cd00271703

AN3212
Application note
5 W to 7 W high power factor offline LED driver
based on VIPer devices
Introduction
The driving idea behind this application note is to exploit the possibility of implementing an
LED power supply module characterized by a high power factor, based on devices from the
VIPer family in flyback configuration and with a TSM1052 as a constant current controller.
The other key point is to avoid using high voltage electrolytic capacitors, evaluate the
influence of the output bulk electrolytic capacitor on overall performance, and consider its
replacement with much smaller ceramic components, eventually implementing a non
electrolytic configuration.
The STEVAL-ISA120V1 demonstration board has been designed as a platform to perform
this evaluation.
Figure 1. STEVAL-ISA120V1 VIPer27 LED driver module
March 2013
DocID17427 Rev 2
1/36
www.st.com
Contents
AN3212
Contents
1
2
3
4
Main characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1
Initial configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2
Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1
Primary side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2
Secondary side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3
Circuit variants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1
Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2
Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.3
Startup sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.4
Short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.5
Open circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.1
7.0 W NO EL_CAP configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.2
7.0 W EL_CAP configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.3
EMI filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.4
Thermal maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
5
Electrical diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
6
BOM list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
7
7 W transformer specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
8
2/36
7.1
Mechanical specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
7.2
Electrical specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
DocID17427 Rev 2
AN3212
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Figure 33.
Figure 34.
Figure 35.
Figure 36.
Figure 37.
Figure 38.
Figure 39.
Figure 40.
Figure 41.
Figure 42.
Figure 43.
Figure 44.
Figure 45.
Figure 46.
Figure 47.
Figure 48.
STEVAL-ISA120V1 VIPer27 LED driver module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Initial configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Primary side schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
IDLIM vs RLIM - VIPer17 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
IDLIM vs RLIM - VIPer27 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
No feedback on FB pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Feedback on FB pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Feedback voltage V_fb vs. VAC and Vout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Secondary side equivalent schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
V_drain, I_drain at Vin= 75 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
V_drain, I_drain at Vin= 100 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
V_drain, I_drain at Vin= 120 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
V_drain, I_drain at Vin= 162 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
V_drain, I_drain at Vin= 254 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
V_drain, I_drain at Vin= 325 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
V_drain, I_drain at Vin= 391 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
V_drain, I_drain at Vin= 50 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Vin and Iin at Vin = 230 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Vin and Iin at Vin = 115 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
V_out, I_out at Vin = 230 VAC, no El_cap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
V_out, I_out at Vin = 115 VAC, no El_cap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
V_out, I_out at Vin = 230 VAC, 1000 µF El_cap. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
V_out, I_out at Vin = 115 VAC, 1000 µF El_cap. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Startup sequence at Vin = 230 VAC, no El_cap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Startup sequence at Vin = 115 VAC, no El_cap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Startup sequence at Vin = 230 VAC, 1000 µF El_cap . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Startup sequence at Vin = 115 VAC, 1000 µF El_cap . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Short circuit application. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Short circuit removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Open circuit protection Vin = 277 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Open circuit protection Vin = 90 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Open circuit application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Open circuit removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
NO_El_Cap output voltage (average). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
NO_El_Cap output current (average) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
NO_El_Cap output current (peak) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
NO_El_Cap output power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
NO_El_Cap efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
NO_El_Cap power factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1000 µF output voltage (average). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
1000 µF output current (average) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
1000 µF output current (peak) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
1000 µF output power. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
1000 µF efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
1000 µF power factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
EMI (PI filter) 230 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
DocID17427 Rev 2
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List of figures
Figure 49.
Figure 50.
Figure 51.
Figure 52.
Figure 53.
Figure 54.
Figure 55.
Figure 56.
Figure 57.
Figure 58.
Figure 59.
