AN2941 datasheet(18/37 Pages) STMICROELECTRONICS

18 / 37 page

Functional check

AN2941

18/37

Doc ID 15447 Rev 4

Overcurrent and short-circuit protection

In this demonstration board the overcurrent is managed by TSM1014 (U5), a CC/CV

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

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

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

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

error amplifier takes over, thus keeping constant the output current.

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

voltage drops so it is not powered, therefore the primary controller must manage the failure

condition.

In case of output short, there are two different possible situations that the controller has to

handle. If the coupling between the secondary winding and the auxiliary winding is good

enough, 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 L6566A to

disable. The controller stops switching and remains in the off-state until the voltage on the

Vcc pin decreases below the Vcc restart threshold (5 V). Then, the HV startup turns on and

charges the Vcc capacitor. As 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 in 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 0.5 Hz), limiting the current flowing at

secondary side (less than 1 Arms) preventing the power supply from overheating, which

could destroy it.

In case the leakage inductance between auxiliary and secondary winding is not negligible,

some spikes on the auxiliary voltage could keep Vcc above the UVLO threshold for a time

long enough to damage the converter. In this case L6566A detects a short-circuit monitoring

the control pins. When the output voltage drops and consequently pin COMP saturates high,

the soft-start capacitor is charged by an internal current source. When the Vss voltage

reaches an internal disable threshold (5 V), the controller stops switching and starts

operating in hiccup mode as described above.

Figure 15.

Transition full load to no load at 265

Vac-50 Hz

Figure 16.

Transition no load to full load at 265

Vac-50 Hz

CH1: Drain voltage

CH2: Vcc

CH3: Output voltage

CH4: Output current

CH1: Drain voltage

CH2: Vcc

CH3: Output voltage

CH4: Output current

AN2941 Datasheet(PDF) 18 Page - STMicroelectronics

AN2941 Datasheet(HTML) 18 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

Datasheet Download

Link URL