Improved constant on-time controlled buck converter with high output-regulation accuracy Xing Chen, Guohua Zhou✉, Kaitun Zhang and Shuhan Zhou Constant on-time (COT) control has been widely applied in industrial applications due to its fast transient response. However, the conventional COT controlled buck converter has poor output-regulation accuracy. A new control method, called improved COT (ICOT) control, that provides both high output-regulation accuracy and fast transient response for the buck converter is proposed. Circuit simulation results validate the correctness of the theoretical analysis.
Introduction: The control method of switching-mode power supply is an important factor which greatly affects the speed of the transient response and the precision of the output voltage in the switching converter. Constant on-time (COT) control has a fast load transient response and an excellent light-load efficiency for switching converters [1]. However, in the COT controlled buck converter, the valley output voltage is equal to the reference voltage, which deteriorates the outregulation accuracy. In practical applications, to ensure that the average output voltage is approximately equal to the reference voltage it usually subtracts one fixed value, which is a half of output voltage ripple. However, it is not a suitable method when input voltage or load resistance varies. To solve this problem, virtual inductor current-COT (VIC-COT) control is proposed and analysed in [2]. Compared with COT control, VIC-COT control has a lower speed of transient response and a complicated architecture. To improve the output-regulation accuracy and not change the transient performance of the buck converter, a new and simple control method, called the improved COT (ICOT) control, is proposed in this Letter, where the referenced voltage is updated by calculating the output voltage ripple dynamically and thus improves the output-regulation accuracy.
Re
VP vg
R
S2
vo
C
RVCM
vH vref
VREF
vL S/H-1
tL TM
CMP
vs
TON vref
aTON
vref
0.5 TON vH
vH
t on
vL
t on
m2 =
Dvo = m1 TON = 2m1
vo Re L
TON 2
Dvo = VREF − vH + vL 2
t on
TON
TON
b
Fig. 1 ICOT controlled buck converter and its operation waveforms a ICOT controlled buck converter b Operation waveforms
(vg − vo )Re , L
vo = vref +
tH
TON
m1 =
vH –vL
tL
VP
Calculating output voltage ripple and updating reference voltage: When a large ESR of the output capacitor is used, this makes the output voltage ripple waveform look piecewise linear [3]. According to the operational principle of a buck converter, the increasing and decreasing slopes of the output voltage are given as [4, 5] (2)
(3)
(4)
According to (1) and (4), the average output voltage of the ICOT controlled buck converter is given as
vH
vL
vL
(1)
We learn from (1) that if the output voltage ripple can be taken into consideration in updating the reference voltage, the output-regulation accuracy of the buck converter will be improved. With output voltage ripple being calculated in the RVCM, the updated reference voltage vref can be obtained to replace the reference voltage VREF of the conventional COT.
vref = VREF −
a
VREF
Dvo 2
According to (3), if the switch S1 is turned on for the time interval TON/2, the output voltage vo increases to Δvo/2. When the switch S1 is turned on, it is clear from Fig. 1b that the sampling clock tL triggers the S/H-1 block during the time interval αTON (0 ≤ α ≤ 0.5) to sample the output voltage and obtain vL. After delaying the constant time interval ton, the sampling clock tH triggers the S/H-2 block to sample the output voltage and obtain vH. The constant time interval ton is defined as: ton = TON/2. According to the above analysis, the half of output voltage ripple can be obtained: Δvo/2 = vH − vL. The updated reference voltage vref of the ICOT control can be further derived as
S/H-2 tH
delay
vo = VREF +
The output voltage ripple is equal to the increasing value of the output voltage when the switch S1 is turned on. Thus, the output voltage ripple can be obtained as
L
S1
ICOT controlled buck converter: The circuit schematic of the ICOT controlled buck converter and its operation waveforms are shown in Fig. 1, where the controller consists of a timing module, a reference voltage calculation module (RVCM) and a comparator. Compared with the conventional COT controller, the ICOT controller has in addition the RVCM which includes two sample/hold (S/H) blocks, a delay module and an adder module. In Fig. 1a, vg, vo, VREF and vref are input voltage, output voltage, reference voltage and updated reference voltage, respectively; Re is the equivalent series resistance (ESR) of the output capacitor and VP is considered as a driver signal. The sampling clocks of S/H-1 and S/ H-2 are tL and tH, and the corresponding sample values are vL and vH, respectively. It is known that the valley value of the output voltage is equal to VREF in the COT control [1]. We define that Δvo is the output voltage ripple. Therefore, the average output voltage is
Dvo = VREF 2
(5)
ICOT control performance observed from circuit simulation: To verify the analysis results given above, circuit simulation was performed with following parameters: vg = 2.5–6.5 V, VREF = 1.5 V, L = 30 μΗ, C = 470 μF, Re = 50 mΩ, R = 0.5–4.5 Ω and TON = 12 μs. A circuit simulation model was built using MATLAB/SIMULINK software and Fig. 2 shows the simulation results of the steady-state output voltage waveforms of the COT controlled and the ICOT controlled buck converters. In Fig. 2, the output voltage ripples of the buck converter with COT control and ICOT control are almost identical, and the valley output voltage of the COT control and average output voltage of the ICOT control are equal to the reference voltage VREF.
