Troubleshooting ③： Case When, Due to Surge, V_DS2 Rises to Above Secondary-Side MOSFET V_DS Voltage

Problem ① "Case When Secondary-Side MOSFET Suddenly Turns OFF" and problem ② “Case When Secondary-Side MOSFET Turns ON Due to Resonance Under Light Loading” are followed by explanation of countermeasures and points to be noted for problem ③, "Case in which a surge causes V_DS2 to rise above the rated V_DS of the secondary-side MOSFET".

Problem ③： Case When, Due to Surge, V_DS2 Rises to Above Secondary-Side MOSFET V_DS Voltage

Countermeasures

To address this problem ③ there are three countermeasures; but, similarly to the cases of problems① and ②, for each countermeasure there are tradeoffs that must be considered. We begin by listing the countermeasures and points to be noted for each.

Problem ③: Case When, Due to Surge, VDS2 Rises to Above Secondary-Side MOSFET VDS Voltage
Countermeasure	Points to be Noted
Countermeasure ③-1 Insert a capacitance between the drain and source of the secondary-side MOSFET	In the region where resonance occurs (no load to light loading), insertion of a capacitance worsens efficiency, and so characteristics must be checked.
Countermeasure ③-2 Increase the gate resistance value of the primary-side MOSFET	Upon increasing the gate resistance value, the efficiency is worsened, and there is the possibility of heat generation by the primary-side MOSFET M1, so again characteristics must be checked.
Countermeasure ③-3 Reduce the windings ratio Ns/Np of the transformer to lower VDS2	Here it is the VDS1 of the primary-side MOSFET that increases, and so the transformer windings ratio should be adjusted such that VDS1 does not exceed the VDS rating for the primary-side MOSFET M1.

※ Numbers "③-n" are used for countermeasures to indicate that they are meant to address Problem ③.

●Countermeasure ③-1: Insert a capacitance between the drain and source of the secondary-side MOSFET

By inserting a capacitor C_DS2 as the capacitance between the drain and source of the secondary-side MOSFET, overshoot of V_DS2 is smoothed. C_DS2 should have a value in the range 1000 pF to 6800 pF or so (reference values), but the capacitance ultimately used should be selected while checking the effect of the countermeasure. The figure below shows the place at which C_DS2 is inserted in the circuit diagram and the resulting waveform.

※Point to be noted: In the region in which resonance occurs (no load to light loads), an inserted capacitance causes the efficiency to be worsened, and so characteristics must be checked in order to find a point of compromise between smoothing of the overshoot and the operating efficiency.

●Countermeasure ③-2: Increase the gate resistance value of the primary-side MOSFET

By increasing the gate resistance R_GATE1 of the primary-side MOSFET and slowing the rise time (blunting the rise) of V_GS1, surges due to sharp voltage rises are suppressed, and overshoot of the secondary-side V_DS2 can be subdued. The figure below shows the inserted R_GATE1 and a waveform diagram showing the result. Values of R_GATE1 range from about 10 to 470 Ω (reference values).

※Point to be noted: Upon increasing the value of R_GATE1, efficiency may be degraded, or heat generation may occur in the primary-side MOSFET M1, so characteristics must be confirmed. Here too, adjustments must be made that consider the tradeoff between reducing the overshoot and the consequences for efficiency and heat generation.

●Countermeasure ③-3: Reduce the windings ratio Ns/Np of the transformer to lower V_DS2

By reducing the windings ratio of the transformer, the amplitude of V_DS2 is attenuated.

※Point to be noted: V_DS2 is attenuated, but on the other hand the V_DS1 of the primary-side MOSFET increases (see the diagram of the waveform after countermeasure implementation). Hence the transformer windings ratio must be adjusted such that V_DS1 does not exceed the V_DS rating of the primary-side MOSFET M1.

This concludes our discussion of examples of problems that may occur, and countermeasures and important points relating to these problems.