It is no secret that analog engineers have struggled with complexity when designing power supplies that require multiple outputs, coordinated load sharing, hot-swap capability, or extensive fault handling. Using analog circuitry to implement system control functions is not always cost-effective or flexible.
Designing power supplies using analog techniques requires components to be "oversized" to compensate for component variation and component drift. Even after overcoming these design challenges, the power supplies need manual tuning at the end of the manufacturing line. So, what options do analog engineers have for designing power supplies? The engineering answer to this problem lies in being able to implement intelligent digital control of the power-conversion feedback loop.
Recently, microcontrollers have enabled analog designers to implement monitoring, control, communications and even deterministic functions, such as power sequencing, soft start and topology control in power supplies. However, digital control of the complete power-conversion loop has been impractical, due to the lack of cost-effective, high-performance technology.
DSCs in Switch-Mode Power Supply Design
A new breed of digital signal controllers (DSCs) is now making digital conversion possible with features such as intelligent power peripherals that combine counter-based pulse-width-modulation (PWM) modules, analog comparator-based feedback and coordinated analog-to-digital converter (ADC) sampling, coupled with fast multiplication in a single clock cycle. The combination of these features allows DSCs to handle the high execution rates needed for control-loop software.
Before starting a power-supply design, the following decisions need to be made:
- Choose a topology that fits the application's needs:
- Step-up or -down (Boost or Buck)
- Isolated (Forward, Half or Full-Bridge)
- Select a switching technique:
- Soft-switching. Soft-switching techniques such as Resonant Mode or Quasi-Resonant Mode, offer lower switching losses at the expense of more complex circuitry and control.
- Select a Control Methodology:
Voltage-mode control and current-mode control are the two methods of control based on classic analog switch-mode power supply (SMPS) control techniques. In voltage-mode, the difference between desired and actual output voltage (error) controls the time that the supply voltage is applied across the inductor, which indirectly controls current flow in the inductor.