AN-42006 - Fairchild Semiconductor

ML4831

INVERTER LAMP NETWORKPFC

BRIDGE RECTIFIER

EMI FILTER

85-135

VAC +

DIMMING

CURRENT SENSE

George A. Hall

June 1996

Application Note 42006

Low Cost Electronic Ballast System Design

THEORY OF OPERATION

Figure 1 displays the block diagram of the ML4831EVAL

board.

Applying AC line voltage to the EVAL board supplies start-

up power to the ML4831 enabling gate drive for the PFC

boost MOSFET Q1 and inverter FETs Q2 and Q3. PFC

action generates a well regulated 205VDC supply for the

lamp inverter circuit and steady-state supply voltage for

the ML4831. The inverter stage consists of 2 totem pole

configured N-channel power MOSFETs with their

common node supplying the lamp network. The pair of

MOSFETs are driven out of phase by the ML4831 with a

50% duty cycle. The lamp network is a parallel resonant

circuit series-fed by the inverter transistors through a

wave-shaping and current limiting inductor T3. The

inductance of the resonant circuit is formed by T3’s

inductance and the primary inductance of the power

transformer T4. The power transformer also provides

safety isolation from the primary circuit to the bulbs. The

lamp intensity is controlled by sampling the lamp current

with current sensing transformer T5. T5’s secondary

current is converted to a voltage and fed to the ML4831’s

Lamp Feedback error amplifier. The amplifier output

voltage varies in accordance with the amount of intensity

required (set by potentiometer R23), internally adjusting

the switching frequency to the inverter stage. The

impedance characteristics of the lamp network results in

lower lamp currents (and intensity) when the inverter

stage frequency is increased.

GENERAL DESCRIPTION

This application note describes a dimmable ballast system

design using the ML4831 electronic ballast controller IC.

This system can be evaluated using the ML4831EVAL kit.

The ML4831EVALuation board is a low cost, improved

version of Micro Linear’s ML4830 dimmable ballast EVAL

board. Careful attention was given to reducing the

magnetic’s cost of the EVAL board as well as other costly

components. In addition, the design was improved to both

increase and linearize the dimming range, eliminate lamp

shut-off at low intensities, reduce visible standing waves

and simplify the lamp-out protection circuitry. All

components used are inexpensive and easy to obtain.

Operating from 85 to 135VAC line, the

ML4831EVALuation board is a power factor corrected

60W electronic ballast with a dimming range capable of a

20:1 intensity change. Optimized to power two series

connected T8 fluorescent bulbs, the ML4831EVAL board

displays all the features of Micro Linear’s latest ballast

controller IC. The mode of operation used for pre-heat,

striking and dimming of the bulbs is the widely accepted

variable frequency, non-overlapping inverter topology.

This EVAL board may be used with various bulbs other

than T8’s (such as T12’s). See “Powering Other

Fluorescent Lamps.”

BLOCK DIAGRAM

REV. 1.0 10/25/2000

Application Note 40

PERFORMANCE DATA

To measure system performance across the range of

permissible input voltages use a variac or adjustable AC

source.

A typical ML4831EVALuation board will have the

following performance characteristics when operated as

shown in the test conditions:

ML4831EVAL BOARD TEST RESULTS

85VAC 120VAC 135VAC Units

Efficiency 88 90 89 %

THD 2.32 2.12 2.35 %

P.F. 0.995 0.984 0.975 %

Test Conditions: 2 series wired T8 lamps (full intensity), 25°C

Equipment Used: Voltech Digital AC Power Analyzer #PM1000

The ML4831EVALuation board provides testpoints at the

following circuit nodes:

TP1 GND

TP2 VCC

TP3 INHIBIT

TP4 PFC Boost Voltage

TP5 Resonant Network (attenuated by 10x)

TYPICAL WAVEFORMS

Figures 2-5 display typical oscilloscope waveforms taken

at various points on the eval board. A brief description

precedes each figure. Test conditions and oscilloscope

settings are given below each photo. The waveforms were

taken with the eval board powering two series connected

T8 bulbs.

PFC BOOST VOLTAGE (Fig. 2, TP4)

The DC bus for the inverter stage is derived from the

rectified AC line. Note the 120Hz (2x line frequency)

ripple voltage superimposed on the DC voltage. This is the

result of the power factor correction of the AC line

voltage. The peak to peak amplitude of the ripple voltage

increases as the lamp intensity increases.

