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Synchronous rectifier (LTC3901) simulation in LTspice malfunctions

Flyback

Well-Known Member
Hi,
The attached is a LTspice simulation of a full bridge with synchronous rectifiers using LTC3723 and LTC3901 . (LTC3901 is a sync rect controller)
If the 2:1 signal transformer has Lp = 10mH, then the sync rect drive malfunctions. If I increase LP of the signal transformer to 11mH (keeping the 2:1 turns ratio), then it functions properly.
(The signal transformer feeds the SYNC pin of the LTC3901)
Why is it malfunctioning?
The Sync pin waveform on the LTC3901 looks fine when LP=10mH…so why does it malfunction?

LTC3723 datasheet:

LTC3901 datasheet:
 

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alec_t

Well-Known Member
Most Helpful Member
In your schematic (as downloaded) the pins of the LT1693 symbols don't line up with the connections to those ICs. Perhaps there have been revisions of the symbols? My LTS was updated recently.
With the pin misalignment corrected the sim works ok for me with Lp = 10mH.
I note that the output gets regulated properly at 14V when the heavy or light load is active, but rises to 17V when the loads are inactive. That may well replicate real life, since some regulators need a minimum load in order to work correctly.
 
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Flyback

Well-Known Member
Thanks, but do the sync rects switch properly?
With Lp=10mH mine works and gets power to the output but the sync rects dont switch properly.
 

alec_t

Well-Known Member
Most Helpful Member
Please define 'properly'.
 

Flyback

Well-Known Member
Properly means the sync rect only conducts current when the transformer is in the power phase and is biasing its intrisic diode ON...but also the sync rect conducts during the free-wheel phase............

The sim working improperly shows the sync rect switchign on and off during an interval when it should have been on continuously.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
What do you see on the "sync" pin when it's misbehaving? Looking at the data sheet, it appears that is totally responsible for controlling the output switching times.
Other things can force the outputs off, but only sync transitions toggle them.

If you are getting a good clean sync and the outputs are wrong, that implies the model is messed up somehow.
 

Flyback

Well-Known Member
Hi,
The attached does now work once it settles down....the sync rects switch properly....but to get it to work properly i had to divide doen the fed back secondary current sense signal to a ridiculiusly low level......that shouldnt have impacted the sync rect switching, since, as Rjenkinsgb says, the SYNC signal is fine.
 

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