Method and apparatus for balancing currents

1. A method for balancing currents of two or more parallel-connected power semiconductor switches during an on-state of the switches, wherein a control terminal of each switch is driven by a driver unit, the method comprising:

determining ratios between the currents through the switches;

for each switch, controlling a voltage at the control terminal on the basis of the ratios by controlling a level of a supply voltage of the driver unit of that switch; and

for each switch, after a turn-on commutation transient, modulating an output of the driver unit, wherein a duty cycle of the modulation is controlled to reduce a time required for a transition of the voltage at the control terminal from one voltage level to another;

wherein the supply voltage is generated by using a switching converter having a duty cycle responsive to a supply control signal determined on the basis of the ratios; and

wherein controlling the level of the supply voltage of the driver unit comprises generating a first supply voltage level by using a first duty cycle of the switching converter, and after the turn-on commutation transient, generating a supply voltage level by using a second duty cycle of the switching converter.

2. A method as claimed in claim 1 , wherein the switching converter has a period of a third duty cycle between the first duty cycle and the second duty cycle, and

wherein the third duty cycle is set to minimize a transition time from the first supply voltage level to the second supply voltage level.

3. A method as claimed in claim 2 , wherein the output of the driver unit is at an on-state during the first and the second duty cycles of the switching converter, and is modulated during the period of third duty cycle.

4. A method as claimed in claim 3 , wherein the determining of the ratios between the currents of the switches comprises

estimating the currents of the switches on the basis of bond voltages of the switches; and

calculating the ratios on the basis of the estimated currents.

5. A method as claimed in claim 4 , wherein the estimating of the currents of the switches comprises, for at least one current:

integrating a bond voltage for a period of time;

measuring the integrated voltage; and

estimating the current on the basis of the integrated voltage.

6. A method as claimed in claim 4 , wherein the estimating of the currents of the switches comprises:

comparing the bond voltage with a set limit;

measuring the time the bond voltage exceeds the set limit; and

estimating the current on the basis of the measured time.

7. A method as claimed in claim 3 , comprising:

determining the supply control signals by using a balance controller configured to equalize the ratios between the currents.

8. A method as claimed in claim 3 , wherein a switching frequency of the modulation of the output of the driver unit after the turn-on commutation transient is at least the cut-off frequency of the low-pass characteristics of the control terminal.

9. A method as claimed in claim 2 , wherein the determining of the ratios between the currents of the switches comprises

estimating the currents of the switches on the basis of bond voltages of the switches; and

calculating the ratios on the basis of the estimated currents.

10. A method as claimed in claim 9 , wherein the estimating of the currents of the switches comprises, for at least one current:

integrating a bond voltage for a period of time;

measuring the integrated voltage; and

estimating the current on the basis of the integrated voltage.

11. A method as claimed in claim 10 , comprising:

determining the supply control signals by using a balance controller configured to equalize the ratios between the currents.

12. A method as claimed in claim 1 , wherein the determining of the ratios between the currents of the switches comprises

estimating the currents of the switches on the basis of bond voltages of the switches; and

calculating the ratios on the basis of the estimated currents.

13. A method as claimed in claim 12 , wherein the estimating of the currents of the switches comprises, for at least one current:

integrating a bond voltage for a period of time;

measuring the integrated voltage; and

estimating the current on the basis of the integrated voltage.

14. A method as claimed in claim 12 , wherein the estimating of the currents of the switches comprises:

comparing the bond voltage with a set limit;

measuring the time the bond voltage exceeds the set limit; and

estimating the current on the basis of the measured time.

15. A method as claimed in claim 12 , comprising:

determining the supply control signals by using a balance controller configured to equalize the ratios between the currents.

16. A method as claimed in claim 12 , wherein a switching frequency of the modulation of the output of the driver unit after the turn-on commutation transient is at least the cut-off frequency of the low-pass characteristics of the control terminal.

17. A method as claimed in claim 1 , comprising:

determining the supply control signals by using a balance controller configured to equalize the ratios between the currents.

18. A method as claimed in claim 1 , wherein a switching frequency of the modulation of the output of the driver unit after the turn-on commutation transient is at least the cut-off frequency of the low-pass characteristics of the control terminal.