INVERTER VOL TAGE/CURRENT (Fig. 3)

The boosted DC bus voltage is chopped by Q2 and Q3

resulting in the square wave (upper trace) appearing at the

input to the lamp network (Q2, Q3, T3 node). The

resulting current in T3’s primary winding appears in the

bottom trace.

205V

1

Figure 2. PFC Boost Voltage

Scope Setting: 100V/div, Horiz = 5ms/div

Test Conditions: Lamps @ maximum intensity, 120VAC

Equipment Used: Tektronix TDS540 Digitizing Scope

2

1

Ch1 Freq

33.84096KHz

Ch1 Pk-Pk

40.8mV

Ch2 Freq

33.89408KHz

Ch2 Pk-Pk

224V

10.0mVΩ Ch2 100V M 10.0 µs Ch2 126V

Figure 3. Inverter Output Voltage/Current

Scope Setting: Top = 100V/div, Bottom = 0.5A/div, Horiz = 10µs/div

Test Conditions: Lamps @ maximum intensity, 120VAC

Equipment Used: Tektronix TDS540 Digitizing Scope, Tektronix AM503 Current

Probe Amplifier Assy

LAMP NETWORK VOLTAGE (Fig. 4, TP5)

The voltage at the T3, T4 and C19 node is so high as to

warrant the use of an X100 probe for inspection. For

safety and ease of visualization it is attenuated by 10x on

the eval board by resistors R27, R28 and R29. Notice the

positive DC offset voltage caused by the blocking

capacitor C20. (The attenuator may not be needed for

production).

Ch1 Freq

36.95728KHz

Ch1 Pk-Pk

68.80V

Figure 4. Lamp Network Voltage (atten. 10x)

Scope Setting: 10V/div, Horiz = 10µs/div

Test Conditions: Lamps @ maximum intensity, 120VAC

Equipment Used: Tektronix TDS540 Digitizing Scope

REV. 1.0 10/25/2000

Application Note 40

INVERTER/LAMP CURRENT (Fig. 5, T3 Pri, T4 Sec)

A comparison of the inverter current (same as Figure 5,

lower trace) and lamp current is shown below. The phase

difference is typical when an AC current source drives a

parallel resonant network. There is however, no phase

difference between the lamp current (T4 secondary

current) and T4’s primary current. The user will note an

increase in the inverter current when the lamp current

(and intensity) are decreased. This phenomena is a result

of the decrease in total impedance of the lamp network at

higher excitation frequencies and the “negative”

resistance characteristic of the fluorescent lamp.

Ch1 Freq

33.86004KHz

Ch1 Pk-Pk

39.2mV

Ch2 Freq

33.80274KHz

Ch2 Pk-Pk

38.0mV

1

Figure 5. Inverter/Lamp Current

Scope Setting: Top = 0.5/div, Bottom = 0.1A/div, Horiz = 10µs/div

Test Conditions: Lamps @ maximum intensity, 120VAC

Equipment Used: Tektronix TDS540 Digitizing Scope, Tektronix AM503 Current

Probe Amplifier Assy

LAYOUT CONSIDERATIONS

The ML4831EVAL Board contains high impedance, low

level and low impedance, high level circuits and as such

requires extra care in component placement, grounding

and pc trace routing. This board makes use of a ground

plane to achieve stable, noise free operation. When laying

out a PC board for ballasts several precautions must be

observed. The following list serves as a guide to ease the

layout and minimize re-layout revisions.

1. Return the low side of the timing capacitor (C6)

directly to the IC ground pin.

2. Bypass the reference and supply voltage pins directly

to the IC ground pin with a 0.01µFd or greater low

ESR capacitor.

3. Make a direct, low ohmic connection from the IC

ground to the PFC current sense resistor (R1).

4. Return all compensation components directly to the

IC ground pin, keeping the lead lengths as short as

possible.

5. Use a ground plane (if permissible) for all low side

(ground) connection points.

6. Whether using a ground plane or a single point

ground layout, use heavy traces form the sense

resistor/Q1 source node.

7. Separate rapidly changing waveforms; such as Q1’s

drain, from sensitive, high impedance circuits, such as

the timing capacitor, PFC current sense input, error

amplifier input/output, etc.

