Methods and circuits for sensing isolated power converter output voltage across the isolation barrier

1. A control circuit for an isolated power converter, comprising:

a first secondary side circuit that senses a secondary side output voltage of the isolated power converter and produces a pulse width modulation (PWM) signal having a logic high time (T_high) interval that is proportional to a value of the secondary side output voltage;

a first isolator that transfers the PWM signal across an isolation barrier to a primary side of the isolated power converter; and

a first primary side circuit including a processor that receives the PWM signal from the first isolator;

wherein the first primary side circuit comprises a sampling circuit including a capacitor that is charged and discharged according to the T_high interval of the PWM signal and a switch that shapes the PWM signal received from the first isolator by reducing a falling time and rising time of the PWM signal;

wherein the first primary side circuit samples a voltage across the capacitor at a sampling time set by a period of the PWM signal; and

wherein the sampled voltage across the capacitor is used by the processor to determine a gain of the sampling circuit and to control primary side power switching devices of the isolated power converter to regulate the secondary side output voltage at a selected value.

2. The control circuit of claim 1 , wherein the first secondary side circuit comprises a comparator that produces the PWM signal.

3. The control circuit of claim 1 , wherein the first secondary side circuit comprises a secondary side microcontroller that produces the PWM signal.

4. The control circuit of claim 1 , wherein the control circuit further includes a feedback circuit comprising:

a second secondary side circuit that senses the secondary side output voltage of the isolated power converter and uses the sensed secondary side output voltage and a reference voltage to produce an error voltage;

a second isolator that transfers the error voltage across the isolation barrier to the primary side of the isolated power converter;

wherein the first primary side circuit determines a steady state output voltage using the reference voltage and calculates a steady state ga in using the steady state output voltage, and uses the steady state gain to calibrate an actual gain of the first secondary side circuit to determine an actual secondary side output voltage;

wherein the first primary side circuit controls the primary side power switching devices of the isolated power converter to regulate the secondary side output voltage at the selected value.

5. The control circuit of claim 4 , wherein the second secondary side circuit comprises an error amplifier that produces the error voltage.

6. The control circuit of claim 1 , wherein the first primary side circuit uses a duration of the T_high interval of the PWM signal to generate a voltage having a magnitude that is proportional to the value of the secondary side output voltage, measures the magnitude of the generated voltage, and uses the measured magnitude of the generated voltage to determine the value of the secondary side output voltage.

7. A method for controlling a n isolated power converter, comprising:

sensing a secondary side output voltage of the isolated power converter using a first secondary side circuit that produces a pulse width modulation (PWM) signal having a logic high time (T_high) interval that is proportional to a value of the secondary side output voltage;

using a first isolator to transfer the PWM signal across an isolation barrier to a primary side of the isolated power converter; and

using a first primary side circuit to receive the PWM signal from the first isolator;

wherein the first primary side circuit comprises a sampling circuit including a capacitor that is charged and discharged according to the Thigh interval of the PWM signal, and a switch that shapes the PWM signal received from the first isolator by reducing a falling time and rising time of the PWM signal;

wherein the first primary side circuit samples a voltage across the capacitor at a sampling time set by a period of the PWM signal; and

wherein the sampled voltage across the capacitor is used by the first primary side circuit to determine a gain of the sampling circuit and to control primary side power switching devices of the isolated power converter to regulate the secondary side output voltage at a selected value.

8. The method of claim 7 , wherein the first secondary side circuit comprises a comparator that produces the PWM signal.

9. The method of claim 7 , wherein the first secondary side circuit comprises a secondary side microcontroller that produces the PWM signal.

10. The method of claim 7 , wherein the method further comprises:

using a second secondary side circuit that senses the secondary side output voltage of the isolated power converter and uses the sensed secondary side output voltage and a reference voltage to produce an error voltage;

using a second isolator to transfer the error voltage across the isolation barrier to the primary side of the isolated power converter;

wherein the first primary side circuit determines a steady state output voltage using the reference voltage and calculates a steady state ga in using the steady state output voltage, and uses the steady state gain to calibrate an actual gain of the first secondary side circuit to determine an actual secondary side output voltage;

wherein the first primary side circuit controls the primary side power switching devices of the isolated power converter to regulate the secondary side output voltage at the selected value.

11. The method of claim 10 , wherein the second secondary side circuit comprises an error amplifier that produces the error voltage.

12. The method of claim 7 , wherein the first primary side circuit uses a duration of the T_high interval of the PWM signal to generate a voltage having a magnitude that is proportional to the value of the secondary side output voltage, measures the magnitude of the generated voltage, and uses the measured magnitude of the generated voltage to determine the value of the secondary side output voltage.

13. An isolated power converter, comprising:

a first secondary side circuit that senses a secondary side output voltage of the isolated power converter and produces a pulse width modulation (PWM) signal having a logic high time (T_high) interval that is proportional to a value of the secondary side output voltage;

a first isolator that transfers the PWM signal a cross an isolation barrier to a primary side of the isolated power converter; and

a first primary side circuit including a processor that receives the PWM signal from the first isolator;

wherein the first primary side circuit comprises a sampling circuit including a capacitor that is charged and discharged according to the T_high interval of the PWM signal and a switch that shapes the PWM signal received from the first isolator by reducing a falling time and rising time of the PWM signal;

wherein the first primary side circuit samples a voltage across the capacitor at a sampling time set by a period of the PWM signal; and

wherein the sampled voltage across the capacitor is used by the processor to determine a gain of the sampling circuit and to control primary side power switching devices of the isolated power converter to regulate the secondary side output voltage at a selected value.