AN1818 Reducing the Total No-Load Power Consumption of Battery Chargers and Adapter Applications 1 Introduction This paper describes how to reduce the power consumption, under no-load conditions, of battery chargers and adapters by using STMicroelectronics’s TSM family of secondary-side devices. This family of devices provides accurate voltage and current regulation, while incurring very low consumption at noload conditions. In fact, owing to these innovative, integrated devices, the total power consumption for the entire system at no-load conditions can be reduced down to nearly 100mW. With the arrival of new power consumption regulations, this capability is increasingly sought after. The most innovative of ST’s devices are the TSM101x family. These are highly integrated solutions for SMPS applications requiring CV and CC mode, integrating one voltage reference and two operational amplifiers. The voltage reference combined with one operational amplifier makes them ideal voltage controllers. The other operational amplifier, combined with the integrated voltage reference and a few external resistors, can be used as a current limiter. These products family are designed for use in battery chargers with a constant voltage and a limited output current and in adapters.They can be used in every type of application requiring 0.5% and 1% voltage reference precision. 2 Power dissipation under no-load conditions In a typical system for battery charger and adapter applications, different factors contribute to the total power dissipation under no-load conditions. However, in broad terms, the total power dissipation can be divided into the dissipation owing to the secondary-side (Pout) and the power dissipation owing to the primary-side (Pin). Secondary-side power dissipation This article deals with reducing the secondary-side power dissipation, so let us begin by considering a typical schematic of the secondary-side of an SMPS application, shown in Figure 1. Figure1: Typical application using CC-CV Standard in SMPS optocoupler secondary side CC-CV Standard D2 OUT+ Rlimit R3 Rref OP1 R2 Rvc1 Cvc1 OP2 C3 PWM controller C1 C2 R4 Vref Cic1 optocoupler primary side R1 R5 D1 Rsense Ric2 Ric1 OUT- AN1818/1004 Revision 1 1/6 AN1818 Power dissipation under no-load conditions The CC-CV (Constant Current - Constant Voltage) standard is a monolithic IC that includes one independent op-amp and another op-amp for which the non-inverting input is wired to a 2.5V fixed voltage reference. A good example of such a secondary-side device is ST’s TSM103W. Normally the CC-CV voltage reference is “shunted”, meaning that the internal current generator requires an external power supply in order to polarize and fix the voltage reference at 2.5V (Vref = 2.5V). If we assume that Vout is connected to a discharge battery, the resulting characteristic curve, Vout-Iout, is shown in Figure 2. Figure2: Characteristic Vout - Iout for adapter application 25 Vout (V) 20 15 10 5 Vout_min in CC mode 0 0 1 2 3 4 5 6 Iout (A) In Figure 2, we can see that the load charges gradually, by increasing the current and the voltage in order to reach a minimal voltage drop. This gradual increase guarantees a limited and stable current. Following this, the voltage value increases (while the current stays constant) up to the constant voltage value. A typical adapter application will have Vout_max = 20V (at no-load conditions) and Vout_min = 5V (which is the minimal voltage necessary to have a constant current). In order to have Vout_min = 5V, Vcc_min = 5V. If the minimum current value to bias Vref is 1mA, this means that: I = 1m A ref R V –V outmin ref 5V – 2.5V = ---------------------------------------- = ------------------------ = 2.5kΩ ref I 1mA ref Therefore, in order to have Vout_min = 5V, we must fix Rref = 2.5kΩ. Now that we have fixed the value of Rref, let's consider a no-load condition where Vout_max = 20V. It follows that: V –V out ref 20 – 2.5 I = ----------------------------- = ------------------- = 7m A ref R 2.5 ref 2/6 Power dissipation under no-load conditions AN1818 The total value of the secondary-side power dissipation is calculated using: Equation 1 P out = V o ut ⋅I tot where Equation 2 I tot = I cc +I ref +I opto and Iopto is the current needed to drive an optocoupler — typically about 1.5mA. This means that for a typical system at no-load conditions where Vout = 20V, Iref = 7mA, Icc = 1.5mA and Iopto = 1.5mA, giving a secondary-side power dissipation (Pout) equal to: Equation 3 P out = (V out ⋅I tot ) = (V out ⋅ (I ref +I cc +I opto ) ) = ( 20 V ⋅ ( 7mA + 1.5mA + 1.5mA ) ) = ( 20 V ⋅ 10 mA ) = 200 