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Application Note AN-1171
IRPLLED7
90-250VAC Offline LED Driver
using IRS2980
By Peter B. Green
Table of Contents
Page
1. Introduction ......................................................................................2
2. Constant Current Control .................................................................3
3. High Voltage Regulator....................................................................5
4. Current Sense Level Shifter.............................................................5
5. PWM Dimming.................................................................................6
6. IRPLLED7 Circuit Schematic ...........................................................7
7. Bill of Materials.................................................................................8
8. PCB Layout......................................................................................9
9. Test Results.....................................................................................10
10. Design Procedure Summary..........................................................13
Safety Warning!
The IRPLLED7 LED driver does not provide safety isolation. Whe n operating the output drive to
the LEDs can produce pote ntially dangerous voltages. This board is intended for evaluation
purposes only and sho uld be handled by qualified electri cal engineers only!
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EVALUATION BOARD - IRPLLED7
1. Introduction
Solid state light sources are now available that offer viable alternatives to
Fluorescent and HID lamps and far surpass incandescent lamps. Luminous
efficacy expressed in Lumens per Watt has now reached levels enabling LEDs to
be used for general illumination. High brightness LEDs also possess the added
advantages of longer operating life span up to 50000 hours and greater
robustness than other less efficient light sources making them suitable for
outside applications such as street lighting.
High power LEDs are ideally driven with constant regulated DC current,
requiring a "driver" or "converter" to provide the required current from an AC or
DC power source. A simple single stage power converter based around the
IRS2980 LED driver IC provides a controlled current output over a wide AC line
or DC voltage input range.
The IRPLLED7 evaluation board is an off line non-isolated constant current
Buck regulator LED driver designed to supply a 350mA DC output current. The
LED output voltage can be up to 90% of the input voltage, operating from an AC
line input voltage between 90 and 250VAC 50/60Hz or 50 to 300VDC. It also
includes PWM dimming capability from 10% to 100% of light output controlled
by an on board potentiometer.
Important Safety Information
The IRPLLED7 does not provide galvanic isolation of the LED drive output from
the line input. Therefore if the system is supplied directly from a non-isolated
input, an electrical shock hazard exists at the LED outputs and these should not
be touched during operation. Although the output voltage is low this electrical
shock hazard still exists.
It is recommended that for laboratory evaluation that the IRPLLED7 board be
used with an isolated AC or DC input supply. The IRS2980 series Buck topology
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is suitable only for final applications where isolation is either not necessary or
provided elsewhere in the system.
Figure 1: IRPLLED7 Block Diagram
2. Constant current control
The IRS2980 is a hysteretic Buck controller operating in continuous conduction
mode (CCM) and using a low side switching MOSFET as the controlled switch
and a fast recovery diode as the uncontrolled switch connected to the positive
DC bus. This mode of operation is opposite to the IRS25401 and includes a
differential floating high side current sense circuit, which is used to hysteretically
control the output current by sensing the voltage drop across a sense resistor
and regulating the average to 0.5V. The IRS2980 is designed for use in current
regulated circuits and not voltage regulated circuits.
Figure 2: IRS2980 Basic Schematic
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Figure 2 illustrates how the current is sensed by differentially measuring the
voltage between the HV and CS inputs, RF and CF have been added to provide
noise filtering. When the MOSFET (MBUCK) is switched on the current in the
inductor LBUCK rises linearly according to the relationship:
dt
di
LbuckVoutVin .=−
Where Vin is the bus voltage rectified from the AC line voltage and Vout is the
combined series voltage of the string of LEDs making up the load.
When the voltage at HV rises to 0.55V with respect to CS the gate drive to
MBUCK switches off. When the MBUCK is off the inductor current flows instead
through DBUCK. During this period the current decreases linearly according to
the relationship:
dt
di
LbuckVout .−=
When the voltage at HV falls to 0.45V with respect to CS the gate drive to
MBUCK switches on. The cycle repeats continuously to provide an average
current in LBUCK which supplies the LED load. The frequency and duty cycle are
dependent on the input and output voltages and the value of the LBUCK as can
be inferred from the equations.
The output current can be set by selecting the appropriate value of RCS
according to the relationship:
RCS
VCS
avgIout =)(
where VCS is 0.5V, therefore for an RCS of 1.5V, the output current will be
nominally 333mA. In practice there are some additional propagation delays in the
circuit which give rise to a small variation in the current regulation over input
voltage, however the accuracy adequate for LED applications. Accuracy of
regulation and amplitude of the current ripple are tradeoffs against inductor size.
