation in efficiency, the IBA scheme is highly recommended when there are three or more individual loads that require individual POL converters. When the number of individual rails increases, the benefits a designer could get in terms of saving cost and reducing size also increase.
Conclusion
IBA is a new method that a design engineer could use to save PCB space. This approach has lower efficiency that could be overcome by using high efficient IBC. The cost and space-saving benefits are much more significant than experiencing less efficiency.