ELECTRONICS LETTERS 19th February 2015 Vol. 51 No. 4 pp. 359–360
vo, V
1.60
COT
1.55 1.50 1.45
vo, V
1.55
ICOT
1.50 1.45 1.90
2.00
1.95
2.10
2.05
Conclusion: Output-regulation accuracy and transient response performance are two important targets for switching-mode power supply in industrial applications. By comparing the performance of the COT control and the ICOT control, it is found that the proposed ICOT control has better steady-state performance and the same transient performance. In addition, the transient performance of the ICOT controlled buck converter is much better than that of the VIC-COT control. These results provide useful guidance for the design of the switching converter with both high output-regulation accuracy and fast transient response.
time, ms
Fig. 2 Steady-state simulation results of buck converter with different control methods Top: COT control Bottom: ICOT control
Fig. 3 shows the average output voltage of the COT controlled and the ICOT controlled buck converters with variation of input voltage and load resistance. In Fig. 3, as input voltage or load resistance varies, the average output voltage has been changed with the COT control, but is unchanged with the ICOT control where it is always equal to the reference voltage. This means that the output-regulation accuracy with ICOT control is much better than with COT control. Moreover, it is shown that COT control cannot improve the precision of the output voltage using the reference voltage minus the fixed value. 1.54
ICOT
1.50 1.48
COT
1.52
1.52
2
3
5
4
6
7
ICOT
0
1
2
3
vg, V
R, W
a
b
4
5
Fig. 3 Average output voltage of COT and ICOT controlled buck converter with different parameters a With variation of input voltage b With variation of load resistance
Fig. 4 shows the transient output voltage waveforms of VIC-COT controlled, COT controlled and ICOT controlled buck converters with the load step decreased from 3 to 1.5 A. From Fig. 4, it is obvious that ICOT control and COT control have the same transient response and a faster transient response than VIC-COT control under load variation, which indicates that the proposed ICOT control technique has fast transient performance for a buck converter.
1 Redl, R., and Sun, J.: ‘Ripple-based control of switching regulators – an overview’, IEEE Trans. Power Electron., 2009, 24, (12), pp. 2669–2680 2 Lin, Y.C., Chen, C.J., Chen, D., et al.: ‘A ripple-based constant on-time control with virtual inductor current and offset cancellation for DC power converters’, IEEE Trans. Power Electron., 2012, 27, (10), pp. 4301–4310 3 Wang, J.P., Xu, J.P., and Bao, B.C.: ‘Analysis of pulse bursting phenomenon in constant-on-time-controlled buck converter’, IEEE Trans. Ind. Electron., 2011, 58, (12), pp. 5406–5410 4 Sun, J.: ‘Characterization and performance comparison of ripple-based control for voltage regulator modules’, IEEE Trans. Power Electron., 2006, 21, (2), pp. 346–353 5 Zhou, G.H., Xu, J.P., and Wang, J.P.: ‘Constant-frequency peak-ripple-based control of buck converter in CCM: review, unification and duality’, IEEE Trans. Ind. Electron., 2014, 61, (3), pp. 1280–1291
VIC-COT
1.6
vo, V
Xing Chen, Guohua Zhou, Kaitun Zhang and Shuhan Zhou (School of Electrical Engineering, Southwest Jiaotong University, Chengdu 610031, Sichuan, People’s Republic of China)
References
1.50 1.48
© The Institution of Engineering and Technology 2015 8 December 2014 doi: 10.1049/el.2014.4261
✉ E-mail:
[email protected]
COT
vo, V
vo, V
1.54
Acknowledgments: This research work is supported by the National Natural Science Foundation of China (grant 61371033), the Specialised Research Fund for the Doctoral Program of Higher Education (grant 20130184120011), the Fok Ying-Tung Education Foundation for Young Teachers in the Higher Education Institutions of China (grant 142027), the Sichuan Provincial Youth Science and Technology Fund (grants 2014JQ0015 and 2013JQ0033) and the Fundamental Research Funds for the Central Universities (grant SWJTU11CX029).
1.5 1.4 1.7
vo, V
COT 1.6 1.5
ICOT
vo, V
1.6 1.5 1.4 2.8
3.0
3.2
3.4
3.6
3.8
4.0
time, ms
Fig. 4 Transient simulation results of buck converter with different control methods Top: VIC-COT control Middle: COT control Bottom: ICOT control
ELECTRONICS LETTERS 19th February 2015 Vol. 51 No. 4 pp. 359–360
