7.4.2 Primary-Side Constant-Current (CC) Regulation

When the load current of the converter increases to the predetermined constant-current limit, operation enters CC mode. In CC mode, output voltage regulation is lost and the shunt-regulator drives the current and voltage at FB to minimum. During CC mode, timing information at the VS pin and current information at the CS pin allow accurate regulation of the average current of the secondary winding. The CV-regulation control law dictates that as load increases approaches CC regulation the primary peak current will be at I_PP(max). The primary peak current, turns-ratio, demagnetization time t_DM, and switching period t_SW determine the secondary average output current (see Figure 16). Ignoring leakage-inductance effects, the average output current is given by Equation 5. When the demagnetization duty-cycle reaches the CC-regulation reference, D_MAGCC, in the current-control block, the controller operates in frequency modulation (FM) mode to control the output current for any output voltage at or below the voltage-regulation target as long as the auxiliary winding keeps V_VDD above the UVLO turnoff threshold. As the output voltage falls, t_DM increases. The controller acts to increase t_SW to maintain the ratio of t_DM to switching period (t_DM / t_SW) at a maximum of 0.425 (D_MAGCC), thereby maintaining a constant average output current.

Figure 16. Transformer-Current Relationship

Equation 5.

Fast, accurate, opto-coupled CV control combined with line-compensated PSR CC control results in high-performance voltage and current regulation which minimizes voltage deviations due to heavy load and unload steps, as illustrated by the V-I curve in Figure 17.

Figure 17. Typical Target Output V-I Characteristic