单节锂离子充电座充电器线性AC适配器-Single-Cell

Abstract: The single cell Lithium-ion (Li-ion) battery charger that does not dissipate power (no heat) can also use a low cost linear AC adapter. This applicaTIon note discusses how to design a simple and low-cost Li-ion charger.

IntroducTIonMany handheld devices are using a single-cell Li+ battery, and they demand a simple and economic charging soluTIon. A cradle charger is one choice and it is getTIng more attention from system design engineers because they do not need to worry about built-in charging logic in their handheld devices. The cradle charger provides a complete standalone Li+ charger solution.

This application note describes an inductorless and virtually lossless single-cell Li+ cradle charger using a low-cost linear adapter.

Linear Input Power Supply for the Cradle Charger


Figure 1. Off-line cradle charger block diagram.

As shown in Figure 2, this simple linear AC adapter does not have a voltage and current regulation circuit, but if the AC input voltage variation and the output impedance of the AC linear transformer adapter are known its output characteristics can be predicted.


Figure 2. AC linear transformer adapter block diagram.

Figure 3 shows the equivalent circuit diagram of the Figure 2 linear transformer. Zout is the output impedance of the AC linear adapter.


Figure 3. Equivalent circuit diagram of the linear transformer adapter.

We can design a transformer that has a turn ratio n.

n= Np/Ns= Vin/Vs

If Vin=120Vac and Vs=12Vac, then n is 10. If the Vin's variation is ±10%, the Vs will vary ±10% as well.

The output voltage of the Figure 2 adapter at Rload:

Vout = (1/n) × Vin × Rload/ (Zout + Rload)

For example, if Vout1 = 9V at 1mA and Vout2 = 4.5V at 500mA load under the nominal AC input, the output impedance of the AC linear transformer adapter should be

Zout = (Vout1 - Vout2)/(500mA - 1mA)
= (9 - 4.5)/ 0.499
= 9Ω

Figure 4 shows the output voltage and current characteristics of an example linear adapter, rated input = 120VAC 60Hz and output = 3.7V at 350mA, that was designed as in Figure 2.


Figure 4. Output characteristics of a 3.7V at 300mA linear adapter.

Table 1. Output Load vs. Output Voltage
Output Load