AN10961
Dimmable CFL using the UBA2027X family
Rev. 2 — 8 June 2012
Application note
Document information
Info
Keywords
Abstract
Content
CFL, Triac dimmable, UBA2027X
This application note describes the design of a dimmable Compact
Fluorescent Lamp (CFL) with low dimming level using the UBA2027X.
NXP Semiconductors
AN10961
Dimmable CFL using the UBA2027X family
Revision history
Rev
v.2
Date
20120608
Description
second, updated issue
Modifications:
v.1
20110815
•
Text and drawings updated throughout the document.
first issue
Contact information
For more information, please visit:
http://www.nxp.com
For sales office addresses, please send an email to:
salesaddresses@nxp.com
AN10961
All information provided in this document is subject to legal disclaimers.
© NXP B.V. 2012. All rights reserved.
Application note
Rev. 2 — 8 June 2012
2 of 57
NXP Semiconductors
AN10961
Dimmable CFL using the UBA2027X family
1. Introduction
This application note describes the design of a dimmable CFL with a low dimming level
using the UBA20270 or UBA20271/UBA20272 ICs. The example illustrated is a 20 W
application using the UBA20270 with SPS04N60C3 external MOSFETs. At power levels
below 20 W, the UBA20271/2 with integrated MOSFETs can be used.
The UBA20270 controller can be used with both the 120 V and 230 V mains voltage
applications. Select the UBA20272 for 230 V mains applications and the UBA20271 for
120 V mains applications for optimal performance.
Remark:
Unless otherwise stated all voltages are AC.
A standard commercially available phase-cut wall dimmer is used as the triac wall
dimmer. This type of dimmer is representative of most dimmers for 120 V or 230 V input
mains applications. Some component values in the application need adapting for dimming
compatibility when using other dimmers.
The topology is based on a Voltage Source Charge Pump (VSCP) that is intended to
create the necessary hold current for the triac in the dimmer. An End of Life (EOL) circuit
has been added externally to the main board for evaluation. The circuit senses high lamp
voltage and can shut down the IC in the burn state. The coil saturation protection in the IC
does not monitor any ignition or high ignition voltage.
2. Scope
This application note is organized as follows:
•
•
•
•
Section 3
describes the basic operation of triac dimming
Section 4
describes application design
Section 5
Appendix 1: power calculation equations
Section 7
Appendix 3: inductive mode preheat calculations
AN10961
All information provided in this document is subject to legal disclaimers.
© NXP B.V. 2012. All rights reserved.
Application note
Rev. 2 — 8 June 2012
3 of 57
NXP Semiconductors
AN10961
Dimmable CFL using the UBA2027X family
3. Triac dimming
3.1 Triac dimmer circuit
Figure 1
shows the circuit diagram of a triac wall dimmer used in 120 V (RMS)
applications.
Cfilter
Lfilter
triac
(BT136)
Itriac
V
i
120 V RMS
Rvar
470 kΩ
R
2.7 kΩ
diac
(DB3)
Load
C
47 nF
019aaa332
Fig 1.
Triac wall dimmer with LC filter
The triac shown in
Figure 1
employs forward phase-cut dimming. CFL loads such as
incandescent lamps are only energized during the last part of each power-line half cycle
(α to
π
and
α + π
to 2π). See
Figure 2
for details.
V
i
α+
α
2
ωt
019aaa333
Fig 2.
Triac with phase-cut dimming angle
α
3.1.1 Circuit overview
In
Figure 1,
the capacitor C (47 nF) is charged using a fixed resistor R (2.7 kΩ) and a
variable resistor R
var
(470 kΩ). R
var
sets the phase-cut dimming angle.
When the resistance is low, the capacitor charges quickly. As the voltage across the
capacitor reaches the diac break-over voltage, the triac fires and the current I
triac
flows.
The current in the load must be as high as the triac latching current within the period the
gate of the triac is fired or triggered. This current continues to flow until I
triac
drops below
its minimum hold current I
H
.
If the triac latching current is not reached by the end of its gate trigger pulse, multiple triac
firing can occur in a mains half cycle. Avoid multiple firings because they generate
unwanted audible noise while the lamp is dimmed.
AN10961
All information provided in this document is subject to legal disclaimers.
© NXP B.V. 2012. All rights reserved.
Application note
Rev. 2 — 8 June 2012
4 of 57
NXP Semiconductors
AN10961
Dimmable CFL using the UBA2027X family
When the load is an incandescent lamp (resistive load), there is no phase shift between
lamp voltage and lamp current. The current can rise high enough within the trigger period
of the triac. However, for CFLs, it is not true because of the capacitor in the input filter and
the buffer capacitor. The triac is a bidirectional device which operates in two quadrants
(see
Figure 3).
During the negative half cycle, the same process as described earlier is
repeated.
+I
rated current I
L
breakover
voltage
minimum holding
current I
H
-V
gate trigger voltage
+V
-I
019aaa334
Fig 3.
Typical triac voltage and current characteristics
In the triac application, the L
filter
and C
filter
can produce ringing of the triac current when the
triac latches after a step response. The LC filter in CFL applications incorporates L
filter
and
C
filter
. The triac ringing current must remain above I
H
ensuring the triac remains powered
up.
3.2 Triac wall dimmer with VSCS charge pump and CFL ballast
Figure 4
shows a triac wall dimmer and CFL ballast with a Voltage Source Current Source
(VSCS) charge pump. The half-bridge MOSFETs in the circuit switch the resonant tank
circuit. The lamp current at minimum dimming and lamp voltage at maximum dimming
generate the triac hold current ensuring dimming stability.
AN10961
All information provided in this document is subject to legal disclaimers.
© NXP B.V. 2012. All rights reserved.
Application note
Rev. 2 — 8 June 2012
5 of 57
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