4/36
AN3212
EMI (PI filter) 115 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
EMI (L + PI filter) 230 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
EMI (L + PI filter) 115 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Thermal map at 90 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Thermal map at 115 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Thermal map at 230 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Thermal map at 277 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Electrical diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Coil former mechanical drawing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Transformer assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Transformer electrical drawing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
DocID17427 Rev 2
AN3212
Main characteristics
1
Main characteristics
1.1
Initial configuration
Several demonstration boards already exist which accept the mains input voltage, wide or
local voltage range, and generate a regulated output current to drive an LED “string” with an
output power in the range of 3 W to 7 W, but none are expressly intended to achieve a high
power factor and/or avoid the use of electrolytic capacitors.
For this reason a “standard” flyback configuration was developed, based on a VIPer device
and with a TSM1052 as the constant current controller, then, some changes were
introduced in order to address the key points indicated above.
Figure 2. Initial configuration
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1.2
Requirements
The design was started taking the following key points into account:
•
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
DocID17427 Rev 2
5/36
Circuit description
AN3212
2
Circuit description
2.1
Primary side
In order to keep the part count to a minimum, the primary side of the converter is based on a
device from the VIPer family, a VIPer17 for the 3.5 W and a VIPer27 for the 7 W version.
As can be seen in Figure 3, the circuit is similar to a standard flyback, with:
•
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
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The more “unusual” points are:
•
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.
6/36
DocID17427 Rev 2
AN3212
Circuit description
Care should be taken in designing the EMI filter due to the constraints indicated above. In
Section 4: Measurements two versions are presented, with their different responses.
The main drawback to this configuration is that it lacks a bulk capacitor which stores energy
on the primary side, and then the output current is affected by a high ripple, unless a large
electrolytic capacitor is used on the secondary side.
The second point is the true way to get a good power factor.
Referring to the VIPer17/27; Off-line high voltage converters datasheets, the cont pin is the
control that allows reducing the MOS peak current setting from the internally fixed point to
about 1/10 of that value. This can be accomplished by means of a resistor Rlim connected
between this pin and ground. Figure 4 and 5 represent the current ratio iDlim/(iDlim @ 100k)
as a function of Rlim. As can be seen, changing Rlim from 100 Kohm to a few kohms
progressively limits the corresponding MOSFET peak current.
Figure 4. IDLIM vs RLIM - VIPer17
Figure 5. IDLIM vs RLIM - VIPer27
An equivalent function can be implemented connecting the cont pin to a variable voltage
through a fixed resistor; in this way the peak current can be modulated simply varying the
control voltage: reducing the voltage, lowers the current.
Then, if the rectified mains voltage is scaled and applied to the cont input, the resulting
MOSFET peak current, and also the corresponding average input current, are shaped just
like Vin, obtaining the required high power factor.
The resistor array made up of R2, R6 and R13 implements this function, where R13 is the
lower practical value that fixes the minimum peak current, and R2 + R6 come out as a
consequence to guarantee a sufficient power transfer to the output (the lower the value, the
higher the output power).
On the other hand, to maintain a constant (average) output current, some kind of regulation
is required and for this reason, on the secondary side, there is an error amplifier which
senses the LED current and drives an optocoupler (see Section 2.2). On the primary section
the corresponding phototransistor is connected to the “FB” pin, and through this input the
voltage of the VIPer's PWM comparator is modulated.
In this way, the MOSFET peak current envelope follows the shape of Vin until it is somehow
limited by the clipping action of the feedback.
It is worth noting that the bandwidth of this loop must be very low, otherwise it would
counteract the Vin modulation.
DocID17427 Rev 2
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Circuit description
AN3212
Figure 6 and 7 show the results of a simulation which represent the behavior of the circuit.
Figure 6 is in the case of no feedback on the FB pin: only Vin is applied to the cont pin. The
average output current is 1.12A.
Figure 7 represents the condition when also the feedback is forced on the FB pin (Iout_avg
= 0.7A). Please note that it is a rough approximation to show how the configuration works.
Figure 6. No feedback on FB pin
Figure 7. Feedback on FB pin
One limit of this solution is that the voltage applied to the cont pin is directly proportional to
the AC input voltage, and then at higher Vin the “clipping” is more evident and the PF is
worse.
To overcome this, a possible solution may be to feed part of the FB voltage to the cont pin; in
this way an offset voltage that is higher at lower AC input is provided, obtaining a more
constant modulation shape, and a better PF.
