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Old 09-12-2010, 10:17 AM   #3760 (permalink)
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You hit on my exact point. The high voltage inductive feedback hits the drain when the mosfets open (at which time the capacitors on the wiki would be effectively disconnected from the drain circuit due to opening of the mosfet switch). Curiously, the wiki circuit doesn't show any way of smoothing out this high voltage feedback on the mosfet drain except for the flyback diode. Without any dampening circuitry on the drain, the voltages can reach many times the supply voltage (as shown on the O-scope waveforms above) and easily blow any mosfet. If we're using 200V mosfets and 120V battery supply, the inductive feedback may reach 800 Volts or more, blowing the 200V mosfets. As the O-scope waveforms show, the flyback diode has a very limited ability to dampen the inductive feedback hitting the drain after the mosfets open. We need more than just the diode if we're going to prevent mosfet failure. Also, the diode tries (only partially successfully according to the O-scope waveforms) to shunt the high voltage drain voltage to V+. When this energy is directly shunted to V+, it wastes this energy and reduces the efficiency of the system. If we use capacitors to store this inductive feedback voltage between cycles, we INCREASE the efficiency. As I learned from testing, these capacitors cannot be placed in parallel with the inductive load without a series resistor. Without a resistor, there is nothing to dissipate the energy from oscillation from feedback between capacitor and inductor and sets up a vicious oscillatory cycle.

Please note: I edited this post because I had accidentally reversed the words open and closed, causing confusion).

Last edited by princeton; 09-12-2010 at 03:06 PM..
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