An Investigation on Control Strategies for Fast Transient Response of SMPS

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Abstract — This paper presents a thorough literature reviews

of control methods to improve transient response in switch

mode power supply (SMPS). A fast transient response to a step

load change is very essential in a power supply. The transient

response can be improved by means of feedback control

method. There are two feedback control methods widely used

for SMPS that are, voltage mode control (VMC) and peak

current mode control (PCMC). A simulation study on these

feedback control methods has been done using

Matlab/Simulink. It is found that the PCMC helps the SMPS

for a faster transient response compared to the VMC. Based on

all the reviews a new method is being investigated known as

“Voltage Injection Switching Inductor” (VISI) method. This

feedback control method provides a better transient response

to PCMC. The comparison of these simulation results validates

the proposed idea.

I. INTRODUCTION

s the technology grows, the speed, complexity and

performance of modern systems increase, that result to

a harsher loading requirement on power supplies. High

frequency switch-mode pulse width modulated (PWM) ac-

dc and dc-dc power converters have become the power

supplies of choice in most systems because of their superior

efficiency and small size. A switch mode power supply

(SMPS) is a power supply that provides function throughlow loss components such as capacitors, inductors,

transformers and the use of switches that are in one of two

states, either on or off. The main advantage of SMPS is that

the converter’s switches dissipate very little power and

power conversion can be accomplished with minimal power

loss, which equates to high efficiency.

Dc-dc converters which are used in SMPS can be of

isolated or non-isolated. In an isolated dc-dc converter the

input and output are separated mechanically by means of a

transformer. Whereas, a non-isolated dc-dc converter, do not

have transformer to isolate the input and the output circuits.

Isolated dc-dc converter is mainly for safety considerationand to reject common mode voltage. Common-mode voltage

in terms of ac power is the noise signal between the neutral

and the ground conductor that affects output voltage

measurement of the SMPS. Isolation to the output circuitry

This work was supported in part by Pentamaster Instrument Sdn.Bhd,

Malaysia.

Jegandren.J is with Faculty of Engineering, Multimedia University,

Malaysia (e-mail: [email protected]).

Gobbi.R is with Faculty of Engineering, Multimedia University,


Hussain S.Athab is with Faculty of Engineering, Multimedia University,


is required to eliminate this noise. The only disadvantage of

the isolated dc-dc converter is the switching transformer

produces high frequency ripple. A non-isolated dc-dc

converter is low cost and simple, but prone to high amount

of noises besides no short circuit protection, and is

applicable only for low power applications up to 100W.

There are several types of isolated dc-dc converter; they

are flyback converter, push-pull converter, half-bridge

converter and full bridge converter. In this paper only half-

bridge converter is considered as it has many advantages

which are suitable for many practical applications. Each

switch in the half-bridge converter switching circuit has half the voltage stress of push-pull converter, due to capacitive

voltage divider arrangement. The transformer core is

operated in the first and third quadrant of the B-H loop [1].

So the half-bridge primary transformer winding has half the

turns for the same input voltage and power compared to

transformer used in flyback converter. Moreover, the

leakage energy of inductance is returned to the input

capacitor instead of being dissipated in resistive snubbers,

this further increases the efficiency of the half-bridge

converter. Another significant advantage is that the

secondary transformer winding of half-bridge converter

produces a full-wave output rather than a half-wave output.Thus, the square-wave frequency in the half-bridge

converter is twice that of the non-isolated dc-dc converter

and flyback converter, so the associated filter circuits can be

much smaller.

There are two feedback control methods that are widely

used in SMPS; one is the voltage mode control (VMC) that

senses only the output voltage, Vout and another is current

mode control (CMC) that senses both the Vout and the filter

inductor current as the feedback signals.

The solution to meet the increasing demand for future

practical applications is with a fast transient response of

SMPS. In a SMPS, generally transient response is limited by

two circuits that are the feedback control circuit and the

filter circuit. The feedback control circuit can be easily

designed to perform very fast and effective with the help of

high-speed microcontrollers. However, transient response

due to the effect of filter circuit cannot be improved easily.

Fig. 1 shows the block diagram of a typical SMPS.