Figure 8. Feedback voltage V_fb vs. VAC and Vout
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Unfortunately it is not possible to simply connect a resistor between the FB and cont pins: to
adapt the impedance levels it is necessary to buffer the feedback signal before driving the
cont input. To do this, the NPN transistor Q1 is employed in an emitter follower configuration
and the R10 resistor provides the correct balancing between the Vin and V_fb actions.
8/36
DocID17427 Rev 2
AN3212
Circuit description
Reducing its value increases the influence of V_fb, obtaining a better control of the output
current even at the extreme mains and load values; on the other hand, increasing it makes
predominant the influence of Vin optimizing the power factor.
2.2
Secondary side
On the secondary side a TSM1052 is employed as a voltage reference and error amplifier
for the constant current control loop, while the CV operational amplifier is simply used as a
comparator for output overvoltage protection.
The configuration is quite common, with the two op amp outputs tied in wired_or to drive the
optocoupler's photodiode.
The equivalent circuit is represented in Figure 9.
Figure 9. Secondary side equivalent schematic
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The first point which is worth noting is the decoupling of the supply voltage (R3, D5, C5, C7,
etc.) It protects the TMS1052 in the case of overvoltage due to LED “open” fault, and filters
the noise that may eventually be picked up from the output wire connection.
Moreover, it avoids that the output voltage and its ripple modulate the photodiode current
(while this action can be useful in CV applications, in this case it isn't, because it would
introduce a voltage feedback in the current loop path).
The second point is related to the TSM1052 grounding; in this configuration the reference
GND is on the left-hand side of the sense resistor (R16, R17, R18 in parallel), the TSM1052
GND pin and the lower side of R14 are connected to this point.
DocID17427 Rev 2
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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
R5 + R14
Vout coff = ( 1.21V ) ⋅  ------------------------- = 15.97V
 R14 
Slightly higher than the maximum output voltage:
Equation 2
1
Vout max =  VLED avg + --- VLED rip + VRsense pk


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
Vref
Vsense' = Vsense ⋅  ----------------------------------------
 Vref + Vsense
Equation 4
1.21
Vsense' = 0.2 ⋅  ----------- V = 0.2 ⋅ ( 0.858 )V = 0.1716V
 1.41
Equation 5
Vsense'
Rsense = ---------------------I LED
Equation 6
0.1716
Rsense = ------------------ = 0.245Ω
0.7
10/36
DocID17427 Rev 2
AN3212
2.3
Circuit description
Circuit variants
Up to now the “basic” 7 W configuration has been referenced, but as indicated in the
introduction to the document, the goal was also to investigate the influence of the
requirements on the design, with special attention to:
•
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:
Table 1. Changes
Components/power
3.5 W
7.0 W
Rsense
0.5 Ω
0.25 Ω
Transformer primary inductance
2 mH
1.5 mH
VIPer
VIPer17
VIPer27
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.
DocID17427 Rev 2
11/36
Waveforms
3
AN3212
Waveforms
To take a look at the behavior of the board, the 7 W configuration has been selected and
analyzed in the main characteristic conditions, capturing the relevant signals.
3.1
Input
With the first series of waveforms the intention was to give a representation of the VIPer's
drain voltage and current at nominal output (10.5 V/0.7 A) with several input voltages.
Because of the difficulty of taking a stable snapshot of these measurements with an AC
input, use of a DC source was chosen, and the voltage fixed at: 75 V (the minimum level at
which the circuit starts switching), 100 V, 120 V, 162 V, 254 V, 325 V, 391 V, and then 50 V
(the minimum level at which the converter stops switching.
