June 1996 Application Note 42006 Low Cost Electronic Ballast System Design George A. Hall GENERAL DESCRIPTION THEORY OF OPERATION 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. Figure 1 displays the block diagram of the ML4831EVAL board. Applying AC line voltage to the EVAL board supplies startup 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. 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 EMI FILTER PFC INVERTER LAMP NETWORK BRIDGE RECTIFIER 85-135 VAC + ML4831 CURRENT SENSE DIMMING REV. 1.0 10/25/2000 1 Application Note 40 PERFORMANCE DATA INVERTER VOLTAGE/CURRENT (Fig. 3) To measure system performance across the range of permissible input voltages use a variac or adjustable AC source. 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. A typical ML4831EVALuation board will have the following performance characteristics when operated as shown in the test conditions: ML4831EVAL BOARD TEST RESULTS Ch1 Freq 33.84096KHz 85VAC 120VAC 135VAC Units 88 90 89 % THD 2.32 2.12 2.35 % P.F. 0.995 0.984 0.975 % Efficiency Test Conditions: 2 series wired T8 lamps (full intensity), 25C Equipment Used: Voltech Digital AC Power Analyzer #PM1000 2 Ch2 Freq 33.89408KHz Ch2 Pk-Pk 224V 1 The ML4831EVALuation board provides testpoints at the following circuit nodes: TP1 TP2 TP3 TP4 TP5 Ch1 Pk-Pk 40.8mV 10.0mV Ch2 100V M 10.0 s Ch2 126V Figure 3. Inverter Output Voltage/Current GND VCC INHIBIT PFC Boost Voltage Resonant Network (attenuated by 10x) Scope Setting: Top = 100V/div, Bottom = 0.5A/div, Horiz = 10s/div Test Conditions: Lamps @ maximum intensity, 120VAC Equipment Used: Tektronix TDS540 Digitizing Scope, Tektronix AM503 Current Probe Amplifier Assy 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. 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 = 10s/div Test Conditions: Lamps @ maximum intensity, 120VAC Equipment Used: Tektronix TDS540 Digitizing Scope 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 REV. 1.0 10/25/2000 205V 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 1 Ch1 Pk-Pk 39.2mV Ch2 Freq 33.80274KHz Ch2 Pk-Pk 38.0mV Figure 5. Inverter/Lamp Current Scope Setting: Top = 0.5/div, Bottom = 0.1A/div, Horiz = 10s/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. 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. 1. Return the low side of the timing capacitor (C6) directly to the IC ground pin. LAMP LAMP 2. Bypass the reference and supply voltage pins directly to the IC ground pin with a 0.01Fd or greater low ESR capacitor. B 3. Make a direct, low ohmic connection from the IC ground to the PFC current sense resistor (R1). Figure 6. Dual/Single Instant-Start Lamp Connections LAMP EVAL BOARD R B EVAL BOARD R 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. REV. 1.0 10/25/2000 3 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 0.33F, 250VAC, "X", capacitor Panasonic ECQ-U2A334MV 4 C4, 8, 9, 22 0.1F, 50V, 10%, ceramic capacitor AVX SR215C104KAA 2 C5, 21 0.01F, 50V, 10%, ceramic capacitor AVX SR211C103KAA 1 C6 1.5nF, 50V, 2.5%, NPO ceramic capacitor AVX RPE121COG152 2 C7, 12 1F, 50V, 20%, ceramic capacitor AVX SR305E105MAA 1 C10 100F, 25V, 20%, electrolytic capacitor Panasonic ECE-A1EFS101 