An Investigation on Control Strategies for Fast Transient Response

of SMPS

Jegandren.J, Gobbi.R and Hussain S.Athab

A

Proceedings of the 2008 IEEE Conference on Innovative Technologiesin Intelligent Systems and Industrial ApplicationsMultimedia University, Cyberjaya, Malaysia, 12-13 July 2008

978-1-4244-2416-0/08/$20.00©2008 IEEE

110



Bridge

Rectifier

Switching

Circuit

Output

Rectifier Transformer

Feedback

Control

Circuit

Filter

Circuit

Output AC

voltage

DC-DC Converter

Fig. 1. Block diagram of a typical SMPS

II. LITERATURE REVIEW ON APPROACHES TO IMPROVE

TRANSIENT RESPONSE

Generally there are five approaches which have been

investigated to improve the transient response of SMPS.

Each approach manipulates either the feedback control

circuit or the filter circuit to achieve fast transient response.

A. Voltage Injection Control (VIC) Approach

The main idea of voltage injection control (VIC) is to

send a signal to the feedback control circuit to increase or

decrease the pulse width of the converter switch before the

load is applied or removed. The pulse width is varied byinjecting an appropriate value of voltage, called injection

voltage, Vinj into the error voltage, Verror . The Vinj is

calculated offline for each possible load switching and is

stored in a look-up table. Therefore, significant improvement

in transient response can only be obtained if an appropriate

value of Vinj is injected. This approach can only be used for

a SMPS to improve transient response where detail

knowledge of the load characteristics is available [2].

B. Stepping Inductor Approach

This approach varies the effective inductance of the filter

circuit with an aim to vary the time constant of SMPS. This

is done by connecting a low value inductor through a switchin parallel to filter inductor. During steady state load

condition, the low value inductor will be isolated from the

filter inductor. This maintains high time constant of the filter

circuit to produce smooth output current. During transient

load condition, the low value inductor is connected in

parallel to the filter inductor to reduce the effective filter

inductance. This reduces the time constant of the filter

circuit and allows fast recovering of output current. However

this approach causes high current ripple during transient

condition, which causes high root mean square current in the

converter switches and filter components and increases

power losses [3].

C. Linear-Non-Linear Approach

In this approach, an improvement in the feedback control

circuit to achieve a fast transient response. There are two

control methods which are applied to a buck converter to get

fast transient response. One is a linear control method where

the output voltage signal is monitored by the feedback

control circuit which produces PWM signals to the converter

switch. This method is used during steady state operation of

the SMPS. During transient operation, where there are load

changes, the second method called non-linear control

method is employed. Here, the same output voltage signal is

monitored by the feedback control circuit which produces

PWM signals with duty ratio of either 1 for increase of load

or 0 for decrease of load. However this approach is not

applicable to half-bridge and full-bridge converters, because

setting the PWM signals’ duty cycle to 1 during transient

operation creates short circuit across the primary terminals

of the isolated transformer [4]. D. Double Non-Isolated DC-DC Converter Approach

This approach uses two buck converters. One is the main

converter and another is the auxiliary converter with lower

filter inductance. The output power of the SMPS is the total

output power given by both of these converters. Each

converter has its own feedback control circuit which

monitors the output voltage signal. The main converter

works in the similar manner as the linear control method

explained in section 2.3 during steady state operation.

During transient operation, the auxiliary converter’ switches

are switched in the similar manner as non-linear control

method explained in section 2.3. Using this approach a fasttransient response could be achieved. However SMPS which

uses this approach has high switching loss hence lower

efficiency. Beside it has large physical size and costly

compared to SMPS with single buck converter [5].

E. Two Stage DC-DC Power Converter Approach

This approach uses two dc-dc converters. The first

converter is a buck converter and the second is a half-bridge

converter. These two converters are connected in series such

that the output of the buck converter is the input for the half-

bridge converter. The duty ratio of the PWM signals for the

half-bridge converter is always set to 0.5. The output voltage

of the half-bridge converter and the filter inductor current of

the buck converter are measured and send to the feedback

control circuit. In the feedback control circuit, these current

and voltage values are processed based on a current control

method before PWM signal is send to the switch of the buck

converter. The details explanation of the current control

method will be given later in this paper. This approach

however produces additional losses due to the additional

switches [6].

III. FEEDBACK CONTROL METHODS

There are two feedback control methods; one is the

voltage mode control and another current mode control. The

main purpose of these control methods is to adjust the duty

cycle of the PWM to regulate the output voltage. Voltage

mode control method is also called as a single loop control

method because only the output voltage, Vout is sensed and

used in the feedback control circuit. Current mode control

method uses two feedback signals that are the output voltage

and the filter inductor current. Both of these control methods

are described in detail in this section.

