Column: Electric Vehicles
It’s time to eliminate the 12V battery to increase EV performance
By Pat Kowalyk, Field Application Engineer, Vicor
a conventional battery when there’s a 400V or 800V battery already in it to power the motors? Today, 12V and 48V batteries power
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all the other systems in the vehicle, but they add cost and weight, in addition to using valuable space. So, removing the 12V battery and using the 400-800V one instead should be enough to power all the systems in the car, right? The simple answer is that many
automotive systems, especially safety systems, must respond quickly to sudden changes in power, and typically batteries have much better response times than DC-DC power converters – until recently.
ou may be surprised to find a conventional 12V lead-acid battery in electric vehicles (EVs); why does an EV need
A fresh look New EVs consume up to twenty times more power (from 3kW to over 50kW) than combustion engines. To date, “progress” has meant adding more and higher-powered batteries to cars, but it is time for a fresh look. A typical load in a vehicle will have
two types of current draw: one for starting up and another for steady-state operation. This could either be raw or existing power from which a signal can be derived. The loads that use raw power will draw
large amounts of current, either to charge a capacitor or turn an armature. After being energised (start-up), the current drops and the load operates continuously (steady state). This initial current draw is what makes the battery a good option for legacy ICE vehicles but not EVs, where weight dramatically impacts range and
performance. Thus, it makes sense to eliminate the heavy lead-acid or lithium 12V battery and replace it with a lighter, compact and high-performance DC-DC converter.
High-performance power modules Replacing a 12V vehicle battery with a traditional converter may cause the load voltage to drop so low that the load turns off, causing a reboot. A key parameter to look at is the load voltage deviation during a change in current over time, also known as transient response. The lower the voltage deviation, the higher the system performance. A modular power approach combined
with topologies such as Vicor’s Sine Amplitude Converter (SAC) enables a much higher slew rate of a 12V lead- acid battery; see Figure 1. Using SAC can provide thousands of amperes from the high-voltage battery to the load, eliminating dips or loss of regulation. Automobile manufacturers typically
require 250A/ms for their fastest loads, which 12V batteries can achieve (75A/30µs). The Vicor modular approach provides transient response of 75A/10µs, thus creating a “virtual battery” that responds three times faster than 12V. Te SAC’s turns ratio, called the “K
Figure 1: Transient test comparison: 48V-to-12V at 75A, compared to a 12V battery 14 October 2022
www.electronicsworld.com
factor”, is the primary-to-secondary-turns ratio. A key advantage to SAC is that any primary-side capacitance is multiplied by the K factor squared. For a 12V-to- 48V conversion, the K factor is ¼, which means the effective secondary capacitance is 42, or 16 times the primary capacitance. Te Vicor NBM is an ideal converter
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