AND8279 The LCD TVs Standby Power Consumption Reduction Prepared by: Stanislav Raska App. Lab. ON Semiconductor Roznov p.R. http://onsemi.com A typical block schema of the TVs power supply unit (PSU) is shown on Figure 1. A major power loss of the standby supply unit (SSU) during the standby mode consists of the power consumption of all three resistance dividers: power factor corrector (PFC), resonant switcher (RS) and SSU divider. Another no negligible power loss (0 ÷ 100 mW) is made by the variable leakage consumption of bulk capacitors. The typical leakage consumption of the bulk capacitors individually varies according to its quality. To maintain the transparency of the article and to keep the elegancy of the calculations the leakage consumption of bulk capacitors is not further considered. The last but not least power consumer remains the SSU itself. Nowadays, many electrical appliances (e.g. computers, audiovisual or white electronics) uses a standby mode of its switched power supplies, but not all of them fulfill the GreenPoint™ statements. According to the GreenPoint requirements the total power consumption of those appliances in a standby mode should be kept under 1.0 W with 0.5 W efficient load. The presented method reduces the standby power consumption of LCD−TV−SMPS unit. The reduction is achieved through a slight modification of the power supply unit. The main advantages of the presented solution are the fulfillment of the GreenPoint requirements and the final low cost of the modification. The reduction of the standby power consumption of LCD TV is described on article of realized 220 W TV converter, although it is generally applicable in any other power supply units that consists of power factor corrector, resonant switcher or more and standby supply unit. R1 +Vb R3 R5 PFC RS B1 V1 VS SSU 0V U + FB B0 V2 B0 SBY SBY 0V C1 MAIN R4 V3 R6 N +VCC Figure 1. The Block Diagram of Standard LCD TVs PSU © Semiconductor Components Industries, LLC, 2007 January, 2007 − Rev. 0 1 Publication Order Number: AND8279/D AND8279 The Power Consumption of Resistance Dividers in SSU (See Figures 1 and 6) of RS/BO resistance divider R3/R4 1.17 MW is 125 mW @ 265 Vac. Thus the power dissipation on all three resistors makes 200 mW. The value of PFC/FB resistor R1 cannot be increased because it is used for a proportional conversion of the bulk voltage to PFC/FB input current. The resistance of RS/BO divider R3/R4 cannot be increased due to the internal input current hysteresis. The power consumption of 3 MW resistance divider R5/R6 of brownout (BO) SSU is considerably low (cca 47 mW @ 265 Vac) and cannot be easily decreased. Moreover the SSU operates also in standby mode. The Power Consumption of Resistance Dividers in PFC and RS (See Figures 1 and 2) The power consumption of PFC/FB feedback resistor R1 1.92 MW is 73 mW @ 265 Vac and the power consumption 375 V @ SBY Vb 220 mA 335 mA 82 mW 125 mW R1 1M7 8 7 6 5 IC1 NCP1653 VCC FB DRV VCT GND IN VM CS 1 2 3 4 R3 1M12 1.0 V 1.9 V C1 1n R4 3k3 0V 1 2 3 4 5 6 7 8 IC2 NCP1395 RT NIN 16 OUT 15 FM SF 14 DT FF 13 CSS VCC 12 FB B 11 CT A 10 BO GNP 9 GNA C2 1n 0V 0V Figure 2. The Original Circuit of Resistor Bulk Voltage Sensing for PFC and RS The Analysis of FB/PFC and BO/RS Inputs R3/R4 in Q1 emitter serves for RS, the resistor R1 serves for PFC. Whole the voltage follower is supplied by the board supply 15 Vdc. The total resistance of R7/R8 resistor divider between 5 ÷ 10 MW is a compromise between the power consumption and accuracy. From the bulk voltage of 400 Vdc produces ca 13 Vdc on the base of transistor Q1. On the emitter of transistor Q1 is thus ca Ve = 12.5 Vdc. For this value we can simply specify the values of other resistors R1, R3, R4. The value of resistor R4 is the same as original. The value of resistor R3 is