Case Study: The Flyback Converter

3.4.2 OPERATION The Flyback converter is an isolated power converter; therefore the isolation of control circuit is also much needed. The two prevailing control schemes are the voltage mode control and the current mode control (in majority of cases current mode control needs to be dominant for the stability during operation). Both require a signal related to output voltage. There are two common ways to generate this voltage. The first is to use an optocoupler on the secondary circuitry to send a signal to the controller. The second is to wind a separate winding on the coil and rely on the cross regulation of the design.
The first technique involving an optocoupler has been used to obtain tight voltage and current regulation, whereas the alternative approach was developed for cost-sensitive applications where the output did not need to be as tightly controlled but up to 11 components including the optocoupler could be eliminated from the

The step change in instantaneous dc link current occurs even if the ac load at the inverter output is drawing steady power. However, average magnitude of the dc link current remains positive if net power-flow is from dc bus to ac load. The net power-flow direction reverses if the ac load connected to the inverter is regenerating. Under regeneration, the mean magnitude of dc link current is negative.

3.5.2 CURRENT SOURCE INVERTER The circuit of a single-phase Current Source Inverter (CSI) is shown in Fig. 3.11. The type of operation is termed as Auto-Sequential Commutated Inverter (ASCI). A constant current source is assumed here, which may be realized by using an inductance of suitable value, which must be high, in series with the current limited dc voltage source. Figure.3.12. Single phase current source inverter