Power Electronics Circuits
In a forward converter, which of the following correctly explains why a transformer reset winding (or a clamp circuit) is needed?
- ATo switch the transformer turns ratio automatically and lower the output voltage when the secondary current becomes too large, protecting the diodes and switch from overcurrent
- BTo return the magnetizing energy stored in the core while the switch is on during the off-time, resetting the flux and preventing core saturation
- CTo keep a DC current flowing in the primary of the transformer so that the core stays magnetized in one direction
Show answer and explanation
Answer: B. To return the magnetizing energy stored in the core while the switch is on during the off-time, resetting the flux and preventing core saturation
A forward converter delivers power to the output through the transformer while the switch is on, and its output side has the same form as a buck converter with an inductor. While the switch is on, voltage stays applied to the primary winding, so the magnetizing current and the core flux keep increasing. Unless this flux is reset every period, it accumulates, the core saturates and a large current flows. With a reset winding of the same number of turns as the primary, the reset takes as long as the on-time, so the duty cycle is limited to 0.5 or less.
- A. A reset winding is not for switching the turns ratio.
- B. Correct. The flux is reset every period to prevent core saturation.
- C. Magnetizing the core in one direction with DC makes it saturate more easily, which is counterproductive.