Reliable current commutation between switches in matrix converters is more
difficult to achieve than in conventional VSI's since there are no natural free wheeling paths. The commutation has to be actively controlled at all times with respect to two basic rules. These rules can be visualized by considering just two switch cells on one output phase of a matrix converter. It is important that no two bidirectional switches are switched on at any instant. This would result in line-to-line short circuits and the destruction of the converter due to over currents. Also, the bidirectional switches for each output phase should not all be turned off at any instant. This would result in the absence of a path for the inductive load current, causing large over voltages. These two considerations cause a conflict since semiconductor devices cannot be switched instantaneously due to propagation delays and finite switching times.
The two simplest forms of commutation strategy intentionally break the rules
given above and need extra circuitry to avoid destruction of the converter. In overlap current commutation, the incoming cell is fired before the outgoing cell is switched off.
This would normally cause a line-to-line short circuit but extra line inductance slows
the rise in current so that safe commutation is achieved. This is not a desirable method since the inductors used are large. The switching time for each commutation is also greatly increased which may cause control problems.
Dead-time commutation uses a period where no devices are gated, causing
a momentary open circuit of the load. Snubber's or clamping devices are then needed across the switch cells to provide a path for the load current. This method is undesirable since energy is lost during every commutation and the bidirectional nature of the switch cells further complicates the snubber design. The clamping devices and the power loss associated with them also results in increased converter volume.
difficult to achieve than in conventional VSI's since there are no natural free wheeling paths. The commutation has to be actively controlled at all times with respect to two basic rules. These rules can be visualized by considering just two switch cells on one output phase of a matrix converter. It is important that no two bidirectional switches are switched on at any instant. This would result in line-to-line short circuits and the destruction of the converter due to over currents. Also, the bidirectional switches for each output phase should not all be turned off at any instant. This would result in the absence of a path for the inductive load current, causing large over voltages. These two considerations cause a conflict since semiconductor devices cannot be switched instantaneously due to propagation delays and finite switching times.
The two simplest forms of commutation strategy intentionally break the rules
given above and need extra circuitry to avoid destruction of the converter. In overlap current commutation, the incoming cell is fired before the outgoing cell is switched off.
This would normally cause a line-to-line short circuit but extra line inductance slows
the rise in current so that safe commutation is achieved. This is not a desirable method since the inductors used are large. The switching time for each commutation is also greatly increased which may cause control problems.
Dead-time commutation uses a period where no devices are gated, causing
a momentary open circuit of the load. Snubber's or clamping devices are then needed across the switch cells to provide a path for the load current. This method is undesirable since energy is lost during every commutation and the bidirectional nature of the switch cells further complicates the snubber design. The clamping devices and the power loss associated with them also results in increased converter volume.
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