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DC/DC converter output for HEV apps gets a boost with MCU

20 Jun 2014  | Masahiro Nakano

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The market for eco-friendly cars such as hybrid vehicles (HVs) and electric vehicles (EVs) see better days ahead due to concerns about climate change and increasing gas prices.

For propulsion, these vehicles use an electric motor powered by a high-voltage lithium-ion battery. For best efficiency, the powerful DC motor drive voltage is usually boosted to several hundred volts by a DC/DC inverter. Meanwhile, HVs and EVs typically use the same electronic control units (ECUs) as gasoline-powered vehicles for functions like body control and instrument clusters. That means they are designed to run on a 12V power supply. As a result, the DC/DC converter also is used to step down the Li-ion battery's few hundred volts of output to standard 12VDC.

MCU

Figure 1: In an eco-friendly vehicle, an inverter boosts the output of a lithium-ion battery to several hundred volts DC (orange) to run the electric motor and also steps down the output to 12VDC to run the ECU (light blue). The CANbus network is shown in dark blue.

The ECU performs mission-critical tasks, so the DC/DC converter is essential to the safe operation of a vehicle. The converter also must be efficient. It needs to deliver a stable power supply to the ECU, even with any battery voltage fluctuations. The circuit design must be highly accurate, efficient, and compact enough for a space-constrained vehicle system. Above all, it must be reliable, since an ECU malfunction could lead to failure, damage, and even injury or death.

A DC/DC converter is only as good as its controller, however. New technology successfully reduces component count using the digital processing capabilities of an MCU. It flexibly and easily supports system requirements using the software platform. It also can send failure information to the driver via the vehicle network, which simplifies vehicle maintenance.


Digital advantage

To better understand how an MCU can impact the operation and functionality of a digital DC/DC converter, consider the Spansion MB91550. The MCU can enable two simultaneous control loops: for example, one for output voltage and one for the current (see figure 2). Running on a clock speed of up to 200MHz, it can generate a highly accurate PWM waveform that is key to maintaining precise control. Phase-adjusting the cycle of PWM output of the converter allows it to perform power factor control (PFC). The MCU also supports the functional safety requirement defined by ISO 26262.

 MB91550 microcontroller

Figure 2: With the aid of a slope-compensation unit and a comparator, the MB91550 microcontroller can control a digital DC/DC converter to deliver a highly efficient system.

Let's take a closer look at the process. A 12bit A/D converter digitises the voltage, then a 32bit CPU with FPU sets a reference of comparator and a slope compensation value designed to buck the voltage down to the target level (see figure 3). This slope-compensation value set passes through a 10bit D/A converter and is sent to the slope compensation unit of the current feedback loop. After processing by the CPU, the current passes to a comparator that checks for deviation from some preset reference value. Depending on the result, the converter adjusts the duty cycle of the PWM output for the present switching cycle. If the current is above the reference value that signifies an over-current condition, the device switches to the off state while revising the commanded PWM output at the same time.


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