CURRENT ISSUE
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Issue #216 July 2008
Problem 3—How does the oscillator circuit work in the steady state?
Answer 3—The transformer “B” provides the feedback that allows the circuit to oscillate.
Assume that the lower transistor BG2 has just turned on (and BG1 is off). Current is flowing through C5, the lamp tube, the primary of transformer B, and L2. The rising current through B’s primary produces a positive voltage on the lower secondary that drives the base of BG2, and this positive feedback reinforces the fact that it is on. However, this current eventually peaks and begins to decay as C5 charges up to the value of the DC bus. This causes the voltage on the two secondaries of transformer B to reverse polarity, switching off BG2 and switching on BG1.
With BG1 switched on, current is driven the other way through L2, increasing in that direction until C5 has been discharged. At this point, the current begins to decrease, again reversing the voltages at the secondaries of transformer B and starting the cycle over again.
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