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Original question:

UCC28220: Interleaved 3 Switch Forward, FET damage

UCC28220: Over current Protection

younghyun kim
Prodigy 50 points
Part Number: UCC28220

Hi.

I am developing an interleaved 3 switch forward convertor using IC UCC28220

  * Topology: 3 switch forward
  * Input: 380VDC(PFC Output)

  * Ouput: 12VDC, 84A, 1000W


The problem is the overcurrent protection circuit.

The circuit design used a current transformer. Also,UCC28220 is controlled from the secondary side.

Under overcurrent conditions, the output voltage gradually decreases. However, the load current continues to increase.

(See picture below: Vertical axis voltage / horizontal axis current)
 




I think that under overcurrent conditions, the UCC28220 limits the duty, so the output current continues to increase, causing the output voltage to drop.


The problem is overheating of the output circuit.



I want to design the overcurrent protection characteristic curve to be like one of the two pictures below.


 

Please check if my circuit design is incorrect. 

Also, Please let me know if there is a way to make ocp hiccup work.



Attatchment: Schematic - 3 Switch Forward - Output 1000W (12V, 84A).pdf


Schematic - 3 Switch Forward - Output 1000W (12V, 84A).pdf





								


				
				

				
			

		

			
						
									
                			    			                        
						
						
			
			
						
			                        
			
			
				
			  
			

			     
			

		


				    			

						
















	

	

					
    
	
    





		
	
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                          TI__Mastermind
                        27325 points
                      
    
                          
                   		
		
    
					
    UCC28220 is a peak current mode PWM controller that enables cycle-by-cycle current limit as you described: "output voltage gradually decreases. However, the load current continues to increase"
    

    Current foldback as you described with the diagram showing the "knee" is also not an integrated feature of UCC28220. I have designed CCCV converters (LED drivers, battery chargers) that will give the square VI characteristic you've shown. The basic approach I'm familiar with is to use two op amps in the feedback where one is regulating constant voltage (CV) and the other is regulating constant current (CC). Reference design PMP9622 is one example of a PSFB controller using CCCV feedback.
    

    If you choose not to use CCCV regulation, you would need to overdesign the power stage to be able to handle the additional current during OCP or externally detect when the output voltage is decreasing while the converter is in OCP and take corrective action to shut down the controller.
    

    

    Regards,
    

    Steve
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    

	            
                 	
	
    
				
		                
                
                
                
                
                
                
        		
		
				
    
			
		
    
		
		 
		
		
     
    
	
    

    
			
    
			
    
		
    

		
    
	
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