Resistive feed rail generation in a power supply
View Patent ↗A circuit provides current from a high voltage output to a low voltage output through a resistor when a load at the low voltage output is removed. The resistor provides current to keep the VBL voltage above the set point of a switching regulator that provides output power to the VBL output. Accordingly, the regulator's output stage provides a switched 0% duty cycle output and saves power until a load is presented again. Presenting a signal to either a shutdown circuit or a sleep pin shuts down the entire controller until a load is presented again. A zener diode is used to clamp the VBL at a voltage higher than the set point of the controller to ensure that the controller operates with a 0% duty cycle output while in a no-load condition. The circuitry can be configured to operate in either a positive or negative powering scheme.
1. A system for reducing idle-mode power dissipation in a switching power supply having at least a high voltage output and a low voltage output, comprising:
means for clamping the low voltage output to a predetermined non-zero voltage upon the detection of a no-load condition on the low voltage output; and
means for maintaining the predetermined voltage at the low voltage output, the maintaining means being connected between the high voltage output and the low voltage output.
2. The system of claim 1 wherein the clamping means includes a reverse-biased zener diode connected between the low voltage output and ground wherein the output potentials of the power supply are negative with respect to ground.
3. The system of claim 1 wherein the clamping means includes a forward-biased zener diode connected between the low voltage output and ground wherein the output potentials of the power supply are positive with respect to ground.
4. The system of claim 1 wherein the voltage-maintaining means includes a resistor connected between the low voltage output and the high voltage output.
5. The system of claim 1 wherein a switching controller/converter having a predetermined set point is connected between an input to the power supply and the high voltage output.
6. The system of claim 1 wherein switching controller/converter having a predetermined set point is connected between an input to the power supply and the low voltage output.
7. A system for reducing idle-mode power dissipation in a power supply having a low-voltage voltage switching controller/converter connected between an input to the power supply and a low voltage output and a high-voltage switching controller/converter connected between the input to the power supply and a high voltage output, comprising:
means for clamping the low voltage output to a non-zero predetermined voltage upon the detection of a no-load condition on the low voltage output;
means for maintaining the predetermined voltage at the low voltage output, the maintaining means being connected between the high voltage output and the low voltage output, and
means for shutting down the low-voltage voltage controller/converter while the no-load condition is present on the low voltage output.
8. The system of claim 7 wherein the means for shutting down the low-voltage controller/converter includes a control input, a supply input and a supply output.
9. In the system of claim 8 wherein the output potentials of the high voltage output and the low voltage output are negative with respect to ground, the clamping means includes a reverse-biased zener diode connected in series with a forward biased diode between the low voltage output and ground, the anode of the zener diode being connected to the low voltage output and the anode of the forward biased diode connected to ground such that the cathodes of the reverse-biased zener diode and the forward biased diode are connected together at a cathode node.
10. The system of claim 9 wherein the control input to the shut-down means is connected to the cathode node, the supply input of the shut-down means is connected to the power supply input and the output of the shut-down means is connected to the supply input to the low-voltage controller such that when the low-voltage output is in a no-load condition, the cathode node attains the bias voltage of the forward biased diode, such that the cathode node voltage provides a voltage at the control input to the shutdown means that causes the shut-down means to interrupt power from the power supply input to the low-voltage controller input when the zener diode breaks down, while providing for the shut-down means to allow power to transfer from the power supply input to the low voltage controller input when the zener diode does not conduct in the reverse direction.
11. In the system of claim 8 , wherein the output potentials of the high voltage output and the low voltage output are positive with respect to ground, the clamping means includes a reverse-biased zener diode having its anode connected to the base of a common-emitter-connected NPN transistor and its cathode connected to the low voltage output.
12. The system of claim 11 wherein the control input to the shut-down means is connected to the collector of the common-emitter-connected NPN transistor, the supply input of the shut-down means is connected to the power supply input and the output of the shut-down means is connected to a supply input of the low-voltage controller such that when the low-voltage output is in a no-load condition, the collector voltage V CE of the NPN transistor provides a voltage at the control input to the shutdown means that causes the shut-down means to interrupt power from the power supply input to the low-voltage controller input when the zener diode breaks down, while providing for the shut-down means to allow power to transfer from the power supply input to the low voltage controller input when the zener diode does not conduct in the reverse direction.
13. In the system of claim 7 wherein the output potentials of the high voltage output and the low voltage output are negative with respect to ground, the clamping means includes a reverse-biased zener diode connected in series with a forward biased diode between the low voltage output and ground, the anode of the zener diode being connected to the low voltage output and the anode of the forward biased diode being connected to ground such that the cathodes of the reverse-biased zener diode and the forward biased diode are connected together at a cathode node and the cathode node is connected to a sleep pin for turning off the low voltage controller, the system further comprising a pull-up resistor connected between the sleep pin and the low voltage controller supply input.
14. In the system of claim 7 , wherein the output potentials of the high voltage output and the low voltage output are negative with respect to ground, the clamping means includes a reverse-biased zener diode connected in series with a forward biased diode between the low voltage output and ground, the anode of the zener diode being connected to the low voltage output and the anode of the forward biased diode connected to ground such that the cathodes of the reverse-biased zener diode and the forward biased diode are connected together at a cathode node and the cathode node is connected to a sleep pin for turning off the low voltage controller.
15. The system of claim 7 wherein the voltage-maintaining means includes a resistor connected between the low voltage output and the high voltage output.
16. In the system of claim 7 , wherein the output potentials of the high voltage output and the low voltage output are positive with respect to ground, the clamping means includes a reverse-biased zener diode having its anode connected to the base of a common-emitter-connected NPN transistor, the cathode of the zener diode being connected to the low voltage output, the collector of said NPN transistor being connected to a sleep pin for turning off the low voltage controller, the system further comprising a pull-up resistor connected between the sleep pin and the low voltage controller supply input.
17. A method for reducing idle-mode power dissipation in a switching power supply having at least a high voltage output controller/converter for controlling the voltage at a high voltage output and having a low voltage controller/converter for controlling the voltage at a low voltage output, comprising:
detecting a no-load condition on the low voltage output;
providing current from the high voltage output to the low voltage output through a means for maintaining a non-zero predetermined voltage at the low voltage output; and
clamping the low voltage output to the predetermined voltage that is higher in magnitude with respect to ground than an output set point of the low voltage controller.
18. The method of claim 17 , further comprising interrupting power being transferred from a supply input of the power supply to a supply input of the low voltage controller during a no-load condition.
19. The method of claim 18 wherein power is interrupted to the supply input of the low voltage controller by impressing a control signal on a sleep pin.
20. The method of claim 17 wherein the means for maintaining he predetermined voltage at the low voltage output includes a resistor.
21. The method of claim 17 wherein the clamping means includes a zener diode.