chosen to assure ca 1.0 V on BO input. The value of resistor R1 is chosen to inject sensing current ca 200 mA to FB input. During practical evaluation should be required voltage values adjusted by resistors connected in series with resistors R1 and R3. The values of capacitors C2, C3 are the same as original, to keep the same time constant. During a standby mode PFC and RS do not operate by disabling of 15 Vdc board supply. The original circuit with bulk resistor dividers in FB/PFC and BO/RS inputs is shown in Figure 2. The output voltage of PFC is regulated through a current feedback loop. The sensing current 100 ÷ 200 mA flowing through a resistor R1 produces voltage 1.5 ÷ 1.9 V on input FB. From the analysis of IC1 has this voltage evidently the same temperature dependency as the forward voltage of two diodes connected in series. The brownout of RS is controlled through the bulk voltage level adjusted by the resistors R3/R4 divider to 1.0 V, with current hysteresis of ca 20 mA. The threshold level of BO/RS input is practically independent on temperature. The Standby Power Consumption Reduction through the Voltage Follower The circuit decreases the standby power consumption is shown in Figure 3. The main point of this circuit is a Q1 transistor connected as an emitter voltage follower. The transistor Q1 with R7/R8 divider in its base proportionally converts the bulk voltage to the emitter voltage. The divider http://onsemi.com 2 AND8279 VCC = 15 V 400 V (375 V / 29 mW @ SBY) Vb R7 4M7 2 Q1 3 BC817 12.5 V 1 0.5 mA 82 mA 2 mA R3 39k R1 200 mA 51k 8 7 6 5 IC1 NCP1653 VCC FB DRV VCT GND IN VM CS 1.0 V 1.9 V 1 2 3 4 R8 M18 C3 1n R4 3k3 0V 0V 1 2 3 4 5 6 7 8 IC2 NCP1395 RT NIN 16 OUT 15 FM SF 14 DT FF 13 CSS VCC 12 FB B 11 CT A 10 BO GNP 9 GNA C2 1n 0V 0V Figure 3. The Emitter Follower in a Bulk Voltage Sensing Circuit The Temperature Compensation The FB/PFC input voltage has the same temperature dependency as two diodes connected in series. The forward base−emitter voltage of Q1 has the same temperature dependency as forward voltage of one diode. Thus this temperature dependency is the same as of three diodes connected in series. Then a temperature change DT = 55°C produces the bulk voltage change DVbpfc equal to (Equation 1): DVbpfc ^ Vb * 3 * dVfńdT * DT Vefb +* 400 * 3 * 0.002 * 55 + * 12.6 V or " 1.75% 10.5 (eq. 1) The BO/RS input voltage has its temperature dependency derived only from Q1 forward base−emitter voltage. Therefore the BO threshold change recalculated at bulk voltage change DVbrs (Equation 2) is: DVbprs ^ Vb * 31 * íVfńíT * DT Ve +* 400 * 1 * 0.002 * 55 + * 3.52 V or " 0.44% 12.5 (eq. 2) DVbpfc − Bulk voltage change (V) DVbrs − Threshold bulk voltage change (V) Vb − Bulk Voltage (V) Ve − T1 Emitter Follower Voltage (V) Vefb − Voltage between emitter of T1 and FB/PFC input (V) dVf/dT − Forward voltage diode temperature dependency −2.0 mV/°C (V) DT − Temperature Change (°C) T = −10°C ÷ 45°C The uncompensated bulk voltage change DVbpfc should be probably not acceptable. Figure 4. shows subsequently the temperature compensation of the bulk voltage change DVbpfc with low−cost dual diode MMBD7000. The dual diode compensation distributes the residual one diode temperature dependency symmetrically between PFC and RS stages. The PFC bulk voltage temperature change in T = −10°C ÷ 45°C is so three time lowered, DVbpfc = −4.2 V. The recount RS threshold change of bulk voltage has previous value DVbrs = 3.5 V but inverse sign. http://onsemi.com 3 AND8279 VCC = 15 V 3 Q1 BC817 12.5 V 1 400 V (375 V / 29 mW @ SBY) Vb R7 4M7 2 82 mA 2 mA 0.5 mA R3 39k 8 7 6 5 IC1 NCP1653 VCC FB DRV VCT GND IN VM CS 1 2 3 4 MMBD7000 R1 200 mA 51k 1.9 V D1 C3 1n 0V R8 M18 R4 3k3 0V 1.0 V 1 2 3 4 5 6 7 8 IC2 NCP1395 RT NIN 16 OUT 15 FM SF 14 DT FF 13 CSS VCC 12 FB B 11 CT A 