Description

This reference design provides isolated positive and negative voltage rails required for Insulated Gate Bipolar Transistor (IGBT) gate drivers from a single 24-volt DC input supply. IGBTs are used in three phase inverters for variable-frequency drives to control the speed of AC motors. This reference design uses a push-pull isolated control topology and provides isolation compliant to IEC61800-5 and is intended to operate from a pre-regulated 24Vdc input.With a regulated (within 5%) input source: a simple open-loop: free-running oscillator can be implemented with a push-pull PWM controller. This topology is essentially a forward converter with two primary windings used to create a dual-drive winding. This fully utilizes the transformer core's magnetizing current more efficiently than flyback or the forward topologies. Another advantage this configuration has over flyback and forward configurations is that the supply output can be scaled up for higher power drives.This reference design also takes advantage of another benefit of the push pull topology in that multiple transformers can be controlled in parallel from a single controller to generate all the isolated voltage rails required for 3-phase IGBT inverters.Lastly: larger IGBTs for higher power drives sometimes require more gate drive current than what is provided by a typical IGBT gate driver: for which designers often use additional transistors for gate current boosting. This reference design provides +16V on the positive outputs and -8V on the negative outputs to compensate for the added voltage drop in those transistors.