FAN501A Datasheet, PDF (13/16 Pages) Fairchild Semiconductor – Offline DCM / CCM Flyback PWM Controller for Charger Applications

During startup, the internal startup circuit is enabled and
the bulk capacitor voltage supplies the current, IHV, to
charge the hold-up capacitor, CVDD, through RHV. When
VDD reaches VDD-ON, the internal HV startup circuit is
disabled and the IC starts PWM switching. Once the HV
startup circuit is disabled, the energy stored in CVDD
should supply the IC operating current until the
transformer auxiliary winding voltage reaches the
nominal value. Therefore, CVDD should be designed to
prevent VDD from dropping to VDD-OFF before the auxiliary
winding builds up enough voltage to supply VDD.
VBLK
Primary-Side
NP
CBLK
RHV
The supply current drawn from the HV pin charges the
hold-up capacitor. When VDD reaches the turn-on
voltage of 17.5 V, normal operation resumes. In this
manner, Auto-Restart Mode alternately enables and
disables MOSFET switching until the abnormal
condition is eliminated, as shown in Figure 32.
VDS
Power On
VDD
VDD-OVP
VDD-ON
Fault
Occurs
Fault
Removed
IHV HV
VDD-OFF
VDD-HV-ON
Aux. ^DVDD
VDD
NA
CVDD
6V/17V
Figure 30. HV Startup Circuit
I_HV
5.0mA
V_BLK
R_HV
2.0mA
1.2mA
100V
V_BLK
200V
300V
400V
500V
VHV
Figure 31. V-I Characteristic of HV Pin
Operating Current
IDD-OP
IIDDDD--SBTrust
Figure 32. Auto-Restart Mode Operation
When a Latch Mode protection is triggered, PWM
switching is terminated and the MOSFET remains off,
causing VDD to drop. When VDD drops to the VDD turn-off
voltage of 5.8 V, the internal startup circuit is enabled
without resetting the protection and the supply current
drawn from HV pin charges the hold-up capacitor. Since
the protection is not reset, the IC does not resume PWM
switching even when VDD reaches the turn-on voltage of
17.5 V, disabling HV startup circuit. Then VDD drops
again down to 5.8 V. In this manner, Latch Mode
protection alternately charges and discharges VDD until
there is no more energy in DC link capacitor. The
protection is reset when VDD drops to 2.5 V, which is
allowed only after power supply is unplugged from the
AC line, as shown in Figure 33.
AC Disconnected
VDS
Power On
Power
On Again
Protections
The protection functions include VDD over-voltage
protection (VDD OVP), brownout protection, VS over-
voltage protection (VS OVP), internal over-temperature
protection (OTP), and externally triggered shutdown
(SD) protection. The VDD OVP and brownout protection
are implemented as Auto-Restart Mode. VS OVP, OTP,
and SD protections are implemented as Latch Mode.
When an Auto-Restart Mode protection is triggered,
switching is terminated and the MOSFET remains off,
causing VDD to drop. When VDD drops to the VDD turn-off
voltage of 5.8 V; the protection is reset, and next step to
reduced operation current until startup circuit is enabled.
VDD
VDD-ON
Protection
Triggered
VDD-OFF
VDD-Burst
VDD-LH
Operating Current
IDD-OP
IDD-Burst
IDD-ST
Protection
Reset
Figure 33. Latch Mode Operation
© 2014 Fairchild Semiconductor Corporation
FAN501A • Rev. 1.0.0
13
www.fairchildsemi.com