Figure 10. V_drain, I_drain at Vin= 75 V
CH1 (brown)=Vdrain, CH3 (red)= Vout,
CH4 (green)= Idrain
CH1 (brown)=Vdrain, CH3 (red)= Vout,
CH4 (green)= Idrain
Figure 12. V_drain, I_drain at Vin= 120 V
CH1 (brown)=Vdrain, CH3 (red)= Vout,
CH4 (green)= Idrain
12/36
Figure 11. V_drain, I_drain at Vin= 100 V
Figure 13. V_drain, I_drain at Vin= 162 V
CH1 (brown)=Vdrain, CH3 (red)= Vout,
CH4 (green)= Idrain
DocID17427 Rev 2
AN3212
Waveforms
Figure 14. V_drain, I_drain at Vin= 254 V
CH1 (brown)=Vdrain, CH3 (red)= Vout,
CH4 (green)= Idrain
CH1 (brown)=Vdrain, CH3 (red)= Vout,
CH4 (green)= Idrain
Figure 16. V_drain, I_drain at Vin= 391 V
CH1 (brown)=Vdrain, CH3 (red)= Vout,
CH4 (green)= Idrain
Figure 15. V_drain, I_drain at Vin= 325 V
Figure 17. V_drain, I_drain at Vin= 50 V
CH1 (brown)=Vdrain, CH3 (red)= Vout,
CH4 (green)= Idrain
DocID17427 Rev 2
13/36
Waveforms
AN3212
And to give an idea of the AC input voltage and current, Figure 18 and 19 show the plot of
these waveforms.
Figure 18. Vin and Iin at Vin = 230 VAC
CH1(brown)= Iin
CH4 (green)= Vin
CH1(brown)= Iin
CH4 (green)= Vin
3.2
Figure 19. Vin and Iin at Vin = 115 VAC
Output
The following images represent the output current and voltage waveforms at nominal load
(10.5 V, 0.7 A) in the case of input voltage of 230 and 115 VAC.
Without an output electrolytic capacitor:
Figure 20. V_out, I_out at Vin = 230 VAC, no
El_cap
CH1 (brown)=Vdrain, CH3 (red)= Vout,
CH4 (green)= Iout
14/36
Figure 21. V_out, I_out at Vin = 115 VAC, no
El_cap
CH1 (brown)=Vdrain, CH3 (red)= Vout,
CH4 (green)= Iout
DocID17427 Rev 2
AN3212
Waveforms
With an output electrolytic capacitor of 1000 µF:
Figure 22. V_out, I_out at Vin = 230 VAC, 1000 µF Figure 23. V_out, I_out at Vin = 115 VAC, 1000 µF
El_cap
El_cap
CH1 (brown)=Vdrain, CH3 (red)=Vout
CH4 (green)= Iout
CH1 (brown)=Vdrain, CH3 (red)=Vout
CH4 (green)= Iout
Of course in the case of no el_cap, the ripple current is much higher, it is up to the
application to decide if it can be tolerated or a capacitor is required.
3.3
Startup sequence
Oscilloscope screenshots were taken at 230 V and 115 V, with the nominal load without and
with - a 1000 µF output capacitor.
Figure 24. Startup sequence at Vin = 230 VAC,
no El_cap
CH1 (brown)=Iout CH3 (blue)= Vfb
CH2 (red)=Vout
CH4 (green)=Vdd
Figure 25. Startup sequence at Vin = 115 VAC,
no El_cap
CH1 (brown)=Iout CH3 (blue)= Vfb
CH2 (red)=Vout
CH4 (green)=Vdd
DocID17427 Rev 2
15/36
Waveforms
AN3212
Figure 26. Startup sequence at Vin = 230 VAC,
1000 µF El_cap
Figure 27. Startup sequence at Vin = 115 VAC,
1000 µF El_cap
CH1 (brown)=Iout CH3 (blue)= Vfb
CH2 (red)=Vout
CH4 (green)=Vdd
CH1 (brown)=Iout CH3 (blue)= Vfb
CH2 (red)=Vout
CH4 (green)=Vdd
The circuit is relatively under damped, but this is intentional in order to guarantee a sure
startup, while minimizing the short-circuit detection time and providing a good average
current regulation.
Increasing R12 leads to a better startup current envelope and lower overshoot, but care
should be taken regarding the overload protection and DC current regulation, which slightly
worsens.
With output capacitors of very high value, it is possible that the circuit doesn’t start at the
“first shot”, in this case it is enough to increase the Vaux capacitors C16 and C17, but also in
this case the short-circuit protection must be evaluated very carefully.
