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<p>下面摘译自Audio Power Amplifier Design Handbook</p><p>1.最简单的电流限制电路</p><p>
功放输出短路如何对待?
<br/></p><p>TR1分流了TR2的基极电流,但Tr7必须加上限流电路,否则保护时,Tr7死。<br/>这种电路功耗线是一个矩形,例如40V,保护电流5.5A,那么保护时功耗达到220W,大大超出功率管的SOA范围<br/>还有以下缺点:<br/> It has to be faced that this arrangement often shows signs of HF instability<br/>when current limiting, and this can prove difficult or impossible to<br/>eradicate completely. (This applies to single and double-slope VI limiting<br/>also.) The basic cause appears to be that under limiting conditions there are<br/>two feedback systems active, each opposing the other. The global voltage<br/>feedback is attempting to bring the output to the demanded voltage level,<br/>while the overload protection must be able to override this to safeguard the<br/>output devices. HF oscillation is often a danger to BJT output devices, but<br/>in this case it does not seem to adversely affect survivability. Extensive tests<br/>have shown that in a conventional BJT output stage, the oscillation seems<br/>to reduce rather than increase the average current through the output<br/>devices, and it is arguable that it does more good than harm. It has to be<br/>said, however, that the exact oscillation mechanism remains obscure<br/>despite several investigations, and the state of our knowledge in this area is<br/>far from complete.</p><p><br/>2.单斜线 VI limiting</p><p>
功放输出短路如何对待?
<br/><br/>由于上面的对功率管的SOA要求很高,因此提出单曲线VI功率限制,加入R3,这样Vce增大时,保护电流闸值减少,Vce小时,保护电流闸值增大。<br/>不同的R3得到不同斜率的负载线。例如下面图的斜率1线就是R3=15K,2线就是R3=10K,2线短路时,能将短路电流降到2A,</p><p>
功放输出短路如何对待?
<br/></p><p>3.双斜线电流限制<br/>电路如如图</p><p>
功放输出短路如何对待?
</p><p>功耗线由两条斜率不同的线组成,Vce过高时,Zd1起主要作用,Vce低时,由R1 R4起主要作用。功耗线如下,能更接近SOA线,</p><p>
功放输出短路如何对待?
<br/><br/></p><p>4.最小负载阻抗保护</p><p>下面电路中的R13 R14 VD2 VT3以及功率管Re构成最小负载阻抗保护.<br/> <img alt="" src="http://www.hifidiy.net/bbs/UploadFile/2006-9/200692911574756698.jpg" border="0"/></p><p>将电路简化变成如下</p><p><img alt="" src="http://www.hifidiy.net/bbs/UploadFile/2006-9/200692914474347703.png" border="0"/></p><p>4148补偿Q1的Vbe压降,这样当电桥两臂不平衡,左面高于右面,就导通。</p><p>左面的分压比固定在2K/150=13.3,如果右面分压比小于13.3,那么左面电压就高于右面电压,管子就导通保护,按Re=0.2欧的话,如果负载小于0.2*13.3=2.6欧就保护了,不过当然输出电压至少要有一定幅度才保护。</p><p>这个电路仅仅有短路保护而没有过流保护,但因为负载不会小于2.6欧,因此就没必要进行限流保护了</p><p>以上1-3适用于BTL OCL,但第4种仅仅适合于OCL.,</p><p>完善的保护其中温度保护也是必须的,因为发生故障时,全部功耗都在功率管上,即使在功率管的SOA范围以内,也是不很安全,因此最后一度防线就是温度保护了。</p>
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