AN4496
Application note
How to get a high power factor with the HVLED815PF device
Stefano Signoria
Introduction
The standard application of the HVLED8xxPF (HVLED807PF, HVLED815PF) is a constant
current (CC) LED driver. The average output current I
OUT
, as described in the HVLED815PF
datasheet (section 4.5 Constant current operation), does not depend on the value or the
waveform of the input voltage V
IN
, then can be used in standard or high power factor
implementation.
Two methods of implementing the high power factor (HPF) based on the HVLED8xxPF
family are current in use: the current sense modulation and ILED modulation. In either case
the input voltage after rectification is not smoothed and the waveform on the bulk capacitor
is a semi-sinusoidal waveform. The voltage on the bulk capacitor, V
RECT
, contains
information about the phase and waveform of V
IN
.
With the first method, the additional circuitry in
Figure 1
applies a DC offset and
a modulation, both proportional to the V
RECT,
to the CS pin and permits to obtain an HPF.
This solution is described in detail in
1.
of
Section 3.
The second one, shown in
Figure 2,
applies a modulation of the ILED pin proportional to the
V
RECT
and features HPF in a single range application (refer to the AN4129 for further
details). This application note describes how to modify the basic circuit to support wide input
range applications.
Figure 1. CS HPF; additional components
1
Figure 2. ILED HPF; additional component
September 2014
DocID026370 Rev 1
1/18
www.st.com
18
Contents
AN4496
Contents
1
ILED pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1
1.2
1.3
DC analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
High power factor modulation analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
High power factor implementation in wide input range application . . . . . . 6
2
Designing a high PF wide range LED driver
with the HVLED815PF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
2.10
Input specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Transformer design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Drain source breakdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Current sense resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Primary inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Secondary/auxiliary turn ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Feed forward . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
OVP protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
AC modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3
4
Supporting material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2/18
DocID026370 Rev 1
AN4496
ILED pin
1
ILED pin
The ILED pin voltage (V
ILED
) is the constant current (CC) loop reference. This voltage
divided by two is used as the reference for the MOSFET's peak drain current during the CC
regulation. An external capacitor is used for filtering the ILED pin current with an appropriate
time constants (
ILED
>> 1 / 2 *
* f
IN
).
In this configuration the peak of the drain current remains constant during the semi-period
but the t
ON
of the primary MOSFET increases when the instantaneous input voltage
decreases and with the mains near the zero-crossing the MOSFET remains in the ON state
until the mains voltage becomes enough to source the peak drain current.
Then near the zero-crossing the mains current has a peak as in
Figure 3
A.
Figure 3. ILED voltage and input current waveform
1.1
DC analysis
With a DC input voltage (V
IN
) and the device working in the transition mode (TM), the
voltage on the ILED, V
ILED
, is:
Equation 1
This voltage changes in function of mains voltage to maintain constant the output current
and V
ILED
reaches the maximum value at the minimum mains voltage.
DocID026370 Rev 1
3/18
18
ILED pin
AN4496
In case of sinusoidal mains voltage, if the input current is in phase and with the same
waveform of the input voltage (i.e.: HPF and THD > 30%), the
Equation 1
becomes:
Equation 2
Where V
ILEDavg
is the average voltage on the ILED pin, obtained by integration
(
ILED
>> 1 / 2 *
* f
IN
) of the ILED current in an external capacitor.
Figure 4
shows the calculated and measured values of the voltage V
ILED
vs. V
IN
with
V
R
= 102 V; the measurements are made on the board EVLHVLED815W10F at V
OUT
nominal (22 V, V
R
= 102 V). The difference between the curves, when the V
IN
increases, is
due to increasing of switching frequency and then greater power transferred in valley
skipping (not TM) .
Figure 5
shows the calculated and measured values of the voltage V
ILED
vs. V
IN
with V
R
= 70 V and V
R
= 122 V; the measurements are made on the same board
EVLHVLED815W10F changing the number of the LED and then V
OUT
and V
R
.
Figure 4. V
ILED
at V
R
= 102 V (V
OUT
= 22 V)
Figure 5. V
ILED
at V
R
= 70 V (V
OUT
= 15 V)
and V
ILED
at V
R
= 122 V (V
OUT
= 26 V)
4/18
DocID026370 Rev 1
AN4496
ILED pin
1.2
High power factor modulation analysis
A solution to obtain the high power factor and low THD, as shown in
Figure 2 on page 1,
is
the AC coupling of the ILED pin with the rectified voltage. In this way the average of the
voltage V
ILEDavg
is generated by the internal loop, which regulates the average output
current, while the waveform is modulated through the voltage divider by V
RECT
(see
Figure 3
B).
The average output voltage of an ideal single-phase full wave rectifier is:
Equation 3
Then the AC modulation is optimal when:
Equation 4
To ensure accurate regulation, the peak voltage on the ILED pin (V
ILEDpeak
) must be smaller
than its maximum headroom voltage V
ILEDx
(1.5 V). From
Equation 2
and
Equation 4
can be
estimated the maximum reflected voltage:
Equation 5
The effect of a reflected voltage greater of V
Rmax
is the reduction of the output current when
V
IN
is lower than:
Equation 6
The equivalent input resistance of the ILED pin (R
inILED
) is 50 K when V
ILEDavg
= 1 V and
the voltage divider must drive the ILED pin with equivalent resistance R
AC
<< R
inILED
. The
phase rotation, at the input frequency, introduced from the capacitor CAC impacts on the
power factor, then the value of the capacitor must be:
Equation 7
DocID026370 Rev 1
5/18
18
微控制器 MCU
京公网安备 11010802033920号
Copyright © 2005-2026 EEWORLD.com.cn, Inc. All rights reserved
电子工程世界
