Over voltage protection of power adapter

Release time:2018-11-26Preview times:5702

In case of failure, the output voltage of most power adapters may be higher than the specified value or requirement. In the power adapter without protection circuit, high output voltage may damage the internal or external equipment of the power adapter. In order to protect the power adapter equipment in this abnormal working condition, the common method is to add over-voltage protection circuit into the power adapter.
Because TTL circuit and other logic circuit are easy to be damaged by over-voltage, it has become a common practice to provide over-voltage protection for the output of this kind of circuit. Other output voltage protection functions are usually added according to the needs of system engineers or users.

Types of overvoltage protection
Over voltage protection technology can be roughly divided into the following three categories
The first kind: simple thyristor over-voltage sharp protection.
The second category: overvoltage protection based on voltage clamp technology.
The third category: overvoltage protection based on voltage limiting technology.
The selection basis of overvoltage protection technology is: power adapter structure, power adapter performance and cost.

First: thyristor management over-voltage sharp protection
Just like the meaning of his name, the over-voltage sharp protection can short-circuit the output terminal of the power adapter when it affects the output overvoltage of the power adapter. If the output voltage of the power adapter exceeds the preset value within the specified time, the short-circuit device (generally the thyristor) will act.
When the thyristor acts, it will short-circuit the output of the power adapter, so the output voltage will drop. In the figure, a typical thyristor overvoltage protection circuit is shown, which is connected to the output terminal of the linear regulator. In the design of thyristor over-voltage sharp protection circuit, it is very important to realize that the bypass function of QCC over-voltage sharp protection may not provide a long-term load protection.
In addition, the bypass device must have enough power to bear the short-circuit current. For the short-circuit short-circuit current beyond the specified time or external current limit, the fuse must be able to work, so as to protect the thyristor from damage.
For linear regulator, thyristor over-voltage sharp protection is a common method. The simple application circuit often used is shown in the figure. The working process of linear regulator and over-voltage sharp protection is as follows:
The unadjusted DC input voltage VH drops after passing through a channeling transistor Q and provides a lower but adjusted output voltage v. Amplifier A1 and resistors R1 and R2 provide a voltage control for the regulator. Transistor Q2 and current limiting resistor R1 are used for current limiting protection.
Most catastrophic circuit failures are caused by the short circuit of the channeling regulator Q1. In this case, a higher, unadjusted input voltage VH will appear at the output. In this case of failure. The whole circuit has lost the function of voltage control and current limiting. At this time, the thyristor which plays the role of over-voltage sharp protection must work and short-circuit the output terminal.

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