The IRS2980 incorporates a frequency limiting function that prevents the
frequency from exceeding approximately 150kHz. This is necessary in order to
limit the VCC current consumption since the internal high voltage regulator can
supply only a limited current (ICC) which is dominated by gate drive current. Gate
current charges and discharges the MOSFET gate capacitance during each
switching cycle and therefore increases with frequency.
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3. High Voltage Regulator
The IRS2980 contains an internal high voltage regulator to supply VCC from the
high voltage DC bus. Figure 2 shows that pin 1 (HV) is connected directly the DC
bus. Current is supplied to the VCC supply at pin 2 through an internal current
source capable of operating up to 450V. The internal regulator can supply up to
3mA, which is sufficient to supply VCC for most MOSFET gate capacitances and
frequencies normally required in an LED driver. ICC can be reduced by selecting
a MOSFET with a low gate capacitance (25nC or less) and selecting an
inductance (LBUCK) that will allow the regulator to operate at a reduced
frequency. A regulator operating at 60kHz for example will require much less ICC
than one operating at 120kHz. As explained earlier this is a tradeoff against
inductor size. It is also important to consider the temperature rise of the IRS2980.
Since the internal regulator operates linearly the associated power loss is
dependent on bus voltage and ICC.
More care must be taken at higher bus voltages to minimize frequency and ICC
to minimize the IC operating temperature. The addition of heat sinking in the form
of large areas of copper on the PCB or thermally conductive potting compounds
can significantly reduce temperature. Inductor values are generally larger for
220V off line AC applications than for 120V in order to reduce switching
frequency, which lowers power dissipation in the circuit.
4. Current sense level shifter
The IRS2980 uses a floating differential current sense circuit to measure the LED
current in the high side of the supply circuit. The Buck regulator configuration
uses a low side switch, which is opposite to the IRS25401. In order to realize
average current control the current must be sensed both when the MOSFET
(MBUCK) is switched on and when it is switched off and therefore must be
sensed at the high side. In order to accomplish this the hysteretic current sensing
circuitry within the IRS2980 is situated within a floating high side well constructed
by means of International Rectifiers HVIC technology. A floating supply voltage
(nominally 8V) for the circuitry contained within this well is developed between
the HV and VS pins of the IC. The supply is provided by a current source located
between VS and COM.
The high side contains a comparator with defined hysteresis connected to a
-0.5V reference with respect to HV. The output from the comparator is
transferred through high voltage level shift circuitry to the gate driver circuitry,
which is referenced to COM. The incorporation of the floating high side well
allows the LED current to be sensed at voltages up to 450V above COM.
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5. Dimming
The IRS2980 includes a PWM dimming oscillator that provides a linear ramp
waveform at the RAMP pin with the frequency determined by an external
capacitor to COM (CRAMP). The IRPLLED7 demo board uses a passive valley
fill circuit comprising two electrolytic capacitors, three diodes and one resistor to
provide a high power factor between 0.8 and 0.9 depending on line voltage and
load, without the additional cost of an additional active stage. This circuit
(C2,C5,D2,D3,D4 and R3) can be seen in the schematic shown in section 6. The
passive valley fill circuit however, creates a high ripple on the DC bus at twice the
line frequency (50-60Hz). The constant current Buck regulator is easily capable
of compensating for this, however in PWM dimming designs it means that the
PWM frequency needs to be significantly higher than 120Hz in order to avoid
visible flicker of the LEDs. The PWM dimming frequency in the IRPLLED7 demo
board is approximately 800Hz determined by a CRAMP value of 10nF. The
dimming ramp varies between 0 and 2V and is compared with a DC dimming
control voltage from 0 to 2V applied to the ADIM input at pin 5.
The IRPLLED7 board includes a pot which adjusts the ADIM input over the 0 to
2V range to provide the full range of dimming.
Figure 3: IRPLLED7 PWM Dimming
Figure 3 shows the output current to the LED load at a dimming level of about
30%. It can be seen that the amount of current ripple varies slightly due to the
DC bus voltage created by the passive valley fill circuit. At this PWM frequency
there is no noticeable flicker during dimming.
The IRS2980 is designed for PWM dimming. It can also be used in a linear
dimming mode with the addition of a few components.
