AN3212

AN3212 STMICROELECTRONICS | Alldatasheet

Document overview

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• PDF pages: 36

Technical content

Datasheet sections

• 1 Main characteristics
• 1.1 Initial configuration
• 1.2 Requirements
• 2 Circuit description
• 2.1 Primary side
• 2.2 Secondary side
• 2.3 Circuit variants
• 3 Waveforms
• 3.1 Input
• 3.2 Output
• 3.3 Startup sequence
• 3.4 Short-circuit protection
• 3.5 Open circuit protection
• 4 Measurements
• 4.3 EMI filter
• 4.4 Thermal maps
• 5 Electrical diagram
• 6 BOM list
• 7.1 Mechanical specifications
• 7.2 Electrical specifications
• 8 Revision history

5 W to 7 W high power factor offline LED driver

VIPer family in flyback configuration and with a TSM1052 as a constant current controller. Figure 1. STEVAL-ISA120V1 VIPer27 LED driver module

1 Main characteristics

1.1 Initial configuration

power factor and/or avoid the use of electrolytic capacitors. introduced in order to address the key points indicated above. Figure 2. Initial configuration

1.2 Requirements

• Input voltage: 100 to 264 VAC
• Power factor: > 0.9 @ 115 V and 230 V
• Output power: 3.5 W to 7 W (3 x 1 W / 3x 2.5 W LED series)
• Output current (average): 0.35 A to 0.7 A
• Input/output isolation
• No high voltage electrolytic capacitors
• Possibility of no low voltage electrolytic capacitors
• Open/short-circuit protection
• Minimal part count
• No dimming required !-V !#?IN !#?IN 2SENSE $#?0 $#?. 6)0ER 0RIMARY -ODULE 43- 3ECONDARY -ODULE

2 Circuit description

2.1 Primary side

device from the VIPer family, a VIPer17 for the 3.5 W and a VIPer27 for the 7 W version.

• Input section with X2 capacitor, diode bridge, EMI filter
• RCD snubber in parallel to the primary winding of the transformer
• Auxiliary power supply
• Optocoupler insulated feedback loop
• VIPer converter

Figure 3. Primary side schematic

• The relatively small values of the EMI filter capacitors
• The circuitry related to the VIPer “cont” pin The first is dictated from the high power factor requirement; usually these capacitors have a much higher value in order to get a low output ripple and reduced EMI emissions, but this inevitably leads to a poor power factor. For this reason their value must be set as a compromise starting with usual values and reducing them until the required PF can be reached. !-V !#?IN !#?IN # # # 2 # 2?2 2 # CONT FB 6CC 6)0ER?X !UXILIARY0OWER 6IN )NPUT3ECTION 3NUBBER

Section 4: Measurements two versions are presented, with their different responses. electrolytic capacitor is used on the secondary side. The second point is the true way to get a good power factor. progressively limits the corresponding MOSFET peak current. control voltage: reducing the voltage, lowers the current. like Vin, obtaining the required high power factor. voltage of the VIPer's PWM comparator is modulated. limited by the clipping action of the feedback. counteract the Vin modulation. Figure 4. ID

predominant the influence of Vin optimizing the power factor.

2.2 Secondary side

comparator for output overvoltage protection. The equivalent circuit is represented in Figure 9. Figure 9. Secondary side equivalent schematic the noise that may eventually be picked up from the output wire connection. introduce a voltage feedback in the current loop path). GND pin and the lower side of R14 are connected to this point.

Circuit description AN3212 Looking at the component values, it can be noted that:

