188

AMSE JOURNALS-2016-Series: Modelling A; Vol. 89; N°1; pp 188-204

Submitted July 2016; Revised Oct. 15, 2016, Accepted Dec. 10, 2016

Research of a High-Precision High-Power-Factor Switching Power

Supply

*Zhi Cui, **Xianpu Cui

* School of Communication and Electronic Engineering, Hunan City University

China, No. 518, East Yingbin Road, Yiyang, (zhicui@yeah.net)

** School of Communication and Electronic Engineering, Hunan City University

China, No. 518, East Yingbin Road, Yiyang, (cuixianpu@sohu.com)

Abstract

We design a novel type of switching power supply which is an integration of flyback type

and half-bridge resonant typology. Based on signal flow graph and division of functional modules

of the circuit, we elaborate on the design principle, functions of different modules and working

process of the switching power supply. By using three-terminal adjustable shunt regulator TL431,

LD7535 and L6599, the voltage control of the power supply and voltage stabilizing are realized

by regulating the pulse width and pulse frequency, respectively. The power factor is increased by

adopting active power factor correction. Experiment shows that the switching power supply has

good voltage stabilizing performance, with small ripple and high power factor as well as high

voltage regulation and load regulation.

Key words

Switching power supply, fly back, half-bridge resonant typology, high precision

1. Introduction

Switching power supply is a DC voltage-stabilizing power supply which uses the switching

regulator [1]. It regulates the output voltage by adjusting the switching frequency or duty cycle.

Because of its small size, light weight and high frequency, the switching power supply is applied

in nearly all electronic devices and plays an irreplaceable role in today’s electronic information

industry [2]. In the meantime, the requirement on the switching power supply is also rising in

other new fields. Addressing the defects of low precision and low power factor in ordinary

189

switching power supply, we design a novel type of switching power supply that is the

combination of flyback type and half-bridge resonant typology. By designing closed-loop

feedback control and employing active power factor correction, the precise control of the output

voltage and the improvement of power factor are realized.

Switching power supply consists of input circuit, power factor correction circuit, pulse

control circuit, power conversion circuit, output circuit, and feedback circuit. In our design, the

first module is the input circuit. The 220V AC current passes through the protection circuit, EMI

suppression filter and bridge rectifier, and the unstabilized DC voltage is obtained. This voltage is

subject to power factor correction in the second module, which maintains the same phase

between the input current and the input voltage. The third module is the power conversion circuit,

which uses the switching tube to convert DC voltage into a pulse waveform with certain

frequency and to transmit the energy to the output terminal. The fourth module is the output

circuit, where the square wave pulse voltage is rectified, filtered and converted into DC voltage.

The fifth module is the feedback control circuit, where the output voltage passes through the

voltage divider and the sampler and is compared against the reference voltage and amplified. The

feedback circuit incorporates the precision shunt regulator TL431 and optical coupler PC817C.

Upon receiving the output voltage feedback, the control chip will output the pulse width

modulation (PWM) signals, thus achieving high-precision voltage stabilizing.

2. Functional requirements and technical indicators

The purpose of this paper is to design a novel type of switching power supply which is an

integration of flyback type and half-bridge resonant typology.

2.1 Efficiency and power factor of the power supply

Within the given range of input voltage and temperature, the table below presents the target

range of output efficiency and power factor of the power supply. To calculate the overall

efficiency, the output power is first obtained based on the product of output current and output

voltage under the rated input voltage and output full load. Then the ratio of the output power to

the input active power on the power meter is calculated as the overall efficiency. The power

factor is the ratio of the input active power to the output apparent power [3].

190

2.2 Output voltage and current

Within the given range of input voltage and temperature, the table below presents the target

values of output current, voltage, ripple and noise. Ripple is a component synchronized with the

input frequency and switching frequency between the output terminals. Ripple is usually

expressed as the peak-to-peak value and should be below 0.5% of the output voltage. Noise is

another high-frequency component besides the ripple and should be about 1% of the output

voltage [4].

Tab. 1 Efficiency and power factor of power supply

Input voltage Output load Efficiency PF value

90VAC rating ≧84% ≧98%

264VAC rating ≧88% ≧93%

Tab. 2 Output voltage and current

V1 V2 V3 V4

Output Voltage +12V +24V +5V +5Vsb

Peak Current 2.5A 5A 3.5A 1A

Rated Current 2A 4A 3A 0.5A

Voltage

Regulation

Factor

±5% ±5% ±5% ±5%

Ripple 60mV 180mV 30mV 30mV

Ripple and

Noise 120mV 240mV 50mV 50mV

3. Detailed Process of Design

In this section, we will present the detailed process of design.