A. Voltage Mode Control (VMC)

The most extended dc-dc converter uses voltage mode

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control (VMC), where there is a single feedback loop with

the output voltage used as the feedback signal. Fig. 2 shows

the block diagram of feedback control circuit employing the

VMC. Output voltage, Vout from the dc-dc converter is

sensed and compared to a reference voltage Vref . The

difference between the Vref and the Vout is known as Verror .

Verror is then compared with a fixed frequency triangular

waveform, and the output is a pulse-width modulated(PWM) signal that is used to control the switch. When Verror

is positive, the PWM duty ratio is decreased so that the ON

time of the switch is for a longer period. Whereas when the

Verror is negative, the PWM duty ratio is increased and the

switch is ON for a shorter period. When Verror is zero, the

previous PWM duty ratio is maintained.

Vout

Vref

PWM

Logic

Circuit

Clock

Signal

To

Switching

Circuit

PCMC

Verror

Triangular Wave

Fig. 2. Block diagram of Feedback Control Circuit employing VMC

Since, the half-bridge converter employs two switches, the

single PWM pulse is converted to two 180° out-of-phase

pulses of the same width. This is done with a clock signal

and logic circuitry as shown in Fig. 2.

Fig. 3 shows a clearer picture on the positioning of pulses

during the operation of VMC. Fig. 3a) shows the interaction

of Verror and the triangular waveform. As seen at t1 to t3 the

PWM pulse is low all the while, this is the time it takes for

the Verror to reach from a desired voltage to the triangular

wave amplitude. The PWM pulse width variates only when

the Verror regulates between the triangular wave amplitude.Logic circuit that is incorporated in the VMC feedback

control circuit contains NOR logic and NOT logic. The logic

circuit gets its input from a fixed frequency clock signal and

PWM pulse. In the logic circuit the fixed frequency clock

signal is separated into 2 clock signals with 180° out of

phase signals using NOT logic as shown in Fig. 3c). Each of

these clock signals are supplied to logic NOR gate, together

with PWM pulse. The output of each of the NOR gate is

supplied as an input to the switching circuitry of the dc-dc

converter. Fig. 3d) shows the pulses from the logic circuit.

d)

c)

b)

Verror

Triangular Wavea)

t9t8t7

PWM pulse

Clock1

Clock 2

Pulse to switch 2

Pulse to switch 1

t6t5t4t3t2t1

Fig. 3. Pulses in VMC

There are two main advantages of the VMC. One is VMC

requires simple hardware implementation due to the single

feedback loop that is easier to design and analyze. Second, a

large-amplitude triangular waveform used in the feedback

control circuit provides a good noise margin for a stable

modulation process. Hence, VMC can be considered for

noisy applications [7].

Slow transient response is one of the drawbacks of the

VMC. This is because, for sudden changes in load, the filter

components provide a time delay before the output voltage

response to the load change. Another drawback is the VMCis prone to unbalanced switching of both switches in the

half-bridge converter during load change. The unbalanced

switching would lead to saturation in the transformer.

Besides, in VMC there is no cycle-by-cycle load over

current protection since the output current is not monitored

by the feedback control circuit.

B. Current Mode Controller (CMC)

As a brief introduction, when a constant current flows in a

fixed load resistor, it generates a constant voltage. But if the

load resistor changes in order to maintain the same constant

voltage, the current level has to change. That is exactly what

current mode control of switching converters is all about.The idea behind the current mode control is to create a

voltage - controlled ideal current source. This current source

is to ensure a constant voltage at the output of the dc-dc

power converter regardless of load current changes.

Current mode controller (CMC) is implemented through

two control loops. A current control loop (inner loop), which

monitors the filter inductor current information, creates the

voltage-controlled current source. The second loop is a

voltage loop (outer loop), which monitors the converter's

output voltage and constantly controls the current source to

regulate the output voltage at a given set point [8].

The PWM used in current-mode control can be operatedeither at fixed frequency or at variable frequency. Fixed

frequency current controllers are such as peak current mode

controller (PCMC), valley current mode controller (VCMC)

and average current mode controller (ACMC). Variable

frequency current controller is such as hysteric current mode

controller (HCMC). Generally the controllers would be

named based on the type of filter inductor current

information being sensed or how the information is used to

control the switches.

PCMC is a powerful feedback controller when it comes

to fixed frequency current mode controllers. It senses the

peak filter inductor current information when the switch is

on, then uses it to turn off the switch. Controlling the turn-

off event of the switch is commonly referred to as "trailing

edge modulation". The advantage of PCMC to other fixed

frequency current controllers are it has a faster response to

load and line changes, simpler loop compensation

requirements as well as inherent peak current limit

protection [9].

