AC-DC battery charger (constant current with voltage limit) using the MC33364 and the MC33341

Transcription

1 Order this document by /D Motorola Semiconductor Application Note A-D battery charger (constant current with voltage limit) using the M33364 and the M33341 By Petr Lidak Application Engineer Industrial System Application Laboratory Motorola, Roznov p/r, zech Republic Introduction This application note presents a way of designing an A-D battery charger using the M33364 on the primary side (operating in the critical conduction current mode) and the M33341 on the secondary side (controlling the output voltage and the output current). The first section describes overall information about the frequently used methods to charge batteries. The second section presents a two-stage battery charger outline. The third section is the design of a real A-D battery charger. Motorola, Inc., 1996

2 harge method summary harge method summary During the last decade, rechargeable batteries have little improved in terms of increased capacity. Only proper battery maintenance allows the full use of available battery capacity. One of the most important parts of the maintenance of the batteries is the battery charging. To charge a battery one needs to have a power supply with some kind of characteristic. The most common three types of battery charger output characterics are: constant voltage source constant current source constant power source The constant voltage source has one major drawback in that big current stresses when charging an empty battery. The current flowing through the battery is regulated only by the battery internal resistance. A representative output characteristic can be seen in Figure 1a. The constant current source solves this problem with current stress but now the output voltage does not depend (by theoretical definition) on the current and it has an infinite value if no load is connected. A representative output characteristic can be seen in Figure 1b. The constant power source tends to have a similar disadvantage as the constant voltage source concerning the current and as a constant current source regarding the voltage. A representative output characteristic can be seen in Figure 1c. nom 0 I 0 I nom 0 a) b) I c) Figure 1. Output haracteristics of Typical Battery hargers I The best output characteristic of a battery charger seems to be a combination of the constant current and constant voltage characteristic. In other words, the most suitable charger should behave like constant current source with voltage limit or (if one MOTOROLA

3 Two-stage Battery harger Outline prefers) a constant voltage source with current limit.the output characteristic of such a battery charger will then have a rectangular shape as can be seen in Figure. nom 0 I nom I Figure. Rectangular Output haracteristic of an Ideal Battery harger Two-stage Battery harger Outline The best way to build a battery charger with the maximum efficiency possible is to use a switched mode power supply. Since we are going to use a battery charger supplied from the mains we will use an A/D converter. An typical A/D converter is composed of two basic stages: A/D non-regulated converter Output voltage sensor Due to safety requirements the information from the output voltage sensor to the A/D non-regulated convertor is transferred through an optoisolator feedback path. Since only voltage is sensed, the A/D converter behaves just like a constant voltage source. For reaching the required rectangular volt-ampere output characteristic it is necessary to rebuild the output control circuit by adding an output current control function. A representative block diagram of a two stage battery charger can be seen in Figure 3. Stage 1 Stage Line A/D nonregulated converter oltage & current control circuit Optocoupler Figure 3. A block diagram of a two stage battery charger MOTOROLA 3

4 Two-stage Battery harger Outline For the A/D converter a variable frequency current mode flyback converter using the M33364 can be used. The M33364 is a control I developed for small output power applications like power supplies for consumer electronics and battery chargers where on a minimum external passive components is important. A simplified diagram of such an A/D converter can be seen in Figure 4. in Rectified & Filtered Line Start-up & Supply ircuit Zero rossing Detector S R Auxiliary Rectifier & Filter Q M33364 Frequency lamp Leading Edge Blanking Isolation boundary oltage & current control circuit Figure 4. Simplified M33364 based A/D converter Detailed information about the A/D converter are available in the application note AN1594. For the second stage a battery charger regulation circuit using the M33341 is selected. This device creates a rectangular output characteristic by cotrol the A/D converter through the optoisolator feedback. A simplified block diagram of the second stage can be seen in Figure 5. In accordance with recommended circuit connections a low side current sensing configuration is used,(current sensor resistor located in the negative path) because only this configuration is able to work down to a zero output voltage (if the cc is secured from an external supply). Figure 5 depicts the extracted main blocks of the M33341 which work in the low side configuration. The circuit operation is as follows: the output voltage is sensed by the resistor divider R1,R and its divided value is sent to voltage control pin of the transconductance amplifier. The transconductance amplifier is a special operational amplifier which converts a differential input voltage to an output current. The ratio between the input voltage and the output current is given by the parameter named "transconductance" specified in the datasheet in mho or Siemens units. 4 MOTOROLA