POWERING OTHER

FLUORESCENT LAMPS

The ML4831EVAL Board design was optimized to power

T8 lamps with cathodes requiring pre-heating prior to

ignition. With little or no circuit modifications, other

lamps can be driven with this board. For example, this

EVAL board was used to power T12 lamps. Due to the

different impedance of these lamps, the board delivers

about 8 watts (4 watts/lamp) less.

For higher wattage lamps the PFC boost voltage can be

increased by either increasing the value of R12 and R9 or

decreasing the value of R13. Use extreme caution when

attempting this as C11’s voltage rating of 250V may be

exceeded resulting in venting or catastrophic failure of the

capacitor!!!

Lower wattage bulbs may not require any circuit

modification, however, because of different lamp

impedance characteristics, it may be necessary to

decrease R5’s (RSET) value to allow lower lamp

intensities. Increasing T5’s primary turns may also be

necessary to achieve lower lamp intensities.

For rapid start lamps, adjusting the value R15 and C13

will shorten the pre-heat time while removing these

components will eliminate the pre-heat time. See the

ML4831 data sheet for details.

Instant start lamps have no cathode(s) and therefore no

need for pre or sustained heating. If desired, remove R15

and C13 and employ the connection technique shown in

Figure 6. For operator safety and to avoid circuit failure

insulate any remaining wires from the EVAL board.

LAMP

EVAL BOARDB R

LAMP

EVAL BOARDB R

Figure 6. Dual/Single Instant-Start Lamp Connections

REV. 1.0 10/25/2000

Application Note 40

TABLE 1: PARTS LIST FOR THE ML4831EVAL EVALUATION KIT

CAPACITORS

QTY. REF. DESCRIPTION MFR. PART NUMBER

2 C1, 2 3.3nF, 125VAC, 10%, ceramic, “Y” capacitor Panasonic ECK-DNS332ME

1 C3 0.33µF, 250VAC, “X”, capacitor Panasonic ECQ-U2A334MV

4 C4, 8, 9, 22 0.1µF, 50V, 10%, ceramic capacitor AVX SR215C104KAA

2 C5, 21 0.01µF, 50V, 10%, ceramic capacitor AVX SR211C103KAA

1 C6 1.5nF, 50V, 2.5%, NPO ceramic capacitor AVX RPE121COG152

2 C7, 12 1µF, 50V, 20%, ceramic capacitor AVX SR305E105MAA

1 C10 100µF, 25V, 20%, electrolytic capacitor Panasonic ECE-A1EFS101

1 C11 100µF, 250V, 20%, electrolytic capacitor Panasonic ECE-S2EG101E

1 C13 4.7µF, 50V, 20%, electrolytic capacitor Panasonic ECE-A50Z4R7

3 C14, 15, 17 0.22µF, 50V, 10%, ceramic capacitor AVX SR305C224KAA

1 C16 1.5nF, 50V, 10%, ceramic capacitor AVX SR151V152KAA

1 C19 22nF, 630V, 5%, polypropylene capacitor WIMA MKP10, 22nF, 630V, 5%

1 C20 0.1µF, 250V, 5%, polypropylene capacitor WIMA MKP10, 0.1µF, 250V, 5%

1 C23 0.068µF, 160V, 5%, polypropylene capacitor WIMA MKP4, 68nF, 160V, 5%

1 C24 220µF, 16V, 20%, electrolytic capacitor Panasonic ECE-A16Z220

1 C25 47nF, 50V, 10%, ceramic capacitor AVX SR211C472KAA

1 C26 330pF, 50V, 10%, ceramic capacitor AVX SR151A331JAA

1 C27 22µF, 10V, 20%, electrolytic capacitor Panasonic ECE-A10Z22

RESISTORS:

