Frequency control method for self-oscillating circuit

A method for controlling a load current through a DC load in a secondary circuit wherein the secondary circuit is isolated from a primary circuit by an isolation transformer, the primary circuit including a DC-to-AC inverter, the DC-to-AC inverter including at least a first semiconductor switch and a second semiconductor switch, the two switches controlled by a switch controller to produce a switched input signal to a primary winding of the isolation transformer, the switch controller controlling the first and second semiconductor switches at a variable operating frequency responsive to a magnitude of a control current flowing out of a control terminal of the switch controller, the control current determined by a current control circuit, the secondary circuit including a secondary winding of the isolation transformer coupled to an AC-to-DC rectifier, the AC-to-DC rectifier providing the load current at a magnitude responsive to the switching frequency, the method comprising:

• sensing the load current to generate a sensed load current magnitude;
• comparing the sensed load current magnitude to a target load current magnitude;
• generating a feedback signal responsive to a difference between the sensed load current magnitude and the target load current magnitude;
• applying the feedback signal to a voltage superposition circuit to generate a superposition voltage responsive to the feedback signal;
• and
• applying the superposition voltage to an input node of the current control circuit to vary the control current flowing into the current control circuit from the control terminal of the switch controller to thereby vary the switching frequency of the switch controller, wherein the method further comprises:
• generating an increased superposition voltage when the sensed magnitude of the load current is less than the target load current magnitude;
• and
• applying the increased superposition voltage to the input node of the current control circuit to generate a decreased control current flowing into the current control circuit from the control terminal of the switch controller, the switch controller reducing the switching frequency in response to the decreased control current, to thereby increase the magnitude of the load current.

A system for controlling the current through a DC load, the system comprising:

• a switch controller having a first output and a second output, each output having an active state and an inactive state, the switch controller configured to turn on only one of the outputs to the respective active state at any time, the switch controller further configured to switch the first and second outputs at an operating frequency, the switch controller responsive to a control current on a control input to vary the operating frequency, wherein:
• the switch controller is responsive to the minimum current flowing out of the control input to operate at a minimum variable frequency;
• and
• the switch controller is responsive to the maximum current flowing out of the control input to operate at a maximum variable frequency;
• a first semiconductor switch having a control input connected to the first output of the switch controller, the first semiconductor switch having a first terminal connected to a first voltage rail and having a second terminal connected to a common switch node;
• a second semiconductor switch having a control input connected to the second output of the switch controller, the second semiconductor switch having a first terminal connected to the common switch node and having a second terminal connected to a second voltage rail;
• an isolation transformer having a primary winding AC-coupled between the common switch node and the second voltage rail, the isolation transformer having a secondary winding connected to an AC-to-DC rectifier, the AC-to-DC rectifier providing a load current that flows through the DC load;
• a current sensor that generates a sensor signal having a magnitude responsive to a magnitude of the load current;
• a comparator having a first input coupled to receive the sensor signal and having a second input that receives a reference magnitude, the comparator having an output that generates a feedback signal responsive to a difference between the magnitude of the sensor signal and the reference magnitude;
• a current control circuit coupled to the control input of the switch controller, the current control circuit comprising at least a first resistor in series with a second resistor between the control input and a voltage reference, the current control circuit further comprising an input node at a junction between the first resistor and the second resistor;
• and
• a voltage superposition circuit having an input and an output, the output of the voltage superposition circuit connected to the input node of the current control circuit, the input of the voltage superposition circuit connected to the output of the comparator, the voltage superposition circuit comprising an optocoupler having an input stage coupled to the input of the voltage superposition circuit, the optocoupler having an output stage having a variable impedance responsive to the feedback signal, the output stage of the optocoupler connected in series with at least one resistor between a supply voltage and the input node of the current control circuit to superimpose a variable voltage on the input node of the current control circuit, wherein:
• the voltage superposition circuit has at least a first state when the output stage of the optocoupler has a minimum impedance;
• the voltage superposition circuit has at least a second state when the output stage of the optocoupler has a maximum impedance;
• the superposition circuit applies a maximum superposition voltage to the input node of the current control circuit when the superposition circuit is in the first state, the current control circuit responsive to the maximum superposition voltage to enable a minimum current to flow out of the control input of the switch controller;
• and
• the superposition circuit applies no superposition voltage to the input node of the current control circuit when the superposition circuit is in the second state, the current control circuit responsive to the absence of the superposition voltage to enable a maximum current to flow out of the control input of the switch controller.