AN1132 APPLICATION NOTE
90W SMPS FOR MONITORS WITH STANDBY FUNCTION
by Claudio Adragna
Purpose of this note is to provide a brief summary of the specifications and the functionality of the application board implementing a 90W multioutput SMPS for monitors, based on the L5991, current mode PWM controller. Evaluation results are also presented so as to underline the benefits offered by the L5991 in such a new generation of SMPS that requires a superior efficiency in standby conditions, aiming at compliance with energy saving standards.
Design Specifications Table 1 summarises the electrical specification of the application. The complete electrical schematic is shown in fig. 1 and the bill of material is listed in Table 2. Table 1. Design Specification
Input Voltage Range (Vin) Mains Frequency (fL) Maximum Output Power (Pout) Horizontal Deflection 88 to 264 Vac 50/60 Hz 90W Vout = 200V Iout = 0.325A Full load ripple = 1% Vout = 80V Video Amplifier Outputs Vertical Deflection Iout = 0.125A Full load ripple = 1% Vout = 15V Iout = 0.33A Full load ripple = 1% Vout = 6.3V Heater Iout = 0.8A Full load ripple = 2% Switching Frequency in Normal Mode (fosc) Switching Frequency in Suspend / OFF mode (fSB) Target Efficiency (@ Pout =90W, Vin =88 ÷264 Vac) () Maximum Input Power (@ Pout = 0.5 W, Vin =88 ÷ 264 Vac) 40kHz 18kHz > 80% 2W
The selected topology is flyback. The operation mode (@ Pout = 90W ) is CCM (Continuous Conduction Mode) at low mains voltage, DCM (Discontinuous Conduction Mode) at high mains voltage. This design choice relieves the stress on the power components at low mains voltage, compared with a full DCM solution. The application will benefit from the features of the L5991 PWM controller in order to minimise the power drawn from the mains under light load conditions: low start-up and quiescent currents, and Standby function.
September 2000 1/12
2/12
C01 4700pF 4KV C02 RP1 KBU4G R01 R31 4.7M R32 4.7M 1 18 D07 BYT11-800 200V 65W 17 D08 STTA106 H1 1H C15 22F 100V 80V 10W GND 16 R23 C12 4 7 D9 BYW100-100 C16 1000F 16V 8 C04 47F 14 8 R12 22 10 11 MF01 STP7NB60FI R13 1K 13 C05 100pF R14 0.47 7 1 VR1 100K 6 2 R27 470K C21 330pF OP1 TPS5904 4 3 R28 4.7K
D99IN1070B
F01
88 to 264 VAC C03 220F 400V R21 47K 3W D06 STTA106 R03 56K D05 1N4148 R22 22 R08 330K 15 14 C13 220F 100V C14 100F 250V C10 47nF 250V
Figure 1. Electrical Schematic.
AN1132 APPLICATION NOTE
D01 1N4148
R02 56K
R07 47K
6 .3 V 5W
C23 10nF R09 22 T1 13 12 D10 BYW100-100
C09 56nF 9 C17 470F 25V C18 470F 25V
7
15
+15V 5W
3
C07 0.1F
R34 4.7K
4
IC1
10 D11 BYW100-100
-15V 5W R24 47
C06 12 R15 0.47 R26 2.7K 11 R16 100 5 C08 3.3nF 6
R20 12K R25
L5991
2
6800pF
C19 47F 25V
R19 10K
Q3 BC337
16
R33 9.1K
C20
C22 1nF
R29 330K
R30
AN1132 APPLICATION NOTE
Table 2. Component List of the circuit of fig. 1.
