Analysis of MOSFET damage at the moment of power failure of isolated power supply

1: The fault phenomenon

The equipment carries out POWER ON/OFF In the experiment , The equipment is out of order , Power Supply 2 The circuit cannot be started .

2 Phenomenon analysis

Due to the aging of the power circuit , Use a multimeter to measure and find the input 24V The power supply is normal , Output 9V The power supply is abnormal , Therefore, it is determined as 24V turn 9V There is a problem with the isolated power loop , Some schematic diagrams are as follows .

  Further analyze the fault and find the switching power supply chip 8 No feet OUT Damaged and MOSFET damage .

3 Cause analysis

because MOSFET And the switching power supply is damaged , Therefore, use an oscilloscope to test the waveform of relevant pins , I found that when the power was off VDS A spike voltage will be generated , The waveform is as follows .

chart 2 Power on waveform

     

chart 3  Drop the wave shape

  No. 1 Watch bar is a switch chip FB The signal , Number two is VDS, Number three is 24V.

According to the above phenomenon MOS The tube is damaged because of the instant of power failure VDS Greater than MOSFET Withstand voltage , Lead to MOSFET breakdown . Switching Mode Power Supply 8 The pin is damaged because MOSFET After damage, the large voltage reaches MOSFET Grid （G）, As a result, the switch chip is damaged .

4 The phenomenon reappears

During repeated power on , It was found that no circuit damage was found during power on , After many power cuts , The circuit is damaged , Thus supporting the above speculation .

5 In depth analysis of the problem

In the process of power failure, the output voltage will drop with the drop of input , At this time, the system still has the power stored by large capacitors , Then the feedback loop works normally , Because the switch chip is PWM type , In the process of output voltage drop ,OUT The duty cycle of the pin will increase , be MOSFET The conduction time will be longer , In this way, the current passing through the transformer will increase , Due to the characteristics of the transformer , stay MOSFET When it's turned off DRC This job , Thus in VDS A spike voltage is generated on .

 6 Problem rectification

programme 1：

Because the peak voltage in the process of power failure comes from the output capacitance of the power module and its own input capacitance , Therefore, the output capacitance of the power module and its own input capacitance can be reduced , The waveform after rectification is as follows .

programme 2：

When analyzing the problem, it is found that the high voltage is caused by the large duty cycle of the switching power supply , Then we limit the duty cycle of the switching power supply chip , Paste the dummy part or on the 5th DTC One corner in parallel 71K Resistance to ground , Get the waveform .

 

programme 3：

The high voltage caused by the high current generated by the transformer , Then add current limiting resistance on the main circuit , It can also achieve the purpose of adding and modifying , The waveform is as follows .

 

programme 4： You can also adjust DRC Absorption loop to solve this problem .

programme 5： It can also be in MOSFET Of DS End use TVS Clamp .

The above scheme is selected after comprehensive evaluation 2

programme 2 Lieutenant general DTC Pin parallel 70KΩ The resistance changes less and the cost is relatively low , And control the problem from the source , about EMC And other follow-up tests also have some support .

Retest POWER ON/OFF No problem occurred , Problem solving .

7 Reflection on Problems ：

1： When designing circuits , Surge current and voltage protection are not provided for external access power supply , This will cause the equipment to be different when using different power supplies , Thus, the damage risk of the product is increased .

2： When reducing cost, be careful when deleting mature circuits , Consider the whole , Multiple angles, dimensions and application scenarios should be considered , After the change, it also needs to be verified repeatedly , The goal of reducing cost without reducing quality has been achieved .