| 摘要 |
A method of improving modulation resolution in a flyback converter by pulsewidth- and/or position-modulation of a subtractive constant current with specific timing considerations is applicable to boost or buck/boost topologies. A typical system of this type uses a pulsewidth-modulated (PWM) circuit to provide a control pulsewidth in response to an incoming data stream. The control pulsewidth is used to drive a switching device which, when energized, charges an inductor. When the switching device is released the energy stored in the inductor "flies back" through a diode into a capacitor and load resistance. According to this invention, a constant current sink, gated under control of the PWM controller, is used to improve modulation resolution through pulsewidth- and position-modulation of a subtractive constant current with specific timing considerations. In the preferred embodiment, the constant current sink is implemented using a transistor in conjunction with a resistor network. When enabled, the current sinks diverts current from the capacitor receiving the energy from the inductor through the diode. This action changes the slope of the discharge of the capacitor. The activation of the current sink is preferably centered on the discharge slope, with the duration being variable as a function of demand for the subtractive modulation in accordance with the incoming data stream. Where constant modulation resolution is not required, however, position modulation of constant current pulsewidths can be used to increase the available dynamic range.
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