AN2331 Datasheet, PDF (9/24 Pages) STMicroelectronics – This note describes the performances

AN2331
Circuit description and test results
2.2
Stand-by and No-load performance
The circuit has been tested for light load and zero load operation, thus simulating a load
disconnection as shown below. The input power at zero load is always below 2W. In this
condition the resonant circuit works at its maximum frequency, while the PFC works
skipping switching cycles according to the load.
During the no load operation at low mains, the PFC controller is supplied by the self-supply
winding of the resonant, because at low load and low mains the PFC self supply cannot
deliver the suitable voltage value for correct operations.
Table 3. Stand-by consumption - Vin=115Vac
Vout [V]
Iout [A]
Pout [W]
23.90
0.1
2.39
23.90
0.022
0.53
24.00
0
0
Pin [W]
5.1
2.03
1.5
Table 4. Stand-by consumption - Vin=230Vac
Vout [V]
Iout [A]
Pout [W]
23.90
0.1
2.39
23.90
0.022
0.53
24.00
0
0
Pin [W]
4.9
2.16
1.7
2.3
Short circuit protection
The circuit protects itself in case of an output short circuit. The primary current flowing into
the resonant capacitor provides a proportional voltage drop across it that is detected and fed
into the pin unlatched enable pin (#9). In case of over current, if the voltage on pin 9 exceeds
the internal threshold the soft start capacitor is discharged and the circuit is pushed to work
at maximum frequency, thus limiting the current flowing at primary side and the power
delivered to the secondary. In short-circuit condition, the average value of the output current
is kept constant around 6.5A and the input power is limited to 17W only. At short-circuit
removal, the circuit will automatically restart via a soft-start cycle.
2.4
Over voltage protections
Both circuit stages, PFC and resonant, are equipped with their own over voltage protection.
The PFC controller L6563 is internally equipped with a dynamic and a static over voltage
protection circuit sensing the error amplifier via the voltage divider dedicated to the feedback
loop to detect the PFC output voltage. If the internal threshold is exceeded, the IC limits the
voltage to a programmable, safe value. Moreover, in the L6563 there is an additional
protection against loop failures using an additional divider (R11, R12, R13) and a dedicated
pin (PFC_OK, #7) for protecting the circuit in case of loop failures or disconnection. Hence
the PFC output voltage is always under control and in case a fault condition is detected, the
PFC_OK circuitry will latch the L6563 operations and, by means of the PWM_LATCH pin
(#8), it will latch the L6598 as well.
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