Method for controlling a power converter using an auxiliary resonant commutation circuit

• US 5,047,913 A
• Filed: 09/17/1990
• Issued: 09/10/1991
• Est. Priority Date: 09/17/1990
• Status: Expired due to Term

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: De Doncker, Rik W. A. A., Lyons, James P. F.
• Primary Examiner(s): Beha, Jr., William H.
• Assistant Examiner(s): STERRETT, JEFFREY L
• Application Number:
  US07/583,910
• Time in Patent Office: 358 Days
• Field of Search: 363/95, 363/96, 363/98, 363/132, 363/136, 363/137, 363/138
• US Class Current: 363/95
• CPC Class Codes: H02M 7/4811 having auxiliary actively switched resonant commutation circuits connected to intermediate DC voltage or between two push-pull branches
• CPC Class Codes: H02M 7/4826 operating from a resonant DC source, i.e. the DC input voltage varies periodically, e.g. resonant DC-link inverters
• CPC Class Codes: H02M 7/5152 with separate extinguishing means
• CPC Class Codes: H02M 7/5236 in a series push-pull arrangement

Abstract

A method for controlling a power converter and an associated auxiliary commutation circuit ensures soft-switching of all switching devices employed therein. Such a power converter includes an inverter with at least two main switching devices per phase. Across each main switching device are connected an antiparallel diode and a relatively large snubber capacitor. The auxiliary resonant commutation circuit comprises two antiparallel-coupled auxiliary switching devices coupled in series with a resonant circuit including an inductor and the snubber capacitors. The gating and conduction times of the main and auxiliary switching devices are controlled so as to add boosting energy to the resonant operation, thus ensuring that the inverter output voltage at least reaches the positive and negative inverter rail voltages during each resonant commutation cycle. As a result, the control achieves commutation of the converter pole with substantially no switching losses.

Claims

1. A method for controlling a power converter of the type including an inverter having at least one phase, said inverter having at least two series-coupled main switching devices per phase, said series-coupled main switching devices being coupled to positive and negative rails of a dc supply, the junction between said main switching devices being coupled to a load, said main switching devices each having a snubber capacitor coupled in parallel therewith and an antiparallel diode coupled thereacross, said power converter further including a pair of series-coupled bus-splitting capacitors coupled between said positive and negative rails of said dc supply, said power converter further including an auxiliary resonant commutation circuit coupled between the junction joining said main switching devices and the junction joining said bus-splitting capacitors, said auxiliary resonant commutation circuit comprising a pair of antiparallel-coupled auxiliary switching devices coupled in series with a resonant circuit, said resonant circuit including an inductor and said snubber capacitors, said method comprising commutating load current between one of said main switching devices and the opposing one of said antiparallel diodes coupled across the other one of said main switching devices, when commutation is initiated with load current in said opposing one of said antiparallel diodes, by:

(a) applying a forcing potential of substantially one-half the dc supply voltage across said inductor by turning on one of said auxiliary switching devices;

(b) allowing current in said inductor to increase to the load current level and displace the load current flowing through said opposing one of said antiparallel diodes;

(c) turning on said other one of said main switching devices so as to enable the current through said inductor to increase to a predetermined boost current threshold above the load current level;

(d) turning off said other one of said main switching devices when current therethrough reaches said predetermined boost current threshold, thereby allowing said resonant circuit to resonate from the voltage level of one of said rails of said dc supply to the voltage level of the opposite rail thereof;

(e) clamping the output voltage of the respective inverter phase to the voltage level of the opposite rail of said dc supply by forward biasing the other one of said antiparallel diodes;

(f) turning on said one of said main switching devices when the voltage thereacross is substantially zero;

(g) allowing the current through said inductor to decrease to substantially zero; and

( (h) turning off said one of said auxiliary switching devices when the current therethrough is substantially zero;

whereby the load current flows through said one of said main switching devices.

2. A method for controlling a power converter of the type including an inverter having at least one phase, said inverter having at least two series-coupled main switching devices per phase, said series-coupled main switching devices being coupled to positive and negative rails of a dc supply, the junction between said main switching devices being coupled to a load, said main switching devices each having a snubber capacitor coupled in parallel therewith and an antiparallel diode coupled thereacross, said power converter further including a pair of series-coupled bus-splitting capacitors coupled between said positive and negative rails of said dc supply, said power converter further including an auxiliary resonant commutation circuit coupled between the junction joining said main switching devices and the junction joining said bus-splitting capacitors, said auxiliary resonant commutation circuit comprising a pair of antiparallel-coupled auxiliary switching devices coupled in series with a resonant circuit, said resonant circuit including an inductor and said snubber capacitors, said method comprising commutating load current between one of said main switching devices and the opposing one of said antiparallel diodes coupled across the other one of said main switching devices, when commutation is initiated with load current in said one of said main switching devices, when the load current is less than a high current threshold, by:

(a) applying a forcing potential of substantially one-half the dc supply voltage across said inductor by turning on one of said auxiliary switching devices;

(b) allowing current in said inductor to increase to a predetermined boost current threshold;

(c) turning off said one of said main switching devices when current therethrough reaches said predetermined boost current threshold above the load current level, thereby allowing said resonant circuit to resonate from the voltage level of one of said rails of said dc supply to the voltage level of the opposite rail thereof;

(d) clamping the output voltage of the respective inverter phase to the voltage level of the opposite rail of said dc supply by forward biasing said opposing one of said antiparallel diodes;

(e) allowing the current in said inductor to decrease to substantially zero; and

(f) turning off said one of said auxiliary switching devices when the current therethrough is substantially zero;

whereby the load current flows through said opposing one of said antiparallel diodes.

