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Original question:

LM5022: Isolated Boost converter with simple transformer (only 2 winding primary/secondary), optoisolator, few component...

LM5155: Designed; LM51551DSSR (HICCUP Mode Version) Step-Up Flyback Isolated Main Supply IN:12V= OUT:55,5V=

Michele Bonollo
Prodigy 170 points
Part Number: LM5155

Hello, Thanks to your suggest i have designe the my flyback power supply with the more complete and last LM51551

Can you give me your opinion about the layout, connections, BIAS, VCC, SS, COMP, Opto protections and others... about the projects. Before i complete the sample ??

Design attached, Many thanks

Michele

  • 0
    TI__Guru* 75805 points

    Hi Michele,

    Thank you for designing with our product.  Would you please refer to our flyback EVM, available at the following link?

    http://www.ti.com/tool/LM5155EVM-FLY

    Best Regards,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Prodigy 170 points

    Hi Youhao and thank you for your suggest,

    Yes, i have already saw and evaluate this schematic flyback and all others TI application notes.

    I think my project is similar and correct like this, i'm not afraid of component values that i can change in my pcb develop sample...

    just these doubts:

    1) I haven't the Auxiliary winding of the transformer (The my voltage request is opposite like evaluation board 12Vin <> 55,5Vout) Therefore the VCC pin2 isn't connet with power supply input, but leave floating with only 1uF bypass capacitopr (N.M. == Not mounting components in my draw)  Is correct ??

    2)  The Optoisolaton feedback is made with the VCC (about 6/7 Volt) of LM51551 and 5K11 resistor, this is the best solutions?? The my Input voltage does not fall below 10 Volt, or is better connect VCC with BIAS pin ??

    3) The my applications isn't critical about start-up speed. May be better put a large capacitor in SS pin 9 from 22nF to 47nF or 100nF or... ??? or there is critical situations about HICCUP mode or others ??

    4) Others connections, layout questionable ??

    

    Thank you very much
    Michele


    



    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    

	            
                	
	
    
				
		                
                
                
                
                
                
                
        		
		
				
    
			
		
    
		
		 
		
		
     
    
	
    

    
			
    
			
    
		
    

		
    
	
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						in reply to Michele Bonollo
					
									
    
	
    
	                                    
                      
    
                        
                          TI__Guru*
                        75805 points
                      
    
                          
                   		
		
    
					
    Hi Michele,
    

    1. You don't have to have the aux winding.  The VCC can work with the internal VCC regulator.  It is right that you need the 1uF at VCC to PGND.
    

    2. No, you connected to VCC.
    

    3. First, the LM5155 does not have hiccup mode.  It is the LM51551 (pin-to-pin compatible) that has the hiccup mode.  In the flyback configuration, we usually don't use the SS to control the soft-start. There is the secondary circuit for soft start.  In this case, we usually reduce the SS capacitor to yield to the secondary circuit control.
    

    4. If you follow the EVM layout, you should be okay.
    

    

    Thanks,
    

    Youhao 
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    

	            
                 	
	
    
				
		                
                
                
                
                
                
                
        		
		
				
    
			
		
    
		
		 
		
		
     
    
	
    

    
			
    
			
    
		
    

		
    
	
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