1 C11 100F, 250V, 20%, electrolytic capacitor Panasonic ECE-S2EG101E 1 C13 4.7F, 50V, 20%, electrolytic capacitor Panasonic ECE-A50Z4R7 3 C14, 15, 17 0.22F, 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 0.1F, 250V, 5%, polypropylene capacitor WIMA MKP10, 0.1F, 250V, 5% 1 C23 0.068F, 160V, 5%, polypropylene capacitor WIMA MKP4, 68nF, 160V, 5% 1 C24 220F, 16V, 20%, electrolytic capacitor Panasonic 1 C25 47nF, 50V, 10%, ceramic capacitor AVX SR211C472KAA 1 C26 330pF, 50V, 10%, ceramic capacitor AVX SR151A331JAA 1 C27 22F, 10V, 20%, electrolytic capacitor Panasonic ECE-A16Z220 ECE-A10Z22 RESISTORS: 4 1 R1 0.33, 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 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 22, 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 SMA4-20K-1 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 1 R18 4.7K, 1/4W, 5%, carbon film resistor Yageo 4.7K-Q 1 R21 33, 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 SMA4-10K-1 DIODES: 4 D1, 2, 3, 4 1A, 600V, 1N4007 diode (or 1N5061 as a substitute) Motorola 1N4007TR 2 D5, 6 1A, 50V (or more), 1N4001 diodes Motorola 1N4001TR 1 D7 3A, 400V, BYV26C or BYT03 fast recovery or MUR440 Motorola ultra fast diode GI 8 D8, 9, 10, 11 12, 13, 14, 15 0.1A, 75V, 1N4148 signal diode Motorola 1N4148TR IC1 ML4831, Electronic Ballast Controller IC Micro Linear ML4831CP 3.3A, 400V, IRF720 power MOSFET IR BYV26C IC's: 1 TRANSISTORS: 3 Q1, 2, 3 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 5 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, 600H, DC resistance = 0.45 Prem. Magnetics SPE116A F1 2A fuse, 5 x 20mm miniature F948-ND FUSES: 1 2 Littlefuse Fuse Clips, 5 x 20mm, PC Mount F058-ND HARDWARE: 6 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 N G R14 C12 C5 120V F1 D6 D5 L2 L1 R1 C3 D2 D1 R4 R6 D4 D3 R2 R3 R5 C25 C26 C4 C2 C1 C6 R24 3 1 C7 D9 D8 2 4 C9 C8 12 11 10 8 9 13 14 15 16 17 18 C10 + 7 6 5 4 3 2 1 R16 R10 R17 T1 R7 ML4831 L + C13 R15 R9 R11 + R8 C11 C14 C24 C15 Q1 D7 + C16 R13 R12 R21 C17 8 5 T2 D11 Q3 R22 4 1 Q2 D12 C22 1 2 10 T3 7 6 8 3 5 4 C19 9 4 1 5 8 R23 C20 T4 T5 8 1 C21 D13 5 4 C23 B B R R Y Y Application Note 40 Figure 7. Circuit Schematic of the ML4831EVAL Evaluation Kit REV. 1.0 10/25/2000 7 W G B L2 C3 D3 D4 D1 D2 L3 REV A ML4831 EVAL C2 C1 R7 R6 + Q1 PFC C11 1994 D7 R18 1994 MICRO LINEAR +C10 1 4 5 R11 R8 VCC +C24 D6 T1 8 C8 D8 D9 C9 R12 D5 C5 R1 R15 ML4831 C16 C15 C14 INHBT C12 R14 C26 C25 R13 D10 GND + C6 C13 R25 Q3 C4 R3 R5 R24 R2 C7 C17 C27 R17 D14 + Q2 T2 5 4 R10 R26 D15 R9 R16 R22 R21 REV. 1.0 10/25/2000 R23 DIM 8 1 8 5 TP5 C21 D13 8 R14 F1 2A/250V C20 D11 C22 T5 R29 R28 T3 D12 L1 1 4 R27 1 10 C23 1 8 C19 5 6 4 5 T4 Application Note 40 Figure 8. ML4831EVAL Component Silkscreen Layer 49/11/5-11 SRLM Application Note 40 LAVE 1384LM Figure 9. ML4831EVAL Ground Plane and Bottom Trace Layer REV. 1.0 10/25/2000 9 Application Note 40 Figure 10. ML4831EVAL Top Trace Layer 10 REV. 1.0 10/25/2000 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. LIFE SUPPORT POLICY FAIRCHILD'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES 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 2000 Fairchild Semiconductor Corporation