View AN1134_247279.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

(R)
AN1134 APPLICATION NOTE 45W AC-DC ADAPTER WITH STANDBY FUNCTION
by Claudio Adragna
Purpose of this note is to provide a brief summary of the specifications and the functionalit y of the evaluation board implementing a 45W, wide-range mains AC-DC adapter, 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 equipment 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 ) Output 88 to 264 Vac 50/60 Hz 45W Vout = 18V Iout = 2.5A Full load ripple = 2% Normal Operation Switching Frequency (fosc) Light Load Switching Frequency (fSB) Target Efficiency (@ Pou t = 45W, Vin = 88 / 264 Vac) () Maximum Input Power (@ Pout = 0.5W, Vin = 88 / 264 Vac) Maximum Input Power (Open load, Vin = 88 / 264 Vac) 70kHz 18kHz >80% 2W 1W
The selected topology is flyback. The operation mode (@ Pout = 45W ) 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 maximum duty cycle will be limited below 50%, thus no slope compensation is needed. 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. Evaluation Board Functionality The outstanding feature of this application board is the so-called Standby Function, directly available from the L5991. When the load is such that the power demanded of the mains is greater then about 13W the switching frequency of the converter is set at fosc = 70 kHz (by means of the capacitor C5 and the parallel of R12 and R13). When the input power falls below about 8.5W the L5991 automatically changes the oscillator frequency to fSB =18 kHz (by disconnecting R13 internally and charging C5 through R12 only). These thresholds are "static" values, that is are related to slow load variations. In case of step-load changes the output of the error amplifier will experience undershoots and overshoots, thus the "dynamic" thresholds will be different. Namely, the dynamic threshold for the transition f osc fSB will be
December 1999 1/9
greater than the static one, whereas the dynamic theshold for the transition fSB fosc will be lower than the static one (see tables 10 and 11). The gap between static and dynamic thresholds can be reduced to some extent by slowing down the control loop, although this goes to the detriment of the dynamic response of the system.
2/9
R22 C17 BD1 DF04M T1 C15 100 nF J2 D5 BYW29-200
F1 T2A250V
NTC1
J1
88 to 264 Vac C1 100 F 400 V R1 56 k D1 BZW06-154 N1 N2 R2 56 k R4 2.2 M D2 STTA106 GND R3 2.2 M C9 330 F 25 V C10 330 F 25 V C11 330 F 25 V
Figure 1. Electrical Schematic
18V/2.5A
AN1134 APPLICATION NOTE
C3 100 nF R6 330 k D3 1N4148 R7 4.7 D4 1N4148 C12 4.7 nF 1kV
R5 47 k
R8 5.6 k R10 22 R24 DIS OUT 14 9 8 10 Q1 STP7NB60 R17 4.3 k OP1 VCC VC R11 10 N3
C2 47 F 25 V
R9 6.8 k
VREF
DCC
ST-BY
4
3
16
IC1
ISEN 13
R14 1 k
R18 2.2 k
L5991
7 11 COMP R16 100 C7 220 pF PGND VFB SS 6 C8 100 pF R15 0.47 1/2 W
R12 24 k R13 8.2 k
TPS5904
1
2
R19 1.2 k 2
RCT
15
12
5
C14 470 nF
C4 100 nF C6 56 nF
DC-LIM
SGND
7 C13 4
C5 3.3 nF
R20 5.6 k
6
3
R21 348
AN1134 APPLICATION NOTE
Table 2. Component List of the circuit of fig. 1
Symbol R1, R2 R3, R4 R5 R6 R7 R8, R20 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R21 R22, R23, R24 C1 C2 C3, C4, C15 C5 C6 C7 C8 C9, C10, C11 C12 C13, C16, C17 C14 D1 D2 D3, D4 D5 IC1 IC2 T1 OP1 Q1 BD1 NTC1 F1 Value 56k 2.2M 47k 330k 4.7 5.6k 6.8k 22 10 24k 8.2k 1k 0.47 100 4.3k 2.2k 1.2k 348 - 100F 47F 100nF 3.3nF 56nF 220pF 100pF 330F 4.7nF - 470nF BZW06-154 STTA106 1N4148 BYW29-200 L5991 - See specs TPS5904 STP7NB60FI DF04M - T2A250V 8A/200V Ultrafast, ST PWM controller, ST Not assembled RDT 20001, RD Elettronica Milano (Tel. +39 02 66106489) Optocoupler + TL431, TI 1.2/600V, ST GI, or equivalent 1A, 400V Not assembled (shorted) 2A, 250V ELU Not assembled 400V, electrolytic, Rubycon MXR or equivalent 25V, electrolytic plastic film plastic film plastic film plastic film plastic film 25V, electrolytic, Panasonic HFZ or equivalent 1kV Not assembled plastic film 154V/600W peak Transil, ST 1A/600V Turboswitch, ST 5% metallic film 5% 5% 5% 5% 5% Note
Notes: - if not otherwise specified, all resistors are 1/4W, 1% - Q1 and D5 are provided with a 15C/W heatsink
3/9
AN1134 APPLICATION NOTE
Table 3. Transformer Specification (Part Number RDT20001, supplied by RD Elettronica).
