Non-magnetic AC/DC Power Supplies

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Non-magnetic AC/DC Power Supplies [Copy link]

One of the most common challenges when creating industrial power supplies is converting AC voltage power to DC voltage power. Almost all applications require changing AC voltage to DC voltage, from charging a cell phone to powering a microcontroller in a microwave oven. Typically, this conversion is done using a transformer and rectifier, as shown in Figure 1. In this circuit, the voltage is stepped down by a transformer (one times the transformer primary and secondary turns ratio).

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Figure 1 : Simplifying AC to DC conversion using a transformer and LDO

There are several disadvantages to magnetic solutions. You probably know that transformers work by converting magnetic flux into electric current. Because of this conversion, transformers generate a lot of electromagnetic interference (EMI). The output voltage of the transformer is also extremely noisy, requiring large capacitors to filter out the noise. For low-power applications, a simpler and cost-effective approach can be used to eliminate the magnetic components. Just like how two resistors form a voltage divider, you can use a capacitor to create an AC impedance (reactance) that drops the voltage before it reaches the power supply. This configuration is often called a cap-drop solution.

The basic capacitor drop solution requires a Zener diode to sink the current required by the application when the load is not switched on. This Zener diode is required so that the input voltage to the linear regulator (LDO) does not exceed the absolute maximum rating.

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Figure 2: Basic capacitive dropout circuit with LDO for 110 V _AC , 5 V _DC , and 30 mA

One disadvantage of the capacitive drop topology is that it is not very efficient because much of the power is dissipated as heat in the resistor and LDO. Even if the LDO is not regulating, the efficiency is still not ideal due to the energy dissipated in the Zener diode.

To improve the efficiency of this system, you need to optimize the voltage drop of three main components – the surge resistor, the Zener diode, and the LDO. Equation 1 shows how to calculate the efficiency of the basic capacitor drop solution shown in Figure 2.

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Because capacitive drop solutions are a common power configuration in industrial applications such as electronic metering and factory automation, Texas Instruments has developed a component focused on optimizing the efficiency and solution size of the capacitive drop architecture. The TPS7A78 integrates many of the discrete components required to implement a capacitive drop circuit, such as an active bridge rectifier. Designed specifically for use with capacitive drop circuits, the TPS7A78 integrates multiple features and improves overall system efficiency. For example, the TPS7A78 integrates a switched capacitor stage that drops the input voltage by a factor of four, thereby reducing the input current at the same ratio and facilitating the use of smaller capacitive drop capacitors. This feature enables a smaller solution size, lowers system cost, and reduces standby power consumption.

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Figure 3: 30mA capacitor drop solution at 30mA using the TPS7A78

To understand the efficiency of using the TPS7A78 over a cap drop stage and a linear regulator, we can compare the traditional solution shown in Figure 2 to the TPS7A78 solution shown in Figure 3. In the traditional dropout solution using a linear regulator, the system efficiency is 11%. When configured to power the same load, the TPS7A78 is able to achieve > 40% efficiency due to the reduced input current of the switched capacitor and the need for a smaller surge resistor .