這才叫無線充電? 新 iPhone 充電距離達 - 手機討論

By Ethan
at 2016-02-26T01:02
at 2016-02-26T01:02
Table of Contents
本人鍵盤電力電子工程師, 來分享一些個人的看法.
文長, 底下有結論, 可以END.
仔細看看Engerous公司所提出的無線充電方案, 確實十分具有革命性
跟顛覆性, 然而, 問題依然很多.
// ======================== 無線充電種類介紹 ========================
首先, 無線充電可以分成兩種:
- 近場無線充電 (Near-field):
就是近距離的那種. 這類技術其實就只是把一個變壓器分成兩半,
一半放在充電器, 另一半放在手機, 最終等效成一個汽隙(Airgap)
很大的變壓器. 也就是透過磁路(Magnetic Circuit)來連結兩邊.
能量的傳遞原則上是不發射電磁波的.
- 遠場無線充電 (Far-field):
這類型就是利用電磁波來傳遞能量. 電磁波會實際被發射出去,
接收端再來接收這電磁波. 也就是, 理論上來說, 這種技術的
接收端能夠從Wi-Fi訊號中擷取能量.
目前所有現行的無線充電技術都是近場的方式, 然而這次Engerous
公司所提出來的技術, 也就是謠傳蘋果要採用的技術, 卻是遠場的
技術.
// ================== Engerous的無線充電技術說明 ====================
如果要比較簡單的了解遠場無線充電的技術, 那我們可以用光來比喻.
遠場無線充電的充電器, 就好像一顆燈泡, 而手機就好像有一個
太陽能板一般. 燈泡發光照亮太陽能板, 太陽能板發電給手機充電.
問題是, 只有照到太陽能板的光是有效的, 其他都浪費掉, 所以說,
充電端必須要能夠做到像鐳射光一樣, 太陽能板到哪裡, 就把光集中
到哪裡.
今天Engerous做到的就是這個, 他們使用了一陣列(翻譯: 很多很多的)
的天線, 讓他的電磁波發射可以只往某一個方向發射. 還可以讓他發射
的方向跟著物體一起移動. 當然, 這種技術一直都存在, 只是沒有人把
他拿來做無線充電而已.
因此Engerous的技術基本上就是運用5GHz的ISM頻段(跟WiFi, 802.11n/ac
所使用的5GHz頻段一樣), 利用天線陣列發射集中跟高度方向性的
電磁波, 跟隨著追蹤著手機的方向, 來讓手機接收充電.
順便提一下Engerous官方所提出的充電數據:
0-5 ft (0-1.5m) 4W (等效於5V0.8A)
5-10 ft (1.5-3m) 2W (等效於5V0.4A)
10-15 ft (3-4.5m) 1W (等效於5V0.2A)
// =================== 為什麼這東西問題很大 =======================
首先, 把電磁坡集中起來最大的問題就是, 被集中的區塊電磁坡很強,
就好像同樣都是50W, 一個50W的LED燈沒什麼, 但是一個50W的雷射是
可以輕鬆的切開木板的.
所以, 要能夠在4.5公尺外蒐集到1W, 那個電磁坡強度會是非常強的.
這個強度是強到各國政府都不會允許的強. 各國在規範電磁波強度時,
用了一個叫做EIRP的數據, 這個數據就是抓電磁波最強的方向的電磁波
強度.
例如美國FCC在特殊點對點的狀況下, 允許5.8GHz ISM Band最高53 dBm
(約200W). 然而如果我們計算[email protected]公尺這種狀況下的最低EIRP, 是
1W*4*pi*(4.5m)^2/(16cm*8cm)=19,880W (你沒看錯, 約兩萬瓦)
( 1W: 接收功率, 4*pi*(4.5m)^2: 4.5公尺半徑的球形面積,
16cm*8cm: 手機接收面積 )
這已經超標了一百倍. 很明顯的, 別說健康什麼的, 法規這種最低限度
都無法通過. 這技術的可行性如何自然不用說.
順帶一題, 一般WiFi發射功率大約20~30dBm, 也就是一瓦以內, 所以
這東西大概比WiFi強了兩萬倍左右.
也許你會問, 如果法規放寬了呢? 法規之所以存在都是有原因的, 會
限制發射功率不只是健康的問題, 還有電磁干擾的問題. 我們一般的
電器, 上市前都需要自我檢測電磁相容, 確保不會被其他設備干擾.
問題是20kW(兩萬瓦)/73dBm這種發射功率, 根本跟電磁砲一樣, 一個
不小心可能就會把其他設備給打暴, 如果電磁波發射途徑中剛好有
5GHz的設備, 這種功率很有可能輕鬆的打暴該設備的RF Frontend.
再來, 這種指向性的天線陣列造價不斐, 還需要搭配大量的Power
Amplifier, 所以, 恐怕要量夠大, 才不會造成充電器比手機貴的景象.
充電速度就不說了, 4.5公尺外充電功率只有1W. iPhone 6 Plus從0%
充到滿(11.1Wh), 最少要11.1個小時(11.1Wh/1W=11.1h), 這還是假設
手機內部能量轉換效率100%的狀況下.
// ======================== 總結 ============================
1. 電磁波最少超標100倍. 比WiFi還強約上萬倍.
2. 就算法規放寬, 這東西根本電磁砲, 很容易就造成電磁干擾.
3. 成本高, 充電器價格加一個0還不夠.
4. 充電慢. 十小時.
綜合以上, 我認為, 這無線充電最多在實驗室玩玩, 或是拿來炒炒
股票, 目前還沒啥實用價值.
--
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