Load current detection in electrical power converters
A resonant converter comprises switching circuitry ( 1 ) for supplying pulses, at a controllable frequency, to a resonant circuit so as to power the primary circuit ( 3, 12 ) of a transformer ( 4 ). The secondary winding ( 5 a, 5 b ) of the transformer ( 4 ) delivers an AC signal which is rectified and then produces a load current. On the primary side, the converter has a resistor ( 13 ) for deriving a first electrical signal representative of the current in the primary circuit ( 3, 12 ), and an auxiliary winding ( 14 ) which is closely coupled to the secondary winding ( 5 a, 5 b ) and across which an auxiliary voltage is induced as a consequence of the close coupling of the winding ( 14 ) to the secondary winding ( 5 a, 5 b ). A resistor/capacitor combination ( 16, 17 ) integrates the auxiliary voltage with respect to time to derive a second electrical signal. Computational circuitry ( 19 ) combines the first and second signals so as effectively to subtract the second signal from the first signal to derive a difference signal which is representative of the load current reflected onto the primary side of the transformer ( 4 ) and is accordingly representative of the actual load current.
1. An electrical power converter comprising a transformer having a primary circuit and a secondary circuit, the primary circuit being energisable by an AC signal to induce a secondary AC signal across the secondary circuit for delivering a load current, wherein the converter has detecting circuitry for deriving an electrical signal representative of the load current, wherein the detecting circuitry comprises a circuit element for deriving a first electrical signal representative of the current in the primary circuit, auxiliary circuitry for deriving a second electrical signal representative of the magnetising current flowing in the transformer and computational circuitry for combining the first and second signals so as effectively to subtract the second signal from the first signal to derive a difference signal which is representative of the load current reflected onto the primary side of the transformer and is accordingly representative of the actual load current, wherein the auxiliary circuitry comprises an auxiliary winding which is closely coupled to a secondary winding of the secondary circuit and across which an auxiliary voltage is induced as a consequence of the close coupling to the secondary winding, and integrating circuitry for integrating the auxiliary voltage with respect to time to derive the second electrical signal.
2. A converter according to claim 1 , wherein the computational circuitry is arranged to either subtract the second signal from the first signal, or add the first signal to a reversed polarity value of the second signal.
3. A converter according to claim 1 , wherein the converter comprises first converter circuitry for converting a DC input into the AC signal to energise the primary circuit.
4. A converter according to claim 3 , wherein the converter also comprises second converter circuitry for converting the secondary AC signal into a DC output voltage for delivering the load current.
5. A converter according to claim 1 , wherein the circuit element is a resistor in series with the primary circuit of the transformer.
6. A converter according to claim 1 , wherein the integrating circuitry comprises a resistor/capacitor combination.
7. A converter according to claim 1 and including a synchronous rectifier for rectifying the difference signal to derive the signal representative of the actual load current.
8. A converter according to claim 1 and including an absolute value detector for deriving the modulus of the difference signal to derive the signal representative of the actual load current.
9. A converter according to claim 1 and including compensating circuitry for compensating for a DC current component in a magnetising inductance of the transformer.
10. A converter according to claim 9 , wherein the compensating circuitry is operable to generate an error signal, based on the difference between the first signal and the second signal, during a time window when the load current is at or close to zero, the error signal being fed back to generate an offset signal to alter the magnitude of the first signal or the second signal fed to the computational circuitry.
11. A method of deriving an electrical signal representative of load current produced by an electrical power converter having a transformer with a primary circuit and a secondary circuit, the method comprising deriving a first electrical signal representative of the current in the primary circuit, deriving a second electrical signal representative of the magnetising current flowing in the transformer using an auxiliary circuitry comprising an auxiliary winding which is closely coupled to a secondary winding of the secondary circuit and across which an auxiliary voltage is induced as a consequence of the close coupling to the secondary winding, wherein the auxiliary voltage is integrated with respect to time to derive the second electrical signal, and combining the first and second signals to derive a difference signal which is representative of the load current reflected onto the primary side of the transformer and is accordingly representative of the actual load current.
12. A method according to claim 11 , wherein the difference signal is representative of the variation of load current as a function of time.
13. The converter of claim 3 , wherein the converter is a resonant converter, and wherein the AC signal is fed into a resonant circuit, which comprises a magnetising inductance of the primary circuit, a capacitor, and a second inductance coupled to the magnetising inductance and to the capacitor.
14. A resonant electrical power converter comprising a transformer having a primary circuit and a secondary circuit, the primary circuit being energisable by an AC signal to induce a secondary AC signal across the secondary circuit for delivering a load current, wherein the converter has detecting circuitry for deriving an electrical signal representative of the load current, wherein the detecting circuitry comprises a circuit element for deriving a first electrical signal representative of the current in the primary circuit, auxiliary circuitry for deriving a second electrical signal representative of the magnetising current flowing in the transformer, computational circuitry for combining the first and second signals so as effectively to subtract the second signal from the first signal to derive a difference signal which is representative of the load current reflected onto the primary side of the transformer and is accordingly representative of the actual load current, and compensating circuitry for compensating for a DC current component in a magnetising inductance of the transformer, wherein the compensating circuitry is operable to generate an error signal, based on the difference between the first signal and the second signal, during a time window when the load current is at or close to zero, the error signal being fed back to generate an offset signal to alter the magnitude of the first signal or the second signal fed to the computational circuitry.
15. A resonant converter according to claim 14 , wherein the computational circuitry is arranged to either subtract the second signal from the first signal, or add the first signal to a reversed polarity value of the second signal.
16. A resonant converter according to claim 14 , wherein the converter comprises first converter circuitry for converting a DC input into the AC signal to energise the primary circuit.
17. A resonant converter according to claim 16 , wherein the AC signal is fed into a resonant circuit, which comprises a magnetising inductance of the primary circuit, a capacitor, and a second inductance coupled to the magnetising inductance and to the capacitor.
18. A resonant converter according to claim 16 , wherein the converter also comprises second converter circuitry for converting the secondary AC signal into a DC output voltage for delivering the load current.
19. A resonant converter according to claim 14 , wherein the circuit element is a resistor in series with the primary circuit of the transformer.
20. A resonant converter according to claim 14 , wherein the auxiliary circuitry comprises an auxiliary winding which is closely coupled to a secondary winding of the secondary circuit and across which an auxiliary voltage is induced as a consequence of the close coupling to the secondary winding, and integrating circuitry for integrating the auxiliary voltage with respect to time to derive the second electrical signal.