3.4
Short-circuit protection
The primary application of this board is as a “bulb replacement”, and therefore short-circuit
failure is not critical: provided that the circuit survives without any damage. There are no
stringent requirements on maximum output current during shorts, so simplicity and minimal
part count are privileged at the expense of higher current pulses.
As the feedback circuit is too slow to react to the short-circuit, the protection is based on the
fact that, in case of overload, the output voltage drops, and as a consequence, also the
auxiliary power Vdd reaches the shutdown voltage of the controller (8 V nom).
The time required for the intervention is directly proportional to the capacitance of C16 and
C17, and then, the lower their value, the shorter the output current pulses. But, on the other
hand, it cannot be reduced too much, otherwise the startup sequence becomes critical.
16/36
DocID17427 Rev 2
AN3212
Waveforms
Figure 28. Short-circuit protection
CH1 (brown)=Iout
CH2 (blue)=Vout
CH3 (red)=Vfb
CH4 (green)=Vdd
As the most severe condition appears at the highest input voltage, the snapshot is taken
with VAC = 277 V, and with a total capacitance value (C16 + C17) of 44 µF.
As can be seen, even though the current pulses are quite high (6.6 A max) the output
voltage and the repetition rate are low, for this reason also the power involved is not critical
(181 mW average) and therefore, this condition can be sustained indefinitely.
Figure 29 and 30 show the conditions when the short-circuit is applied and removed.
It is worth noting that the very short and high current pulse, which appears when the short
circuit is forced, is due to the discharge of the ceramic output capacitors; even though no
electrolytic is present this current can reach a very high value.
Figure 29. Short-circuit application
CH1 (brown)=Iout
CH2 (blue)=Vout
CH3 (red)=Vfb
CH4 (green)=Vdd
Figure 30. Short-circuit removal
CH1 (brown)=Iout
CH2 (blue)=Vout
DocID17427 Rev 2
CH3 (red)=Vfb
CH4 (green)=Vdd
17/36
Waveforms
3.5
AN3212
Open circuit protection
As already indicated, the TSM1052 in the secondary section contains an op amp that
senses the output, and in the case of overvoltage, drives the optocoupler photodiode, which
in turn forces the VIPer's FB pin to ground. As a result the VIPer stops switching, and enters
the burst mode if Vfb drops below the 0.6 V threshold. If Vdd goes under 8 V (VAC higher
then 140 V), a restart cycle is initiated.
Figure 31. Open circuit protection Vin = 277 VAC
CH2 (blue)=Vout
CH3 (red)=Vfb
CH4 (green)=Vdd
Otherwise a continuous burst mode is sustained.
Figure 32. Open circuit protection Vin = 90 VAC
CH2 (blue)=Vout
CH3 (red)=Vfb
CH4 (green)=Vdd
18/36
DocID17427 Rev 2
AN3212
Waveforms
Figure 33. Open circuit application
CH1 (brown)=Vdrain CH4 (green)=Iout
CH3 (red)=Vout
Figure 34. Open circuit removal
CH1 (brown)=Vdrain CH4 (green)=Iout
CH3 (red)=Vout
DocID17427 Rev 2
19/36
Measurements
4
AN3212
Measurements
For all the board configurations, a common test setup was defined with:
•
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
VAC.
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.
20/36
DocID17427 Rev 2
AN3212
Measurements
Usually, in making these evaluations on standard power supplies, it is enough to take the
average values, as read out from the Voltmeter (V_Led) and Ammeter (I_Led) and simply
calculate the output power as their product.
This approach is correct whenever these values are constant, but in this application, due to
the high ripple present, especially if no electrolytic capacitor is employed, these waveforms
cannot be considered DC values at all. Therefore a more accurate way to take the
measurements, at least from the AC-DC converter point of view, is to connect a true
wattmeter also to the output.
For this reason, as indicated above, the two Keithley 2000s were replaced with a WT210
then the input/output power measurements were repeated and the efficiency evaluated.
