PWM controller with constant output power limit for a power supply
A PWM controller has a line voltage input that allows using a start-up resistor for both start-up and power-limit compensations so that it can save the power consumption, ease the PCB layout, and shrink the power supply size. In the integrated circuit, a current switch used for both start-up and line voltage sensing is composed of a diode and a switch transistor. A current multiplier is used to improve the precise by canceling the impact of the integrated resistor's absolute value, which is composed of a transistor loop, a constant current and a reference current. Thus, by properly selecting the value of the start-up resistor, an identical output power limit for low line and high line voltage input can be achieved.
1. A PWM controller for controlling an output power limit of a power supply, comprising:
a current switch unit, having a first terminal connected to a line input voltage of the power supply via a start-up resistor, a second terminal coupled to a power supply voltage, a third terminal connected to an under-voltage lockout unit, and a four terminal;
a current multiplier, having a first terminal coupled to the four terminal of the current switch unit, and a second terminal;
an adder, having a negative input terminal coupled to the second terminal of the current multiplier, a positive input terminal coupled to a reference current, and an output terminal;
a resistor, having a first terminal coupled between the third terminal of the adder, and a second terminal coupled to a ground voltage;
a buffer, having a input terminal coupled between the output terminal of the adder and the first terminal of the resistor, and an output terminal;
a first comparator, having a positive input terminal coupled to the output terminal of the buffer, a negative input terminal, and an output terminal;
a second comparator, having a positive input terminal coupled to the supply voltage, a negative terminal coupled to the negative terminal of the first comparator, and an output terminal;
a NAND gate, having two input terminals coupled to the output terminals of the first and second comparators, and an output terminal; and
a flip flop, coupled to the output terminal of the NAND gate; and an oscillator, coupled to the flip flop.
2. The PWM controller of claim 1 , wherein the current switch unit comprising:
a diode, having a anode connected to the input voltage, a cathode connected to the power supply voltage; and
a switch transistor, having a gate terminal connected to the under-voltage lockout unit, a drain to the input voltage, and a source terminal connected to the adder.