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LTC9103 Recommended for New Designs
The LTC9101-1/LTC9102/LTC9103 chipset is a 48-port power sourcing equipment (PSE) controller designed for use in IEEE 802.3at Type 2, 802.3bt Type 3 and...
Datasheet
LTC9103 on Analog.com

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Transformer and Transistor

		
	
				

			

									
					    ninaleechie
					
							

            
            			
				
									
											
			
			
				
			
			
			    	
			                    
                
				                on Apr 23, 2026 
				
			
			
		

		
		                    	    

        	    
Thread Summary

                The user inquires about substituting the PSMN075-100MSEX MOSFET with the BUK9M60-100LX and using a transformer with common mode chokes (S558-5999-T7-F) instead of the recommended Wurth 7490100143. The final answer suggests verifying the MOSFET's SOA against the LTC9102/LTC9103 foldback curves and testing the circuit. Common mode chokes are problematic for the LTC9103 as they can distort low-frequency/DC signaling, leading to signal integrity issues or communication failure. Alternative MOSFETs tested and approved by ADI include the SI7322ADN-T1-GE3 and DMT10H015LFG-7, available at Digikey.
                        	    

                
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Category: Hardware

    		        		            		    
Product Number:  LTC9103  

    		        		    		    

		    		    
		    			

			    
The datasheet strongly suggests PSMN075-100MSEX for the transistor and a 10/100 Mbps transformer without common mode chokes for isolation.  Wurth 7490100143 is given as an example.


Both of these components are difficult to source.


BUK9M60-100LX from Nexperia seems to be very similar in parameters to PSMN075-100MSEX.  Would this be a valid alternative?

For the transformer, S558-5999-T7-F from Belfuse is the same footprint, but it has common mode chokes.  Why are common mode chokes and issue for this application?

			

			
									
		                             
                        

    

                                            
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                 LTC9103
                
                                                    
                 Power-over-Ethernet (PoE) Interface Controllers
                
                                
            

    
    
		


				
				        
        		

			
						
									
												
			
			
                        
                                    
                        
                                                    
                        
            
			            
						
						
			
			
			
			
		


        
        
        
    
					

			





















	

	

					



			








	
	
	
	



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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Apr 23, 2026 12:21 PM
            
            
						
			            		
    
		
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    We cannot test all variations of FETs in an application, so we can't say with any certainty that a particular FET would work in an application.  That said, the specs look close enough.  It's something you'll have to test.
     
    As for the transformer, the short answer is: because the LTC9103 uses low‑frequency/DC signaling over the Ethernet magnetics, and a transformer with integrated common‑mode chokes can distort or attenuate that signaling. The disadvantage of using a transformer with common‑mode chokes is reduced signal integrity, timing errors, or outright communication failure of the controller’s isolated interface.
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
						
				
				
				
		
		
    
			
		
    
	
    

    

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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Apr 23, 2026 12:21 PM
            
            
						
			            		
    
		
										0
					
	
    

		
		
    
		
    
                                
                                                            
    
					
    We cannot test all variations of FETs in an application, so we can't say with any certainty that a particular FET would work in an application.  That said, the specs look close enough.  It's something you'll have to test.
     
    As for the transformer, the short answer is: because the LTC9103 uses low‑frequency/DC signaling over the Ethernet magnetics, and a transformer with integrated common‑mode chokes can distort or attenuate that signaling. The disadvantage of using a transformer with common‑mode chokes is reduced signal integrity, timing errors, or outright communication failure of the controller’s isolated interface.
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
						
				
				
				
		
		
    
			
		
    
	
    

    

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