LM5046 Used in Current Fed Phase Shift Full Bridge DC Dc Converter

Mahesh,

The LM5046 is a new part, so we have not created a lot of different reference design for it.  There are example on the web of a 3.3V @ 30A, a 12V @20A, and a 720W 12V output using a current doubler. The LM5046 and LM5045 are pin for pin compatible, so if you search the TI website look for either part to see what is available. The main difference in the applications between the LM5405 and the LM5046, with the LM5046  you need to increase the transformer leakage inductance to achieve resonant soft-switching.

Terry

Hello,

Can someone explain the purpose of some of the feedback elements for the LM5045 application circuit?

1. Why are there 10 ohm resistors in the feedback resistor divider (one at the top, R19, and one at the bottom, R31)?  Does the resistor ratio have to be tuned beyond what is available in standard 1% values, e.g. R22 and R29?

2. What does D16 protect for? (BAT54WS-7-F Schottky)  Isn't the anode already clamped to 1.2V by U5?  How can the anode go above the cathode which is tied to Sbias?

3. Why is Vx needed to power the opto coupler and the op amp (U4)?  It looks like it set to 4.7V (D17), but there is already Sbias, which is 5.1V.

We tend to try design parts into our systems from the app notes, assuming they are proven designs and we understand them clearly, but there doesn't seem to be much avaiable explanation for these parts.  To reduce cost and component count, we'd like to remove as many as possible.

Thank you,

David Mulligan

Global Embedded Technologies

1-The 10 ohms resistors are  in the circuit in case a customer wanst to do remote output voltage sensing.

Normally you don't need to use resistors with a lower tolerance than 1%, it really deepens on what you need the output voltage regulation to be.  There is a tolerance stack-up between the resistors, and the voltage reference.  If you refer to the application note the load and line regulations is about 0.2%.

2-D16 is in the circuit to discharge C38 (the secondary side soft-start capacitor) when power is removed from the board.  If you don't discharge C38 and quickly re-apply VIN you may not get a secondary side soft-start and the result may be a non-monotonic start-up.

3-If you connect the anode of the opto (through a resistor) to the output, there are two feedback paths, one is from the error amplifier (U4) the second, which is typically called the fast lane, is through the opto.  If you think about it, if the output voltage quickly rises, or drops, the error amplifier to try to correct, but the opto will also  have more or less current, so it will responced faster than the error amplifier. Some times this is a good thing, but you need to account for it.  To simply things and eliminate the affects of the fast lane, a regulated voltage is connected to the anode of the opto.

Terry