What is the isolated power supply using a buck regulator IC?
An isolated voltage output is usually required to provide galvanic isolation and enhance safety and noise immunity. Common applications include programmable logic controllers (PLC), intelligent power metering, and IGBT drive power supplies. Flyback and push-pull converters are two common and economical solutions. However, flyback converters usually require optocouplers or auxiliary windings to regulate the output voltage. In addition, flyback switches are affected by high voltage spikes, so RCD snubbers are usually required. Push-pull DC transformers operate at a fixed 50% duty cycle, which may affect the output voltage regulation. Sometimes additional LDOs are required to achieve accurate output regulation.
In the above bipolar power supply, an additional output voltage output is achieved by using an inductor to add a magnetically coupled winding in a buck or inverting buck-boost converter. By simply isolating these two output loops, an isolated voltage output can be achieved (see Figure 1, this method is becoming more and more commonly used.
Isolated power supply with single isolated voltage rail
Figure 1 Simplified single-isolated voltage rail using buck method (a) or inverting buck-boost method (b)
Two methods of using a buck regulator to generate an isolated voltage output are shown in Figure 7. These configurations are similar to the bipolar power supply shown in Figure 4, except that the two output circuits (reference) are separated. Unlike a bipolar power supply with a transformer turns ratio of 1: 1, this method can set the required output voltage on the secondary side by optimizing the turns ratio of the isolated power supply. In addition, the controller can be adjusted to run at the optimal duty cycle.
Isolated power supplies with buck regulators have multiple advantages. As shown in Figure 7 (a), this step-down method is used as an example to illustrate its advantages. First, it removes the optocoupler and auxiliary freewheeling circuit required in the flyback converter. Secondly, compared to the flyback converter, the buck configuration is provided on the primary side FET to provide low voltage stress, low voltage FET means lower on-resistance and higher efficiency. Third, the primary side output (VOUT1) is well regulated, and the isolated output (VOUT2) corresponds to VOUT1, which provides good output voltage regulation on the secondary side over a wide input voltage range. Compared with a push-pull DC transformer without additional LDO, better voltage regulation can be achieved. Highly integrated buck regulator ICs, such as the ISL85410 with internal compensation, can easily implement the above methods in power supply design.
In Table 2, the advantages and disadvantages of the bipolar power supply design of buck conversion and inverting buck-boost conversion are also applicable to isolated power supplies using buck regulator ICs. Power supply designers should choose the most suitable for their specific applications Methods.
Isolated power supply with multiple isolated voltage outputs
Multiple isolated voltage outputs can be achieved by adding more coupled windings, the working principle of which is similar to a single isolated voltage output.
Figure 2 Multiple isolated voltage outputs using buck method (a) or inverting buck-boost method (b)
in conclusion
The highly integrated buck regulator IC can more easily achieve different power conversions and meet different application requirements. This article explains how these buck regulator ICs can be used to generate inverting power supplies, bipolar power supplies, and single or multiple isolated power supplies. The highly integrated ISL8541x series buck regulator ICs have a wide input voltage range, integrated start-up diodes and internal compensation. Inverting, bipolar, and isolated power solutions designed with these buck regulator ICs have many important advantages such as a small number of external components, a small overall solution size, and ease of use.
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