Method and apparatus for reducing standby power in power supplies
First Claim
1. A method of reducing the power consumption under low-load or no-load conditions of a flyback power converter using a pulsed current switch to control current flow through the primary winding of the flyback transformer, comprising the steps of:
- a) maintaining the OFF time of said switch constant when the load on said converter exceeds a predetermined level, and b) increasing the OFF time of said switch as an inverse function of said load when said load is below said predetermined level.
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Accused Products
Abstract
Power consumption of a low-power flyback power converter under standby-load or no-load conditions is reduced by modulating the OFF time of the flyback transformer'"'"'s main switch as a function of the feedback control current when the load drops below a predetermined level. This modulation overcomes the conventional frequency-increasing modulation of the ON time at low output power levels so as to reduce the switching frequency, and hence the switching losses, to minimal levels as the load is reduced. Excessive frequency reduction can optionally be avoided by a clamping feature that limits the expansion of the OFF time.
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Citations
10 Claims
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1. A method of reducing the power consumption under low-load or no-load conditions of a flyback power converter using a pulsed current switch to control current flow through the primary winding of the flyback transformer, comprising the steps of:
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a) maintaining the OFF time of said switch constant when the load on said converter exceeds a predetermined level, and b) increasing the OFF time of said switch as an inverse function of said load when said load is below said predetermined level. - View Dependent Claims (2, 3)
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4. A method of reducing the power consumption under low-load or no-load conditions of a flyback power converter using a pulsed current switch to control current flow through the primary winding of the flyback transformer, comprising the steps of:
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a) producing a control signal representative of the load on said converter;
b) applying said control signal to a ramp generator to cause said ramp generator to generate a ramp having first and second stages, the first stage having a steep, substantially constant slope, and the second stage having a shallower, variable slope which is a function of said control signal;
c) comparing said ramp to a fixed reference; and
d) turning said switch ON when said ramp exceeds said reference. - View Dependent Claims (5)
e) comparing said control signal to a signal representative of the current through said switch; - and
f) turning said switch OFF when said switch current representative signal exceeds said control signal.
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6. A circuit for saving power in a power supply at low load or no load, comprising:
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a) a switch;
b) a transformer having a primary winding so connected as to store energy when said switch is ON and to release it when said switch is OFF;
c) a control signal source arranged to provide a control current signal which is an inverse function of the load on said power supply;
d) circuitry arranged to turn said switch OFF when current through said primary winding exceeds said control current signal;
e) a capacitor connected to turn said switch ON when said capacitor is charged above a predetermined level; and
f) circuitry arranged to charge said capacitor at a single, substantially fixed rate when said control current signal is below a predetermined level, and at a two-stage rate when said control current signal is above said predetermined level;
g) said two-stage rate including a first stage in which the charging rate is fixed, and a second stage in which the charging rate is slower and is variable as a function of said current control signal. - View Dependent Claims (7, 8, 9, 10)
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Specification