Symbol R1 R2, R3 R7 R8, R29 R9, R12, R22 R13 R14, R15 R16 R19 R20 R21 R24 R26 R27 R28 R31, R32 R33 VR1 C1, C2 C3 C4, C19 C5 C6 C7 C8 C9 C10 C13 C14 C15 C16 C17, C18 C21 C22 C23 D1, D5 D6, D8 D7 D9, D10, D11 IC1 T1 OP1 MF1 RP1 Q3 F1 H1 M1, M2, M3 Value NOT USED (shorted) 56k 47k 330k 22 1k 0.47 100 10k 12k 47k 47 2.7k 470k 4.7k 4.7M 9.1k 100k 4.7nF 220F 47F 100pF 6.8nF 100nF 3.3nF 56nF 47nF 220F 100F 22F 1000F 470F 330pF 1nF 10nF 1N4148 STTA106 BYT11-800 BYW100-100 L5991 ETD4407 TPS5904 STP7NB60FI KBU4G BC337 1H Note
metallic film
3W
multiturns, Bourns 3296W or equivalent 1kV 400V, electrolytic, Panasonic TSUP or Roederstein EYS 25 V, electrolytic plastic film ceramic multilayer plastic film plastic film plastic film 250V, polypropylene o polystyrene film (Siemens-Matsushita) 100 V electrolytic, Roederstein EKE or equivalent 250 V, electrolytic, Roederstein EKS or equivalent 100 V, electrolytic, Roederstein EKE or equivalent 16V, electrolytic, Panasonic FA or equivalent 25 V, electrolytic, Panasonic HFZ or equivalent ceramic or plastic film ceramic or plastic film plastic film ST, TurboSwitch ST, Ultrafast ST, Ultrafast ST ITACOIL, see Table 3 TI ST GI, or equivalent 4A rectifier bridge 5A fuse axial inductor connectors
Notes: - if not otherwise specified, all resistors are 1/4 W, 1% - the MOSFET is provided with a 9.5 C/W heatsink - components indicated in the PCB and not quoted in table 2 are not assembled
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AN1132 APPLICATION NOTE
Table 3. Transformer Specification (Part Number ETD4407, supplied by ITACOIL).
Core Bobbin Air gap Leakage inductance Winding Pri1 Sec1 Sec2 Windings Spec & Build Sec3 Sec4 Sec5 Pri2 Aux Wire 4xAWG29 AWG25 AWG25 AWG25 AWG25 AWG26 4xAWG29 AWG29 S-F 2-4 17-18 15-16 13-14 11-12 10-11 1-2 8-7 Philips ETD44, 3C85 Material Horizontal mounting, 18 pins 1 mm for an inductance 1-4 of 380 H < 10H Turns 19 48 32 3 6 6 19 8 Evenly spaced Bifiliar with Sec5 Bifiliar with Sec4 Evenly spaced Notes
Figure 2. PCB layout: Component side and bottom layer (top view); 1:1.33 scale
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AN1132 APPLICATION NOTE
Application Board Functionality The outstanding feature of this application board is the so-called Standby Function, directly available from the L5991. When the power demanded by the load is roughly included between 40 and 90 W (Normal mode) the switching frequency of the converter is set at 40 kHz. When the monitor enters in lowconsumption mode (Suspend or OFF mode), the power demanded by the load will be much lower, few Watts. The L5991 will automatically recognise this new operating condition and change the oscillator frequency to 18 kHz. The capacitor C6, along with the parallel of R19 and R20, sets fosc ; fSB is set by C6 and R20. If the user would like to fine tune the power level that causes the switching frequency to be moved from fosc to fSB (PinSB), he or she can add a fixed DC offset (typically in the range 0-200 mV) on the current sense pin (13, ISEN). This can be accomplished by means of R17, currently not used. The offset will be the partition of the reference voltage (pin 4, VREF) through R17 and R13. To change the power level that causes the switching frequency to be moved from fSB to fosc (PinNW), the ratio fosc / fSB should be changed. R10 and R11 allow to provide an additional DC offset on the current sense which depends on the supply input voltage. This can be used for compensating L5991's delay to output. In the present case the delay is not compensated (R10 and R11 are not assembled) and the effect is a slight dependence of PinSB and PinNW on the mains voltage (see table 7). This is reinforced by the slope compensation circuit (Q3 and R33), which adds a little offset (variable with the duty cycle) on the current sense pin. Additionally, the board includes some protection functions tipically required, not only in monitor applications, such as overvoltage (OVP) and overcurrent protection (OCP). OCP is inherent in the functionality of the L5991: the controller provides both pulse-by-pulse and "hiccup" mode current limitation (see Application Information