I am trying to find out a way to improve the rise /start up time of a series LC circuit placed in a full bridge (or half bridge) circuit.
Here is a simplified schematic. The circuit works very well, the voltage across the inductor reaches 500V, the resonant frequency is at 100kHz+.
Image A shows what happens to the voltage across the inductor when the bridge starts to oscillate (PWM inputs activated).
For the existing LC values this translates into approximately 0.5 ms of build up time.
I discovered the following:
If the bridge is stopped and Q1/Q4 are set to on while Q2/Q3 are off then the capacitor stays charged.
If POINT A is briefly grounded (using a piece of wire) then when the grounding is removed the capacitor discharges into the coil thus creating an instantaneous rise of the voltage across the coil – see Picture B.
This showed me that in theory is possible to reduce the oscillator rise time to zero if there is a way to discharge previously stored energy into the LC tank at start up.
Does anyone know to to address this issue?
Best Answer
This seems to work pretty well, but...
\$ \style{color:red;}{ CAVEAT \ \ !}\$
The purpose of this simulation was to determine whether a circuit topology was viable, and the components were selected to keep them from blowing up, but with little regard for optimization.
Basically, you generate a magnetic field around L2 by turning Q5 ON, and then when the current through L1 has built up sufficiently you turn Q5 OFF abruptly and start the MOSFET drive at the same time. L1 is tightly coupled to L2, so when the current through L2 stops, the field breaks down quickly, transferring most of its energy to L1, a la flyback transformer, immediately starting oscillation at its maximum amplitude and at the frequency determined by L1C1 and maintained by the MOSFET drive, which is tuned to operate on the same frequency.
Here's the LTspice circuit list just in case you want to play with the circuit: