这是一个系列，预计5篇。
关键词：
快速启动
高压启动
加速启动
无损启动
有源启动
恒流充电电路
高压LDO
高压调节器
静态功耗
待机功耗
fast start up
HV start up
speed start up
active start up
quick start up
standby power
今天这个来源于Microchip的一个AN中的节选：
包括电流检测采样+Vcc启动电路部分（后续再讲）
https://ww1.microchip.com/downloads/en/Appnotes/00002122B.pdf
BOOTSTRAP DESIGN 自举供电设计
In some power-supply applications, the Pulse-Width Modulator (PWM) controller is powered from an auxiliary winding, tapped off the power stage’s transformer. This technique is used to reduce power loss. The only drawback is at start-up as the capacitor needs to be trickle charged from the rectified voltage, as shown in Figure 47. Another discrete technique to start-up the circuit is depicted in Figure 48.
These techniques work well but they have the following disadvantages:
• Inefficiency – Draws current continuously from the high-voltage source
• Poor dynamic range – bias must be set for the minimum input voltage (at high-input voltage, the current drawn is higher)
• Poor regulation
• No current limit
• No overtemperature protection
• Requires large power resistor and Zener diode
The schematic in Figure 49 depicts a simplified offline switching power supply’s auxiliary using the LR8 for start-up. This IC solves the problem of continuous current draw by entering in a Standby mode after the auxiliary voltage rises higher than the set output voltage. All current is then supplied from the bootstrap circuit rather than from the high-voltage source, increasing overall efficiency. The output voltage of theLR8 should be set high enough above the minimum operating voltage of the PWM controller; yet, low enough to ensure the bootstrap circuit takes over after start-up. If boot time is short, die temperatures will not reach the overtemperature protection trip point.
The advantages of using this technique are the following:
• LR8 goes in to Standby mode after supply has bootstrapped, drawing no current from highvoltage input
• Good regulation
• Built-in current-limiting
• Overtemperature protection
The output voltage of the LR8 can be calculated as shown in Equation 46. Figure 50 depicts the selected bootstrap design for the project. R1 of 1 kΩ and R2 of 5.1 kΩ are selected, thus Equation 47 results from Equation 46:
Most PWM controllers have an undervoltage lockout (UVL) circuit or programmable start-stop voltages so that when the supplied voltage reaches the turn-on threshold, the device will start consuming current. Thus, if the consumed current is greater than the current delivered by the LR8, the output voltage will fall. If a large enough capacitor (C2) is added, the auxiliary will start to supply current before the reach of the turnoff voltage.
Microchip LR8：高压线性调节器
这个可以参见耗尽型MOS启动电路相关知识。
Onsemi也有类似的料：NCP78X
Supertex（被Microchip收购）LR745
工作时序：
Stage I
Once a voltage is applied on VIN, the LR745 will start to charge the VCC capacitor, C1. The VCC voltage will start to increase at a rate limited by the internal current limiter of 3.0mA. The PWM IC is in its start-up condition and will typically draw 0.5mA from the VCC line. The VCC voltage will continue to increase until it reaches the PWM IC’s start threshold voltage of typically 16V.
Stage II
Once VCC reaches 16V, the PWM IC is in its operating condition and will draw typically 20mA, depending on the operating frequency and size of the switching MOSFET. The output of the LR745, VOUT, is internally current limited to 3.0mA. The remaining 17mA will be supplied by C1 causing the VCC voltage decrease. When VCC decreases to 13.25V, the LR745 will turn off its output, thereby reducing its input current from 3.0mA to 10s of microamperes. At this point, all 20mA will be supplied by C1. The PWM IC can now operate to a minimum VCC voltage of typically 10V.
Once the switching MOSFET starts operating, the energy in the primary winding is transferred to the secondary outputs and the auxiliary winding, thereby building up VAUX. It is necessary to size the VCC storage capacitor, C1, such that VAUX increases to a voltage greater than 10V before VCC decreases to 10V. This allows VAUX to supply the required operating current for the PWM IC.
If for some reason the auxiliary voltage does not reach 10V, VCC will continue to decrease. Once VCC goes below 10V, the PWM IC will return to its start-up condition. The PWM IC will now only draw 0.5mA. VCC will continue to decrease but at a much slower rate. Once VCC decreases below 7.0V, the LR745 will turn the output, VOUT, back on. VOUT will start charging C1 as described in Stage I.
Stage III
At this stage the LR745’s output is turned off and the PWM IC is operating from the VAUX supply. The auxiliary voltage, VAUX, can be designed to vary anywhere between the minimum operating VCC voltage of the PWM IC (10V) to the maximum auxiliary voltage rating of the LR745 (22V).
其他相关快速启动的电路内容：
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其中 片语杂谈 主要是用来记录自己的一些随感之类...
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关于本人：
Eric Wen (文天祥)
IEEE Senior Member, 高级工程师职称（副高级），中国电子学会物联网青年专业技术组通信委员(AIoT),中国照明学会高级会员，中国电源学会照明电源专业委员会委员,中国电源学会青年工作委员会委员等
个人其他信息参见公众号：Aladdin阿拉丁
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