Common mode noise reduction apparatus and method

1. An apparatus comprising:

a common mode choke disposed between an input dc source and a primary side network of an isolated power converter;

a balance inductor coupled between an upper terminal of a primary winding of the isolated power converter and a negative terminal of the input dc source;

a first capacitor coupled between the upper terminal of a primary side of a transformer and an upper terminal of a secondary side of the transformer of the isolated power converter; and

a second capacitor coupled between a lower terminal of the primary side of the transformer and a lower terminal of the secondary side of the transformer of the isolated power converter.

2. The apparatus of claim 1 , wherein

the first capacitor is a first lumped inter-winding capacitor of the transformer; and

the second capacitor is a second lumped inter-winding capacitor of the transformer.

3. The apparatus of claim 1 , wherein the balance inductor is coupled with the common mode choke to form an integrated magnetic device.

4. The apparatus of claim 1 , further comprising a first balance capacitor connected in parallel with the first capacitor.

5. The apparatus of claim 1 , further comprising a second balance capacitor connected in parallel with the second capacitor.

6. The apparatus of claim 1 , further comprising an auxiliary inductor coupled between the primary side of the transformer and the balance inductor.

7. The apparatus of claim 1 , wherein the isolated converter is an LLC resonant converter.

8. A system comprising:

an isolated power converter comprising a primary side network coupled to an input dc source;

a transformer coupled to the primary side network;

a rectifier coupled to a secondary side of the transformer; and

an output filter coupled to the rectifier; and

a balanced impedance network comprising

a common mode choke disposed between the input dc source and the primary side network of the isolated power converter;

a balance inductor coupled between an upper terminal of a primary side of the transformer of the isolated power converter and a negative terminal of the input dc source;

a first capacitor coupled between the upper terminal of the primary side of the transformer and an upper terminal of the secondary side of the transformer of the isolated power converter; and

a second capacitor coupled between a lower terminal of the primary side of the transformer and a lower terminal of the secondary side of the transformer of the isolated power converter.

9. The system of claim 8 , further comprising an integrated magnetic device formed by the common mode choke and the balance inductor.

10. The system of claim 8 , wherein an inductance ratio between the balance inductor and the common mode choke is equal to a capacitance ratio between the first capacitor and the second capacitor.

11. The system of claim 8 , further comprising an auxiliary inductor disposed between the primary side of the transformer and the balance inductor, wherein a sum of the balance inductor and the auxiliary inductor is configured such that an inductance ratio between the sum and the common mode choke is equal to a capacitance ratio between the first capacitor and the second capacitor.

12. The system of claim 8 , further comprising a first balance capacitor connected in parallel with the first capacitor, wherein a sum of the first balance capacitor and the first capacitor is configured such that an inductance ratio between the balance inductor and the common mode choke is equal to a capacitance ratio between the sum and the second capacitor.

13. The system of claim 8 , further comprising a second balance capacitor connected in parallel with the second capacitor, wherein a sum of the second balance capacitor and the second capacitor is configured such that an inductance ratio between the balance inductor and the common mode choke is equal to a capacitance ratio between the first capacitor and the sum.

14. The system of claim 8 , wherein the isolated converter is an LLC resonant converter comprising:

a pair of switches connected in series;

a primary side winding coupled to the pair of switches; and

a secondary winding magnetically coupled to the primary side winding.

15. The system of claim 14 , wherein the secondary winding is a center-tapped winding.

16. A method comprising:

determining a first capacitance value of a first parasitic capacitor coupled between an upper terminal of a primary side of a transformer and an upper terminal of a secondary side of the transformer of an isolated power converter;

determining a second capacitance value of a second parasitic capacitor coupled between a lower terminal of the primary side of the transformer and a lower terminal of the secondary side of the transformer of the isolated power converter;

determining a inductance value of a common mode choke coupled between the primary side of the transformer and an input dc source; and

coupling a balance inductor between the upper terminal of the primary side of the transformer and a negative terminal of the input dc source, wherein an inductance value of the balance inductor is configured such that a ratio between the inductance value of the balance inductor and the inductance value of the common mode choke is equal to a ratio between the first capacitance value and the second capacitance value.

17. The method of claim 16 , further comprising:

coupling a first balance capacitor connected in parallel with the first parasitic capacitor.

18. The method of claim 16 , further comprising:

coupling a second balance capacitor connected in parallel with the second parasitic capacitor.

19. The method of claim 16 , further comprising:

coupling an auxiliary inductor between the upper terminal of the primary side of the transformer and the balance inductor.

20. The method of claim 16 , further comprising:

coupling the balance inductor with the common mode choke to form an integrated magnetic device.