3.1 Input protection circuit

Input protection circuit consists of tube fuse, negative temperature coefficient (NTC)

thermistor and voltage dependent resistor. When the power supply has just started up, NTC

thermistor has low temperature and high resistance, offering instantaneous restraint to the

charging current [5]. As the heat dissipated by the current increases, the resistance of NTC

191

thermistor decreases rapidly. Thus NTC thermistor is started up and the power consumption is

reduced.

Voltage dependent resistor absorbs the voltage surges from the grid, which are generated

due to disturbances from the electrical equipments in the grid or natural lightning. Voltage surges

can take place within a very short time and reach very high values, causing the fuse and other

components in the power supply to burn out. Therefore, it is necessary to apply the voltage

dependent resistor across the two ends of the input voltage, so as to divide and absorb the voltage

and to protect the circuit.

1

3

CN1 AC INPUT

1 2

F1 T5A-L250V

T

NTC15D-11/13

CX1

0.47uFK/275V-X2

L

N

ZNR1 14D561K

Fig.1. Input protection circuit

3.2 EMI suppression filter

EMI. suppression filter is usually designed with a common mode inductor and a filter

capacitor. Common mode inductor is composed of two winding resistors with equal inductance in

a closed magnetic circuit [6]. The phase difference is 180 degrees due to the magnetic flux

generated by the frequency components of the power supply. Since the two resistors have equal

number of windings, they counteract each other and the inductance of the frequency components

of the power supply is zero. However, for the common mode noise, the effective permeability is

very high, leading to large attenuation.

14

23

LF1

*CY1

470pF/400VAC-X1Y1

CY2

470pF/400VAC-X1Y1

-4

AC

3

+1

AC

2 BD1

GBJ1006

R3

3.3M

R4

3.3M

R1

3.3M

R2

3.3M

14

23

LF2

*

CX2

0.22uFK/275V-X2

ZNR2

*14D561K

Fig.2. EMI suppression filter

192

3.3 Active power factor correction circuit

Power factor is the ratio of active power to apparent power [7]. In the electronic devices

containing AC/DC converter, the power supply for the DC/DC converter or DC/AC converter is

usually the DC voltage obtained by rectifying AC mains power and large-capacitance filtering.

The filter capacitor makes the output voltage smooth and the output current a spike pulse. If there

is no filter circuit after the rectifier circuit but only the resistive load, the input current will be the

sine wave having the same phase as the voltage of the power supply and a power factor of 1. The

basic principle of the active power factor correction circuit is to isolate the rectifier from the filter

capacitor, thus turning the capacitive load into resistive load in the rectifier circuit.

Power factor correction falls into two categories, active and inactive [8]. Active power factor

correction (APFC) circuit has an active power controller connected in series between the rectifier

and the output capacitor. As a result, the input current and input voltage of AC/DC converter will

be sine waves having the same frequency and the same phase. The input current is forced to go

with the input voltage, thus realizing unity power factor. APFC can improve the power factor and

overall efficiency of the switching power supply and prevent harmonic pollution of the grid. We

use analog integrated circuit L6562 for APFC.

193

HV1

C92

39 uF450V

D90

STTH10LCD06FP

HV C88

1uF50V

C93

39uF450V

HV1

R83

22K

C86

47pF1 KVR8910K

R840R( 470R)

NC6

FB4

MOS3

O

O

GND5

HV1

NC2

L81

SKY-DY201C

R8568K( 20K)

C810. 047uF200V

HV

G

DS

Q81

STP14NM50ZFP

C2CBB21-1uF/450V

INV1

COMP2

MULT3

CS4 ZCD 5

GND 6

GD7

VCC8

U81

L 6562D

C84

22pF50V

C83

0.01uF50V

R81

9K1( 10K)-F

VCC

R81M( 750K)

R101M(750K)

R91M( 750K)

C82

1uF50V

R8210K

C85

0.47uF50V

R97510K

R99510K

R98510K

R93

1R

R94

1R

R95

1R

R96

*1R

R9010R(4R7)

2 R92

0. 075R/2W

L82

FB

C90

100pF1KV

C91

39uF450V

1

R88

1R/1W

R86470R

Q82

PM3906

R9122R

R8747R

L 90

FB

Q 41

Fig.3. L6562 circuit

As shown in Fig. 3, the output voltage HV of the APFC passes through the sampling resistor

and then into the inverting input of the error amplifier through the pin INV. The reference voltage

of 2.5V is input to the non-inverting terminal. After amplification, it is input into the multiplier

