Why is the output of the MAX22256 much higher than expected under no load?

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Accepted Answer

The MAX22256 is an unregulated transformer driver, so it is normal that the floating voltage of the output under no load will be higher than designed as you can see from the below plot:

The magnitude of the no load voltage directly correlates to the parasitics in the circuit, namely the leakage inductance of the transformer windings. Another source is the rectifying diodes, choosing fast, low capacitance rectifying diodes over Schottky diodes can also help. 
 
 Lastly, a snubber or clamp circuit can be used on the output to control the maximum voltage under no load conditions. The example circuits below (MAX256 part is used, but is functionally the same) show how a Zener Diode can be used to clamp the output voltage to a known value, protecting the downstream LDO in this instance:

Please note that a Zener diode's clamp voltage is dependent on the current, so care must be taken to choose a Zener diode that will not only clamp the voltage to an acceptable level but also ensuring not too much power is dissipated in the Zener diode itself during clamping.

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MAX22256: FAQ