| Abstract |
A bootstrap power supply is used to reduce the number of DC/DC converters needed for supplying the gate drivers. The advantage of a bootstrap is the reduction of size and cost for gate driver circuitry. The disadvantages is a modulation index limit. A modulation index limit can be compensated for, by increasing the DC-link voltage and thereby raising switching losses. It is therefore critical to accurately determine the modulation index limit, to prevent excessive losses, obtain safe operation and to evalu- ate the applicability of a bootstrap. The design rules of a bootstrap currently relies on rules of thumb estimations, generally underestimating the modulation index limit. In this paper a new generic method for calculating the modulation index limit is presented. The method takes on-state resistance, switching states and output current into consideration. The method is validated based on a proposed bootstrap cir- cuit for a three level T-type converter. The proposed circuit achieves equivalent charging states as a half bridge bootstrap. |