M2. The AC voltage which has passed through the full-bridge rectifier is then sampled by the

voltage divider of the sampling resistor. It is input into multiplier M1 through the MULT pin. The

output voltage of the multiplier is proportional to the product of M1 and M2. The series

connected source resistor of power MOSFET is responsible for the sampling of the peak current

of drain voltage-increasing inductor L. It is input into the error amplifier via CS pin and

compared with the output voltage of the multiplier. When the voltage of the CS pin reaches the

threshold value, that is, when the current reaches the peak in L, the PWM comparator will stop

driving the gate of MOSFET.

194

3.4 Flyback typology

Flyback circuit refers to the following condition. When the switching tube is conducted,

driving the primary side of the pulse transformer, the secondary side of the transformer does not

supply power to the load [9]. This is the alternating conduction and disconnection of the primary

and secondary side. But due to leakage inductance of the transformer, the primary side will have

voltage spikes, causing the breakdown of the switching device. Therefore, it is necessary to

install the RCD clamp circuit. Single-terminal flyback driver circuit can satisfy the requirement

as a small-power high-frequency switching power supply. Furthermore, since the transformer in

the flyback typology switching power supply plays the dual roles of inductor and transformer,

only the filter capacitor needs to be selected, but not the filter inductor. The circuit structure is

simple. The typical isolated flyback driver circuit is shown in Fig. 4.

Trans

DC

GND

VOUT

LD7535

Fig.4. Isolated flyback driver circuit

LD7535, the pulse-width modulator, outputs the PWM signal to control the conduction and

disconnection of MOSFET. The working frequency is 50-130KHz and it is adjusted by grounding

a resistor through pin 3. The working frequency is 100K, and the corresponding switching

frequency is 65KHZ. When MOSFET is conducted, it stores energy in the primary coils of the

transformer. The diode connected to the secondary side of the transformer is in reverse biased

state and so the diode is disconnected. No current flows through the secondary circuit of the

transformer, and therefore no energy is supplied to the load. When MOSFET is disconnected, the

voltage polarity in the secondary coils of the transformer is reversed, thus conducting the diode

and charging the output capacitor. In the meantime, the current flows through the load.

3.5 Half-bridge resonant typology

Resonant power supply represents the new trend of switching power supply. The sine wave

is generated by the resonant circuit, and the switching tube is turned on and off during zero

195

crossing of the sine wave. Therefore, the MOS tube of the half-bridge circuit is alternately

conducted and disconnected. By regulating the switching frequency, the average output voltage

on the secondary side of the transformer can be changed. The half-bridge resonant typology

combined with APFC can achieve a power factor above 0.95, thus greatly inhibiting harmonic

pollution of the grid [10].

C701uF50V

R650R(1K)

C600.1uF50V

CSS1

DELAY2

CF3

RFMIN4

LVG11

STBY5

ISEN6

DIS8

LINE7

PFC_STOP9

NC13

OUT14

HVG15

VBOOT16

VCC12

GND10

U61

L6599A

R6712K

C68330pF(100pF)1KV

D66BAV99

HV

R71A0R(24K)

R7118K(24K)

G1

D2

S3

Q61

SVD7N60F

R7710K

G1

D2

S3

Q62

SVD7N60Fn

c

ZD61LBZX84C16LT1

nc

D63LMBD914LT1

VCC

C670.01uF50V

R63

2K4+330R(4K7)

HV

R6415K(24K)

C69

CBB21-0.022uF/630V

C63470pF50V

R681M

R701M

R691M

R66220R(820R)

C640.1uF50V

C65NC(100uF35V)

R614K7(12K)

C611uF50V

R62

750K(1M)

C620.47uF25V

R78A

NC(1K2)

Q64NC(S8550LT1)

R7847R(150R)

R75

10KR76A

NC(1K2)

Q63NC(S8550LT1)

R75ANC(20R)

R7647R(150R)

R77ANC(20R)

R720R(20R)

C71

Fig.5. L6599 circuit

L6599 is a double-ended controller specific for half-bridge resonant typology, outputting

signals with phase difference of 180° and 50% duty cycle. Unlike PWM controller, energy is

transmitted by adjusting the duty cycle. For the half-bridge resonant typology, the duty cycle is

fixed, so the energy transmission is controlled by the switching frequency. The output voltage is

controlled by the working frequency. As shown in Fig. 5, a resistor RF min is grounded via pin 4

for configuring the lowest oscillation frequency. Resistor RF max is grounded via pin 4. Photo-

coupled grounding is controlled by the feedback circuit and the oscillation frequency of the

controller is adjusted along with the output voltage. RF max is for configuring the highest

working frequency.

