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Design and Implementation Challenges of Digital Controlled DC-DC Converters
1Oladimeji Ibrahim, 2Nor Zaihar Yahaya, 3Nordin Saad
123 Department Electrical and Electronics Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610 Perak, Malaysia.
https://doi.org/10.26782/jmcms.2019.03.00004
Abstract
DC-DC converters are used at the front stage of multiple stage inverters for multiple energy sources integration and voltage regulation. The design has been dominated by conventional analogue techniques until recently that decline in the price to performance ratio of digital signal processor arose interest in digital control.
Digital control offers high flexibility, programmability, less part number, monitoring and auto diagnosing capability. This paper presents a technical overview and design constrains of digitally controlled DC-DC power converter towards achieving fast and improved system dynamics. An insight is provided on the limitations of practical implementation of digital control DC-DC converters which includes the digital PWM resolution, the ADC sampling delay and limited control bandwidth of digital compensator.
Keywords : Dc-Dc Power Converter, Digital Control, Digital PWM, Distributed Generation, Voltage Regulation
I. Introduction
Power electronics devices such as AC to DC (rectifier), DC to AC (Inverter), AC to AC (converter) and DC to DC (converter) have been playing a major role in distributed energy (DE) deployment by providing necessary power adaptation and control between the source and the loads (Chakraborty, Simões, & Kramer, 2013;
Ibrahim, Yahaya, & Saad, 2015). The power interface scheme provides the necessary controls for energy generations by managing variability, intermittency, reactive power control, harmonic minimization, frequency control, and phase matching for smooth synchronization (England & Truewind, 2009). Recent advancement in power electronics devices led to the development of fast semiconductor switches such as the MOSFET/IGBT capable of handling high power at high frequency with reduced switching losses. Also, real-time microprocessor capable of implementing advanced control algorithms with low power consumption are now readily available (Carrasco et al., 2006). These state of the art devices have contributed to the development of
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cost-effective grid-friendly power converters that serve in small and large distributed energy deployment.
The conventional approach for integrating distributed generation such as renewables is by direct connection, which generators are connected to the loads with limited power electronics interface (Applebaum, 1987). This approach was faced with the challenges of poor power quality delivery. Increasing usage of power electronic device in renewable energy systems led to the development of specific power converter for different energy sources such as a converter for fuel cell generator, a converter for solar PV generator , and converter for wind turbine generator (Todorovic, Palma, & Enjeti, 2008; Yazdani & Iravani, 2006). The quest for high power density and improve efficiency led to the development of a series of multi- input DC-DC power converters that now allowed the combination of two or more complementary generating units like wind and solar using the same control link.
DC-DC power converters are electronics semiconductor device that changes electrical energy from one voltage, or current level to another. It comprised of the switching and storage elements for power conversion and the control circuitry for power flow regulation. Until recent times, analogue control circuitry has been widely used in converter applications due to its design simplicity, high control bandwidth and low cost. Digitally controlled power converters are now on the increase, credit to the availability of high-speed digital signal processors with less computational power at reduced cost. Digital control offers the advantages of high flexibility where protection, monitoring and prevention circuitry can easily be implemented and updated based on the state of the system in real time. (Chang & Lai, 2010).
This paper presents the architecture of digital controlled DC-DC converter and its implementation constrains toward achieving tight output voltage regulation. An insight is provided on the limitation of practical implementation of digital control DC-DC converters including the limited digital PWM resolution, the ADC sampling delay and limited control bandwidth of digital compensator. A review of fundamental challenges with real-time implementation and continuous effort on the exploitation of digital technology towards achieving improved dynamics of power converters are discussed.
II. DC-DC converter topology and control techniques
A power conversion process involves the use of electronics semiconductor device to change electrical energy from one voltage, current or frequency level to another. The two common techniques for achieving this are the linear regulators and the switching mode power supply (SMPS) (Rashid & Press, 2010). The conventional DC-DC power converters adopt the linear regulation technique of varying resistance in accordance to load demand. There are enormous heat dissipation and high power losses associated with the system, resulting in low output efficiency between 30 % - 50 % (Kularatna, 1998). The quest for smaller, lighter and highly efficiency led to the development of switching mode power supply. The SMPS achieves power regulation by varying the control signal duty cycle (D) to the converter switching elements. This approach offers the improved efficiency of greater than 90 % with an increased operating frequency leading to significant reduction in the size of converter storage components like the inductors and capacitors for high power density. The
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disadvantages of this technique are the issues of ripples, noise on the output DC, conducted and electromagnetic interference (EMI) which needs to be carefully managed.
