Gain compensation for optocoupler feedback circuit
A feedback circuit has an optical coupler with a feedback gain control. The feedback gain control includes an active element connected to vary current flow depending on changes in gain of the optical coupler.
1. A power supply, comprising:
a power transformer having an input side and an output side;
a rectifier connected to said output side of said power transformer and having an output at which is available an output voltage;
a operational amplifier connected to said output of said rectifier and connected to a first reference value, said operational amplifier having an output;
a first active element having a control input connected to said output of said operational amplifier;
an optical coupler having an input and an output isolated from said input, said input being connected to be controlled by said first active element;
a control circuit having an input connected to said output of said optical coupler and an output connected to said input side of said power transformer;
a voltage divider connected to receive at least a portion of a current through said first active element and said input of said optical coupler;
a second active element connected to selectively draw current from said first active element and said input of said optical coupler, said second active element operating relative to a second reference value to hold a gain through said optical coupler substantially constant.
2. A power supply as claimed in claim 1 , wherein said operational amplifier is a first operational amplifier, and further comprising:
a second operational amplifier connected to receive a voltage from said voltage divider and to receive said second reference value, said second operational amplifier having an output connected to a control input of said second active element.
3. A power supply as claimed in claim 1 , wherein said second active element is a shunt regulator having an input connected to said voltage divider.
4. A gain controlled feedback circuit, comprising:
an optical coupler having an input and an output isolated from said input;
a first transistor connected to said input of said optical coupler;
a first operational amplifier having an output connected to a control input of said first transistor, said first operational amplifier having first and second inputs connected to a first reference value and to a value to be controlled;
a voltage divider connected to said first transistor;
a second operational amplifier having first and second inputs connected to a second reference value and to said voltage divider; and
a second transistor having a control input connected to an output of said second operational amplifier, said second transistor having a controllable current path connected to said first transistor to draw off current from said voltage divider depending on gain variations of said optical coupler.