8.3.10 I_PEAK Adjust vs. Line

The controller applies a line-dependent reduction in the peak-current demand to correct for the current overshoot due to the PWM and gate drive propagation delay, with the aim of delivering a constant peak current versus line at a given power level. This maintains approximately constant switching frequency versus line for a given power level (until the operation enters into CCM), improves regulation, reduces audio noise, and allows lower standby power at high line. If not corrected, the current overshoot could become significant at high line, where the inductor current di/dt is higher. This overshoot would cause a pronounced increase in transferred power per switching cycle at high line, because power is proportional to I_PK². The effect of the delay on the peak-current overshoot is illustrated in Figure 32.

Figure 32. Peak-Current Demand Adjustment vs V_BULK to Correct Prop Delay Overshoot

For different power stage designs, the combination of primary magnetizing inductance L_PRI, current sense resistance R_CS and external MOSFET gate turn-off delay t_OFF(ext), must be verified against Equation 17, to ensure that the internal peak-current compensation gain range is satisfied. The K_LINE(adj) factor should be within the range indicated. If the external turn-off delay is too long, then the internal I_PEAK adjustment factor is too low, and the adjustment at high line is not able to achieve the required level of over-shoot compensation. As noted previously, this could result in regulation difficulties at no-load, and may cause poor line and load regulation, or require an increase in output pre-load power.

Equation 17.

where

• where t_PROP(gate) is the internal controller gate-drive turn-off propagation delay, given in Table 7.