Solutions For Digital Power Conversion Engineers Answer Your Questions

- Dec 27, 2019-

Analog engineers used to struggle with complexity when designing power supplies with multiple outputs, dynamic load sharing, hot-swap, or extensive fault handling capabilities. Using analog circuits to implement system control functions is not always cost-effective or flexible. Using analog technology to design a power supply requires the use of excessively large components to address component variations and component drift. Even after overcoming these design difficulties, these power supplies require manual adjustment at the end of the production line.

So, what should an analog engineer choose to design a power supply? Engineering's answer to this question is to use intelligent digital control of the power conversion feedback loop to achieve the above functions. The microcontroller has enabled analog designers to implement monitoring, control, communication, and even deterministic functions (such as power-up sequencing, soft-start, and topology control in power supplies). However, due to the lack of cost-effective, high-performance technology, digitally controlling the entire power conversion loop is not yet practical.

DSC Design in Switching Power Supplies

Now, the advent of a new type of digital signal controller (DSC) enables digital conversion with functions such as smart power peripherals, because this device uses counter-based pulse width modulation (PWM) modules and analog comparator-based feedback And coordinated analog-to-digital converter (ADC) sampling for fast multiplication in a single clock cycle. The combination of these features helps DSC handle the higher execution speed required by control loop software.

Before starting a power supply design, you need to make the following choices.

1. Choose a topology suitable for the application: boost or buck (boost or buck), and isolation (forward, half-bridge, or full-bridge).

2. Choose a switch technology: hard switch or soft switch. Soft switching technology (such as resonant mode or quasi-resonant mode), at the cost of increasing the complexity of the circuit and control, in exchange for less switching loss.

3. Choose a control method: voltage mode or current mode.

Voltage mode control and current mode control are two control methods based on traditional analog switching power supply (SMPS) control technology. Under voltage mode conditions, the difference (error) between the desired output voltage and the actual output voltage is used to control the time that the power supply voltage is applied to the inductor, and indirectly control the current in the inductor. Under current mode control conditions, the difference (error) between the desired output voltage and the actual output voltage is used to create a threshold value for the analog comparator to set the peak inductor current, thereby controlling the average inductor current. Voltage mode provides higher stability in noisy environments or wide operating ranges; current mode control enables cycle-by-cycle current limiting and faster transient response, and it prevents the inductor from saturating and causing disaster Increased MOSFET current due to a faulty MOSFET failure.

4. Select the PWM operating frequency. High-frequency PWM facilitates the use of smaller inductors and capacitors, but at the cost of additional switching losses.

5. Determine the required control bandwidth. This depends heavily on the load transient response expected by the application.

6. Allocate processor resources based on estimated control bandwidth requirements. Although there are multiple control algorithms, the commonly used techniques are proportional, integral, and derivative (PID) methods. Using a common PID algorithm, the control loop will need to run at eight times the speed of the required control system bandwidth to ensure sufficient phase margin. When estimating the delay of the control loop, all delays in the control loop must be taken into account (see the section Calculating the Delay of the Control Loop).