RCC工作原理，附英文版本講解（直流電源工程師必懂）
2017-5-26 10:56:47??????點擊：
RCC變換器通常是指自振式反激變換器。它是由較少的幾個器件就可以組成的高效電路，已經廣泛用于小功率直流電源電路離線工作狀態。由于控制電路能夠與少量分立元件一起工作而不會出現差錯，所以直流電源電路的總的花費要比普通的PWM反激逆變器低。一方面，當其控制電流過高時就會出現一種間歇振蕩現象，從而使得直流電源電路的振蕩周期在很大范圍內變化，類如例如從數百赫茲到數千赫茲之間變化，因而在較大功率輸出時將引起變壓器等產生異常的噪音，所以需要抑制這種現象的產生。另一方面，當直流電源電路的輸出功率輸出較小時，卻可以利用這種間歇振蕩，使開關電路處于低能耗狀態。當需要直流電源電路工作時，只需給電路一個信號脈沖即可。
最近無意看到一些推送的直流電源RCC原理講解，文章大多從網上轉載，內容真實性不說，原理講解也很勉強，本人嘗試把所學與各位分享。
如下為一款量產過百萬的RCC電路，雖然現在RCC電路采用者相對較少，但是作為直流電源研發精英，如果講不清RCC工作原理，未免是一種遺憾。
如上是一款經典的RCC直流電源應用圖，輸出功率為2W左右，這是一個改良過的RCC電路，直流電源可以做到恒流，恒壓的輸出，無需光耦，不輸于一般的PSR芯片，應用可以稱得上絕妙！
RCC是一種自激式震蕩電路，直流電源電路簡單，可靠，其拓撲原理等同反激式電路拓撲，變壓器的設計等同反激式，此文不再討論。
本文中講述RCC如何自激的過程，以及直流電源如何實現恒流恒壓。
如上圖，將整個原理圖局部放大，我們從RCC啟動開始講解：
直流電源上電后，Q1的B極從兩顆啟動電阻（分別1.5M）得到啟動電流，Q1三極管開始 導通。啟動電阻應盡量大，直流電源將啟動電流維持在1MA以上即可，降低待機功耗。
Q1導通后，輔助繞組得到感應電壓，感應電壓通過向C6充電，然后驅動三極管的B極，加速Q1導通。
搞清楚Q1三極管如何截止是理解RCC自激的關鍵。C6獲取充電后，對Q1的B極放電，Q1的Ic在瞬間是逐步線性增加，同時Ib是逐步減小的，Q1的Ice逐步增加到Ic=Ib*B(beta)時，其實可以理解成Ib電流減小到不足以支撐Ic這個點時，Q1三極管Vce開始增加，即開始截止，此時Lp上的電壓減小，同步輔助繞組電壓也減小，輔助繞組最終形成的負壓可以穩定的將Q1截止。Q1截止期間，次級繞組開始放電，直到電量放完，Q1又開始啟動打開的循環。
C6的作用，是控制Q1開關的關鍵，但不是自激震蕩的電路部分。C6值的大小，可以決定Q1三極管導通的時間，電源起機能力。
該直流電源電路電壓和電流的穩定，主要通過Q2和ZD1組成的負反饋控制，ZD1的電壓為5.1V，所以直流電源的輸出電壓也會鉗位在5V 左右（因為有R8），當輸出電壓過高時，電壓會擊穿ZD1，從而導致Q2導通，從而降低Q1的IC電流，達到控制輸出電壓的目的。ZD2的作用主要是做高壓補償，當交流輸入過高后，ZD2同樣形成負反饋的作用，限制電源的輸出功率。
如有興趣探討具體計算過程，可以關注本公眾號參與直流電源工程師討論群討論。
英文好的同學，可以看看老外的解釋：
一、直流電源RCC工作原理圖RCC基本模型：
Components in Circuit:
1. TR1: three windings SMPS Transformer:
Primary winding (PRI): 5-6,
Secondary winding (SEC): 1-4,
Auxiliary winding (AUX): 7-8
2. Q1: switching power transistor
3. RS: resistor for start-up current
4. RB: resistor to limit the base current of Q1
Working process
Q1 transistor acts as a switch in circuit, it is off at the beginning,
When the power is applied on the circuit, a small start-up current will flow through RS resistor into the base pin of Q1, the Q1 is partially on.
Then, the PRI winding (5-6) gets positive voltage difference (Vin – Vce), and the AUX winding (7-8) also gets same polarity positive voltage, this AUX winding voltage will generate current flowing through RB resistor into the base pin of Q1, so the base current increases , and Q1 turns on more.
It is positive feedback process, the Q1 will be fully on quickly.
As Ip=Vp/Lp * t, the current in PRI winding (5-6) path begins increasing linearly, and Ip will reach the critical point finally – Ip=Ib*B (beta), beyond this point, according to transistor characteristic curves,
A little Ic (equals to Ip here) increasing will cause a bigger Vce increasing, this means the Q1 will leave saturation region, and Vce will increase dramatically.
Therefore, the voltage on PRI winding will begin to decrease, the voltage on AUX winding will decrease too, this will make transistor base current decreasing, it is positive feedback process again, and will lead to transistor turn-off quickly.
When transistor is off, the AUX winding will get negative voltage, and draw the current from Q1 base to keep it off firmly.
The SEC winding (1-4) will get polarity reversed voltage, D1 diode becomes conducted and energy stored on PRI winding transfers to SEC winding, then to load.
After all stored energy is released, the current on every winding is zero, and voltage on every winding is zero too, then this cycle starts again.
Notes
1. This circuit doesn’t regulate output voltage
2. RCC SMPS works exactly at the point between continuous mode and non-continuous mode
3. The cycle frequency depends on B (beta) value of transistor which is not reliable
二、直流電源RCC工作原理圖改良電路
Function of capacitor C2
Let’s start from the point when Q1 is fully on , then C2 begins to charge according to RC time constant curve, at same time the current in PRI winding will increase linearly and energy is stored.
As C2 keeps charging, the voltage on resistor RB will decrease, and the current to Q1 base will decrease too.
So Ip keeps increasing and Ib keeps decreasing, then a critical point will be reached where Ip=Ib * B (beta), after that, the Ib keeps decreasing, then the Vce will increase, this means the voltage on PRI winding will decrease, and the voltage on AUX winding will also decrease, this will speed up Ib decreasing, and finally turn off Q1,
After Q1 turns off, the PRI winding will get a reversed voltage, so does AUX winding, this will draw away current from Q1 base, and C2 begins to discharging, the energy stored on PRI winding begins to release to load through SEC winding,
After all stored energy is released, the circuit returns to starting point, and the cycle will repeat all over again.
Notes
a. It is clear here that capacitor C2 can control the cycle-on time and frequency.
According to RC time constant curve, small value capacitor can decrease cycle on time, and provide less power to output every cycle, so circuit will have a higher cycle frequency; a large value capacitor will increase cycle on time, and provide more power to output every cycle, so circuit will have a lower cycle frequency
b. Relation between Load and Frequency:
Heavy load => less cycle off time (larger duty cycle) => higher frequency
Light load =>more cycle off time (smaller duty cycle) =>lower frequency
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