Plain Half-Bridge SMPS is Zero-Voltage -Switched?

[grizedale]

Hi,

The Half-Bridge SMPS is a Zero-Voltage-Switch-ON converter.

-Because when a FET turns off, the primary magnetising current continues to circulate, and does so by forward biasing the opposite FETs parallel diode....

...then when this FET is turned ON, it obviously has zero volts across it , because its diode is conducting.

So why is the Humble Half-Bridge SMPS never , ever advertised as a "Zero Voltage Switched "technology?

 

[WimRFP]

The above is only valid when each half cycle the transformer current changes direction. If not, the diode of the fet that is not about to switch carries the current.

 

[grizedale]

the fet that is "not about to switch" is the one thats just turned off.

...its diode couldnt possibly start carrying the current.....it would be carrying the current the wrong way.

An inductive current will do all it can to keep flowing, in the same direction.

 

[mtwieg]

The symmetric half bridge does not inherently achieve ZVS. It can be made to be ZVS if engineered properly (just like the flyback can be made quasiresonant), but that only works under certain conditions. If the duty cycle is short, then the primary current will fall to zero before the next switch turns on.

Also, I doubt most half bridge designs work at high enough frequencies for ZVS to provide a significant boost to efficiency.

 

[grizedale]

Hi,

Surely the magnetising current will persist for as long as the fet had just been on for?

....so at high duty cycle, we'll get ZVS, which is good, because high duty corresponds to high load, so we reduce switch on losses at high load which surely is great?