This article is part 2 of "Some limitations of power supplies"----What will happen when output current exceed a power supply's range?We will discuss the potential failures that may happen when the output currents exceed power supplies' allowable range.
Part 1:What Can Happen When Input Voltage Exceed a Power Supply's Range?
Part 3:What Can Happen When Working Temperature Exceed a Power Supply's Working Temperature Range?
Before starting this article we need make OCP(Over Current Protection) clearly to understand next contents better.
When selecting power supplies,the output current is one of the most important points need to be considered.It plays a large role in determining power supplies cost and size,so designers maybe only choose power supplies that have right enough current to meet their requirements to save cost and size.
In some conditions,some products for example motors require a little bit higher current than normal operating current to start(starting current),at the same time the components value has deviations,and superpositions of many components deviations are visibly large.Due to the mentioned causes,when designing power supplies,we always make the power supplies are able to ouput a little higher currents than rated ones.
Normally we set power supplies maximum output current around 1.2-1.5 times of rated ones,we call this maximum output current OCP(Over Current Protection) point.
Next we will analysis different problems performance when the power supplies output current exceed the rated ones.
When operating power supplies between their rated output current and OCP point,their efficiency,regulations and EMC will be affected.
The output power increase along with output currents go up.The additional current shall lead to a linear increase of power supplies' power dissipation and this added power loss will cause components temperature rise which lead to life span decrease and following thermal failures.
Another point needed to be considered is load regulation. The load regulation manifests that the output voltage can be changed along with output current changes between unload and full load.
Most power supplies output voltage drops along with output current increases.If output current exceeds rated one much,the output voltage may drops too much to drive the applications working.
Few power supplies output voltage increases along with output current increases.If output current exceeds rated one much,the output voltage may increases too much and destroys applications.
For some small rating power power supplies which are only with simple control schemes and without 431 or Optocoupler,they have rated min. output current,if the output current is less than the rated ones,the power supplies efficiency will be very low and components temperature will increase which lead to thermal failures.
As we know,there are some filters inside power supplies which help to control the EMI to meet regulatory requirements.normally the fitlers value are just enough to assure the power supplies can pass the required test,if the output current exceeds rated ones,the filters will work beyond their loads which may lead to EMC test failures.
As stated at the beginning,we set OCP for power supplies,if the output current is over rated one much and arrive the OCP point,the protection will works and power supplies will be no output.
In summarize,for some small watts power supplies,if the output current is lower than the rate min. current,it will lead to low efficiency and thermal failures then components failures.For most power supplies,if the output current is higher than the rate maximum current,the efficiency will be also lower than expected,and power components for example MOSFET,Diode,Filters,Inductors,Chocks,transformers will work under big stress,this will lead to high temperature of components and reduce life span then failures.