5 Two-stage Battery harger Outline The transconductance amplifier compares the divided output voltage with the reference voltage of 1.. The transconductance amplifier output drives the optocoupler LED and consequently the A/D converter. The transconductance amplifier in the M33341 also has other inputs for the output current sensor. The output current sensing is provided by a shunt resistor R cs which converts the current to a voltage drop across it. The unity amplifier delivers the voltage drop on the current sensor to the transconductance amplifier input where it is compared with the reference voltage of 0.. The voltage difference is then converted (the same way as for the output voltage control part) to the transconductance amplifier output current. The primary influence of the current control path is when the output voltage of the battery charger is below the output voltage control threshold (i.e. when a heavy load or discharged battery is applied). On the other hand, when the output current of the battery charger is lower than this limit, then the output voltage is controlled. The capacitor c connected to the transconductance amplifier output provides the overall frequency compensation of the feedback loop. A/D unregulated convertor cs R cs 0. Reference Unity Amplifier I M33341 c Feedback compensation Feedback optoisolation Transconductance Amplifier/Driver R 1 R Figure 6. M33341 Application Outline Now, by exploitation of the recommended circuit connections of the M33364 and M33341, we can build a battery charger. MOTOROLA 5

6 Battery harger Demonstration Board Battery harger Demonstration Board This battery charger has the following performance and maximum ratings: Output Input voltage range 0.8Amp max 90A - 70A T 0. A 1 N41 48 D8 R9 5 J1 1 line F1 10u F/ 350 D1 B5 0R 1 D 18 B ZX8 4/ 18 R1 0 0 uf 1N D3 R 4k7 D4 1N4148 k R3 47k R6 47k R T1 9 7 R M URS 30 T3 100uF D7 6 1uF 5 4k7 U D9 BZ X84 /5 1 D O 8 7 S B 6 M S 5 R14 k R1 3 8 k J 1 U D6 MU RS 160 T3 1n F S A 1 T A M P 3 G N D 4 M L I N E GN D R E F 4 Z D GAT E S F B 3 6 1N R D5 R7.7 Q1 MT D1N 60 E R R 7 33nF 0. 5 R1 1 R1 10 k n F IS O1 5 M O Figure 7. Battery harger Schematic 6 MOTOROLA

7 Battery harger Demonstration Board Table 1. Parts List Designator Quantity alue/rating Description µF/350 apacitor, Electrolytic 1 0µF/5 apacitor, Electrolytic 3 100nF apacitor, eramic (SMD) 4 1 1nF/1k apacitor, eramic µF/5 apacitor, Electrolytic 6 1 1µF/50 apacitor, Electrolytic nF apacitor, eramic (SMD) D1 1 A, 500 Rectifier bridge D 1 BZX84/18 Zener diode 18 (SMD) D3,4,5, A,100 Diode, 1N4148 (SMD) D6 1 1A, 600 Diode, MURS160T3 (SMD) D7 1 3A, 40 Diode, MURS30T3 (SMD) D9 1 BZX84/51 Zener diode 5.1 (SMD) J1, J onnector Q1 1 1A, 600 MOSFET, MTD1N60E (SMD) R1 1 0Ω, 1/4W Resistor (SMD) R 1 4.7kΩ, 1/4W Resistor (SMD) R3 1 kω, 1/4W Resistor (SMD) R Ω, 1/4W Resistor (SMD) R5, R6 47kΩ, 1/4W Resistor (SMD) R7 1.7Ω, 1/W Resistor R Ω, 1/4W Resistor (SMD) R kΩ, 1/4W Resistor (SMD) R Ω, 1/4W Resistor (SMD) R Ω, 1/W Resistor (SMD), (4*1Ω in parallel) R1 1 10kΩ, 1/4W Resistor (SMD) R13 1 8kΩ, 1/4W Resistor (SMD) R14 1 kω Trimmer T1 1 Transformer U1 1 M33364 I U 1 M33341 I ISO1 1 Optoisolator, MO810 MOTOROLA 7

8 Battery harger Demonstration Board ircuit operation A/D convertor: The circuit operation is as follows: the rectifier bridge D1 and the capacitor 1 convert the A line voltage to D. This voltage supplies the primary winding of the transformer T1 and start-up block in U1 through pin 8. The primary current loop is closed by the transformer s primary winding, the TMOS switch Q1 and the current sense resistor R7. The switch Q1 is driven from pin 6 of U1 through the resistor R4 and the diode D5. The resistor R4 smooths the switch-on of Q1. The diode D5 ensures a hard switch-off. The resistors R5,R6, diode D6 and capacitor 4 create a clamping network that protects Q1 from spikes on the primary winding. The battery charger should also work driving a short circuit on its output. Since the flyback direction voltage is not present on the auxiliary winding during that condition, one has to build a circuit which will use a forward direction voltage to make a cc supply voltage for U1. This circuit consists of the diodes D3 and D4, capacitor and a reverse (to the obvious flyback connection) connected auxiliary winding. The diode D4 is not a main part of the cc supply but together with resistor R creates an inverter to get an inverted signal (from the auxiliary winding) for the zero crossing detector. Since the forward voltage varies significantly with the load and the input voltage, the zener diode D limits cc voltage and resistor R1 reduces the current. The resistor R3 reduces the current flow through the internal clamping and protection zener diode of the zero crossing detector (ZD, pin1) within U1. 3 is the decoupling capacitor of the reference voltage. Output voltage and current control circuit: The diode D7 and the capacitor 5 rectify and filter the output voltage. The same method is used (as in the A/D converter) to obtain the cc voltage for U. The cc supply voltage circuit consists of the resistor R8, diode D8, capacitor 6, resistor R9 and zener diode D9. The device U drives the primary side through the optoisolator ISO1 to enforce the rectangular volt-ampere output characteristic. The output voltage information is delivered to the voltage control part of U by a resistor divider that consists of resistors R1, R13 and variable resistor R14 (used for tuning the output voltage). The output current information is delivered to current control part of the U by a shunt resistor R11. During the start-up of the battery charger the output current can exceed the nominal value because U does not yet work. For this reason the resistor R10 reduces the excessive current flow through the internal clamping and protection diode (voltage drop on the current sensor exceeds the diode threshold) of the current sensor pin1 of the U. The capacitor 7 provides frequency compensation of the feedback loop. 8 MOTOROLA