In recent times, different DC-DC power converter topologies have been deployed in DE applications for integrating single and multiple energy sources with improved power quality delivery (Rehman, Al-Bahadly, & Mukhopadhyay, 2015).
DC-DC converters are broadly classified as isolated and non-isolated type. The non- isolated converters are used in low voltage applications where voltage needed to be stepped up/down by relatively small ratio. Common isolated converters are the buck, boost, buck-boost, and Cuk converters. The isolated converters are used for medium and high-power application in the range of 500 W and above. These categories mostly found their applications in renewable energy and the popular isolated converters are the forward, flyback, push-pull, half-bridge, and full-bridge converters (Kularatna, 1998). The choice of converter topology for a particular application depends on the required power level, the need for isolation, high output current and the multiple input/output voltage requirements (Wai & Jheng, 2013). After the topology choice, the control circuit is also an important part of the system that plays an important role in the system dynamic performance for high-quality power delivery.
Power converter controls encompass signal monitoring, measurement, regulation, drive signal generation (PWM) and protection of the power module. The aim of the control unit is to keep the desired output voltage of power converter constant using feedback loop irrespective of variation in the line voltage, load current and circuit parameters (Rashid & Press, 2010). The output voltage largely depends on input voltage, duty cycle, load current and the converter circuit component values. In the conventional PWM converters, power flows from the source to the load is regulated by adjusting the duty cycle of PWM gating control to the power switching devices.
The two main control architecture for PWM DC-DC power converter control are the voltage mode and current mode control. The voltage mode control is a single loop control system with only output voltage been sensed for the converter power regulation. Since its single loop, it is easy to implement and good for constant load application but, has issues of slow response to line-load variation. On the other hand, the current mode control is a multi-loop control technique with a current feedback loop in addition to the outer voltage feedback loop. This control approach offers improved transient, robustness and high reliability. The multi-loop architecture naturally comes with extra sensor demand, and has the disadvantages of instability at a duty cycle above 50 % and does present electromagnetic interference (EMI) if not well managed (Ang & Oliva, 2005).
III. Digital control DC-DC power converter
The design and Implementation of DC-DC power converter control unit has been dominated by analogue circuitry due to design simplicity and low cost. The large part number, complex hardware configuration, and high sensitivity to the environment in terms of thermal and ageing effect are the disadvantages(Ang &
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Oliva, 2005). The advent of high-speed and low-cost digital microprocessor arose interest in digital control of power converters (Liu & Sen, 2005). Digital control offers numerous advantages like less part count, online monitoring and auto diagnosing, less sensitivity to parametric variation, and high flexibility as they can be reprogrammed to a different function. Also, the complexity of control is moved from hardware to software base with better noise immunity, ease of implementing complex and sophisticated control algorithm for achieving improved overall system dynamics.
ic(t) Vc(t)
+
-
iout(t)
C L D
Compensator
DPWM
H
Digital Signal Processor Q1 PWM
Vo(t)
Vref
e ADC Vo(t)
RL
iL(t)
Vin
d(t) iin(t)
Figure 1. Digitally controlled DC-DC boost converter
A typical schematic diagram of a digital controlled DC-DC boost converter is presented in Figure 1. The sensed output voltage (Vo) is transformed to discrete equivalent using an analogue digital converter (ADC) and compared with the desired reference voltage (Vref) to generate an error signal (e). The error is processed by a compensator block to determine the duty cycle of PWM control signal to the converter switching device. During supply voltage transient or load variation, the ability of the closed loop controller to accurately adjust the duty cycle of the PWM signal is critical to the tight output voltage regulation. High speed computational engine such as digital signal processor (DSP), microcontrollers (µC), digital signal controllers (DSC), and field programmable gate arrays (FPGA), are used for implementing ADC, compensator algorithm and generation of digital PWM control signal (Buccella, Cecati, & Latafat, 2012).
III.i Analogue to Digital Converter (ADC)
The primary function of the ADC is to convert the sensed output voltage (Vo) or primary inductor current (iL) to digital equivalent which is then compared with the reference value. The ADC resolution (nA/D) and sampling frequency determine the feedback accuracy which in turn affects the converter dynamics. As reported in the literature, to accurately meet the tight output voltage regulation, the ADC resolution error must be lower than the quantization output voltageVo change (Prodic, Maksimovic, & Erickson, 2001).
/ max /
. 2A D
o A D
n
o o
V V
V H V
, ref
o
H V
V (1)
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max / / int[log2 A D. o ]
A D
ref o
V V
n V V
(2)
Where, VmaxA D/ - is ADC voltage range
H- voltage sensor gain
int [] –upper rounded integer of the argument.
