原创 为什么有时候电源会失效（保护或者炸管），当我变化负载的时候（zz）

当负载变化时，成为重载后，电源就不工作了，或者保护，或者开关管炸掉。

根本原因是因为 开关管离开了安全工作区或者 是控制电路失效（由于开关管的EMI）。

由于MOSFET的快速的开关，会在电路板上产生电压或者电流的震荡。这种震荡可能会有很大的峰值，造成ＭＯＳＦＥＴ损坏。

这种震荡会产生ＥＭＩ进入控制电路，导致控制电流失效

重载时的大电流，会产生更大的ＥＭＩ和开关损耗，使结温超过允许值，炸管。

抑制电路可以ｄａｍｐ这种震荡，使电压电流在ＳＯＡ区。

抑制电路可以减少开关ＥＭＩ，减少开关损耗，和抑制ＭＯＳＦＥＴ开关损耗，防止结温过高。

find a good websitean about switching mode power supplies(SMPS)

http://www.smpstech.com

Why does my power supply fail when I increase the load?

My power supply is working fine up to 25% load, but it fails when I try to go to full load. (Usually by failure of the power transistors, power diodes, or losing control.)

The failure of the power switches is usually due to leaving the safe operating area
of the power switch when the load is increased. But it can also be loss
of control due to electromagnetic interference (EMI) from the switches
getting into the control electronics causing the power supply to lose
control.

The solution to both problems is usually the proper use of snubbers.

The rapid switching of power switches in switching-mode power supplies can excite parasitic resonances
in other components and the layout of the power circuit and cause
ringing on the voltages and current waveforms. The effect of this
ringing is three fold. First, ringing can drive voltage and current
waveforms outside the forward biased (turn-on) and reverse biased
(turn-off) safe operating areas of the power switches causing device
reliability degradation (long term failure) or near instant failure of
the power semiconductors and other components. Second, the ringing of
high voltage and current waveforms can create local EMI that enters and
upsets the low power control circuitry, in the worse case, commanding
the circuit to destroy the power supply. Third, as the load increases,
the increased switching current can cause more EMI and power dissipated
in the power semiconductors can drive the junction temperatures beyond
their ratings.

Snubber circuits are the usually solutions to these problems.
They damp the ringing and shape the waveforms to stay within safe
operation areas. They greatly attenuate the EMI generated by the power
switches, although careful design and layout practices are also needed
around the control circuits to desensitize them from EMI. Finally
snubbers can be used to reduce the switching power losses of the total
design or divert the losses from overheating semiconductors to snubber
resistors.

But snubber circuits are not as simple as they seem.
A February 16, 2008 search of the IEEE Xplore data base yielded 1,075
IEEE papers on the subject and through the linked GlobalSpec data base,
3,464 application notes. Virtually every text book on power supply
design discusses some aspects of snubber design, but not all aspects.
This should give a clue that they are not simple circuits in their
application. Yet surprisingly, there is no book devoted to the subject.
But that has changed with the release of Rudy Severns ebook on the
subject, Snubber Circuits for Power Electronics, the first book every devoted to this complex subject.