3. The method of claim 2 wherein said high current threshold is determined according to the following expression:

##EQU5## wherein I_T represents said high current threshold, C_r represents the effective capacitance of the parallel combination of said snubber capacitors, V_dc represents the dc supply voltage, and t_max represents the maximum time for commutation of each respective one of said main switching devices.

4. The method of claim 2 wherein said high current threshold is sufficiently less than the peak output current from said inverter such that the sum of said high current threshold and said predetermined boost threshold current does not exceed said peak output current.

5. A method for controlling a power converter of the type including an inverter having at least one phase, said inverter having at least two series-coupled main switching devices per phase, said series-coupled main switching devices being coupled to positive and negative rails of a dc supply, the junction between said main switching devices being coupled to a load, said main switching devices each having a snubber capacitor coupled in parallel therewith and an antiparallel diode coupled thereacross, said power converter further including a pair of series-coupled bus-splitting capacitors coupled between said positive and negative rails of said dc supply, said power converter further including an auxiliary resonant commutation circuit coupled between the junction joining said main switching devices and the junction joining said bus-splitting capacitors, said auxiliary resonant commutation circuit comprising a pair of antiparallel-coupled auxiliary switching devices coupled in series with a resonant circuit, said resonant circuit including an inductor and said snubber capacitors, said method comprising commutating load current between one of said main switching devices and the opposing one of said antiparallel diodes coupled across the other one of said main switching devices, when commutation is initiated with load current in said one of said main switching devices, when the load current is greater than a high current threshold, by:

(a) turning off said one of said main switching devices;

(b) allowing the load current to drive the output voltage of the respective inverter phase from the voltage level of one of said rails of said dc supply to the voltage level of the opposite rail thereof;

(c) clamping the output voltage of the respective phase of said inverter to the voltage level of the opposite rail of said dc supply by forward biasing said opposing one of said antiparallel diodes;

whereby the load current flows through said opposing one of said antiparallel diodes.

6. The method of claim 5 wherein said high current threshold is determined according to the following expression:

##EQU6## wherein I_T represents said high current threshold, C_r represents the effective capacitance of the parallel combination of said snubber capacitors, V_dc represents the dc supply voltage, and t_max represents the maximum time for commutation of each respective one of said main switching devices.

7. The method of claim 5 wherein said high current threshold is sufficiently less than the peak output current from said inverter such that the sum of said high current threshold and said predetermined boost threshold current does not exceed said peak output current.

8. A method for controlling a power converter of the type including an inverter having at least one phase, said inverter having at least two series-coupled main switching devices per phase, said series-coupled main switching devices being coupled to positive and negative rails of a dc supply, the junction between said main switching devices being coupled to a load, said main switching devices each having a snubber capacitor coupled in parallel therewith and an antiparallel diode coupled thereacross, said power converter further including a pair of series-coupled bus-splitting capacitors coupled between said positive and negative rails of said dc supply, said power converter further including an auxiliary resonant commutation circuit coupled between the junction joining said main switching devices and the junction joining said bus-splitting capacitors, said auxiliary resonant commutation circuit comprising a pair of antiparallel-coupled auxiliary switching devices coupled in series with a resonant circuit, said resonant circuit including an inductor and said snubber capacitors, said method comprising commutating load current between one of said main switching devices and the opposing one of said antiparallel diodes coupled across the other one of said main switching devices, when commutation is initiated with load current in said opposing one of said antiparallel diodes, when the load current is greater than a diode boost threshold, by:

(a) applying a forcing potential of substantially one-half the dc supply voltage across said inductor by turning on one of said auxiliary switching devices;

(b) allowing current in said inductor to increase to the load current level and displace the load current flowing through said opposing one of said antiparallel diodes;

(c) allowing said resonant circuit to resonate from the voltage level of one of said rails of said dc supply to the voltage level of the opposite rail thereof;

(d) clamping the output voltage of the respective inverter phase to the voltage level of the opposite rail of said dc supply by forward biasing the other one of said antiparallel diodes;

(e) turning on said one of said main switching devices when the voltage thereacross is substantially zero;

(f) allowing current in said inductor to decrease to substantially zero; and

(g) turning off said one of said auxiliary switching devices when the current therethrough is substantially zero;

whereby the load current flows through said one of said main switching devices.