7.0 W NO EL_CAP configuration
Figure 36. NO_El_Cap output voltage (average)
2XWSXW9ROWDJH9
4.1
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DocID17427 Rev 2
21/36
Measurements
AN3212
2XWSXW&XUUHQWP$
Figure 37. NO_El_Cap output current (average)
ϳϬϱ
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Figure 38. NO_El_Cap output current (peak)
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22/36
ϴ͘ϳϱs
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!-V
DocID17427 Rev 2
AN3212
Measurements
Figure 39. NO_El_Cap output power
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2XWSXW3RZHU:
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Figure 40. NO_El_Cap efficiency
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DocID17427 Rev 2
23/36
Measurements
AN3212
Figure 41. NO_El_Cap power factor
ϭ
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Ϭ͘ϵϴ
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24/36
ϴ͘ϳϱs
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!-V
DocID17427 Rev 2
AN3212
7.0 W EL_CAP configuration
The following measurements were taken with a 1000 µF electrolytic capacitor connected to
the module output.
Figure 42. 1000 µF output voltage (average)
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Figure 43. 1000 µF output current (average)
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4.2
Measurements
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DocID17427 Rev 2
25/36
Measurements
AN3212
Figure 44. 1000 µF output current (peak)
2XWSXW&XUUHQWP$
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Figure 45. 1000 µF output power
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26/36
ϴ͘ϳϱs
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!-V
DocID17427 Rev 2
AN3212
Measurements
Figure 46. 1000 µF efficiency
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Figure 47. 1000 µF power factor
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DocID17427 Rev 2
27/36
Measurements
4.3
AN3212
EMI filter
The first version of the EMI filter has been implemented with the classic PI cell:
•
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).
Figure 48. EMI (PI filter) 230 VAC
Figure 49. EMI (PI filter) 115 VAC
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 50. EMI (L + PI filter) 230 VAC
28/36
Figure 51. EMI (L + PI filter) 115 VAC
DocID17427 Rev 2
AN3212
4.4
Measurements
Thermal maps
The following images were taken with a thermo camera under the following conditions:
•
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
Figure 52. Thermal map at 90 VAC
110.0
°C
98.7
110.0
°C
98.7
87.4
87.4
76.1
76.1
64.8
2
1
Min: 44.4°C
Max: 56.5°C
53.5
42.2
Min: 42.4°C
Max: 69.4°C
30.9
64.8
2
1
53.5
3
Min: 35.0°C
Max: 70.2°C
Figure 53. Thermal map at 115 VAC
Min: 34.7°C
Max: 68.2°C
Min: 44.1°C
Max: 55.5°C
42.2
3
Min: 41.8°C
Max: 67.7°C
30.9
19.6
19.6
Figure 54. Thermal map at 230 VAC
2
Figure 55. Thermal map at 277 VAC
110.0
°C
98.7
110.0
°C
98.7
87.4
87.4
76.1
76.1
64.8
1
53.5
3
Min: 38.0°C
Max: 57.3°C
53.5
42.2
30.9
Min: 32.8°C
Max: 61.2°C
Min: 41.0°C
Max: 69.9°C
64.8
2
1
Min: 36.2°C
Max: 76.8°C
Min: 46.4°C
Max: 61.0°C
42.2
3
Min: 45.0°C
Max: 71.6°C
30.9
19.6
19.6
Table 2. Components max. temperature
Device
T[0C]@ 90 V
T[0C]@ 115 V
T[0C]@ 230 V
T[0C]@ 277 V
VIPer27
70.2
68.2
61.2
76.8
Transformer
56.5
55.5
57.3
61.0
STPS3L60
69.4
67.7
69.9
71.6
As can be seen, the circuit is well within safe conditions.