1 R1 0.33Ω, 5%, 1/2W, metal film resistor NTE HWD33

1 R2 4.3K, 1/4W, 5%, carbon film resistor Yageo 4.3K-Q

2 R3, 26 47K, 1/4W, 5%, carbon film resistor Yageo 47K-Q

1 R4 12K, 1/4W, 5%, carbon film resistor Yageo 12K-Q

1 R5 20K, 1/4W, 1%, metal film resistor Dale SMA4-20K-1

1 R6 360K, 1/4W, 5%, carbon film resistor Yageo 360K-Q

1 R7 36K, 1W, 5%, carbon film resistor Yageo 36KW-1-ND

3 R8, 22, 11 22Ω, 1/4W, 5%, carbon film resistor Yageo 22-Q

1 R9 402K, 1/4W, 1%, metal film resistor Dale SMA4-402K-1

1 R10 17.8K, 1/4W, 1%, metal film resistor Dale SMA4-17.8K-1

1 R12 475K, 1/4W, 1%, metal film resistor Dale SMA4-475K-1

1 R13 5.49K, 1/4W, 1%, metal film resistor Dale SMA4-5.49K-1

REV. 1.0 10/25/2000

Application Note 40

TABLE 1: PARTS LIST FOR ML4831EVAL EVALUATION KIT (Continued)

RESISTORS: (Continued)

QTY. REF. DESCRIPTION MFR. PART NUMBER

4 R14, 17, 24, 25 100K, 1/4W, 5%, carbon film resistor Yageo 100K-Q

1 R15 681K, 1/4W, 5%, carbon film resistor Yageo 681K-Q

2 R16, 29 10K, 1/4W, 1%, metal film resistor Dale SMA4-10K-1

1 R18 4.7K, 1/4W, 5%, carbon film resistor Yageo 4.7K-Q

1 R21 33Ω, 1/4W, 5%, carbon film resistor Yageo 33-Q

1 R23 25K, pot (for dimming adjustment) Bourns 3386P-253-ND

1 R27 48.7K, 1/4W, 1%, metal film resistor Dale SMA4-48.7K-1

1 R28 41.2K, 1/4W, 1%, metal film resistor Dale SMA4-41.2K-1

DIODES:

4 D1, 2, 3, 4 1A, 600V, 1N4007 diode Motorola 1N4007TR

(or 1N5061 as a substitute)

2 D5, 6 1A, 50V (or more), 1N4001 diodes Motorola 1N4001TR

1 D7 3A, 400V, BYV26C or BYT03 fast recovery GI BYV26C

or MUR440 Motorola ultra fast diode

8 D8, 9, 10, 11 0.1A, 75V, 1N4148 signal diode Motorola 1N4148TR

12, 13, 14, 15

IC’s:

1 IC1 ML4831, Electronic Ballast Controller IC Micro ML4831CP

Linear

TRANSISTORS:

3 Q1, 2, 3 3.3A, 400V, IRF720 power MOSFET IR IRF720

MAGNETICS:

1 T1 T1 Boost Inductor, E24/25, 1mH, Custom Coils P/N 5039 or Coiltronics P/N CTX05-12538-1

E24/25 core set, TDK PC40 material

8-pin vertical bobbin (Cosmo #4564-3-419),

Wind as follows:

195 turns 25AWG magnet wire, start pin #1, end pin #4

1 layer mylar tape

14 turns 26AWG magnet wire, start pin #3, end pin #2

NOTE: Gap for 1mH ±5%

1 T2 T2 Gate Drive Xfmr, LPRI = 3mH, Custom Coils P/N 5037 or Coiltronics P/N CTX05-12539-1

Toroid Magnetics YW-41305-TC

Wind as follows:

Primary = 25 turns 30AWG magnet wire, start pin #1, end pin #4

Secondary = 50 turns 30AWG magnet wire, start pin #5, end pin #8

REV. 1.0 10/25/2000

Application Note 40

TABLE 1: PARTS LIST FOR ML4831EVAL EVALUATION KIT (Continued)

MAGNETICS: (Continued)

QTY. REF. DESCRIPTION MFR. PART NUMBER

1 T3 T3 Inductor, LPRI = 1.66mH, Custom Coils P/N 5041 or Coiltronics P/N CTX05-12547-1

E24/25 core set, TDK PC40 material

10 pin horizontal bobbin (Plastron #0722B-31-80)

Wind as follows:

1st: 170T of 25AWG magnet wire; start pin #10, end pin #9.

1 layer of mylar tape

2nd: 5T of #32 magnet wire; start pin #2, end pin #1

1 layer of mylar tape

3rd: 3T of #30 Kynar coated wire; start pin #4, end pin #5

4th: 3T of #30 Kynar coated wire; start pin #3, end pin #6

5th: 3T of #30 Kynar coated wire; start pin #7, end pin #8

NOTE: Gap for 1.66mH ±5% (pins 9 to 10)

1 T4 T4 Power Xfmr, LPRI = 3.87mH, Custom Coils P/N 5038 or Coiltronics P/N CTX05-12545-1

E24/25 core set, TDK PC40 material

8 pin vertical bobbin (Cosmo #4564-3-419)

Wind as follows:

1st: 200T of 30AWG magnet wire; start pin #1, end pin #4.

