LT1952 Operation

Question

I'm designing a forward converter using the LT1952 and I'm trying to understand how does the IC prevent the transformer from saturation without a reset winding/circuit. The spec sheet said the programmable volt second clamp can do this but I just don't understand how the transformer can reset during the FET off time without providing any path back to the source. 
 I'm also looking at some of the demo circuits and saw there is an active reset circuit as shown below 
 
 Do I actually need that? The simulation example for the part on the website doesn't have a reset circuit either. 
 Best 
 Thien Nhien Huynh

ADIApproved · Accepted Answer

Hi Thien Nhien Huynh, 
 There is always a reset. It is either resonant or active reset. 
 In resonant reset the transformer magnetizing current drives the MOSFET drain voltage above Vin to reset the transformer. There may be a cap added across the MOSFET to reduce the peak voltage of MOSFET drain voltage. In active reset the transformer charges the reset cap network on the drain of Q13. The Q13 applies this voltage across the transformer winding during MOSFET OFF time. 
 The difference between the two methods is lower peak MOSFET voltage when ACTIVE reset is used. 
 
 You can read more about resonant reset here: Designing Single-Switch, Resonant-Reset, Forward Converters (maximintegrated.com))

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LT1952 Operation