Core Bobbin Air gap Leakage inductance Winding Pri1 Sec (a) Sec (b) Windings Spec & Build Pri2 Aux Wire AWG27 AWG25 AWG25 AWG27 AWG32 S-F 2-4 11-7 12-8 4-6 3-1 Philips EFD30x15x9, 3C85 Material or equivalent Horizontal mounting, 12 pins 0.7 mm for an inductance 2-6 of 400 H < 10H Turns 25 12 12 25 10 Evenly spaced Bifiliar with Sec (b) Bifiliar with Sec (a) Notes
Note: sec (a) and sec (b) are paralleled on the PCB
Figure 2. PCB layout: Silk + component side and bottom layer (top view); 1:1.25 scale.
If the user wants to decrease 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 L5991's current sense pin (13, ISEN). This can be accomplished by means of R24, currently not used. The offset will be the partition of the reference voltage (pin 4, VREF) through R24 and R14. Consider that applying the offset may require the sense resistor R15 to be reduced, as shown in table 4. Increasing R15 is instead the way to increase PinSB. Table 4. Adjustment of the static standby thresholds
R24 R15[] DC offset[mV] P inSB[W] PinNW[W] open 0.47 0 8.3 13.3 100k 0.43 50 6.2 11.9 47k 0.43 100 5.1 12.3 33k 0.43 150 3.6 11.1 24k 0.39 200 2.5 11.5
4/9
AN1134 APPLICATION NOTE
The power level that causes the switching frequency to be moved from fSB to fosc (PinNW) is proportional to the ratio fosc / fSB and depends only slightly on the offset. Thus to reduce PinNW, fSB needs reducing and vice versa. If, instead, P inNW is increased there is no risk of transformer saturation: the primary peak current is limited by the sense resistor (R15) and cannot definitely exceed the full load value. The thresholds are expressed in terms of input power (PinSB, PinNW); the relevant output power levels (PoutSB, PoutNW) can be obtained by multiplying by the efficiency. R2 and R3 provide an additional DC offset on the current sense which depends on the supply input voltage. This is used for compensating L5991's delay to output and also minimises the dependence of PinSB and PinNW on the mains voltage (see table 7). Additionally, the board includes some protection functions tipically required in AC-DC adapters, 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 R5-R6 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 (e.g. optocoupler'sfailure). At maximum load and minimum mains voltage the converter operates at about 48% duty cycle (this is why slope compensation is not required), however the maximum duty cycle of the L5991 is limited at about 55% to make allowance for load transients. This implies that during transients resulting from a large enough step-load change at minimum mains voltage, subharmonic oscillations are likely to arise. It is, however, acceptable, this being a condition lasting few milliseconds. To set the maximum duty cycle at 55%, L5991's pin3 (DC) is biased through R8 and R9 at about 2.26V. Please refer to Application Information in L5991's datasheet for the calculation of the voltage divider. The evaluation board is supplied with a start-up circuit simply made up of a dropping resistor (R1+R2), in series with a diode (D3), that draws current from upstream the bridge rectifier. This circuit, really inexpensive, dissipates about 300 mW @ 264 Vac. The typical wake-up time is 2.8 s at 88Vac and 0.8 s at 264 Vac. Should the wake-up time or the consumption become an issue, a more expensive solution would be adopted. The PCB is also able to accommodate a high-voltage start-up IC (IC2), the LR745N3 available from SUPERTEX and housed in a small TO92 package. In that case R1, R2 and D3 would be removed and the consumption of the start-up circuit would be of few mW. The wake-up time would be about 0.2 s independently of the mains voltage. To enhance light load efficiency, the EVAL5991-45 board is supplied with the clamping network (for the leakage inductance spike) made up of a Transil diode (D1) instead of the usual RCD type. The PCB is able to accommodate the RCD clamp anyway (R23 and C16). The use of the Transil, although slightly worsens efficiency at full load, allows to save over 100 mW that would have been dissipated on R23 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 J1 (close to the bottom left-hand corner). The 18VDC output (connector J2) is located few centimeters on the right of J1. 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. There is a number of test points where significant signals can be probed: TP1: Q1 drain voltage; TP2: pin 6 of the L5991: output of the error amplifier; TP3: pin16 of the L5991: standby indicator; TP4: pin 2 of the L5991: local oscillator; TP5: pin 1 of the TPS5904: anode of the LED of the optocoupler.
