Fast startup control circuit for switch mode power supply
A control circuit for a switch mode power supply (SMPS) includes a power switch for coupling to a primary winding of the power supply and a startup resistor coupled to an external input voltage and to a control terminal of the power switch. The control circuit also includes a controller. During startup, the controller is configured to cause the power switch to amplify a startup current from an external input voltage through the startup resistor and provide a startup power to the controller. During normal operation, the controller is configured to provide a power switch control signal to turn on and off the power switch for controlling a current flow in the primary winding and regulating an output of the power supply. The controller is configured to provide a current signal for driving an NPN power switch and to provide a voltage signal for driving an NMOS power switch.
1. A controller for a switch mode power supply (SMPS) having a power switch coupled to a primary winding for regulating an output of the power supply, wherein the power switch is either an NPN power switch or an NMOS power switch, wherein the controller is configured to provide a current signal for driving the NPN power switch, and the controller is also configured to provide a voltage signal for driving the NMOS power switch.
2. The controller of claim 1 , comprising:
a pulse generation circuit for providing a pulsed control signal; an RC pump circuit including a resistor coupled to a capacitor at a common node, the resistor being coupled to a first bias voltage, the capacitor being configured to receive the pulsed control signal; and
a drive transistor having a gate coupled to the common node, and a source of the drive transistor coupled to the NPN or NMOS power switch for providing the current signal or the voltage signal, respectively, for regulating an output of the power supply.
3. The controller of claim 2 , further comprising:
a current mirror coupled between the drive transistor and a voltage supply; and
a switch transistor coupled between the drive transistor and a ground.