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UCC256403: UCC256403A design calculator CTR selection

Ruian Yu
Prodigy 10 points

Part Number: UCC256403

I‘m using the calculator to desgin a LLC conveter based on UCC256403A, I choose the PC187B for my OPTO, But the common CTR of PC187B is 130%, so when I select  130%, the cross frequency goes to 63khz! It's hard for me to design the loop circuits, so in this situation, please teach me what should I do, replace a other OPTO? when I choose CTR 10% the Fc decrease to a normal value. image.png 

  • 0
    TI__Intellectual 1480 points

    Hi Ruian Yu,

    The internal IFB current source for UCC256403 is approximately 164uA and this limits the maximum LED current for the optocoupler.

    Optocoupler CTR is dependent of the bias current of the LED, in this case it's very small. So you have to always put CTRmin in your calculator, which can be found from the curve (CTR vs Ibias) of the optocoupler datasheet.

    Regards,

    Pritam

  • 0
    Prodigy 10 points

    Hi Pritam,
    Thank you very much for the reply, I use the PC817B for the FB Circuit, But I read the datasheet of PC817B,The curve (CTR vs IF)  was start at the IF=1mA, so I don't know the CTR when the current is under 1mA, what should I do, I can't find a opto datasheet that has the curve(CTR vs IF) start at a small current. could you tell me what should I do? just put a normal  value such as 40% to the calculator? 

    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    

	            
                	
	
    
				
		                
                
                
                
                
                
                
        		
		
				
    
			
		
    
		
		 
		
		
     
    
	
    

    
			
    
			
    
		
    

		
    
	
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						in reply to Ruian Yu
					
									
    
	
    
	                                    
                      
    
                        
                          TI__Intellectual
                        1480 points
                      
    
                          
                   		
		
    
					
    Hi Ruian Yu,
    

    Yes I saw the datasheet curves are limited to 1mA. One thing can be done to linear extrapolate the curve upto 0.1mA, in that case for the given datasheet curve the value of %CTR will be 20% for 0.1mA.You can use that value in your calculation.
    

    Regards,
    

    Pritam
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    

	            
                 	
	
    
				
		                
                
                
                
                
                
                
        		
		
				
    
			
		
    
		
		 
		
		
     
    
	
    

    
			
    
			
    
		
    

		
    
	
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						in reply to Pritam Banerjee
					
									
    
	
    
	                                    
                      
    
                        
                          Prodigy
                        10 points
                      
    
                          
                   		
		
    
					
    Hi Pritam，

    Thank you very much for the support !

    Regards,
    Ruian
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    

	            
                	
	
    
				
		                
                
                
                
                
                
                
        		
		
				
    
			
		
    
		
		 
		
		
     
    
	
    

    
			
    
			
    
		
    

		
    
	
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