Real Time Waveform Analysis Helps Optimize Charger Performance

Mar 1, 2011 12:00 PM
Jerry Zheng, Yong Li and Zahid Rahim iWatt

Real-time cycle-by-cycle waveform analysis shows that controllers can deliver optimum charger performance and cost to end users, while providing cell phone manufacturers with a robust design that is insensitive to manufacturing processes and component tolerances.

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Today, cell phone charger designs face many technical challenges, including increasingly stringent technical, cost and size requirements driven by end users, OEMs of handheld devices, cell phone charger manufacturers, and regulatory agencies. This has created an opportunity for the development of next generation power supply controller technologies that take full advantage of real-time signal examination and control techniques.

Cell phone chargers allow normal operation of the handheld device while connected to the ac voltage source. Fig. 1 shows the typical cell phone operating modes that provide a regulated constant voltage (CV) output, V_OUT. Here, output voltage is sensed directly and the feedback signal transferred to the primary controller via an opto-isolation device.

Besides the “operating mode,” cell phone chargers manage the charging function of the battery (typically Li-ion) of the handheld device. Again referring to Fig. 1, the “charging mode” provides a regulated constant current (CC) output I_OUT, which requires sensing of the output current. Again, this can be provided by current sense circuit on the secondary side. Alternatively, primary-only constant current regulation can be achieved using two well known methods: the power balance equation (Equation 1), or the time average equation (Equation 2).

Where:

f = Function of

I_OUT = Output current in Amps

L_M = Magnetizing inductance in Henries

I_PK = Primary peak current in Amps

V_OUT = Output voltage

Where:

f = Function of

N = Transformer turns ratio

t_RESET = Transformer rest time in µsec

T = Switching period in µsec

Using the power balance equation, output voltage regulation is critical because it has a direct bearing on constant current performance. Likewise, using the time average requires an accurate measurement of transformer reset time.

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