AN1620 APPLICATION NOTE 25W OFF-LINE AUTORANGING BATTERY CHARGER WITH L5991A
by Domenico Arrigo
INTRODUCTION This note describes an off-line autoranging (85Vac-264Vac) battery charger designed to operate in three different functional modes: constant voltage constant current constant power These requirements are typical of battery chargers for notebook computers. The resulting output characteristic of the system is shown in fig 1. The A-B portion of the V-I characteristic is a constant voltage mode. This situation happen when the battery pack is almost completely charged. Vo has to be set at the final value of the battery pack. From point B to C the charger operates in constant power mode, reducing the output voltage while increasing the output current. The benefit of introducing the constant power mode is that the maximum output power is lower with respect to the power output at point E. This reduces the power managed by the primary circuitry and in consequence reduces the total dissipation power. This is a useful improvement considering that the complete system is packaged in a plastic case with a very limited capability of dissipating heat. From point C to D the adapter works in constant current mode until the output voltage reaches 0V. In practice it is difficult to have a constant current characteristic below an output voltage of 2-3V because the primary auxiliary supply voltage drops at the controller turn-off threshold (internally fixed at 7.6V typ.) and the V-I characteristic exhibit a foldback to zero. If mandatory to operate in constant current mode till 0V, a secondary post-regular or other tricky solutions on the windings, primary or secondary, are required. Fig 2 shows the V-I characteristic obtained with the circuit of Fig 3. ELECTRICAL SPECIFICATION: Input voltage Regulated output voltage Maximum output power Switching frequency Target efficiency @ full load Topology Fig 3 shows the electrical circuit. Figure 1. V-I output characteristic Figure 2. Practical V-I characteristics
VO
D96IN439
85Vac to 264Vac 18V 25W 100kHz >80% discontinuous current mode flyback
Vin=220VAC
V A B E
15 Vin=110VAC
C
10
5
D96IN437
D
I
0 0 0.5 1.0 1.5 2.0 IO
November 2002
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D7 STPS8H100D T1 L2 5H C3 C1 50pF C5 120pF R16 40m 5T D11 C4 82F 400V C13 2x 470F D3 STTH1L06 L1 470H B1 + D4 R1 91K STTH1L06 0.5W R20 10K 41T C14 330F
THERMISTOR
AN1620 APPLICATION NOTE
85 264 Vac
FUSE C0
LF
18V/2A 25W
R28 2.7M Q3 BC337 Q4 BC337 R12 10K 6T D5 18V D8 9V VC 9 10 OUT R6 10 Q1 STP4NL80Z R13 240 1 IC2 R19 13K VREF V CC Csen ISEN R30 1K 8 1 C7 22F C18 1F 3T C15 22F C6 22F R4 20K R26 10 VCC 8 D2 D6 R14 82
R17 20K R18 4.7K R22 1K
R3 10K
R9 5.1K R10 36K DIS 14
Figure 3. AC-DC converter electrical schematic
DCC
3
R21 12K
R05 10K
VREF
4
R7 5.1K
RCT
C2 1F 2
2
L5991A
13 6 11 PGND R29 0.5 C8 100pF SGND C12 1nF 5 12
TSM 101A
7 2 OUTPUT C16 220nF R24 C17 220nF R23 100K R25 2.2K 6 3 CCREF 4 5 GND Crin Vrin
C9 1F
C10 2.2nF
SS
7
5
R8 5.1K
FB
COMP
C11 10nF
TCDT 1101GB 4
C18 2.2nF
IN537_mod
IC2
AN1620 APPLICATION NOTE
The realisation of the three different functional modes requires dedicated controls. The voltage and current controls are located on the secondary side of the transformer, and the error signal is transferred to the primary controller via an optocoupler in order to have an isolated feedback. The TSM101A a dual op/amp, with an internal 1.24V reference is used. One op/amp is dedicated to the voltage mode control and the second one to the constant current control. The two output are OR-ed and the common point drives the optocoupler. The current signal is taken across the 40mOhm current sense resistor, R16, while the R18 potentiometer adjusts the output voltage. The constant power characteristic is easy to achieve with discontinuous mode operation, since constant primary peak current means constant output power. The value is programmed by adding a proper offset voltage, defined by R3 value (fig 4), to the current sense ramp at pin 13. Figure 4. Schematic diagram for output constant power.
4 10K
2.7M
L5991A
10 10 1K 13 100pF
IN438_mod
0.5
The value of this resistor is related to the desired output power Pout by the below relationship: 4 R 3 = R 30 ----------------------------------------------------------------------2 1 ------------------- R S P o u t L f 1 where L1 is the primary inductance, is the expected efficiency, f is the switching frequency and Rs is the current sense resistance. The L5991A introduces a delay, td @ 100ns, at turn-off, on the current loop, and due to this, the primary peak current value increases according to its slope. The consequence is that the regulated output power is a function of the input voltage. R28 is introduced in order to compensate this error. Its value can be calculated with the following formula: ( R 30 / /R 3 ) L1 R 28 = ------------------------------------t d RS A primary auxiliary winding is required to supply the IC after turn-on, and it has to be designed to generate a dc voltage within the limits of the IC supply voltage range. It should be loosely coupled with the secondary winding in order to minimise the reflected secondary-to-primary effects when the output voltage is going down towards short circuit. A second auxiliary winding, on the secondary side transformer, forward coupled with the principal primary winding, has been added. When the system is in constant current mode, and the output voltage is reduced, this winding provides the supply voltage for the TSM101A. The Table A summarises the efficiency performance of the complete system, and Table B shows the electrical system performance.
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AN1620 APPLICATION NOTE
Table A. System efficiency.
Vin (Vac) Iout = 1A Vout (V) 85 110 220 264 17.97 17.94 17.89 17.89 83.9% 85.1% 84.4% 82.0% Iout = 2A, Pout = 36W Vout (V) 17.96 17.96 17.91 17.89 82.2% 83.7% 83.5% 83.0% Iout = 1.8A, Pout = 25W Vout (V) 13.82 13.86 14.31 14.41 82.6% 83.7% 83.8% 82.7%
Table B. System performance (Pout limited to 25W)
Line regulation Load regulation Maximum efficiency Output ripple Vin = 85 to 264 Vac, Iout = 1A Iout = 0.5 to 1.8A, Vin = 85V Vin = 264V Vin = 180 Vac, Iout = 1.8A Vin = 85 to 264 Vac, Iout = 1.8A 80mV 10mV 10mV 85.2% < 200mV
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