Accuracy of a volt-second clamp in an isolated DC-DC converter
View Patent ↗A novel system and methodology for providing a volt-second clamp. A DC/DC conversion system configured for producing an output voltage in response to an input voltage has a transformer with a primary winding responsive to the input voltage and a secondary winding for producing the output voltage. The conversion system has a power switch coupled to the primary winding of the transformer and controlled with a converter control signal, such as a PWM control signal. The power switch is further controlled by a comparator that compares an input value supplied to its input with a variable reference value so as to prevent magnetic flux density of the transformer from increasing to an undesired level. The input value of the comparator is produced by a comparator input circuit as a function of the input voltage and an on-time of the power switch. A reference circuit produces the reference value that varies as a function of the input voltage.
1. A DC/DC conversion system configured for producing an output voltage in response to an input voltage and having a transformer with a primary winding responsive to the input voltage and a secondary winding for producing the output voltage, comprising:
a power switch coupled to the primary winding of the transformer and controlled with a converter control signal,
a comparator for comparing an input value supplied to the comparator with a variable reference value to limit on-time of the power switch so as to prevent the magnetic flux density of the transformer from increasing to an undesired level,
a comparator input circuit including an RC circuit supplied with the input voltage for producing the input value of the comparator representing a product of the input voltage and the on-time of the power switch, and
a reference circuit for producing the variable reference value that varies as a function of the input voltage.
2. The system of claim 1 , wherein the reference circuit is configured for producing the variable reference value based on a reference adjustment value inversely proportional to the input voltage.
3. The system of claim 2 , wherein the reference circuit is configured for producing the reference adjustment value by dividing a pre-selected constant value by a value of the input voltage.
4. The system of claim 3 , wherein the reference circuit is configured for subtracting the reference adjustment value from a fixed reference value to produce the variable reference value.
5. The system of claim 1 , wherein the comparator is arranged on an integrated circuit and the comparator input circuit includes a capacitor arranged externally with respect to the integrated circuit.
6. The system of claim 5 , wherein the comparator input circuit further includes a switch responsive to an output signal of the comparator to control charging of the capacitor.
7. A system for controlling a circuit having a transformer arranged between input circuitry and output circuitry to galvanically isolate the output circuitry from the input circuitry, the system comprising:
a power switch controlled to apply voltage to the transformer,
a switch control circuit responsive to a difference between a variable control value and a variable reference value to limit on-time of the power switch so as to prevent the magnetic flux density of the transformer from increasing to an undesired level,
a control value generating circuit including an RC circuit supplied with the voltage applied to the transformer, the RC circuit being configured for producing the control value representing a product of the voltage applied to the transformer and the on-time of the power switch, and
a reference circuit for producing the reference value that varies as a function of the voltage applied to the transformer.
8. The system of claim 7 , wherein the reference circuit is configured for producing the variable reference value based on a reference adjustment value inversely proportional to the voltage applied to the transformer.
9. The system of claim 8 , wherein the reference circuit is configured for producing the reference adjustment value by dividing a pre-selected constant value by a value of the voltage applied to the transformer.
10. The system of claim 9 , wherein the reference circuit is configured for subtracting the reference adjustment value from a fixed reference value to produce the variable reference value.
11. The system of claim 7 , wherein the switch control circuit is arranged on an integrated circuit and the control value generating circuit includes a capacitor arranged externally with respect to the integrated circuit.
12. The system of claim 11 , wherein the control value generating circuit further includes a switch responsive to an output signal of the switch control circuit to control charging of the capacitor.
13. A method for limiting magnetic flux density in a transformer used to galvanically isolate an output of a DC/DC converter from an input voltage, the method comprising the steps of:
producing a control value representing a product of the input voltage and an on-time of a switch that controls the DC/DC converter,
producing a variable reference value as a function of the input voltage, and
based on a difference between the control value and the variable reference value, limiting the on-time of the switch to prevent the magnetic flux density of the transformer from increasing to an undesired level.
14. The method of claim 13 , wherein the variable reference value is produced based on a reference adjustment value inversely proportional to the input voltage.
15. The method of claim 14 , wherein the reference adjustment value is produced by dividing a pre-selected constant value by a value of the input voltage.
16. The method of claim 15 , wherein the variable reference value is produced by subtracting the reference adjustment value from a fixed reference value.
17. A DC/DC conversion system configured for producing an output voltage in response to an input voltage and having a transformer with a primary winding responsive to the input voltage and a secondary winding for producing the output voltage, comprising:
a power switch coupled to the primary winding of the transformer and controlled with a converter control signal,
a comparator for comparing an input value supplied to the comparator with a variable reference value to control the power switch so as to prevent the magnetic flux density of the transformer from increasing to an undesired level,
a comparator input circuit for producing the input value of the comparator as a function of the input voltage and an on-time of the power switch, and
a reference circuit for producing the variable reference value that varies as a function of the input voltage,
wherein the reference circuit is configured for:
producing the variable reference value based on a reference adjustment value inversely proportional to the input voltage,
producing the reference adjustment value by dividing a pre-selected constant value by a value of the input voltage, and
subtracting the reference adjustment value from a fixed reference value to produce the variable reference value.
18. A system for controlling a circuit having a transformer arranged between input circuitry and output circuitry to galvanically isolate the output circuitry from the input circuitry, the system comprising:
a power switch controlled to apply voltage to the transformer,
a switch control circuit responsive to a difference between a variable control value and a variable reference value to control the power switch so as to control the voltage applied to the transformer,
a control value generating circuit for producing the control value as a function of the voltage applied to the transformer and an on-time of the power switch, and
a reference circuit for producing the reference value that varies as a function of the voltage applied to the transformer,
wherein the reference circuit is configured for:
producing the variable reference value based on a reference adjustment value inversely proportional to the voltage applied to the transformer,
producing the reference adjustment value by dividing a pre-selected constant value by a value of the voltage applied to the transformer, and
subtracting the reference adjustment value from a fixed reference value to produce the variable reference value.
19. A method for limiting magnetic flux density in a transformer used to galvanically isolate an output of a DC/DC converter from an input voltage, the method comprising the steps of:
producing a control value as a function of the input voltage and an on-time of a switch that controls the DC/DC converter,
producing a variable reference value as a function of the input voltage, and
based on a difference between the control value and the variable reference value, controlling the supply of the input voltage to the transformer,
wherein the variable reference value is produced based on a reference adjustment value inversely proportional to the input voltage,
the reference adjustment value is produced by dividing a pre-selected constant value by a value of the input voltage, and
the variable reference value is produced by subtracting the reference adjustment value from a fixed reference value.