1 layer of mylar tape

2nd: 300T of 32AWG magnet wire; start pin #5, end pin #8

NOTE: Gap for inductance of primary: (pins 1 to 4) @ 3.87mH ±5%

1 T5 T5 Current Sense Transformer, Custom Coils P/N 5040 or Coiltronics P/N CTX05-12546-1

Toroid Magnetics YW41305-TC

Wind as follows:

Primary = 3T 30AWG kynar coated wire, start pin #1, end pin #4

Secondary = 400T 35AWG magnet wire, start pin #5, end pin #8

INDUCTORS:

2 L1, 2 EMI/RFI Inductor, 600µH, DC resistance = 0.45ΩPrem. SPE116A

Magnetics

FUSES:

1 F1 2A fuse, 5 x 20mm miniature Littlefuse F948-ND

2 Fuse Clips, 5 x 20mm, PC Mount F058-ND

HARDWARE:

1 Single TO-220 Heatsink Aavid Eng. PB1ST-69

2 Double TO-220 Heatsink IERC PSE1-2TC

3 MICA Insulators Keystone 4673K-ND

REV. 1.0 10/25/2000

Application Note 40

1

2

3

4

5

6

7

8

9

18

17

16

15

14

13

12

11

10

ML4831

D1 D3

D2 D4

L

G

N

F1

120V

L1

L2 C2

C1 C3

D8

T1

41

3 2

R6

R14

C12 C5 D5

D6 R1 R4 R2 R3 R5 R24

C25 C26 C4 C6 C7

R16

R10

R17

C10

+

R7

R11

Q1

D7 C11

+ R12

R13

R9 R8

C13 C14 C24 C15 C16

+ +

R15

D11 D12

C22

Q2

Q3

T2

5

8 4

1

C17

R21 R22

T3

2

1

4

5

3

6

8

7

10 9

C20

C19

T4

1

4

8

5

4 1

85

T5

C23

Y

R

B

D13

C21

R23

C8

C9

D9

Figure 7. Circuit Schematic of the ML4831EVAL Evaluation Kit

REV. 1.0 10/25/2000

Application Note 40

1994

MICRO LINEAR

F1

2A/250V

C3

L1

L2

C2

C1

D1D2

D4 D3

R7

R6

Q1

L3

ML4831 EVAL

REV A PFC

C11 +C10

T1 8 5

41

D7

+

B

G

W

C8

D8

D9

C9

R18

R12D5

R1

R11

D6

R13

D10

GND

R14 C25

C26

R14 C5C12

INHBT

VCC

+C24

R8

C16

C15

C14

ML4831

R15 R25

C6

R10

R26

D15

R9

R16

R22

R21

R3

R5

R24

R2

C7

C4 C27

D14

R17

C13+

C17

R23

DIM

C21

D13

Q3 Q2

5 8

T2

4 1

T5

D11

C22

8

5

1

4

TP5 R28

R29

C20 8 5

1 4

R27

C19

C23

10 6

1 5

T3

D12

T4

+

Figure 8. ML4831EVAL Component Silkscreen Layer

REV. 1.0 10/25/2000

Application Note 40

ML4831 EVAL

MLRS 11-5/11/94

Figure 9. ML4831EVAL Ground Plane and Bottom Trace Layer

REV. 1.0 10/25/2000

Application Note 40

Figure 10. ML4831EVAL Top Trace Layer

REV. 1.0 10/25/2000

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OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR

CORPORATION. As used herein:

1. Life support devices or systems are devices or systems

which, (a) are intended for surgical implant into the body,

or (b) support or sustain life, and (c) whose failure to

perform when properly used in accordance with

instructions for use provided in the labeling, can be

reasonably expected to result in a significant injury of the

user.

2. A critical component in any component of a life support

device or system whose failure to perform can be

reasonably expected to cause the failure of the life support

device or system, or to affect its safety or effectiveness.

www.fairchildsemi.com

DISCLAIMER

FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO

ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME

ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN;

NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.



2000 Fairchild Semiconductor Corporation