• The time constant of the voltage op_amp is quite short (R9 = 0 Ω, C10 = 560 pF); this is because the circuit has to react as fast as possible to output overvoltage
• The time constant of the current op_amp is very long (R12 = 5.6 kΩ, C20 = 1 µF), as already stated, the reason for this is in the way in which the current control is implemented; while Vin modulates the cont pin cycle by cycle, the current feedback op_amp simply evaluates the average output current and drives the FB (and cont) pins with a voltage that varies very slowly. For the same reason, also the capacitor C21, on primary side, has a very high value of 10 µF
• The resistor on the optocoupler's photodiode anode (R4) is a mere 220 Ω, this is in order to achieve a high DC loop gain, and so a good current regulation
• The voltage divider, made up of R5 and R14, is dimensioned in order to fix an overvoltage cut-off of: Equation 1 Slightly higher than the maximum output voltage: Equation 2 But not too high, so as to avoid the possibility that Vaux too could reach a critical voltage.
• The sense resistor is implemented with R16, R17 and R18 in parallel. Due to the configuration with GND on the “transformer side” of Rsense, its value must be evaluated taking into account that the threshold level is 172 mV instead of 200 mV. Equation 3 Equation 4 Equation 5 Equation 6 Vout coff 1.21V() R5 R14+  15.97V=⋅= Vout max VLED avg 2---VLED rip VRsense pk++ = Vsense ' Vsense Vref ⋅= Vsense ' 0.2 1.21 Rsense Vsense ' ILED Rsense 0.1716

2.3 Circuit variants

• Output Power: 3.5 W/7.0 W
• Input voltage: wide range (90 V - 277 VAC) / European range (170 V to 277 VAC)
• Power factor: > 0.7/>0.9
• Electrolytic capacitors: yes/no (ripple current) Output power: to change this, it is enough to change the value of some components: Even though, to obtain the best performance also at 3.5 W, some kind of fine tuning may be required in the current shaping circuitry and in the EMI filter section, and probably a smaller transformer would be sufficient. Input voltage range: this impacts the voltage rating of the devices directly connected to the rectified input voltage. The demonstration board is provided with the indicated components to sustain the max value of Vin = 277 V, and, of course, in the case of a 90 V - 130 V range they can be derated. On the other hand, the max input current occurs at the lower input voltage and then the transformer must be dimensioned as a consequence; for this reason, if the board is targeted to the high line range, the transformer may be reduced (to be carefully verified). That is to say that the wide range is the worst condition, and the demonstration board design reflects this fact. Power factor: if it is sufficient to reach a PF > 0.7, the transistor Q1, and the associated R8, R10, and C13, can be avoided. In any case, if this parameter must be optimized, R2+R6, R13, and R10 must be modified, even though it's not a straightforward task, because the best shape of the peak current envelope must be found, as a function of input and output voltage ranges. Electrolytic capacitors: the question is slightly more complicated; as LEDs have a very long life, also the electronics should have a comparable MTBF, but el_caps with this property, despite being very expensive, are difficult to find, for this reason they should be avoided, but without them, in this configuration, the output current ripple is inevitably high. Therefore, special care must be taken in selecting the LEDs: their max. allowed current must be higher than the output peak current. Moreover, this ripple is almost equivalent to a sort of dimming at twice the line frequency which should be carefully considered from the optical point of view. In any case the board allows all these variations in order to carry out the tests without any major changes.

Table 1. Changes

3 Waveforms

analyzed in the main characteristic conditions, capturing the relevant signals.

3.1 Input

drain voltage and current at nominal output (10.5 V/0.7 A) with several input voltages. (the minimum level at which the converter stops switching. Figure 10. V_drain, I_drain at Vin= 75 V F igure 11. V_drain, I_drain at Vin= 100 V Figure 12. V_drain, I_drain at Vin= 120 V F igure 13. V_drain, I_drain at Vin= 162 V

3.2 Output

Figure 18. Vin and Iin at Vin = 230 VAC Figure 19. Vin and Iin at Vin = 115 V AC Figure 20. V_out, I_out at Vin = 230 VAC, no Figure 21. V_out, I_out at Vin = 115 VAC, no

application to decide if it can be tolerated or a capacitor is required.

3.3 Startup sequence

with - a 1000 µF output capacitor. Figure 22. V_out, I_out at Vin = 230 VAC, 1000 µF Figure 23. V_out, I_out at Vin = 115 VAC, 1000 µF Figure 24. Startup sequence at Vin = 230 VAC, Figure 25. Startup sequence at Vin = 115 VAC,

this case the short-circuit protection must be evaluated very carefully.