in the datasheet), which fully protect the converter in case of overload or short circuit. The OVP, in this specific case, is realised by sensing the supply voltage of the L5991 (generated by the auxiliary winding) through the divider R7-R8 and feeding this partition into pin 14 (DIS). The divider ratio is such that the OVP is tripped when the supply voltage exceeds 20V. This protection is particularly effective in case of feedback disconnection. At maximum load and minimum mains voltage the converter operates at about 55% duty cycle (this is why slope compensation is required) but no limitation is imposed on its maximum value: L5991's pin3 (DC) is shorted to pin 4 (VREF). If desired, it is possible to set the maximum duty cycle by adding the divider R34-R35. Please refer to Application Information in L5991 datasheet for calculation of the voltage divider. The application board is supplied with a start-up circuit simply made of a dropping resistor (R2+R3) that draws current from upstream the bridge rectifier. Figure 3. Low-consumption start-up circuit (not currently implemented)
88 to 264 VAC
R05 33K R04 2.2M Q01 STK2N50 R06 10 K D02 20V Q02 BC337
D04 1N4148
4
8
7
L5991
12 11 8
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AN1132 APPLICATION NOTE
This circuit, really inexpensive, dissipates about 300 mW @ 264 Vac. The typical wake-up time is 2.8 s at 88 Vac and 0.8 s at 264 Vac. Should the wake-up time become an issue, a more expensive solution would be adopted. The PCB is also able to accommodate an active start-up circuit that, under the same conditions, dissipates less than 10 mW and provides 0.7 s and 0.2 s wake-up times respectively. The schematic is shown in fig. 3 (R2 and R3 will be removed). A further improvement of light load efficiency can be achieved by replacing the RCD clamp (C10, R21) with a Transil. The suggested part is a 1.5KE150A. This slightly worsens efficiency at full load but allows to save about 200 mW, currently dissipated on R21, at light load. Application board evaluation: getting started The AC voltage, from an AC source ranging from 88 VRMS to 264 VRMS, will be applied to connector M1 (close to the top left-hand corner). The 200VDC and 80VDC outputs are located in connector M2 (top right-hand corner) while 15VDC and 6.3VDC outputs are available at connector M3, near the bottom right-hand corner. Like in any offline circuit, extreme caution must be used when working with the application board because it contains dangerous and lethal potentials. The application must be tested with an isolation transformer connected between the AC mains and the input of the board to avoid any risk of electrical shock. Application board evaluation: results In the following tables the results of some bench evaluations are summarised. Some waveforms under different load and line conditions, as well as system's transient response are also shown for user's reference and to illustrate the operation of the standby function. Table 4. Full load measurements
VAC [V] Pin [W] Vout [V] 88 105.4 199.9 79.43 14.27 -14.41 6.65 Pout [W] [%] 89.75 85.2 110 103.5 199.9 79.43 14.26 -14.4 6.65 89.74 86.7 160 101.5 199.9 79.4 14.29 -14.36 6.65 89.72 88.4 220 100.6 199.9 79.36 14.26 -14.39 6.65 89.7 89.2 264 100.3 199.9 79.36 14.26 -14.38 6.65 89.7 89.4
Load conditions: 200V: 630; 80V: 600; 15V: 80; 6.3V: 8
Table 5. Consumption from the mains in Suspend mode (PO = 5.5W)
VAC [V] Pin [W] 88 6.9 110 7 160 7 220 7.1 264 7.2
Load conditions: 200V: open; 80V: open; 15V: 0.5W; 6.3V: 8
Table 6. Consumption from the mains in OFF mode (PO = 0.5W)
VAC [V] Pin [W] 88 1.3 110 1.4 160 1.5 220 1.6 1.4 264 1.8 1.5
Pin [W] (*) 1.2 1.2 1.3 Load conditions: 200V: open; 80V: open; +15V: 0.5W; 6.3V: open (*) With the active start-up circuit of fig.3 6/12
AN1132 APPLICATION NOTE
Table 7. Standby function: transition thresholds in terms of input power
VAC [V] PinSB [W] 88 10 110 11 160 12.5 220 14 264 15
37 38 39 41 42 PinNW [W] Note: there is no risk of premature current limitation or transformer saturation when the system operates at fSB up to 60W input power. To reduce PinNW, increase fosc or reduce fSB.