mW Primary-side power dissipation Now to turn our attention to the primary side of a typical adapter application. The primary side of an SMPS application consists of a number of functional blocks (such as the PFC and PWM), each contributing to the overall power consumption of the device. But in general, the overall power dissipation owing to primary side functions can be assumed to be about 80mW under no-load conditions (for battery chargers and adapters in the range 5W). The key factor concerning the overall power consumption on the primary-side is its efficiency rating, which is at best about 50%. This means that to transfer 1mW of power to the secondary side, 2mW of power are generated in the primary side. Returning to our calculation in Equation 3 above, this means that if we require 200mW of power on the secondary-side in no-load conditions, 400mW will be generated on the primary side, and an additional 80mW will be necessary to drive the primary-side functions. This relationship means that for any given reduction in no-load power dissipation on the secondary side, the beneficial effect will be twice as great on the primary side. 3/6 AN1818 3 Optimizing your system Optimizing your system Typically, when one designs a battery charger or adapter application, a target power consumption is set for the application under no-load conditions. This target may be 500mW, or even 300mW, but until recently, it seemed impossible to reach a no-load total power consumption of as low as 100mW. In this section, we will look at three example systems: a A typical secondary side configuration, as seen in Figure 1, and using a general-purpose secondary-side integrated circuit, such as ST’s TSM103. b An advanced secondary side circuit schematic (see Figure 3), using an integrated circuit such as ST’s TSM1011 which includes a self-polarizing voltage reference. The inclusion of a self-polarizing voltage reference permits the elimination of the resistor Rref, and by consequence, elimination of Iref. c An advanced secondary-side schematic (see Figure 3) using a very advanced integrated circuit, ST’s TSM1012, which draws only 150µA under no-load conditions. Figure3: Advanced secondary-side schematic for battery-charger/adapter application Rlimit optocoupler secondary side D2 TSM1012 Vref 1 Vcc 8 R3 2,5V R2 28V CV+ CV 5 Out Rvc1 Cvc1 7 C3 R4 C1 PWM controller CC+ C2 R5 CC- optocoupler primary side 2 Gnd Ric2 D1 4 CC 3 CV- 6 Cic1 R1 Ric1 Rsense In addition to considering the typical (Figure 1) and advanced (Figure 3) secondary side schematics, there are now on the market optocouplers that need as little as 0.5mA. As we will see below, using such an optocoupler can allow power consumption to drop very low indeed. 4/6 Optimizing your system AN1818 The following table summarizes the parameters for each of the above systems: System A “Typical Configuration” System B “Optimized Configuration” System C “Highly-Optimized Configuration” TSM103 TSM1011 TSM1012 Vref 2.5V (externally polarized) self-polarizing self-polarizing Rref 2.5kΩ none none Iref 7mA none none Icc 1.5mA 1mA 0.1mA 1mA 1mA 1mA Itot = Iref + Icc+ Iopto 9.5mA 2mA 1.1mA Vout under no-load condition 20V 20V 20V Pout 190mW 40mW 22mW Pin 380mW 80mW 44mW Primaryfunction power dissipation 80mW 80mW 80mW Total Power Consumption 460mW 160mW 124mW <500mW <200mW <150mW 0.5mA 0.5mA 0.5mA 9mA 1.5mA 0.6 Pout 180mW 30mW 12mW Pin 360mW 60mW 24mW Primaryfunction power dissipation 80mW 80mW 80mW Total Power Consumption 440mW 140mW 104mW <500mW <150mW ~100mW Secondary-side IC Iopto (standard) Target consumption window Using a low-power optocoupler: Iopto (low power) Itot Target consumption window We can see from the above comparison that by using the TSM1012 and a low-consumption optocoupler, an economy of nearly 80% in power consumption can be obtained relative to the “typical” CC-CV secondary side solution. And most importantly, it is possible with this advanced system to descend to only 104mW in no-load conditions. However, we can also see that only improving the optocoupler and retaining a standard secondary-side device (such as the TSM103) does not significantly improve the overall power consumption. 5/6 AN1818 4 Conclusions Conclusions The new family of TSM101x secondary-side power management devices from STMicroelectronics for adapter and battery charger applications open new horizons in total power consumption reductions. These devices, coupled with innovative application schemes permit access to extremely efficient solutions. 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. 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