PWM from HCPL-3120 gate driver

Question

I'm still in my Zeta Converter Project (Link to previous questions)

I can run the project in Proteus and already built one prototype but there is a problem. The gate driver output PWM is not smooth

Here is my schematic

If i'm using the same power source as converter to the gate driver, the PWM from gate driver is significantly better than isolated. You can see here is PWM from gate driver after using the same power source. But i'm curious why the current flow to converter is so big?

Answer 1

\$Q_2\$'s gate-source voltage is not being controlled. \$V_{GSQ2}\$ must be positively greater than the threshold voltage to turn \$Q_2\$ on significantly. Depending on the FET this can be greater as much as 10V. Since the source voltage is \$V_{OUT}\$, \$V_{GQ2}\ge V_{OUT}+10V\$.

To turn off \$Q_2\$, \$V_{GSQ2}\$ must be zero volts ( less than the threshold but zero is best). So \$V_{GQ2}=V_{OUT}\$.

So your next challenge is to find a way to control \$V_{GSQ2}\$. The current circuit controls \$V_{GQ2}\$ with noconsideration of the \$Q_2\$'s source voltage.