Column: Electric Vehicles
Utilising zero-current, zero-voltage switching (ZCS/ZVS), the
BCM converters operate at higher frequencies than conventional converters; for example, the BCM6135 operates at 1.2MHz and, unlike a conventional ZV/ZC resonant converter, it operates within a narrow-band frequency. Te BCM’s high-frequency operation provides a fast response to changes in load current and a low-impedance path from input to output. Fixed-ratio conversion, bi-directional operation, fast transient response and a low impedance path are the set of qualities that enable the BCM to make a 384V battery appear like a 48V battery – we’ll call this “transformation”. Tis ability to transform a power source is both the key benefit and key difference when compared to conventional converters.
Power source transformation Te BCM transforms an input voltage into an output voltage by a fixed ratio scaling, mathematically expressed as VOUT
Figure 2: Functional block diagram of BCM converter
= K ∙ VIN
.
Consider a 48V power distribution system drawing power from a high-voltage battery charged to 384V. Te loads on the 48V bus have an input voltage range that is a fixed fraction of the battery’s output. An isolated BCM (1/8) converts the output of the high- voltage direct current (HVDC) battery into a range of voltages compatible with 48V distribution. Due to the fast response time of the BCM, from the perspective of any load on the low side, the 384V battery appears like a battery discharging at 48V. Now consider the same application but with a conventional
converter, which regulates the input in the voltage range to a specific output voltage that is decoupled from changes on the input. Voltage fluctuations on the input do not propagate to the regulated output. Te low bandwidth of the regulated converter prevents the distribution system from delivering power as quickly as a direct connection to the battery. From the point of view of the low side, there is only an idealised 48V supply voltage. While there is utility in this conversion, there are two relative weaknesses: First, the lower bandwidth necessitates some additional intermediate energy storage (either capacitative or an extra battery) to supply current during a high dI/dt discharge event. Second, a regulation stage is unnecessary because the input voltage of the loads on the low side is a fixed fraction of the battery voltage on the high side. Te conventional converter is needlessly regulating – wasting energy, adding cost and reducing overall system efficiency. In addition, the limited bandwidth of the regulated converter degrades the reaction time needed by fast power draws in the distribution system. Designing a system where all voltage ranges – for sources, loads
and various distribution paths – are fixed ratios of each other allows optimal selection of best-in-class technology for power storage, power distribution and subsystem capabilities needed in high-performance EV power architectures. Tese systems use Li-ion batteries (arrayed for capacity and high voltage for fast charging time), distribute power at 48V (per the LV148V specification for SELV power distribution) and use a mix of legacy 12V cost-effective subsystems alongside the latest 48V powered AI technology. BCMs bridge all these voltages into a single high- efficiency system.
Figure 3: Transformation of a high-voltage battery
Figure 4: Decoupling of a 48V source from a high-voltage battery
Figure 5: EV power architecture
A virtual 48V battery architecture EV power architectures can use BCMs to create a highly efficient and lightweight power system. Te high-voltage battery arrays, which are the primary energy storage unit, are stepped down
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