BW6562A Datasheet, PDF (7/15 Pages) Bruckewell Technology LTD – High PFC LED Driver

BW6562A High PFC LED Driver
where θ is phase shift between input voltage and current.
Effective value of input voltage and current are defined by
I_TH
(11)
as follows :
(12)
To quantify degree of current waveform distortion, THD
(2) _{is written by}
(3) _THD
(13)
where v_s(t) is instantaneous value of input voltage,
(14)
i_s(t) is instantaneous value of input current,
T is the cycle.
So,
Therefore, P_AV is written as :
PF
(15)
avg [ v(t) i(t) ]
(4)
If the current and voltage are in phase, then θ 0, which
(5) ^{will lead to cos(θ) 1, and the PF will be simplified as}
And apparent power is defined by
PF
(16)
V_RMS I_RMS
(6)
where I_RMS is the root mean square (RMS) value of i_RMS.
Therefore, based on the above equations, obtained
power factor (PF) is defined by the ratio of average power
and apparent power :
The major cause of this THD distortion is the inability of
the system to transfer energy effectively when the
instantaneous line voltage is very low. This effect is
magnified by the high-frequency filter capacitor placed
after the bridge rectifier, which retains some residual
voltage that causes the diodes of the bridge rectifier to be
reverse-biased and the input current flow to temporarily
stop.
To overcome this issue, the circuit section designed in the
BW6562A forces the PFC regulator to process more
energy near the line voltage zero-crossings, as compared
to that commanded by the control loop. This results in
both, minimizing the time interval when energy transfer is
lacking, and fully discharging the high-frequency filter
capacitor after the bridge.
The BW6562A is designed with a special circuit that
reduces the conduction dead-angle occurring to the AC
Based on the equation 16, if THD is very small, then it
will get near unity power factor.
is shown in Typical Application Circuit on page 1. During
the start-up transient, the V_CC is lower than the UVLO
threshold voltage (V_CC(ON)) thus there is no gate pulse
produced from the BW6562A to drive power MOSFET.
Therefore, the current through R6 will provide the start-
up current and to charge the capacitor C2. Whenever
the V_CC voltage is high enough to turn on the BW6562A
and further to deliver the gate drive signal. Once the
BW6562A is in normal operation, the supply current is
switched to and provided from the auxiliary winding of
the PFC choke (transformer). Lower start-up current
requirement on the PFC controller will help to increase
the value of R6 and then reduce the power consumption
on R6. By using CMOS process and the special circuit
design, the maximum start-up current of the BW6562A
is only 60µA. If a higher resistance value of R6 is
chosen, it usually takes more time to start-up. To
carefully select the value of R6 and C2 will optimize the
power consumption and start-up time.
© 2012 Bruckewell Technology Corp., Ltd.
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