Primary-side sensing arrangements for power converters
A sensing arrangement and method a sense winding is used to provide a voltage which represents the voltage appearing across an in-circuit magnetic component. In a flyback phase, when the component is supplying the output, that voltage represents an output voltage and in a supply phase, the supply voltage. This arrangement provides a solution to the problem of the disparity in magnitude of sense winding output during the two phases by proving a pull-up resistor arranged to apply bias to the voltage measured, the pull-up being to a first level during the supply period and to a second value during the flyback period, the first and second levels being selected such that the voltage across the sense winding is scaled differently during the supply period and the flyback period. The invention is suitable for use in a transformer based flyback power converter in which the magnitude disparity problem may be exacerbated by a turns ratio.
1. A sensing arrangement for a circuit comprising a magnetic component arranged such that the component is driven during a supply period and provides an output during a flyback period when the component is not supplied, the arrangement comprising:
a sense winding coupled to the magnetic component;
an interface network coupled to the sense winding and arranged to provide a circuit node at which a voltage may be measured as indicative of the voltage across the sense winding; and
a fixed value pull-up resistor arranged to apply bias to the voltage measured, wherein the fixed value is chosen so that the circuit node is always positive over a range of input voltage;
the pull-up is to a first level during the supply period and to a second level during the flyback period;
the first and second levels being selected such that the voltage across the sense winding is scaled differently during the supply period and the flyback period.
2. A sensing arrangement as claimed in claim 1 , and wherein the scaling is such that a value measured during the supply period and a value measured during the flyback period are closer together than would have been the case had the pull-up been constant.
3. A sensing arrangement as claimed in claim 1 , and adapted to directly measure the voltage across the sense winding during both the supply period and the flyback period.
4. A sensing arrangement as claimed in claim 1 , and including control circuitry, the control circuitry having a drive output to which the pull-up resistor is connected and arranged to drive the drive output to the first level during the supply period and to the second level during the flyback period.
5. A sensing arrangement as claimed in claim 4 , and wherein a supply for said controller circuit is derived from said sense winding.
6. A sensing arrangement as claimed in claim 4 , and wherein said measurements are sensed at a same input pin of said control circuitry.
7. A sensing arrangement as claimed in claim 1 , and wherein the pull-up resistor is isolated from the interface network during one of said periods.
8. A sensing arrangement as claimed in claim 7 , and wherein said isolation is provided by a diode.
9. A sensing arrangement as claimed in claim 7 , and wherein said isolation is provided during said supply period.
10. A power converter embodying the sensing arrangement of claim 1 .
11. A power converter as claimed in claim 10 , and including a transformer to which the sense winding is coupled, wherein a drive output of a control circuitry drives a primary winding of said transformer.
12. A method of sensing for a circuit comprising a magnetic component arranged such that the component is driven during a supply period and provides an output during a flyback period when the component is not supplied, the arrangement including:
providing a sense winding coupled to the magnetic component;
providing an interface network coupled to the sense winding and arranged to provide a circuit node at which a voltage may be measured as indicative of the voltage across the sense winding; and
providing a fixed value pull-up resistor arranged to apply bias to the voltage measured, wherein the fixed value is chosen so that the circuit node is always positive over a range of input voltage; the
pulling-up to a first level during the supply period and to a second value during the flyback period; and
selecting the first and second levels such that the voltage across the sense winding is scaled differently during the supply period and the flyback period.
13. A method as claimed in claim 12 , and wherein the scaling is such that a value measured during the supply period and a value measured during the flyback period are closer together than would have been the case had the pulling-up been constant.
14. A method as claimed in claim 12 , and including the steps of measuring the voltage across the sense winding during both the supply period and the flyback period.
15. A method as claimed in claim 12 , and including providing control circuitry, the control circuitry having a drive output to which the pull-up resistor is connected and arranged to drive the drive output to a first level during the supply period and to a second level during the flyback period.
16. A method as claimed in claim 15 , and including deriving a supply for said controller circuitry from said sense winding.
17. A method as claimed in claim 15 , and including sensing said measurements at a same input pin of said control circuitry.
18. A method as claimed in claim 12 , and including isolating the pull-up resistor from the interface network during one of said periods.
19. A method as claimed in claim 18 , and including isolating the pull-up resistor from the interface network by means of a diode.
20. A method as claimed in claim 18 , and including isolating the pull-up resistor from the interface network during said supply period.