NCL30051LEDGEVB ON Semiconductor, NCL30051LEDGEVB Datasheet - Page 7

• Manufacturer Part Number: NCL30051LEDGEVB
• Description: Power Management IC Development Tools 90-265VAC 60W ISO CC EVB
• Manufacturer: ON Semiconductor
• Type: Power Factor Correctionr

Specifications of NCL30051LEDGEVB

• Rohs: yes
• Product: Evaluation Boards
• Tool Is For Evaluation Of: NCL30051
• Input Voltage: 90 VAC to 265 VAC
• Output Voltage: 35 V to 50 V
• Output Current: 1.5 A

Resonant Half−Bridge Transformer Design (T1)
frequency, symmetrical duty ratio, the design becomes very
straightforward. A half−bridge converter switches 1/2 of
bulk voltage across the transformer primary due to the
capacitive divider network formed by resonant capacitors
C6 and C7. By choosing the maximum bulk voltage at about
500 Vdc and assuming a maximum output voltage of 50 V,
the turns ratio on the transformer will be:
V
and one of the half’s of the−secondary(note: push−pull
output rectification!). All that is required now is to
determine the minimum number of primary turns necessary
to avoid core saturation and then ratio the secondary turns
from this point. The selected core is a PQ−2020 with a cross
sectional core area (Ae) of 0.6 cm
design relationship:
Where:
60 W / 250 Vdc x 0.95 = 228 mA assuming close to 95%
converter efficiency. The rms value will actually be a little
higher but AWG # 28 magnet wire will easily handle this and
96 turns can comfortably be wound over 3 layers with 32
turns per layer.
so 19 turns will be close enough. It turns out that due to the
center tapped secondary, two strands of #26 magnet wire
wound bifilar on top of the primary will make the secondary
easily handle up to 1.5 A output current. The primary
leakage inductance is the only unknown factor that was not
bulk
Np +
Since the half−bridge transformer operates in a fixed
So, a turns’ ratio of 5:1 is required between the primary
The average primary current will be a little more than
The number of secondary turns will be 96 / 5 = 19.2 turns
+ 96.7 turns
/2 divided by 50 V
4
V
F
D
Np is the minimum primary turns
needed
V is the max voltage across the
primary (with a little margin)
F is the switching frequency
Bm is the maximum flux density in the
ferrite core
Ae is the cross sectional area of the
core
BM
10
8
Ae
out
+
= 250/50 = 5
4
35 kHz 3200G
20 V
2
. Using the transformer
1
10
8
0.6 cm
http://onsemi.com
2
7
actually purposely designed in, however, with the three layer
primary and adequate insulating tape between the primary
and secondary there should be adequate leakage inductance
that will facilitate a resonant capacitor with a common value
that will obtain a reasonable resonant frequency that can be
accommodated by the internal half−bridge clock in the
NCL30051 controller. By shorting the transformer
secondary pins out with very short wires, the primary
leakage can be measured with an inductance meter. In this
design the leakage inductance worked out to be between 90
and 100 mH, sufficient to produce a resonant frequency of
36 kHz with a pair of 0.1 mF capacitors (in parallel
effectively) for C6 and C7. It turned out that a clock timing
capacitor of 1 nF for C10 sets the switching frequency to
about 36 kHz which provided the optimum tuning as
displayed in primary current waveform of Figure 2. The
design summary of the transformer T1 is shown in Figure 4.
controlling the PFC bulk voltage, the value of the bulk
voltage will be directly proportional to V
ratio of the transformer. For example, if we have an LED
string with a nominal forward voltage of 40 V, the bulk
voltage will be regulated at: V
= 400 Vdc (where two represents the fact that the
half−bridge primary switches only 1/2 of the bulk voltage).
Herein highlights a limitation of this topology in cases
where the string voltage may be very low. For a V
the bulk voltage will be 320 Vdc and this puts a limit on the
maximum line voltage in which the PFC boost converter can
function. The bulk voltage must always be higher than the
peak of the line voltage for the boost converter to work, so
at 230 Vac input, the line peak is 1.4 x 230 = 322 V so now
we have reached the lower limit of the LED forward voltage
range. Obviously at 120 Vac (V
feasibly allow the output V
without any problems (25 Vdc x 5 x 2 = 250 Vdc bulk which
is still higher than Vac peak). Careful analysis of the
throughput voltage conversion and proper selection of the
transformer turns ratio will allow optimization for a given
LED application. A maximum operating PFC bulk voltage
of 510 Vdc is recommended for adequate safety margins.
Examples and further discussions of the circuit limitations
are addressed below under “Topology Limitations”.
Since the output current and/or voltage is regulated by
f
to go even as low as 25 Vdc
out
peak
x Np/Ns x 2 = 40 x 5 x 2
= 170 Vdc) we could
out
via the turns
f
of 32 V,