5/9
AN1134 APPLICATION NOTE
Evaluation board performance: bench 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 5. Typical application performance
Parameter Regulated Output Voltage (Vin = 220 Vac, Iout = 2.5A) Normal Operation Switching Frequency Stanby Switching Frequency Line regulation (Vin = 88 to 264 Vac, Iout = 0.5A) Load regulation (Vin = 220 Vac, Iout = 0 to 2.5A) Full Load peak-to-peak output ripple (Vin = 88 Vac, Iout = 2.5A) Maximum Efficiency (Vin = 160 Vac, Iout = 2.5A) Value 18.27 67.8 17.6 5 15 100 87.3 Unit V kHz kHz mV mV mV %
Table 6. Full load efficiency (%)
VAC [V] Iout [A] 2.5 2 1.5 1 0.5 0.5* 88 86.2 86.3 86.2 86 83.9 83.5 110 86.8 87 86.6 86.2 84.2 83.8 160 87.3 87 86.5 86 83.1 82 220 87.2 86.8 85.9 84.6 80.2 77.7 264 86.2 85.7 85.1 83.5 79.1 76.3
(*) @ fSW = fosc (0.5A applied after opening the load)
Table 7. Light load consumption (@ Pout = 0.5W), with and without standby function
V AC [V] Pin [W] * Pin [W] ** 88 1 1.15 110 1.05 1.2 160 1.15 1.4 220 1.3 1.7 264 1.5 2
(*) @ fSW = fSB (**) @ fSW = fosc (R13 connected to pin 4 instead of pin 16)
Table 8. Zero Load consumption from the mains
V AC [V] Pin [W] 88 0.4 110 0.5 160 0.6 220 0.7 264 0.9
Table 9. Wake-up time
V AC [V] TWAKE [s] 88 2.8 110 2.2 160 1.4 220 1 264 0.8
6/9
AN1134 APPLICATION NOTE
Table 10. Transition from normal operation to standby mode
VAC [V] PinSB / PoutSB [W]* PinSB / PoutSB [W]** 88 8.3/6.9 13.3/11.2 110 8.3/6.9 13.3/11.2 160 8.4/6.9 13.2/10.9 220 8.5/6.9 13.2/10.7 264 8.7/6.9 13.4/10.7
Note: (*) Load current decreased manually by -1.2mA steps (**) Negative step-load change from 2.5A with 0.25A/s rate of fall
Table 11. Transition from standby mode to normal operation
VAC [V] PinNW / PoutNW [W]* PinNW / PoutNW [W]** 88 13.3/11.2 11.5/9.6 110 13.3/11.1 11.5/9.6 160 13.3/11 11.6/9.6 220 13.4/10.9 11.8/9.5 264 13.7/10.9 11.9/9.5
Note: (*) Load current decreased manually by 1.2mA steps (**) Positive step-load change from 0.4A with 0.25A/s rate of rise
Figure 3. Drain voltage at full load (left: Vin = 100 VDC, right: Vin = 300 VDC)
Figure 4. Drain voltage at zero load (left: Vin = 100 VDC, right: Vin = 300 VDC)
7/9
AN1134 APPLICATION NOTE
Figure 5. Load transient (0.1-2.5A)
Figure 6. Load transient (0.1-2.5A)
8/9
AN1134 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 (c) 1999 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
9/9

▲Up To Search▲

Price & Availability of AN1134

	To Download AN1134 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .