Method and apparatus for matching turn-off times of parallel connected semiconductor switching devices

In a power switching system of the type employing a plurality of switching devices connected in parallel between two electrical power sources, one of which comprises a DC current source, said devices configured to be simultaneously conductive or simultaneously non-conductive and being rendered individually non-conductive in response to an application of respective turn-off control signals, a method of controlling turn-off of said devices so as to force a time of turn off of all said devices to be the same where individual turn-off times of the respective devices are not necessarily the same, comprising the steps:

• (a) generating individual current signals respectively representative of the current magnitude within each of said switching devices;
• (b) generating an average current signal representing the average current magnitude within said plurality of switching devices;
• (c) generating a turn-off control signal for said switching devices;
• (d) generating a reference signal from said average current signal;
• (e) generating a first signal when the magnitude of a selected one of said individual current signals is less than said reference signal;
• (f) generating a second signal when the magnitude of said average current signal is less than said reference signal;
• and
• (g) adjusting a time of application of the turn-off control signal for a following conduction cycle of said devices as a function of a time difference between said first or second signal.

In a system including a power converter having at least one set of switching devices comprised of a plurality of devices connected in parallel for transferring electrical power between two electrical power sources, a turn-off balance control circuit for rendering the devices of said set of switching devices non-conductive simultaneously, comprising:

• (a) means for generating individual current signals respectively representative of a current and magnitude within each of said switching devices;
• (b) means coupled to said current signals for generating an average current signal representing an average current magnitude within said plurality of switching devices;
• (c) means for generating a turn-off control signal for said switching devices;
• (d) means coupled to said average current signal for generating a reference signal;
• (e) means coupled to a selected one of said individual current signals, said average current signal and said reference signal for generating a first signal when the magnitude of said one individual current signal is less than said reference signal;
• (f) means coupled to said one individual current signal, said average current signal and said reference signal for generating a second signal when the magnitude of said average current signal is less than said reference signal;
• and
• (g) means coupled to said first and second signals for adjusting a time of application of the turn-off control signal for a following conduction cycle of said devices as a function of a time difference between said first or second signals.