| 摘要 |
The control range of a flyback converter switched-mode power supply is limited, inter alia, by the limiting resistor in the base circuit, which limiting resistor is provided to protect the power switching transistor, since the control current which is required for full switching-through is no longer reached when the supply voltage is below a specific threshold. According to the teaching of the invention, a significant expansion of the control range is achieved in such a manner that a constant-current source is connected in the base circuit of the power switching transistor instead of a fixed current-limiting resistor. The power switching transistor thus remains completely switched through, even at a very low supply voltage, down to the residual voltage of the constant-current source, which is in general designed as a transistor, that is to say down to a voltage drop of only a few tenths of a volt. At the same time, there is no risk whatsoever of current overloading in the base circuit at a very high supply voltage, because the constant-current source limits the control current even better than the otherwise normal fixed resistor.
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