I am making a 55A58V charger, which requires 0-100% output voltage and current, so I use MCU and UCC28950 to build the control framework. The MCU generates the softstart/enable siganl to SS/EN pin and reference voltage to EA+ pin for UCC28950. The UCC28950 generates the PWM pulses for PSFB. The operational amplifier inside UCC28950 is designed as a voltage follower. I will adjust the voltage on EA+ to affect the phase shift angle to adjust the output voltage.
The detailed design of UCC28950 is shown as following pic.
Q1: If the MCU provides SS/EN and reference voltage at the same time, the oscilloscope shows that the voltage of EA+ (cyan curve) quickly rises to the set voltage, while the voltage on SS/EN(blue curve) gradually rises to the maximum voltage. I think that due to the SS/EN voltage, the phase shift angle should gradually increase from zero to the angle set by EA+. But in reality, the phase shift angle reaches its maximum value, which means that the output reaches its maximum value and then decreases to the angle set by EA+.wHY?
Q2: If I give the start signal first, delay for a period of time (such as 100ms), and then give the reference voltage of EA+, there will be no situation encountered in question 1. The phase shift angle will jump directly from zero to the angle set by EA+. WHY?
Q3:From the above waveforms, it can be seen that the voltage on the SS/EN pin rises linearly from 0V at first, but it will suddenly rise to full scale later. Why?