Design Guide for Bulb and T8 Tube Universal Non-Isolation LED Modules

Key Features

l  Active PFC function by One Cycle Control (OCC)

l  Continuous Inductor Current Mode (CICM)

l  PF > 0.95 over full voltage range

l  ±3% output LED current accuracy by Average Current control

l  High efficiency > 0.85 (typ.)

l  Fixed frequency at 45KHz with Spread Spectrum to reduce EMI filter cost

l  Very low BOM cost

l  Linear dimming on DIM pin

l  Open Loop Protection (OVP)

l  Short Circuit Protection (SCP)

l  Over Current Protection (OCP)

l  Over Temperature Protection (OTP)

l  Available in SOT-26 package

l  RoHS compliant and Pb free

l  Application for LED lamp for decorative lighting,E26/E27 bulbs and T8 tube LED lighting

Introduction
This design note is to ease the engineering effort to select the key components under operating conditions. The selection of the PFC resistors, input bulky capacitor, LED DC output voltage, output capacitor, inductor, ultra-fast rectifier and current sensing resistor can be easily calculated by the table on Page 5 for a universal AC off-line in E26/E27 bulbs or T8 tubes applications.
Description
To start with a new design, double click the calculation Table and enter parameters in red area, then the output parameters will be automatically generated in blue entries. One standard reference circuit is also presented with corresponding components on Figure 5 as follows :
1.  Selecting LED DC output voltage (VLED)
The LED DC output voltage should be set low enough to ensure at least a 50% conduction angle at the lowest line voltage. A conduction angle of less than 50% will result in very poor PF and very high peak currents, and will degrade efficiency at lowest line. Conduction angle (θCOND(%)) can be calculated as a percentage of the AC line half-cycle (or indeed full-cycle) as follows :
θCOND(%)cos-1
Figure 1 shows the trade-off between the level of the LED output voltage and the conduction angle.
Figure 1. Buck PFC Line Current
Conduction Angle

Based on the Figure 2, power module designer can select applicable LED DC output voltage to get better PF value.
Figure 2. Conduction Angle
versus VLED at the Different AC Line
/ 2.Selecting output bulk capacitor (CO)
CO can be calculated as follows :
CO
where
POUT is the output load power drawn.
ΔVLED is the output ripple voltage at any operating point. In general, ΔVLED can be set at 30% of VLED.
θCOND(%) is the conduction angle at the AC line of interest (as a decimal percentage of total cycle - e.g., a 50% conduction angle expressed as 0.5).
fL(MAX) is the AC line maximum frequency.
3. Setting sensing resistor (RCS)
The output LED average current (ILED(AVG)) is shown in Figure 3 which follows input waveform for high PF in the CICM operation. The RMS value of the ILED as represented by the green line ILED(RMS) in Figure 3.
Figure 3. ILED Current Waveforms

The peak current of the ILED(AVG) can be derived by
ILED(AVG)_PK1.15ILED(RMS)
If the peak-to-peak ripple current through the LED is set at 30% of ILED(AVG)_PK, the sensing resistor should be as follows :
RCS
RCS
Be noted that VCS(AVG) is used in finding RCS because ILED(RMS) is interested. And
VCS(PK) VCS(AVG)
4. Selecting inductor (L1)
Assume that the efficiency is η and the ripple current is selected to be 30% of nominal current ILED(AVG), then the required minimum inductor value can be obtained by,
DMIN
DMAX
tON(MIN)
tON(MAX)
L1≧
5. Selecting ultra-fast freewheel diode (D1)
Normally, a less than 35ns fast recovery time diode can be used with good result. The power module designer must follow the below rules to achieve the best performance.
(1). IF > ILED(RMS)
(2). VRRM > VHV
(3). tRR ≦ 35ns
(4). Minimum CJ
where / IF is average rectified forward current of D1.
VRRM is the maximum repetitive reverse voltage of D1.
tRR is the reverse recovery time and defined on Figure 4.
CJ is the reverse recovery junction capacitance of D1
Figure 4. Reverse Recovery Time Characteristic

6. Start-up Resistor and Biasing Zener Selections
SQ6211 requires as low as 8μA and 15V (VDD(ON)) at VDD pin to start the circuit so that the power consumption can be minimized. At the same time, the VDD pin capacitor is charging. The start-up time can be adjusted by selecting correct combination of resistor and capacitor. Usually, a 10μF capacitor and a higher than 500KΩ resistor are used. Be noted, if the resistor is too small, the start-up power consumption will be higher, and if the resistor is too high, the operating input voltage cannot go to too low. VDD(OFF) is set at 8V. This hysteresis is to allow the capacitor to have enough energy to supply the power to the IC.
After successful initial power up, VDD is then biased from LED output. For proper operation, the Zener diode in Figure 12 needs to have clamping voltage set by
VZ = VLED – 18V
7. COMP pin Capacitor Selection
The capacitor at the COMP pin is very important in PFC control loop because it will impact the system stability and the total harmonic distortion (THD). In order to keep VCOMP constant during an operating cycle, a very small bandwidth filter is required. Usually, it is set at less than 10Hz. The simplest compensation circuit uses a 1μF - 4.7μF capacitor at the COMP pin can achieve the goal.
10. Dimming
The SQ6211 supports linear dimming function. This linear dimming is implemented between 0.3V to 3V range. If the voltage at DIM pin is lower than 0.3V, the GATE drive is disabled. If the DIM pin voltage is higher than 3V, the SQ6211 operates in normal condition. / 11. Checking spike
The total parasitic capacitance (CP) can be expressed as below :
CPCDRAIN + CPCB + CL1 + CJ
where
CDRAIN is the MOSFET Q1 drain capacitance.
CPCB is the printed circuit board traces capacitance
CL1 is the parasitic capacitance of the L1 inductor
CJ is the junction capacitance of D1
For CL1, it can be calculated through SRF (Self-Resonance Frequency) which can be found from data book and is associated with each inductor and it can be expressed by the equation;
SRF
The coil capacitance can then be calculated by
CL1
The spike time (tSPIKE) can be calculated the following equation :
tSPIKE
< tBLANK (Minimum 160ns)
where ISAT is the saturation current of external MOSFET Q1.
Calculation Table

Figure 5. Reference Circuit for Universal Input LED Lamp Driver Using the SQ9971

© 2009 Sequoia Microelectronics Corp. - 6 - Rev. 0.2

DN-6211_V0.2 2014, Feb 014