Designing Buck Converters with Isolated Outputs

Virtually in any electronic system, low-voltage bias supplies are necessary in order to power devices like microcontrollers, sensors, ADCs, DACs, digital isolators, transceivers, etc. For this, common bus voltages like 48 V, 24 V or 12 V need to be stepped down to the typical bias voltage levels of 5 V, 3.3 V, 1.8 V or 1.2 V that most analog and digital circuitry require to operate. In some applications, galvanic isolation between devices may also be required, either for user protection and compliance with relevant safety standards in high-voltage systems, and/or to avoid the detrimental effects of ground loops and common-mode currents in noise sensitive systems like communications or data acquisition systems. 

With a focus on compact size and low cost, the buck converter topology allows to easily obtain multiple low-power isolated output voltage rails, in addition to its non-isolated output. This is accomplished by simply adding magnetically coupled windings to the power inductor (i.e. using a coupled-inductor or transformer) and using a peak rectifier circuit (diode and capacitor) on each output. With this, each isolated output voltage would indirectly track the well-regulated output voltage of the buck converter, scaled as desired by the corresponding winding turns-ratio. This means that not only a step-down, but also a step-up voltage conversion is possible. Since the isolated/secondary output voltage is only indirectly regulated, it is very important to understand the impact of different parameters on its regulation.

Main topics:

• Review of synchronous buck converter
• Analysis of buck converter with isolated output(s)
• Regulation of isolated/secondary output voltage
• Improving output voltage regulation
• Circuit design and PCB Layout considerations
• Step-by-step validated design example

Download:  ANP017