III.ii. Digital Compensator
Digital compensator is an important part of switching power converters responsible for power flow regulation. It generates the control signal by compensating for the error (e) resulting from the difference between the reference and sensed output voltage (Vo) to determine the next duty cycle (D) of the PWM signal.
The conventional approach for error compensation in DC-DC converter has been the classical PID controller due to design simplicity and ease of implementation. The limitation of PID controller is inability to take care of time-variant and nonlinear nature of converter circuit elements. The continuous effort to achieve improved dynamic performance are on the development of modern nonlinear control laws exploiting features of digital signal processor. The popular advanced control law that have been applied to DC-DC converter includes the adaptive PID, fuzzy logic, state feedback, Posicast, and slid mode control (Algreer, Armstrong, & Giaouris, 2011;
Guo, Hung, & Nelms, 2012; Ibrahim, Yahaya, Saad, & Ahmed, 2017). All have demonstrated significant improvement on converter dynamic with reduced voltage overshoot, and faster transient.
In order to implement a digital compensator, the two major design approaches are the digital redesign (design by emulation) and direct digital design method. Design by emulation employs control design in the continuous time s- domain and then discretize to z-domain using one of the discretization methods such as backward Euler, bilinear and pole/zero matching (Ang & Oliva, 2005). The digital redesign method provides good response with the well-known continuous-time analogue design method but suffers from discretization delay effects. The direct digital controller design is done directly in the z-domain in which a switching action on the converter is considered and modelled as a sampled-data system. The approach provides better transient to load variation, improve phase margin and control bandwidth (Prodic et al., 2001).
III.iii. Digital Pulse Width Modulation (DPWM)
The Pulse width modulation techniques are used for generating gating control signals in power converter switching control. The control signal switches the power devices ‘on’ and ‘off’ by varying the pulse duty cycle ratio as dictated by the compensator signal (O'Malley & Rinne, 2004). In digital control implementation, the DPWM is generated by the microprocessor. The quantized error signal from digital compensator (modulating signal) is compared with ramp signal samples at a regular interval of carrier sequence to generate a PWM signal. The limitation of this conventional approach is that only one pulse is produced in each carrier period. This
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introduce delay that deteriorate the output voltage regulation most especially in a highly varying source like renewable energy.
Table 1. Comparison of DPWM Techniques
DPWM Pros Cons Ref.
Counter based Good time conversion linearity
Large clock frequency, high
delay
(Hwu & Yau, 2009)
Delay line Improve PWM resolution
Large chip area (Wang, Shen, Tan, Yan, & Min, 2011) Hybrid Reduce chip area,
high resolution
Modulation delay (Corradini &
Maksimovic, 2010) Segmented
delay line
High resolution Increase cost (Sun, Tan, & Siek, 2010) Dithered High resolution Susceptible to
EMI
(Yu, Wu, & Wang, 2013) Delta-sigma High resolution Natural EMI
source
(Lukic, Rahman, &
Prodic, 2007) In power converter design, high-resolution DPWM is required for tightly regulated output voltage without limit cycle effect. Recently, some improved modulation techniques have been proposed to deliver high-resolution DPWM at a low clock frequency for tight output voltage regulation of DC-DC power converters. Some of the existing hardware and software DPWM techniques are presented in Table 1.
IV. Implementation challenges of digital control DC-DC converters
The implementation of digitally controlled DC-DC converter design requires the design engineers to apply control stability criteria in the discrete time z-domain (Prodic et al., 2001). The z-domain is a transfer function of a sampled data system or discretized continuous time domain system for predicting the converter behaviour.
Power converters operates in the continuous time domain, therefore analogue signals are discretized before processing which introduces delay in the system. Aside discretization delay, due to the limited word length of a digital processor, there is issues of accuracy most especially if the resolution is not high enough (Chang & Lai, 2007).
Switching power converter controller design has been challenging due to the non-linear nature of switching elements, fluctuation of the input voltage and load currents. The design and practical implementation of a digital controller power converters have constrains at the analogue to digital conversion (ADC) stage in terms of conversion speed and quantization error. The development and implementation of linear and nonlinear control law algorithm and generation of high-resolution digital pulse width modulation (DPWM) demand continuous research effort.
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The other challenges in digital control DC-DC converter includes accuracy of converter model for compensator design, control bandwidth limitation, computational speed of the digital processor, resolution in time and voltage measurements (Buccella et al., 2012; Chang & Lai, 2010). Research efforts are focused on how to further improve the ADC sampling and quantization effect, development of more accurate converter model, application of nonlinear compensator for wider dynamics performance and generation of high-resolution DPWM generation with less delay at low clock frequency. Improvement on identified areas contributes to better dynamics performance of power converters and tight output voltage regulation requirement in distributed energy systems.
