This is a compromise between the lowest possible current consumption (when the lamp is off) and the highest possible duty cycle that is allowed. These lamps use a rectifier and internally they actually operate off DC.A few remarks about the size of R3 and R4. Electronic lamps, such as the PL types, cannot be dimmed with this circuit either. T1 is switched asynchronously with the mains frequency and this can cause DC current to flow. Just to be clear, note that this circuit cannot be used to control inductive loads. At 230 V mains voltage, the voltage across the lamp is only 2.5 V lower, measured with a 100-W lamp. At this value the transistor is just about 100 % in conduction. This value may be considered the maximum duty cycle. At 94 % the voltage of 4.8 V proved to be just enough to cause T1 to conduct sufficiently. A higher duty cycle results in a lower voltage. When the voltage across T1 reduces, the voltage across D1 remains equal to 10 V up to a duty cycle of 88 %. An additional effect is that T1 will conduct a little longer than what may be expected from the PWM signal only. In order to reduce switching spikes as a consequence of parasitic inductance, the value of R2 has been selected to be not too low: 22 kΩ is a compromise between inductive voltages and switching loss when going into and out of conduction. The transistor in the optocoupler is connected to the positive power supply so that T1 can be brought into conduction as quickly as possible. This ensures the safety of the regulator. The optocoupler is anold acquaintance, the CNY65, which provides class-II isolation. R1 also functions as a normal current limiting device so that a ‘hard’ voltage can be applied safely. R1 is intended as protection for the LED in the optocoupler.
An optocoupler and resistor (R2) are used for driving the gate. The voltage across C2 is regulated to a maximum value of 10 V by R3, R4, C1 and D1. R5 limits the current pulses through D6 to about 1.5 A (as a consequence it is no longer a pure peak rectifier).
The power supply voltage for driving the gate is supplied by the voltage across the MOSFET.
0 kommentar(er)