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6. IRPLLED7 Circuit Schematic
1
VAC1
1
VAC2
1VOUT+
1VOUT-
3
1
4
2
BR1
DF10S
D2
S1G-13F
D3
S1G-13F
D4
S1G-13F
C3
0.22uF / 500V
COM
4HV 1
VS
2
VCC
3
ADIM 5
RAMP 6
OUT 7
CS 8
IC1
IRS2980
R3
10 / 1W
R2
1.5 / 1W
R5
10
C6
0.1uF
L2
1mH / 0.5A
M1
IRFR812
C1
100nF / 250V
C7
0.01uF
R4
15K
D1
MURS120-13
+
C2
22uF / 160V
+
C5
22uF/160V
R1
10 / 1W
L1
1mH / 0.5A
C4
1nF R6
1K
R7
5K
C8
22nF
C9
0.1uF
Figure 4: IRPLLED7 Complete Schematic
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7. Bill of Materials
Item Description Part Number Manufacurer Quantity Reference
1 IC, LED Controller IRS2980S International
Rectifier 1 IC1
2 Rectifier ,1A , 400V,
SMA S1G-13-F Diodes Inc 3 D2,D3,D4
3 Diode, 1A, 600V, 35nS,
SMB MURHS160T3G On
Semiconduct
or 1 D1
4 Bridge, 1000V, 1.5A,
4SDIP DF10S Fairchild 1 BR1
5 MOSFET, 500V,
2.2Ohm, DPAK IRFR812 International
Rectifier 1 M1
6 Capacitor, 100nF,
250V, Radial B32520A3104K Epcos 1 C1
7 Capacitor, 1nF, 630V,
10%, 1206 C32160G2J102J TDK 1 C4
8 Capacitor, 0.1uF, 50V,
10%, 1206 C3216X7R1H104
K TDK 2 C6, C9
9 Capacitor, 22uF,
250VDC, 20% EEU-EB2E220 Panasonic 2 C2, C5
10 Capacitor, 0.22uF,
450VDC, 1210 CKG32KX7T2W2
24M TDK 1 C3
11 Capacitor, 22nF, 50V,
1206 CGA5C2C0G1H2
23J TDK 1 C8
12 Capacitor, 0.01uF, 50V,
1206 C3216C0G1H10
3J TDK 1 C7
13 Resistor, 1.5Ohm, 1W,
5%, 2512 ERJ-1TYJ1R5U Panasonic 1 R2
14 Resistor, 10Ohm, 1W,
5%, Axial PR01000101009
JR500 Vishay 2 R1, R3
15 Resistor, 15K, 0.25W,
5%, 1206 ERJ-8GEYJ153V Panasonic 1 R4
16 Resistor, 10, 0.25W,
5%, 1206 ERJ-8GEYJ100V Panasonic 1 R5
17 Resistor, 1K, 0.25W,
5%, 1206 ERJ-8GEYJ102V Panasonic 1 R6
18 Pot, 5K, 0.5W, Single,
Top adjust 3386P-1-502LF Bourns Inc 1 R7
19 Inductor, 1mH, 0.55A,
1.68Ohm B82477G4105M Epcos 2 L1, L2
20 Test point, 0.063"D
Yellow 5009 Keystone 2
21 Test point, 0.063"D
Red 5005 Keystone 1
22 Test point, 0.063"D
Black 5006 Keystone 1
23 PCB IRPLLED7 Rev C 1
24
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8. PCB Layout
Top Overlay Top Copper
Bottom Overlay Bottom Copper
Layout Considerations
It is very important when laying out the PCB for the IRS2980 based LED driver to
consider the following points:
1. CVCC (C6) and CHVS (C8) must be as close to IC1 as possible.
2. The feedback path should be kept to a minimum length and separated as
much as possible from high frequency switching traces to minimize noise
at the CS input.
3. The current sense filter components RF (R6) and CF (C4) should be
located close to the IRS2980 with short direct traces.
4. It is essential that all signal and power grounds should be kept separated
from each other to prevent noise from entering the control environment.
Signal and power grounds should be connected together at one point only,
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which must be at the COM pin of the IRS2980. The IRS2980 may not
operate in a stable manner if these guidelines are not followed!
All low side components associated with the IC should be connected to
the IC signal ground (COM) with the shortest path possible.
5. All traces carrying the load current need to be sized accordingly.
6. Gate drive traces should also be kept to a minimum length.
9. Test Results
Measurements were carried out using a variable DC power supply and a load of
7 white LEDs being driven at a nominal 350mA.
