DocID16493 Rev 2

13/25

AN3089

Functional check

25

In Figure 10 and Figure 11 the transitions from full load to no load and vice versa, at

maximum input voltage, have been checked. The maximum input voltage has been chosen

because it is the most critical input voltage for transition: in fact, at no load the burst pulses

have the lower repetition frequency and the Vcc may drop, causing restart cycles of the

controller. As shown, both transitions are clean and there isn’t any output voltage or Vcc dip.

3.2

Overcurrent and short-circuit protection

In this demonstration board the overcurrent is managed by the TSM1014 (U3), a CC/CV

controller. Inside the IC there is a voltage reference and two Or-ed operational amplifiers,

one dedicated to act as the error amplifier of the voltage loop, and the second dedicated to

act as the error amplifier of the current loop. During normal operation the voltage feedback

loop is working, while in the case where the output current exceeds the programmed value,

the current loop error amplifier takes over, therefore keeping the output current constant.

In case of a dead-short, the current cannot be limited effectively by U5 because the output

voltage drops, and so it is unpowered, therefore the primary controller must manage the

failure condition.

In case of a output short, there are two different possible situations which the controller must

handle: if the coupling between the secondary winding and the auxiliary winding is good, as

soon as the output voltage drops, the auxiliary voltage drops as well and the IC supply

voltage falls below the undervoltage lockout threshold, causing the L6566B to stop

switching. It remains in the off-state until the voltage on the Vcc pin decreases below the

soon as the turn-on threshold is reached, the circuit restarts. If the short is still there, the

circuit just attempts to restart but it stops after a few milliseconds. Restart attempts are

repeated indefinitely until the short is removed. This provides a very low frequency hiccup

working mode (for this board 1 Hz), limiting the current flowing at the secondary side (less

than 1 Arms) preventing the power supply from overheating, which could destroy it.

In the case where the coupling between the auxiliary and secondary winding is poor, some

spikes on the auxiliary voltage may keep Vcc above the UVLO threshold for a period long

enough to damage the converter. In this case the L6566B detects a short-circuit by

monitoring the control pins: in the case of a short, the COMP pin goes high, and an internal

Figure 10. Transition full load to no load

at 265 Vac – 50 Hz

Figure 11. Transition no load to full load

at 265 Vac – 50 Hz

CH1: drain voltage

CH2: Vcc

CH1: drain voltage

CH2: Vcc

CH3: output voltage

CH4: output current

CH3: output voltage

CH4: output current