The main and key reason that digital control of a power converter is a good way forward is that it provides flexibility. And flexibility is useful for a number of reasons. The first is that the function of the product can be changed by changing the firmware or the software. There is always verification and validation effort associated with flexibility but this effort is often worth bearing for a digital controller running in a programmable processor or FPGA. The second reason is the use of common processor parts across a number of products giving purchasing advantage. A third reason is that effectively the production repeat cost of firmware and software is zero. This is because typically the BOM cost of the software or firmware is usually zero rather than the code’s amortized development cost.
There is also flexibility in the control approach. Software or firmware allows switching strategies to be changed for different converter operating conditions. Alongside this the switching instants themselves can be adjusted to minimise the switching loss.
For applications where flexibility is not required then analogue control may be what is required. In these cases digital control may not be the best. Traditional analogue control has many strengths.
So back to digital. There are specific advantages that digital control of power electronics provides when compared to traditional analogue control. These include
- Re-tuning the loop for component variation such as Electrolytic capacitor freeze out at low temperatures
- Management of the non-linearity of the converter
- Self measurement of the loop response in closed and open loop.
- The ability to tune the switching times precisely to minimize the power loss and maximize the efficiency.
These have proved very useful for power converter control.
There are some differences with a digital controlled converter when compared to analogue control. These differences can be problematic if you are not ready for them.
So what should the digital controller be implemented with?
This is a good question that does not have one answer. Many microprocessors are able to control power electronics. Other options are digital signal processors (DSP) and field programmable gate arrays (FPGA). Choosing an appropriate solution means assessing the processing power against the allowable control loop delay for the power converter and the target bandwidth. For increasing loop bandwidth the solution order is microprocessor, DSP and the FPGA. The FPGA can provide the least loop delay as it can process the signals very quickly in parallel.
And what issues are key to take care of to go digital?
It is great idea to use digital control for power electronics. Hopefully it will allow the power electronic converter to be more efficient and more flexible. One key issue is numeric precision.
1. Numeric precision – limited number of bits
The digital controller is made up from an analogue to digital converter (ADC), some digital filters which implement the controller and then a digital to analogue converter typically in the form of the pulse width modulator (PWM) or a variable frequency/period oscillator (VFO/VPO). Each of these has a limited number of bits and has an effect on the precision and noise performance of the entire system. The analogue to digital converter has a limited number of bits. Typically analogue to digital converters have eight, ten, twelve, fourteen, sixteen, eighteen or twenty bits. The increasing number of bits means increasing cost. Determining how many bits are required for the ADC is the first step in designing the digital controller.
The limited number of bits means that the measured output voltage or current is quantized and the dynamic range is limited. This means that the precision to which the output can be controlled is limited to the step size in the ADC. If the precision is not accurate enough then the feedback loop will not be able to measure the difference from the required output value. Effectively the precision of the output control will be limited to the least significant bit step of the ADC. Typically there is a trade-off between dynamic range and precision in the ADC. It is useful to use some precision extension techniques on ten and twelve bit ADC inputs to get both a high dynamic range and accurate output control. Precision extension techniques can include only sampling the error signal created with an analogue summing amplifier or using two ADC channels together to provide the precision and the range.
2. Digital filter coefficients – numeric precision
The internal calculations for the filters involve multiplications and additions. These are the typical MAC (multiply and accumulate) instructions in a DSP. These MACs realize the digital filters that provide the integrators, phase lift networks, differentiators and low pass filters that are used in closing the control loops. Each multiplication by a filter coefficient effectively reduces the precision of the signal. This reduction in precision is especially noticeable in digital integrators and digital filters with narrow bandwidth. This loss of precision can, if it is large, lead to the digital filter failing to operate on small inputs.
Managing the digital precision of the digital filter is done by ensuring that as many bits as possible are retain in all the calculations by using coefficients that are chosen to maximize the retained signal level without clipping in large transients. Another precision extension system is the retention of extra result bits in the internal filter accumulators. This has a remarkably useful effect on reducing digital power control system noise.
Other issues that arise in the digital control of power electronics are
- Timer Precision for PWM
- Converter Non-linearity effects on the digital control
- Bandwidth limits from sampling
- Anti-aliasing filter effects
- Processing delays
First step toward Digital Control of Power Electronics
Taking the first step toward going digital can involve a leap of faith. Alternately it can involve understanding the issues, the disadvantages and the advantages. The primary first choice of processor or controller technology involves determining how much processing power and the number of bits the power converter controller requires.