Power supply with current limiting circuits
A power supply ( 200 ) is provided having a plurality of independent current limiting circuits. A first current limiting circuit ( 6 ) provides protection against a short circuit or other extraneous load conditions, while a second current limiting circuit, using a trace on a PCB as a sensing element, is programmable on the basis of a time constant and a current level.
1. A method for a current limiting function in a power supply, comprising;
providing a first current limiting circuit which comprises:
a resistor, a PWM control, a MOSFET, a shunt regulator, and a first transistor and further comprising,
providing a second current limiting circuit on the secondary side of a transformer, located within the power supply,
using a PCB copper trace as a current sensing element,
further comprising the steps of:
(a) providing a resistor, a zener diode, and operational amplifiers;
(b) sensing a voltage from said PCB copper trace;
(c) comparing said voltage sensed with voltage developed from said zener diode;
(d) providing a second shunt regulator and an opto-coupler comprising a primary LED and a secondary transistor; and
(e) causing the power supply to initiate a shut down sequence when the output of said operational amplifiers increases to a high level thereby causing said secondary transistor to turn on fully and forcing current flow through said opto-coupler causing said PWM control to stop the gate drive to said MOSFET resulting in the shut down of the power supply to a latch-off condition.
2. The method for current limiting function in a power supply, according to claim 1 , further comprising the steps of:
(a) providing a temperature coefficient thermistor and a voltage reference divider network;
(b) inserting said temperature coefficient thermistor within said voltage reference divider network,
Wherein said thermistor eliminates any temperature drift associated with using said PCB copper trace as a current sensing element having a positive temperature coefficient.
3. The method for current limiting function in a power supply according to claim 1 , further comprising a second transistor and third transistor and a capacitor, wherein when said first transistor is operative, said second transistor cannot remain in a latched on state, whereby the current drain to said capacitor is only approximately 200 microamperes as drawn from said PWM control, thereby initiating a start sequence, wherein the power supply will start up and regulate again.