IV.i. DPWM Resolution
Resolution of DPWM is an important factor that affects dynamic performance of digitally controlled DD-DC power converters. Different modulation architecture has been proposed in the literature as presented in Table 1. The higher the DPWM resolution the better the regulated output voltage and ease of eliminating limit cycle oscillation on the output voltage (Bradley, Alarcon, & Feely, 2012).
The Low PWM resolution usually cause a phenomenon called limit cycle oscillations which results from quantization effects of the ADC and the digital PWM resolution. It is a steady state oscillation of output voltage at frequencies lower than that of converter switching frequency. If the desired output voltage, which guarantees zero error signal is not mapped to one of the available discrete values of DPWM, the feedback loop continually swings between the two closest values of the duty cycle.
As investigated in the literature, limit cycle oscillation is eliminated by ensuring that the DPWM resolution nDPWMis higher than that of ADC nA D/ (Peterchev & Sanders, 2001).
/ 1
DPWM A D
n n (3) A demonstration of the quantitative behaviour of limit cycle oscillation effect on DC-DC converter steady-state output voltage when DPWM resolution is less than that of ADC and vice versa is presented in Figure 2 and Figure 3 (Peterchev &
Sanders, 2001).
Figure 2. Vout with DPWM resolution lower than ADC (Peterchev & Sanders, 2001)
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Figure 3. Vout DPWM resolution twice the ADC (Peterchev & Sanders, 2001) IV.ii. DPWM Delay
Modulation delay is a major drawback of most existing DPWM architectures in digital control power converters. It is the time delay when the modulation duty cycle is updated and changes of the switch gating signal (Buccella et al., 2012). The undesirable effect of the modulation delay slows the dynamic response of power converters leading to prolonged output voltage deviation from the reference value.
The larger the DPWM modulation delays, the larger the output voltage deviation.
During the transient, a decrease in line voltage/ load increase leads to output voltage undershoot and, transient increase of line voltage/load decrease causes output voltage overshoots. Renewable energies are characterized by continuous variation, so to achieve better voltage regulation, the modulation delay must be significantly reduced.
A typical turn-off delay effect in a counter based DPWM is simulated in MATLAB Simulink as shown in Figure 4. A counter base DPWM can be trailing- edge modulation (up counter), leading-edge modulation (down counter) and dual- edge modulation (combination of up/down counter). Figure 4 shows a leading-edge counter based PWM where a counter-ramp signal is compared with the duty cycle value from the compensator to generate a pulse signal. The modulator output (PWM signal) remains low when the duty cycle is below the counter, otherwise the counter S-R flip-flop sets PWM signal high (1) and stays till counter reaches zero before resetting to a low state (0). Under transient line or load condition, the duty cycle drops below the counter more than one occasion before the counter completes a cycle, the modulator did not respond thereby introducing turn-off delay (td(off)) in Figure 4 which will lead to output voltage deviation.
Figure 4. DPWM with turn-off delay
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Research efforts to reduce DPWM modulation delay and increase the resolution has led to modifications of existing techniques and development of new ones. Some of the improved digital modulation architectures take the advantage of integrating features of a digital processor such as digital clock manager (DCM) in FPGA resource, while others combine basic techniques such as segmented hybrid DPWM, counter-comparator with multi-bit Delta-Sigma (Lukic et al., 2007), dithered delta-sigma (Cho, Powers, & Santoso, 2010). The trade-off with most of these methods is the difficulty in timing constraint, increased power consumption, increase chip area, and cost. Continuous effort is being made to exploit digital signal processor integrating feature and high computational speed to further improve the transient and dynamic performance of digitally controlled power converters. Digital control of power converter interface scheme has the potential of allowing implementation of a complex and sophisticated control algorithm that will ensure better dynamics performance of DC-DC power converter in the highly varying power source.
V. Conclusions
The share of distributed generation in the global electricity generation has been on an increase in the past few decades. To fully integrate these energy sources without compromising existing grid network demands power interface scheme with advanced control techniques. Digital control of power converters offers the platform for implementing advanced control techniques for delivering improved voltage regulation and better dynamics performance in power converter design. Though with the advantages of the digital control system, there are constraints hindering maximizing its capability which has been itemize in this paper and some of research efforts made so far in digital control of DC-DC converter have been presented.
VI. Acknowledgment (Optional)
This work was supported by Universiti Teknologi PETRONAS through MyRA under Grant 0153AB-J17.
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