Control methods and power converters suitable for TRIAC dimming
Control methods and apparatuses providing a holding current for a TRIAC dimmer are disclosed. A control method is suitable for a power controller powered by an operation power source. A high-voltage device in the power controller is connected between the operation power source and an input power source, from which the high-voltage device drains a conduction current. An operation voltage of the operation power source is detected, and, during a startup procedure, the conduction current is forwarded to charge the operation power source. A detected voltage representing an input voltage of the input power source is provided, and if the detected voltage is below a setting voltage, the conduction current is forwarded to a ground line instead of charging the operation power source.
1. A power converter suitable for converting an input power source into an output power source, the power converter comprising:
a power controller powered by an operation power source with an operation voltage, comprising:
a high-voltage pin coupled to the input power source;
a high-voltage device capable of draining a conduction current from the input power source via the high-voltage pin;
a high-voltage startup control circuit, for detecting the operation voltage;
a cutoff-time detection circuit for detecting an input voltage of the input power source to provide a detected voltage; and
a demultiplexer coupled to the high-voltage device, the high-voltage startup control circuit and the cutoff-time detection circuit, for forwarding the conduction current to the operation power source during a startup procedure and to a ground line when the detected voltage is below a setting voltage;
wherein the conduction current charges the operation power source during the startup procedure; and
the conduction current goes through the high-voltage device and does not charge the operation power source when the detected voltage is below the setting voltage.
2. The power converter of claim 1 , wherein the cutoff-time detection circuit comprises two resistors connected in series between the high-voltage pin and the ground line.
3. The power converter of claim 1 , further comprising:
a transformer with a primary winding, a secondary winding, and an auxiliary winding; and
two resistors connected in series via a joint, and between two ends of the auxiliary winding;
wherein the power controller comprises a resistance detector coupled to the joint for providing the setting voltage.
4. The power converter of claim 3 , wherein the power controller is a pulse-width modulator providing a PWM signal to a power switch controlling a primary-winding current through the primary winding.
5. The power converter of claim 3 , wherein the input power source is an alternating-current input power source, the power converter further comprising:
a bridge rectifier, for rectifying the input power source to provide a direct-current voltage power line coupled to the primary winding.
6. The power converter of claim 1 , further comprising:
a current-limiting resistor, connected between the high-voltage pin and the input power source.
7. The power converter of claim 6 , further comprising:
a bipolar junction transistor with a base, an emitter, and a collector;
wherein the base and the emitter are coupled to two ends of the current-limiting resistor respectively, and the collector is coupled to the ground line.
8. The power converter of claim 1 , further comprising:
a TRIAC dimmer coupled to receive an alternating-current power source and to output the input power source.