DC Input
Voltage
(V)
DC Input
Current
(A)
Output
Voltage
(V)
Output
Current
(mAav)
Ripple
(mApp) Frequency
(kHz) Duty
Cycle (%)
60 0.14 20.1 335 120 98 40
70 0.12 20.1 340 140 99 36
80 0.11 20.1 344 150 99 30
90 0.10 20.1 349 160 97 26
100 0.09 20.1 353 180 95 23
110 0.08 20.1 357 190 94 21
120 0.07 20.1 360 190 92 19
130 0.07 20.1 364 200 89 17.6
140 0.07 20.1 367 200 87 16.2
150 0.06 20.1 370 210 85 15.2
160 0.06 20.1 373 220 83 14.2
170 0.06 20.1 375 230 81 13.4
180 0.05 20.1 377 240 80 12.6
Table 1: IRPLLED7 Test Results
As expected table 1 indicates that the duty cycle is approximately equal to
Vout/Vin, the LED total voltage drop divided by the supply voltage. It can be seen
that the current ripple increases as the duty cycle reduces since the voltage
difference is increasing. This is because:
dt
di
LbuckVoutVin .=−
and therefore di/dt is increasing, which results in more overshoot in the hysteretic
comparator due to inevitable propagation delays in the system. These delays
actually provide an advantage because the operating frequency is decreasing
with higher input voltage which limits ICC and switching losses in both MBUCK
(M1) and DBUCK (D1).
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The gate drive and output current waveforms are displayed in Figure 5:
Input = 60VDC Input = 180VDC
Green Trace = Gate Drive, Blue Trace = Output Current
Figure 5: IRPLLED7 Typical Waveforms
The IRPLLED7 board uses an inductor of 1mA. Increasing this value would
reduce frequency and ripple. Ripple can also be reduced by adding a capacitor to
the output although this is not necessary in most applications and may reduce
the PWM dimming range.
In the example shown in figure 6 below, where a load of fewer series LEDs was
attached:
Yellow = Gate, Green = LED Current
Vin = 60V, Iin = 0.09A, Pin = 5.4W
Vout = 13.87, Iout = 0.338A, Pout = 4.66W
Figure 6: IRPLLED7 at lighter load
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In the following example, an combination of LEDs was connected with a
combined voltage drop of approximately 30V. The board is capable of operating
down to 60VDC input, below which the high voltage regulator does not operate.
In applications requiring a lower input voltage VCC can be supplied directly from
an alternate source, the simplest option being a resistor from the DC bus to VCC.
DC Bus
Voltage (V) Output
Voltage
(V)
Output
Current
(mAav)
Ripple
(mApp) Frequency
(kHz) Duty Cycle
(%)
60 30.76 334 100 150 54.4
70 30.60 330 110 154 46
80 30.55 329 120 155 41.8
90 30.52 329 130 155 37.3
100 30.43 331 140 157 33.0
110 30.43 332 150 157 30.3
120 30.43 335 150 158 27.4
130 30.43 337 160 159 25.2
140 30.44 340 170 159 25
150 30.45 343 170 160 23.1
160 30.47 346 170 160 21.6
170 30.49 349 170 161 20.5
180 30.51 353 170 161 19.6
Table 2: IRPLLED7 Additional Test Results
Since the IRS2980 incorporates an internal high voltage regulator and level
shifting circuitry it dissipates some heat during operation which increases with
frequency and line voltage. It is necessary as with the MOSFET (MBUCK) and
diode (DBUCK) to ensure that these components do not overheat in the
application. This is done by providing additional copper around the components
on the PCB to allow heat conduction from the devices. In 220VAC off line
applications is is necessary to use a suffliciently large inductor (LBUCK) in order
to maintain a low operating frequency in the 30 to 60kHz range. This will
substantially reduce heat dissipation in all of the components mentioned.
Replacing the the 1mH inductor used in the IRPLLED7 demo board with a 3.3mH
part lowers the frequency and reduces heat loss in at 200VAC.
Efficiency varies depending on input voltage, output voltage, output current and
switching frequency.
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10. Design Procedure Summary
1. Determine the systems requirements: input/output voltage and current
needed
2. Calculate current sense resistor
3. Determine the operating frequency required.
4. Select LBUCK so that they maintain supply into the load during t_HO_on.
5. Select the switching MOSFET and diode) to minimize gate drive current
and switching losses.
IR WORLD HEADQUARTERS: 101 N. Sepulveda Blvd., El Segundo, California 90245 Tel: (310) 252-7105
Data and specifications subject to change without notice. 7/31/2012
