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Electric Bicycle Range and DC/DC Converter Technology 2023

업데이트 시간: Mar 25, 2023 독자층: 2905

Warming, scarcity of natural resources and pollution are growing environmental problems that the world is facing. Electric bicycles are a green and energy-efficient alternative to traditional internal combustion engine vehicles and are one of the solutions to alleviate environmental problems.

China is the world's largest market for electric bicycles, with sales growing from 30.5 million units in 2017 to 49.8 million units in 2021, at a CAGR of 13.0%, and accounting for 75.9% of total global sales in 2021.

A direct current-direct current (DC/DC) converter is a direct current (DC) power conversion circuit. In different EV power configurations, at least one DC/DC converter is required to connect the battery to the DC transmission line. By temporarily storing the input power and then releasing it at a different voltage output, the DC/DC converter can change the DC voltage value from a certain voltage value to another different value.

The energy storage device is either a magnetic field type energy storage device (inductor, transformer) or an electric field type energy storage device (capacitor). The power flow between the input and output can be controlled by adjusting the duty cycle (the on/off time ratio of the switching tube).

This is done to control the output voltage, the input current, the output current or to maintain a constant power. In electric vehicles, the converters are primarily used to supply auxiliary load devices such as horns, LED lights and other peripherals.

ST's Advanced DC/DC Converter Empowers the Development of Electric Bicycles

To ease the range anxiety of electric vehicle users, the use of DC/DC converters is an ideal solution. ST has introduced the L3751, a new generation of highly integrated DC/DC converter controller that improves the switching performance of discrete power devices. ST's controller chip utilizes a high-performance gate driver to control the high-speed switching operation of MOSFETs, reducing the size of energy storage components and the amount of radio emission.

The L3751 is a general-purpose DC/DC synchronous conversion controller with an operating voltage range of 6V to 75V for a wide range of applications. With a minimum on-time of 40 nanoseconds and a very high voltage conversion ratio in the 100 kHz - 1 MHz switching frequency range, the new product saves external components of the application circuit and simplifies the development process while reducing the PCB size.

The controller uses pulse-hopping mode to maximize energy efficiency and effectively control output voltage ripple at light loads. The forced PWM (FPWM) mode maintains a constant switching frequency over the load range and minimizes output voltage ripple. The on-chip integrated gate driver minimizes the number of external components and is optimized for standard VTH MOSFETs. To ensure safety in critical applications such as electric bicycles, the controller has comprehensive built-in safety protection, including output overcurrent protection, input voltage UVLO protection, internal voltage monitoring, and thermal shutdown.

typical application circuit schematic

Figure 1: Typical Application Circuit Schematic

simplified block diagram

Figure 2: Simplified block diagram

L3751 Functional Description

The L3751 is a native buck converter controller based on a constant frequency voltage controlled architecture. The chip compares the output voltage VOUT detected by the feedback pin (FB) with the internal reference voltage (0.8V), then provides an error signal on the COMP pin, and finally controls the on-time of the external power switching tube based on the comparison of the COMP voltage value with the internal programmable frequency sawtooth wave. As shown in Figure 2, the main internal modules include

Voltage regulator : Supplies power to the internal circuitry and provides an internal fixed reference voltage. This module also implements a voltage monitoring circuit (UVLO) for monitoring the input voltage and the internal voltage.
Integrated external N-channel MOSFET driver and bootstrap capacitor circuitry: to power the high-side switch driver
Integrated sawtooth generation circuit: adjusts duty cycle according to internal programmable oscillator or input synchronization signal; input voltage feedforward circuit implemented to improve the controller's response to line voltage transients
Voltage mode error amplifier
PWM comparator and associated analog and digital circuitry: Driver for controlling the internal MOSFET
Soft-start circuitry and reference voltage clamp: used to limit inrush current during the start-up phase
Adjustable sensorless or precision valley current limiting sensing modules and hiccup circuits; handles overload and short circuit conditions
Thermal shutdown circuitry to prevent thermal runaway
Output voltage monitoring circuit: sends a PGOOD power good signal if the output voltage measurement is above 87% of the target value

The controller's Diode Emulation Mode (DEM) improves application energy efficiency at light loads. At light load, the DEM mode operates in DCM and pulse-hop mode to maximize light load energy efficiency and control output voltage ripple to minimize conversion losses. Forced PWM (FPWM) mode over the load range keeps the switching frequency constant and minimizes output voltage ripple, and is recommended for low-noise applications such as power supplies for sensors or analog circuits.

L3751 Reference Design

ST offers users a reference design based on the L3751, as shown in Figure 3, for DC-DC converters with input voltage 36-72V DC and output voltage 12 VDC, 10A.

l3751 dc converter reference solution

Figure 3: L3751 DC converter reference solution

reference board schematic

Figure 4: Reference board schematic

Circuit Description

The input stage components include high-capacity capacitors forming the PI network, and filter circuits (C4, C5, and L1) to mitigate high-frequency radio emissions caused by high-frequency switching operations. The input DC bus voltage is connected to the controller's VIN pin (pin 20), with series resistors in the path.

VIN supplies power to the controller's internal circuitry as well as to the internal LDO regulator to produce a Vcc output voltage of 7.5 V. VIN also serves as the power supply for the internal gate driver. A small MLCC capacitor needs to be connected on pin 20. A bias voltage is applied to the enable pin (pin 1) via an input voltage divider circuit to enable the controller at 40V to avoid deep battery discharge.

Once the EN/UVLO pin voltage exceeds 1.2 V typical, the soft-start phase begins, the SS/TRK pin voltage rises, and the controller starts. To allow the controller to operate at the minimum input voltage of 40 V, resistors R1 and R6 are selected with resistance values of 100 K and 4.7 K, respectively.

To ensure that the control loop is absolutely stable and independent of the capacitor type, the controller implements a Type III compensator that provides the developer with two zeros and three poles, ensuring that the phase margin is sufficient to improve the performance of the step load.

Performance and Test Results

We tested the board under various load conditions and obtained energy efficiency and performance test results.

Input Voltage vs. Efficiency and Load Variation

Figure 5: Input Voltage vs. Efficiency and Load Variation

Conclusion

In China, new changes from the policy side, technology side, market side, and demand side have come together to fuel the accelerated development of the e-bike industry. The average number of daily two-wheeled trips in China exceeds 1 billion, and electric bicycles are the choice of 80% of national trips.

According to the 5G IoT Industry Alliance data, the incremental market size of China's electric bicycle market is expected to reach 60 million units in 2021. And new technologies such as high-efficiency DC/DC converters can help e-bike electronic systems meet the demand for smaller, more efficient and better performance, thus driving the market development.

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