However PCMC suffers from sub-harmonic oscillation at

duty cycle which is more than 50%. Furthermore, dc-dc

converter which utilizes PCMC would have a minimum on-

time limitation due to time delays required to properly sense

the peak filter inductor current when the switch is on. When

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low duty-cycle operations are demanded, the trailing-edge

PCMC has a disadvantage. Filter inductor current

information in trailing-edge PCMC is sensed across the

high-side switch. During turn-on of the high-side switch, the

switch node will exhibit a lot of ringing due to parasitic on

that node. Therefore, PCMC usually employ blanking time

before sensing the filter inductor current. Normal values for

blanking time are around 200 ns to 300 ns. Due to the blanking time requirement, trailing-edge PCMC will not be

able to regulate to very low high-side switch on-time

requirements that is less than 200ns because the on time is

shorter than the 200ns blanking time. Fig. 4 shows the

blanking period and the high side switch on time.

IL

Vsense

High side Switch on-timeBlanking

Fig. 4. Blanking Period and High side switch on time.

VCMC senses the valley filter inductor current

information when the switch is off, then uses it to turn on the

switch. Controlling the turn-on event of the switch is

commonly referred to as "leading edge modulation". VCMC

does solve the problem of minimum on time that happens in

PCMC. However it also has some disadvantages and may be

the reason why very few commercial regulators employ this

control method. For VCMC sub-harmonic instability willoccur when the duty cycle is less than 50%. Eliminating this

oscillation would require a compensating ramp which has a

slope greater than one-half of the inductor current up-slope.

Moreover, VCMC has a slower transient response to sudden

input voltage changes when compared to PCMC [10].

ACMC senses average filter inductor current as the

feedback information. The advantage of ACMC is it does

not have the problem of sub-harmonic oscillation as in

PCMC and VCMC, so there is no need for slope

compensation. The disadvantage of ACMC is it gives a

slower transient response to change of load.

HCMC senses both the peak and valley filter inductor

current information to turn the switch on and off. This

feedback control produces high ripple because it runs on a

changing frequency.

In this work a fast transient response to a change of load is

the main aim, so PCMC is used as the feedback control

circuit for the dc-dc converter.

1) Peak Current Mode Control (PCMC)

In this section, a peak current mode control (PCMC) is

explained. Fig. 5 shows the block diagram of PCMC. Here,

the feedback control loop is set, so that Verror is not

compared to a triangular waveform as in VMC, but to a

ramp signal which represents the peak filter inductor current.

As seen from Fig. 5 there are two feedbacks from the dc-dc

converter, one is the current from the filter inductor and the

other is the output voltage, Vout. Basically the Vout is

compared with a Vref and the output is Verror . In VMC the

Verror is actually compared with a triangular wave but in

PCMC the Verror is compared to a peak filter inductor current.

When the peak filter inductor current is equal to the Verror ,

then the comparator outputs a pulse 1 else the comparator

would give an output pulse 0. The pulse from the comparator

is supplied to a logic circuit. The logic circuit consists of RS

flip-flop, AND logic and NOT logic. The RS flip-flop

outputs a PWM pulse based on the clock signal and

comparator output pulse. Fig. 6 shows a clearer picture on

the positioning of pulses in PCMC. A clock signal with a

fixed frequency generates a narrow pulse to the set pin of the

RS flip-flop. At each and every clock pulse the RS flip-flop

is set to 1, causing its PWM pulse, to go high. The duration

of the high time of the PWM pulse is the duration of on time

of one of switch in the switching circuit. During this time the

peak filter inductor current rises due to charging phenomena.

When the peak inductor current IL equals to Verror , the

comparator resets the RS flip-flop and the PWM pulse goes

low. At this particular time the filter inductor appears to be

in the discharging phenomena. The discharging phenomena

occur until the RS flip-flop is set again by the fixed

frequency clock signal for the next cycle.

Vout

Vref Logic

Circuit

Clock

Signal

To

Switching

Circuit

PCMC

Verror

IL

Reset

pulse

Fig. 5. Block diagram of Feedback Control Circuit employing PCMC.

Reset

Filter inductor

current waveform

PWM pulse

t1Set

t9t8t7

Clock1

Clock 2

Pulse to switch 2

Pulse to switch 1

t6t5t4t3t2Verror

Fig. 6. Pulses in PCMC.

