UCC27714: Two-Switch Forward Converter and UCC27714

Hello Tanvi,

I am an applications engineer supporting this device and will provide and explanation regarding your question.

UCC27714 Two Switch Forward Bias Operation.

 

With a two switch forward powering the HB to HS floating bias is different than a half bridge application. Both high side Mosfet and the low side Mosfet must be on at the same time to conduct primary current. For the typical bootstrap bias to work, the HS node must switch close to ground for the Cboot to charge from VDD through the boot diode. This occurs in a half bridge by the low side Mosfet turning on. In a two switch forward, HS will transition to ground after both Mosfets turn off and the polarity of the transformer winding reverses balancing the transformer volt-sec. Just turning on LO will not accomplish HS switching to ground. During initial startup and some operating conditions, the HS may not switch to ground.

Two auxiliary switches, which are not connected to the transformer winding, are used to accomplish guaranteeing the Cboot will be charged by forcing the HS node to switch near ground. The auxiliary switch connected to LO is configured as an open drain connecting to the other auxiliary switch gate. When LO is high the gate of the upper auxiliary switch is pulled near ground and turned off. When LO is low the upper auxiliary switch is connected to VDD thru Rboot which turns on the upper auxiliary switch. With the upper aux switch on, HS is pulled near ground thru the diode connected to the upper auxiliary switch drain. This allows the Cboot to charge from VDD.

Please confirm on the thread if this answers your questions.

Regards

Richard Herring

Hello Tanvi,
Let me address your questions in the last post.
The method for the HS to transition to ground after both Mosfets driving the transformer primary is the following. When both Mosfets are on there is primary current flowing in the transformer which is the sum of reflected load current from the load and current into the transformer primary magnetizing inductance. During higher load operation there is adequate current stored in the transformer winding inductance such that when the Mosfets turn off the transformer winding inductance will cause the transformer winding to reverse polarity and conduct into the diodes connected to Q1 drain and Q2 source. These diodes are for the purpose of limiting the transformer winding to ground and HV when the transformer primary reverses polarity. Under light load conditions, or narrow duty cycle, there may not be enough energy stored in the transformer primary inductance to result in HS fully transitioning to ground however.
For the second question. When LO is high and the gate if the upper auxiliary switch is pulled to ground, this is the normal on time of the power transformer and you do not want HS pulled to ground. When LO is low, and the transformer primary is expected to reverse polarity the HS will be pulled to ground to ensure the HB to HS capacitor can charge. This is to ensure the HB-HS capacitor can be charged for the 1st cycles, and also the narrow duty cycle conditions where the transformer may not fully clamp to ground during the magnetizing inductance current reset.

Regards,
Richard Herring