AN3256 STMICROELECTRONICS | Alldatasheet
Document overview
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- PDF pages: 23
Technical content
Datasheet sections
- 1 Schematic
- 2 Circuit description
- 3 Performance with LED loads
- 3.1 Strings of 4 series-connected 1 W white LEDs
- 3.2 Strings of 6 series-connected 1 W white LEDs
- 3.3 Strings of 8 series-connected 1 W white LEDs
- 3.4 Strings of 10 series-connected 1 W white LEDs
- 3.5 Strings of 12 series-connected 1 W white LEDs
- 4 Graphical data
- 5 FET voltage and current stress
- 6 Thermal stress
- 7 Startup
- 8 Conducted EMI
- 9 Layout
- 10 Bill of material
- 11 Reference
- 12 Revision history
1 Schematic
Figure 3. Schematic
AN3256 Circuit description Doc ID 17790 Rev 1 5/23
2 Circuit description
C1, L1, and C2 provide filtering for conducted EMI. Bridge rectifier BRI feeds the stepdown switching regulator. The regulator appears inverted - the flywheel diode, D8, is connected to the positive rail instead of the negative. Q1 pulls the inductor input negative, rather than positive. Inductor L2 filters the output current, feeding C7 and the LED load. The circuit starts up with a trickle of current into C3 thru R1. It takes about ¼ second to charge C1 to U1's startup voltage. The startup timer in U1 starts the switching cycle by turning on Q1. Current in Q1 and L2 increases from zero to about 800 mA. This current appears on R10 and R11 which drop about 0.6 V. A filtered portion of the rectified AC line (from R9 and C5, added by R14 and R13) is added to this voltage, to compensate out the duty cycle shift due to line voltage. U1 turns off Q1 when it sees about 1 V at pin 4. The LED average current would increase with line voltage if not compensated, due to the decrease in FET ON time. R10 and R11 drop about 0.6V at the current peak. Line voltage compensation is added to this from divider R9- R14-R13. C6 filters the line voltage to DC to avoid further distortion of the line current”. L2's current continues to flow after Q1 turns off, instead flowing in D8. The current ramps toward zero, at which time D8 turns off. L2 and stray capacitance then ring the voltage at D8's anode down to about twice the LED voltage below the positive rail. When the ringing voltage turns up, U1 senses the end of the discharge and turns on Q1. The cycle then repeats. Current in L2's upper winding therefore ramps between zero and twice the load current. When Q1 turns on, D8 has already turned off, so Q1 does not see D8's reverse recovery spike. Housekeeping power is supplied by the auxiliary (lower) winding on L2. This winding steps down the peak-to-peak voltage (the bulk input voltage) applied to the upper winding by 5:1. D1 and D2, with C8, peak detect the transitions on the auxiliary winding. D2 is a Zener diode, clamping the maximum voltage applied to C3 to slightly over 16 V. The 5:1 turns ratio is not optimum - about 7:1 would be better, wasting less power in D2 and R12. Note: It should be noted that this circuit is not capable of providing output open circuit protection. Monitoring of the voltage on C3 no longer provides an indication of the output voltage. The auxiliary winding also provides U1 with timing for the zero-current sensing function, through R7. The current waveform at the circuit's AC input is not sinusoidal, but it is clean enough to give acceptable power factor for residential use in the U.S. Some improvement at the leading and trailing edges is provided by U1's multiplier input, which is allowed to affect the current limit only when the instantaneous line voltage is very low.
3 Performance with LED loads
driver acts as a current source, the LEDs determine the output voltage.
3.1 Strings of 4 series-connected 1 W white LEDs
which this occurs is noted below. LEDs cannot be used without modifying the circuit. Strings of 5 LEDs give sufficient margin).
- Trace colors: – Y ellow= line voltage, 50 V/div ref 0 – Magenta= line current, 100 mA/div ref 0 – Blue= LED voltage, 5 V/div ref -3 div – Green= LED current, 100 mA/div ref -3 div
Table 1. Performance overline voltage range with 4 LEDs per string
3.2 Strings of 6 series-connected 1 W white LEDs
- Trace colors: – Y ellow = line voltag e, 50 V/div ref 0 – Magenta = line current, 100 mA/div ref 0 – Blue = LED voltage, 5 V/div ref - 3 div – Green = LED current, 100 mA/div ref - 3 div
Figure 4. 96 V input, 4 LED load Figure 5. 132 V input, 4 LED load Table 2. Performance overline voltage range with 6 LEDs per string
3.3 Strings of 8 series-connected 1 W white LEDs
- Trace colors: – Y ellow = line voltag e, 50 V/div ref 0 – Magenta = line current, 100 mA/div ref 0 – Blue = LED voltage, 5 V/div ref - 3 div – Green = LED current, 100 mA/div ref -3 div
Figure 6. 96 V input, 6 LED load Figure 7. 132 V input, 6 LED load Table 3. Performance overline voltage range with 8 LEDs per string
3.4 Strings of 10 series-connected 1 W white LEDs
Dropout to flashing, 71.5 V.
- Trace colors: – Y ellow = line voltag e, 50 V/div ref 0 – Magenta = line current, 100 mA/div ref 0 – Blue = LED voltage, 5 V/div ref - 3 div – Green = LED current, 100 mA/div ref -3 div
Figure 8. 96 V input, 8 LED load Figure 9. 132 V input, 8 LED load Table 4. Performance overline voltage range with 10 LEDs per string
3.5 Strings of 12 series-connected 1 W white LEDs
- Trace colors: – Y ellow = line voltag e, 50 V/div ref 0 – Magenta = line current, 200 mA/div ref 0 – Blue = LED voltage, 10 V/div ref - 3 div – Green = LED current, 100 mA/div ref -3 div
Figure 10. 96 V input, 10 LED load Figure 11. 132 V input, 10 LED load Table 5. Performance overline voltage range with 12 LEDs per string
Figure 12. 96 V input, 12 LED load Figure 13. 132 V input, 12 LED load
4 Graphical data
Figure 14. Power factor vs line voltage sensitive to line voltage, lowest at high line. Figure 15. LED current vs line voltage
5 FET voltage and current stress
- Y ellow = FET voltage, 100 V/div
- Blue = FET current, appx. 0.24 A/div The FET sees only the peak line voltage. The diode (D8) has turned off well before the FET turns on - there is no reverse recovery problem. The leading edge spike is small, and is due only to the FET discharging D8's and L2's capacitance, and includes the gate drive leading edge current.
Figure 18. 132 V input, 4 LED load Figure 19. 132 V input, 4 LED load, single Figure 20. 132 V input, 12 LEDs Figure 21. 132 V input, 12 LEDs, single pulse
6 Thermal stress
workbench, in a 24.9 C ambient, 120 V input, 12 LED load. Table 6. Component temperatures
7 Startup
- Y ellow = AC line, 200 V/div
- Green = LED current, 100 mA/div
Figure 22. Time from application of power to LED illumination 450 ms to light, 500 ms to full output.
8 Conducted EMI
Traces shown below are the maximum of 3 successive sweeps (max hold), peak values.
- Red = line 1
- Blue = line 2
Figure 23. Conducted EMI, db µV versus frequency across the choke should help at its parallel self-resonance.
9 Layout
Figure 24. Top side layout and placement Figure 25. Bottom side layout
Figure 26. Bottom side placement
10 Bill of material
Table 7. BOM
11 Reference
- J. Shao, “Single Stage Offline LED Driver ,” presented at Applied Power Electronics Conference and Exposition (APEC), Washington, DC, 2009.
Table 8. Document revision history 23-Sep-2010 1 Initial release.