IV. SIMULATION RESULTS

In this study, a 300W, 25V SMPS simulation model is

developed using Matlab/Simulink simulation tool. The

SMPS is built using the half-bridge converter. Both the

voltage mode and current mode control methods are tested

separately. As suggested, in the previous sections only peak

current mode control method is studied. The main aim of

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this simulation study is to investigate the transient response

of both the control methods. For this, the load is changed

from half load to full load, which is from 6A to 12A and the

results are depicted as in Fig. 7. Table 1 shows the numerical

results based on Fig. 7 for both control methods.

Table 1 Numerical result for VMC and PCMC

VM PCM Maximum voltageundershoot

3.7 0.6V

Settling time 400 µs 200 µs

Maximum spike 2.2 null

0.0098 0.01 0.0102 0.0104 0.0106 0.0108

21

22

23

24

25

26

27

28

Time (sec.)

V o u t

( V )

PCMC Vout

VMC Vout

24.4 V

21.3 V

Fig. 7. Transient response for step change load from 6A to 12A for VMC

and PCMC.

V. DISCUSSION ON NEW CONTROL METHODBased on literature studies and simulation investigations,

following are the important points noted to derive a new

control method for fast transient response of SMPS.

a.) Based on simulation results, PCMC gives a faster transient response to step change of load compared toVMC. Among the several types of CMC explained in

this paper, peak filter inductor current leads to a better transient response of SMPS.

b.) Transient response in SMPS is actually limited by the

feedback control circuit and the filter inductor in thedc-dc power converter. The feedback control circuit

can be designed to be very fast and effective. But theimprovement in the transient response is onlyachievable if the effect of filter inductor is taken intoconsideration when designing control system.However transient response is determined by the

value of the filter inductor.c.) It is noted that lower filter inductance value helps a

lot during transient operation of SMPS. However, it

is not in favor during steady state operation wherehigh amount of load current ripple is produced.

d.) Many reviews have connected a lower inductance

filter inductor in parallel to the main filter inductor

(higher inductance), that serve the purpose as toreduce the effective inductance during step increase

of load. The voltage that is applied to the lower inductance is somehow the output voltage. Thismethod causes high ripple and step increase of load

causes the duty cycle to go beyond 50%, hence thisleads to sub-harmonic oscillations.

e.) Based on all the reviews that a new method is being

investigated known as “Voltage Injection SwitchingInductor” (VISI) method. Basically this methodinjects a high dc voltage from the input to the lower inductance filter inductor during step increase of load

.So when a high dc voltage passes through a smaller effective inductance, the rate of change of currentthat is supplied to the load would be faster. Another

added advantage of VISI is during step up of the loadthe duty cycle of the switch is eventually reducedsmaller than the steady state switching cycle. Hencethere is no problem of sub-harmonic oscillations because the duty cycle does not go more than 50 %.Initial simulation study shows that this method

improves the transient response in SMPS by 88.33%. Fig. 8 shows the result, in which the maximumvoltage undershoot is 0.07V with a settling time of

70µs and null maximum spike. Any how, a prototypeof the SMPS with the VISI as the feedback controlmethod is being developed and the authors of this paper are working towards this research direction.

5.95 6 6.05 6.1

x 10−3

24.88

24.9

24.92

24.94

24.96

24.98

25

25.02

25.04

Time (sec.)

V o u t ( V )

VISI Vout

24.93 V

Fig. 8. Transient response for step change load from 6A to 12A for VISI.

VI. CONCLUSION

In this paper, thorough literature reviews of control

methods to improve transient response in SMPS were

presented. Each review was accompanied with constructive

comments which helped to derive the challenges as far as

control method design for transient response improvement is

concerned. Base on the derived challenges, a new control

strategy was proposed in this paper. Simulation results

comparing the proposed method and two commonly used

control methods, the VMC and PCMC show conversely

improvement in the transient response. However more

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development on the proposed control method is required and

the authors are working towards this direction.

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control for switched-mode power-supply applications”, Proc.IEEElectric Power Applications, 2000, pp.486-490.

[3] Franki Ngai Kit Poon, Man Hay Pong, Joe Chui Pong Liu, “Stepping

inductor for fast transient response of switching converter”, 0ctober

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[7] Abraham I. Pressman “Switching power power supply design”,second edition, 1998, pp.237-243.

[8] Joel P.Gegner , C.Q.Lee, “Linear Peak Current Mode Control: A

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An Investigation on Control Strategies for Fast Transient Response of SMPS

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