3.4 Short-circuit protection

part count are privileged at the expense of higher current pulses. auxiliary power Vdd reaches the shutdown voltage of the controller (8 V nom). hand, it cannot be reduced too much, otherwise the startup sequence becomes critical. Figure 26. Startup sequence at Vin = 230 V Figure 27. Startup sequence at Vin = 115 VAC,

3.5 Open circuit protection

then 140 V), a restart cycle is initiated. Figure 31. Open circuit protection Vin = 277 VAC Otherwise a continuous burst mode is sustained. Figure 32. Open circuit protection Vin = 90 VAC

Figure 33. Open circuit application Figure 34. Open circuit removal

4 Measurements

• A HP6812B programmable AC mains voltage source
• A Yokogawa WT210 wattmeter to measure input voltage, current, power, and PF
• A string of several diodes to simulate the LED load
• A couple of Keithley 2000 multimeters to measure output (average) voltage and current, or alternatively, a WT210 Wattmeter to measure output power and efficiency
• An Agilent E7402A spectrum analyzer plus LISN for EMI conducted emission tests The test procedure consisted of connecting the module output to a string of 10 diodes (STTH108) to emulate an LED load with a forward voltage of approximately 8.75 V, and then taking the measurements while the input voltage was set at several values from 90 to 277 V AC. The procedure was repeated increasing the number of diodes (12 and 14 devices) in order to simulate an LED load with a voltage of about 10.5 V and 12.0 V. The first run was without any electrolytic, and then the measurements were repeated with a 1000 µF capacitor directly connected to the output. In both conditions relevant data were collected and the results summarized in the following graphs: the first shows the output voltage as a function of the input AC voltage with the number of load diodes as the parameter, the others represent:
• the output current (average)
• the output current (peak)
• the output power
• the efficiency
• the power factor as a function of the input voltage and with the output voltage approximately corresponding to 10, 12, and 14 diodes load, as the parameter.

Figure 35. Test setup Special consideration must be paid to output power and efficiency measurements.

calculate the output power as their product. wattmeter also to the output. then the input/output power measurements were repeated and the efficiency evaluated. Figure 36. NO_El_Cap output voltage (average)

Figure 41. NO_El_Cap power factor

4.3 EMI filter

• C4 (22 nF) capacitor before the diode bridge plus C3 (22 nF) capacitor
• L1 coil (1 mH)
• C1 (100 nF) capacitor Figure 48 and 49 show the plots taken at 230 V and 115 V with an LED load (0.7 A/10.5 V). As can be seen, there wasn't a lot of margin, so the inductance value was increased. It is not possible to increment the capacitances, unless at the expenses of a worse PF, and with limited improvement. Then, to be safe, a second coil (L3 -1 mH) was introduced in the AC path, actually adding an L cell just before the diode bridge and the PI section. The result obtained is clearly better and is indicated in Figure 50 and 51.

Figure 48. EMI (PI filter) 230 VAC Figure 49. EMI (PI filter) 115 VAC Figure 50. EMI (L + PI filter) 230 VAC Figure 51. EMI (L + PI filter) 115 VAC

4.4 Thermal maps

• Ambient temperature: 27 °C
• Load: 7 W LED
• AC input voltage: 90 V, 115 V, 230 V, and 277 V The three highlighted areas correspond to the devices: 1. VIPer27 2. Transformer 3. STPS3L60 output diode As can be seen, the circuit is well within safe conditions.

Figure 52. Thermal map at 90 VAC Figure 53. Thermal map at 115 VAC Figure 54. Thermal map at 230 VAC Figure 55. Thermal map at 277 VAC Table 2. Components max. temperature

5 Electrical diagram

Figure 56. Electrical diagram

2 K2 K

6 BOM list

Table 3. BOM 7.0 W version

Table 3. BOM 7.0 W version (continued)

7.1 Mechanical specifications

Figure 57. Coil former mechanical drawing Figure 58. Transformer assembly

7 W transformer specifications AN3212

7.2 Electrical specifications

1. CORE: EE16 AL:1140 +/-25%nH/N*N (T DK PC40 MATERIAL or equivalent)
2. Leakage inductance (P1 - P2): < 50 µH (at 100 kHz, 1 V) with other pins shorted

Figure 59. Transformer electrical drawing

2 Start winding from the pin marked “·” (especially for N5 and point A)

3 Cut off pin 5, 6, 7, 8

4 Attach a copper foil 3M #1245 tape (4 mm width*10 mm length) to the Core,

5 Make sure the foil maintains good contact with the ferrite core, and solder a UL xpvc wire to

Table 4. Transformer winding data

8 Revision history

Table 5. Document revision history 19-Oct-2010 1 Initial release.