9 Battery harger Demonstration Board A/D converter design Predesign consideration D input voltages: in( min)d = in( min)a= ( 90A) = 17D in( max)d = in( max)a= ( 70A) = 38D Maximum input average current: P out I in av( max) ( 9.6W ) = = A η in( min) 0.7( 17) A safe value of the flyback voltage across the TMOS switch: flbk = TMOS in( max) 100= = 118 Since this value is very close to the in(min), it was decided for easier further calculations: flbk = in( min) = 17 The flbk value of the duty cycle is given by: = η.. efficiency flbk ( 17) δ max = = flbk + in( min) ( = 17) + ( 17) 0.5 The maximum input primary peak current: I in av( max) I ppk = = δ max ( 0.108A) = 0.43A 0.5 The desired minimum frequency fmin of operation is 70kHz. Designing the transformer After reviewing the core sizing informations provided by the core manufacturer it was decided to use an EE core of size about 0 mm.the primary inductance value is given by: δ max in min L ( ) 0.5 ( 17) p = = =.1mH I ppk f 3 min ( 0.43A) ( Hz) The manufacturer recommends for that magnetic core a maximum operating flux density of: B max = 0. T The cross-sectional area A of the EF0 core is: A c = 33.5 mm The A L factor is determined by: L p A L = n = p ( B max A c ) L p I = ppk 6 (( 0.T) ( m )) = 115nH 3 (.1 10 H) ( 0.43A) MOTOROLA 9

10 Battery harger Demonstration Board From the manufacturer s catalogue recommendation a core with an A L of 103 nh is selected. The desired number of turns of the primary winding is: n p = L p = A L H = 143turns H The number of turns needed by the +1 secondary is (assuming a ultrafast rectifier is used): ( s + fwd ) ( 1 δ max ) n p n s ( ) ( 1 0.5)143 = = = 15turns δ max in( min) 0.5( 17) Even if the forward direction of the auxiliary winding voltage is used for the cc supply voltage of U1, the auxiliary winding is designed for a flyback voltage of +1 to have a sufficient signal for the zero crossing detection when the battery charger output is shorted. Its number of turns is given by: ( aux + fwd ) ( 1 δ max ) n p n aux = = ( ) ( 1 0.5)143 = 15turns δ max in ( min) 0.5( 17) More details are given in AN1594. The Bulk Input Filter apacitor The approximate value of capacitance needed is: t off I 3 in av( max) ( 5 10 s) ( 0.118A) in = = = ripple ( 50) 10µF urrent Sense Resistor Determining the value of the current sense resistor (R 7 ), one uses the peak current in the predesign consideration. Since within the I there is a limitation of the voltage for the current sensing, which is set to 1., the design of the current sense resistor is simply given by: cs R 7 = = I ppk ( 1.) ( = 0.43A).7Ω cc supply circuit for the U1 The values of the resistor R1 and capacitor are as a result of an experimental trade-off between the power dissipation on the resistor R1 and power consumption of U1 for the whole specified voltage range of the line and for every load on the battery charger output. Zero crossing detection circuit The value of the resistor R3 is given by the U1 datasheet. The value of the resistor R is a maximum value to get a good signal for the zero crossing detector of U1 on its pin1. 10 MOTOROLA

11 Battery harger Demonstration Board Designing the output voltage & current control circuit Output voltage control circuit A value of 10kΩ is used for the lower resistor of the voltage divider. The value of the upper resistor is given by: out ref M33341 R upper = R 13 + R 14 R ( ) = = Ω ( ) = 90kΩ 1. ref( M33341) Since a resistor of 5kΩ is internally connected from the reference voltage to the feedback pin 3 of the M33364, the external resistor for the bias of the optocoupler transistor is not necessary. Output current control circuit The value of the current sense resistor R11 is given by: refi 0. R 11 = = = 0.5Ω I out 0.8A MOTOROLA 11