Figure 4. Drain voltage at full load (left: Vin = 100 VDC, right: Vin = 300 VDC)
Figure 5. Drain voltage in OFF mode (left: Vin = 100 VDC, right: Vin = 300 VDC)
Figure 6. Load transient (0-0.3A) on 200V output
Left -1: 200V output -A1: L5991 pin 16 (St-by) Right -1: L5991 E/A out (pin 6) -A1: L5991 pin 16 (St-by)
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AN1132 APPLICATION NOTE
Figure 7. Load transient (0-0.3A) on 200V output
Left -1: 5991 pin 16 (St-by) -A1: L5991 pin 2 (RCT) Right -1: 5991 pin 16 (St-by) -A1: L5991 pin 2 (RCT)
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AN1132 APPLICATION NOTE
APPENDIX Low-consumption modes management The application board is not provided with the circuits that handle the loads in a monitor SMPS during Suspend and OFF modes. As a result, if the board is connected to a monitor unit "as is", the consumption from the mains will be significantly higher than the values shown in tables 5 and 6. In particular, it will not be possible to meet the "less than 3W" specification required by the current energy saving regulations in OFF mode. This happens because the monitor's circuits, in particular those connected to the high voltage buses, are still powered and have some mA residual consumption, despite they are not operating. The actual load is then heavier than the one assumed in table 5 and 6, where the load conditions in OFF-mode are simulated, provided some "power management" circuit takes care of their reduction. A popular solution used for cutting down the residual loads and minimizing the power consumption in OFF mode is to reduce 8 to 10 times the voltage of all of the outputs, except the one that powers the P governing the entire monitor operation, power management included. In this way the voltage produced by the SMPS will not be enough to power monitor's circuits and their consumption will drop to zero. Additionally, the reflected voltage during switch OFF-time will be much lower, which will reduce switching and capacitive losses. The above mentioned functionality can be achieved in a number of different ways. Figure A1 shows the application board schematic modified with the addition of a circuit (pointed out by the shaded areas) that does the job. A 5V linear regulator (L7805CP), which is supposed to supply the P, has been added for completeness. The operation of the circuit can be described as follows. When the OFF signal is pulled high, Q5 is turned on, the base of Q4 is grounded and Q4 is turned on as well. This connects the 80V winding and the 2.2F capacitor, charged at 80V, to C17+C19 charged at 15V. Being the latter much bigger, the transient voltage change is negligible. The 4.7 resistor in series to Q4's emitter limits the current surge during the transient. By turning Q5 on, the cathode of the TL431, typically at 11V in normal operation, is now forced to drop at about 4V by the 3.3V zener and the decoupling diode. Considering 1V drop across the photodiode and the drop on R26, which changes very little, the voltage on C17+C19 will be fixed at about 8.5V. The volts-per-turn across the windings will drop from 80 / 32 = 2.5 V/turn to 8.5/32 = 0.265 V/turn, that is nearly 10 times less. All of the outputs will be reduced by the same ratio (a higher value can be found because of capacitors peak charging due to load absence). The TL431 is cut out: it sees the drop of the 200V output and would try to correct this by increasing its cathode voltage, which is not possible because this is fixed by the 3.3V zener. The reduction of winding voltages concerns the primary side as well: the voltage generated by the auxiliary winding drops to some 1V and is no longer able to power the L5991. To maintain circuit operation, a second auxiliary winding, stacked on the first one, has been added, with a turn number (40) such that in OFF mode it develops a voltage sufficient to power the L5991. However, during normal operation the voltage it develops will be much higher (close to 120V). This is why Q6 has been added: during normal operation the first auxiliary winding develops more than 15V thus the base-emitter junction of Q6 is reverse biased and Q6 is cut off, thus blocking the high voltage. When entering OFF mode, Q6 is turned on (it does not work as a linear regulator) and lets the second auxiliary winding supply the L5991. As Q5 is turned off because normal operation is to be resumed, also Q4 will be turned off and the output voltages will go back to their rated values after a transient similar to the initial power-up. Table A1 shows the improvement offered by the voltage reduction circuit. A load condition similar to or slightly heavier than that of a real monitor (without any power management circuit) is assumed. The consumption from the mains is shown with and without the additional circuit included in fig. A1. Table A1. Consumption from the mains in OFF mode.