10 BO GNP 9 GNA C2 1n 0V 0V Figure 4. The Temperature Compensated Bulk Voltage Sensing Circuit THE MEASURES OF TEMPERATURE COMPENSATED BULK VOLTAGE SENSING CIRCUIT Vb Hysteresis Vbon + 420 V Vboff + 375 V (eq. 3) The temperature dependence of Vbon, Vboff is negligible. The Temperature Dependence of the Bulk Voltage Vb T(°C) −10 0 25 45 Vb(V) 389.5 389 386.7 385.2 DVb = −4.3 V or $0.54% The Dynamic Behavior of the Modified PSU To maintain the original dynamics of PSU the original time constants and topology of both PFC and RS has to be preserved. The On/Off tests and load tests proved that the dynamics of PSU remained the same. http://onsemi.com 4 AND8279 The Complementary Standby Mode Switch The scheme shown in Figure 5 keeps stable board voltage of the power supply during the operating mode and ensures zero power consumption during the standby mode. 1 VCC = 15 V The SBP/SBN inputs are designed to switch−on and switch−off the PSU. The specific SBP/SBN input can be selected according to the external logic signals. Va = 20 V 3 2 Q1 BC817 R1 0k1 4 1 R2 3 2 3k3 3k3 PC817 D1 MMSZ16 0V 2 0V Q2 BC817 1 R3 33k 0V 3 VS = 5.0 V 0V SBP SBN Figure 5. The Complementary Standby Switch and Board VCC Regulator The Standby Supply Unit (SSU) Efficiency When the SSU is in standby mode unloaded, the SSU switcher has to be operating strictly in a skip mode. This limits number of hard−switching of the SSU switcher and subsequently decreases the total power consumption of the SSU. The skip mode of the SSU switcher is achieved through the lower capacity C49 between R−C pins of the voltage reference device VR2 TL431. (See in Figure 6). The total power consumption of measured TV converter (supplied by 265 Vac main) in standby mode is 179 mW @ 0 W and 828 mW @ 0.5 W. The further diminishing of the standby power consumption of SSU is restricted by the total efficiency of the transformer and switcher used in SSU and the required low cost of the application. http://onsemi.com 5 1M 3 2 + + 1SMB170AT36 R48 C36 1SMB33AT3G 1M R49 1M BO FB 3 4 3 PC817 3 4 R58 7k5 6 C A R55 C38 C51 R54 R57 R65 D19 http://onsemi.com 3k3 1 2 3 4 10k 75k 10u 10u D21 4k7 MMSZ16ET1 10n 18k GND Figure 6. The Final Schema of a Standby Power Supply R66 1 3k3 Q3 2 BC817 3 n0 R37 4n7 2*4u7 R VR2 TL431CLP C49 L6 R50 PC817 OK3 2 MURA120T3 D16 R56 M33 OK2 1 NCP1027 HT 5 RMP OPP 7 56 m47 C29 + 1k 2 2 MBRS260T3 R51 1 V 8 CC GND IC3 Ua = 20 V 4 D15 1k Q2 BC817 1 D17 MURA160T3 87 R52 R6 R47 TSB TR2 1 R53 +15V VCC D20 R5 Vb 3k3 m47 C30 + 3k3 VS SBN SBP 0V 0V 5V AND8279 AND8279 The SSU Power Consumption Measurement The standby power consumption Psby is being measured on TVs PSU as a product of DC bulk voltage Vb and corresponding current Ib: Vb (V) AC/DC 90/120 120/170 150/212 230/325 265/375 Ib @ 500 mW (mA) 5.6 4.1 3.35 2.41 2.21 Psby @ 500mW (mW) 672 697 710 780 828 Ib @ 0mW (mA) 0.48 0.43 0.41 0.44 0.47 58 82 87 143 179 Psby @ 0mW (mW) The auxiliary voltage Vaux is being measured on variable output loads Po in a standby (SBY) and operates (on) mode of the SSU: Po (W) 0 0,5 10 Vaux (V) SBY 18 20 23.7 17.8 18.7 2 Vaux (V) on (@18 mA) Conclusion To maintain the high quality and the total low−cost of the application the ON Semiconductor devices were implemented. This improves the total efficiency of the power supply unit in a standby mode such, that it with reserve fulfils the GreenPoint statements. The presented solution describes the “easy to make” modification of the power supply unit that diminishes the former maximal standby power consumption of the standby power supply unit (>1.1 W) by more than 200 mW to final achieved value 828 mW. GreenPoint is a trademark of Semiconductor Components Industries, LLC (SCILLC). ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. 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