Implementing Small, Efficient Power Subsystems for New Li-Ion E-Bikes

By Thong “Anthony” Huynh, Principal MTS, Industrial Power Applications, and Suhel Dhanani, Sr. Principal MTS, Industrial Business Development, Maxim Integrated

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Table 2: MAX17503 balance design component footprint
Table 2: MAX17503 balance design component footprint

Another DC-DC converter part that has been used in older China e-bikes is a 3.5-V to 60V input, 2.5A step-down converter. Let’s call it Device T, target device for comparison. Using Device T’s online simulation and sizing software, we can estimate size and efficiency of the power system designed with this component. We chose three design options – one design seeking a balance between efficiency and size, another optimized for size and the third focusing on achieving the highest efficiency.

This balanced design runs at 295kHz, yields 85% efficiency, and has a footprint of 407 mm2. This is more than 2.5x the size of the MAX17503 design with a 1.5% lower power conversion efficiency. Note that this Device T design runs at lower switching frequency, thus it requires a larger inductor. The non-synchronous rectification diode also takes some additional space as well.

Let’s redesign this circuit, optimizing for size as follows:

In this compact design, the circuit runs at 489kHz and has a footprint of 217mm2 with even further degradation of efficiency (83.3%).

Redo the design optimized for highest efficiency yields 89%, the converter runs at 100kHz and has a total component footprint of 1315mm2.

For comparison purposes, let’s go back to MAX17503 and create two more designs: one optimized for small size and another for high efficiency.

MAX17503 optimized for small solution size:

MAX17503 optimized for highest efficiency:

Below is the summary of the design in three distinctive design optimizations:
Vin 27VDC to 42VDC,
Vo = 5V @ 2A,
Ta = 30oC

Table 3: Comparison of power designs
Table 3: Comparison of power designs

Power conversion efficiency directly impacts the range of an e-bike. So even a few percentage points difference is crucial. In addition, since this power subsystem needs to fit in a small space (in the tubing or under the seat), the power/heat dissipation must be kept at a minimum to avoid overheating and impact on long-term reliability.

Are a Power IC’s No-Load Quiescent Current and Shutdown Current Important?

There is some confusion on how much a power conversion IC’s quiescent current impacts the driving range since there will be a lot of idle time while using the e-bike in any urban setting. When we run the numbers, we find that even when assuming an excessive amount of idle time the percentage of the battery power that can attributed to the quiescent current of the power IC is virtually negligible.

First, let’s understand how IC manufacturers specify no-load quiescent current:

  • Device T specifies it as “Operating non-switching supply current” at 138uA typical
  • MAX17503 specifies it as “Input Quiescent Current in PFM mode, IQ_PFM” at a comparable value of 162uA typical

Now, let’s take our same bike with 360Wh battery (20-mile range) down Wangfujing Street in Beijing, where there is one intersection at approximately every 0.2 miles. Assume that we will hit one stop for every two intersections, and that the average wait time is two minutes. During the 20-mile ride, we’d stop 50 times and total idle time would be 100 minutes. With the MAX17503 IQ_PFM of 160uA, the energy consumed during the idle time is merely 0.01Wh, or 0.003% of the total battery energy.

If we took the bike down another street where there are twice as many intersections, we’d consume 0.006% of the total battery energy for the same 20-mile trip. This is still an insignificant amount.

Regarding total shutdown current, the MAX17503 IIN_SH is 4.5uA maximum while Device T’s “shutdown supply current” is at a comparable value of 4uA maximum. At 4.5uA shutdown current, MAX17503 will dissipate 0.12Wh, or 0.032% of the total battery energy after one month in storage. Again, this is not a significant amount.

Real problems customers deal with are heat dissipation and size

Conclusion

E-bikes, especially those designed around the lithium-ion battery chemistry, are a growing market worldwide, especially in China. When the power system for e-bike controllers is designed, system designers must pay close attention to the overall power conversion efficiency and the size of the total solution – including the recommended sizes of the passive components like the inductors and capacitors. The range of an e-bike and the size of the system controller box are directly impacted by these choices made.


References
1 http://www.pedegoelectricbikes.com/wp-content/uploads/2016/07/MF-EBIKE-16-Executive-Summary-w-Pedego.pdf

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