VAC [V] Pin [W] (*) Pin [W] (**) 88 4.3 2 110 4.4 2.1 160 4.6 2.2 220 4.8 2.4 264 4.9 2.5
Load conditions: 200V: 40 k; 80V: 20 k; +5V: 47; other outputs open (*) Without voltage reduction (**) With voltage reduction
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10/12
C01 4700pF 4KV C02 BD01 KBU4G R31 4.7M R01 1 R21 47K 3W 17 D08 STTA106 16 4 2.2F 100V C15 22F 100V BYW100-100 H1 1H 80V 10W C14 100F 250V GND 6.3V 5W 4.7 Q4 BC394 +15V 5W 1k C10 47 nF 250V C13 220F 100V 18 D07 BYT11-800 C03 220F 400V R32 4.7M 200V 65W D06 STTA106 R03 56K 22 9 STTA106
10F 100V
F01 AC 250V T3.15A
88 to 264 VAC
AN1132 APPLICATION NOTE
D01 1N4148 15 14 D9 BYW100-100 C16 1000F 16V 13 12 D10 BYW100-100
F 470C17 25V
R02 56K 12k R12 330K 15V R09 22 R22 10 14 C04 47F 8 10 R12 22 13 C05 12 R14 0.47 R16 100 7 1 R15 0.47 R26 2.7k 11 6 C08 3.3 nF 6 4 OP1 TPS5904 3 2 C21 330 pF R28 4.7K 3.3 V R27 470 k C22 1.2 nF 1N4148 VR1 100K 100pF C1 47F 9 25V R13 1k D11 BYW100-100 MF01 STP7NB60FI 10 11
1 470CF8 25V
Q6 BC393
R13 47K 7 D05 1N4148
C23 10nF
15
98
1
C07 0.1F
3
-15V 5W
R34 4 .7 K
L5991
4
L7805CP 2.2F 16V
5V 0.5W
R20 12K
2 7 5
C06 6800pF
Q3 BC337
16
Q5 BC393
Figure A1. Application board Electrical Schematic with OFF-mode management.
R33 9.1K
R19 10K
C09 56nF
3.3k
OFF-MODE
R29 330K
10k
AN1132 APPLICATION NOTE
Alternative Frequency Compensation Network A method alternative to the one illustrated in the previous section for cutting down the residual loads is to physically disconnect the loads by means of series switches. In that case the outputs are actually open. With this approach, if the application board is repeatedly subjected to quick power-off/power-on cycles during OFF mode, it may not start-up. In fact, being the load of the 200V output open, after a power off the output voltage decays very slowly. If the board is powered on again when the output capacitor is still almost fully charged, the output voltage will rise quickly and overshoot the regulated value. The PWM may be stopped so long - to allow the output voltage to decay to its correct value - that the L5991 loses its supply and goes into undervoltage lockout. Next, the L5991 is restarted by R2+R3, the sequence recurs and the system gets stuck in this on-off cycle. To avoid this, it is recommended to use the other feedback configuration provided in the PCB, which makes use of C20 and R30. As shown in figure A2, in that case C22 and R27 will be omitted and the value of C21 will be changed. C20 provides an anticipatory effect that prevents the overshoot and the resulting vicious circle above described. Figure A2. Alternative compensation network to be used with switch-opened loads. Parts added or modified are in bold italics.
+15V out +200V out
R2 4 47 C1 9 47F 25V
R2 6 2.7k pin 6 of L59 91 R16 100 7 1
C08 3.3 nF 6
VR1 100 k 2 R2 9 330 k
C20 8.2 nF 250 V R3 0 1.8k
4 OP1 TPS5904 3
C2 1 6.8 nF
R28 4.7k
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AN1132 APPLICATION NOTE
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specification mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics 2000 STMicroelectronics Printed in Italy All Rights Reserved STMicroelectronics GROUP OF COMPANIES Australia - Brazil - China - Finland - France - Germany - Hong Kong - India - Italy - Japan - Malaysia - Malta - Morocco Singapore - Spain - Sweden - Switzerland - United Kingdom - U.S.A. http://www.st.com
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