DocID17427 Rev 2
29/36
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DocID17427 Rev 2
6S
30/36
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Electrical diagram
AN3212
Electrical diagram
Figure 56. Electrical diagram
!-V
AN3212
BOM list
6
BOM list
Table 3. BOM 7.0 W version
Reference
Part
VL
CN1
Header 2
5.08 mm
CN2
Header 2
3.81 mm
C1
100 nF
400
5.0x13.0
EPCOS B32921
C2
680 pF
1 kV
5.08 mm
1KV CERCAP DIA. 4x7
mm PITCH 5.08 mm
MURATA
C3
22 nF
400
5.0x13.0
EPCOS B32921
C4
22 nF
X2
5.0x13.0
EPCOS B32921
C5,C6,C7,C8,C9,C12,
C14,C15,C16,C17
22 µF
25
1210
C10
560 pF
25
0603
C11,C21
10 µF
25
1210
C13,C23,C24
0.1 µF
16
0603
C18
1 µF
16
0603
C19
4.7 µF
16
0603
C20
1.0 µF
16
0603
C22
2.2 nF
Y1
10 mm
C25
1.8 nF
50
0603
D1
STPS3L60U
SMB
D2,D6
BAS316
SOD-323
D3
MB6S
SOIC-4
D4
STTH1L06U
SMB
D5
MMSZ4687T1
SOD-123
D7
MMSZ4708T1
SOD-123
F1
Fuse
L1,L3
250 V
PCB footprint
note
Y1 SAFETY CAP
DE1E3KX222M
MURATA
8.5x4 mm
800 mA
1.0 mH
5.2x12 mm
0.1 A axial lead
L2
33 µH
1206
Q1
BC847C
SOT-23
R1
330 kΩ
1206
R2
750 kΩ
1206
R3
1.2 kΩ
1206
R4
220 Ω
0805
DocID17427 Rev 2
31/36
BOM list
AN3212
Table 3. BOM 7.0 W version (continued)
32/36
Reference
Part
VL
R5,R8
100 kΩ
0805
R6
680 kΩ
1206
R7,R13
2.2 kΩ
0805
R9
0
0805
R10
10 kΩ
0805
R11
330 Ω
1206
R12
5.6 kΩ
0805
R14
8.2 kΩ
0805
R15
3.0 kΩ
0805
R16
0.82 Ω
1206
R17,R18
0.68 Ω
1206
R19,R20
0
0805
R21
2.2 kΩ
AX/RC05
T1
T1
E16
U1
TSM1052
SOT23-6
U2
VIPer27H
DIP-7
U3
OPTO-PC817-A
4-SMD
DocID17427 Rev 2
PCB footprint
note
AN3212
7 W transformer specifications
7
7 W transformer specifications
7.1
Mechanical specifications
Figure 57. Coil former mechanical drawing
Figure 58. Transformer assembly
DocID17427 Rev 2
33/36
7 W transformer specifications
7.2
AN3212
Electrical specifications
1.
BOBBIN: EE16 (4 +4 pin)
2.
CORE: EE16 AL:1140 +/-25%nH/N*N (TDK PC40 MATERIAL or equivalent)
3.
Primary inductance (P1 - P2): 1.5 mH +/- 10% (at 100 kHz, 1 V)
4.
Leakage inductance (P1 - P2): < 50 µH (at 100 kHz, 1 V) with other pins shorted
Figure 59. Transformer electrical drawing
!-V
Table 4. Transformer winding data
Note:
34/36
Winding
Pins
Wire type
Wire size
# turns
N1
P2----A
2UEW
Φ 0.15 mm * 2
54T 2Layer
N2
P4
Copper foil
0.025 mmT * 7 mmW
N3
FLY1
---FLY2
Triple insulation winding wire
Totoku TIW-3 UL FILE no. E66483
or equivalent
Φ 0.55 mm
N4
P4
Copper foil
0.025 mmT * 7 mmW
N5
A----P1
2UEW
Φ 0.15 mm * 2
54T 2Layer
N6
P3----P4
2UEW
Φ 0.22 mm
14T
10T
1
HIPOT: 3 KVAC primary to secondary 1 min. 10 mA
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
pin 4 as short as possible.
DocID17427 Rev 2
AN3212
8
Revision history
Revision history
Table 5. Document revision history
Date
Revision
Changes
19-Oct-2010
1
Initial release.
15-Mar-2013
2
Replaced part number EVLVIP27-7WLED with STEVALISA120V1.
DocID17427 Rev 2
35/36
AN3212
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