196

3.6 Output rectifier and filter circuit

The output voltage of the switching power supply needs to be rectified and filtered. Schottky

rectifier diode depends on the working of most charge carriers and can be conducted and

disconnected easily. It has a small positive voltage drop, which decreases with higher

temperature. Therefore, the loss arising from conduction is reduced. The following principles

should be adhered to when choosing the parameters of the output rectifier tube. The rated current

should be at least three times that of the maximum output current of the circuit. The working peak

reverse voltage should be higher than the minimum permissible voltage.

Output filter converts the AC square waves into DC current. As shown in Fig. 6, the rectified

waveform is directly input into the filter and smoothed into DC waveform by high-capacitance

filtering. L51 and C55 make up the post-filter that smooths the waveform and reduces ripple.

GND

D51STPS20LCD45CFP

C53

1000uF16V

L51 2uH

GND

C760.1uF50V

GND

C55

470uF16V

GND GND

+12V

C570.1uF50V

GND

C54

NC(1000uF16V)

GND

GND

D71MBRF20100CT

C73

1000(470)uF35V

L71 2uH

GND

R504K7

GND

C75

470uF35V

+24V

GND

C74

1000(470)uF35V

R7310K

R7410K

L52 FB

L72*FB

Fig.6. Output rectifier and filter circuit

3.7 Feedback voltage stabilizing circuit

Closed-loop feedback is used to stabilize the output voltage. Optical coupler is used for

input sampling, signal feedback and driving output. The feedback circuit is designed as shown in

Fig. 7. TL413 is a precision voltage stabilizer that stabilizes the voltage at pin 2 of the optical

coupler PC817C at 2.5V. When the output voltage of 24V increases, the voltage of pin 2 remains

constant, while the voltage of pin 1 increases. As a consequence, the light emitting device is

conducted and gives off light. The light receiver is also conducted and the voltage of pin 4

decreases. On the contrary, when the output voltage of 24V decreases, the light receiver is

disconnected and the voltage of pin 4 increases. The voltage level of pin 4 of PC817C is feedback

to the control chip. By regulating the duty cycle, the voltage is stabilized.

R54 and R56 are the output sampling resistors. They divide the output voltage by controlling

the shunt from the cathode to the anode via the REF terminal of TL431. This current directly

197

drives the light emission of the optical coupler. When the output voltage increases, Vref increases

as well, leading to increased current flowing through TL431. As a result, the light emitted by the

optical coupler becomes stronger and the feedback voltage at the light sensing terminal increases

as well. The PWM control chip, upon receiving this feedback voltage, will change the switching

time of MOSFET and the output voltage will drop.

R513K9

A1

K2

C4

E3

U62

PC817B

R534K7

C510.01uF50V

R566K8-F

GND GND

R1

A2

K3

U63

TL431

R521K

+12V +24V

R5420K-F

R576K8-F

GND

R5582K-F

Fig.7. Feedback circuit

4. The Main Hardware Circuit

The circuit diagram consists of three parts, namely, input circuit and APFC module,

flyback typology, half-bridge resonant typology and PCB design. For each part, the circuit

diagrams are shown in Fig. 8, Fig. 9, Fig. 10, and Fig. 11, respectively.

198

HV1

C92

39uF450V

D90STTH10LCD06FP

HV C88

1uF50V

C93

39uF450V

HV1

R8322K

C8647pF1KV

R8910K

R840R(470R)

NC6

FB4

MOS3

O

O

GND5

HV1

NC2

L81SKY-DY201C

R8568K(20K)

C810.047uF200V

HV

G

DS

Q81

STP14NM50ZFP

C1CBB21-1uF/450V

C2CBB21-1uF/450V

L1

150uH

INV1

COMP2

MULT3

CS4

ZCD5

GND6

GD7

VCC8

U81L6562D

C84

22pF50V

C830.01uF50V

R81

9K1(10K)-F

VCC

R81M(750K)

R101M(750K)

R91M(750K)

C821uF50V

R8210K

C850.47uF50V

R97510K

R99510K

R98510K

R931R

R941R

R951R

R96*1R

R9010R(4R7)

2 R920.075R/2W

L82

FB

C90100pF1KV

C91

39uF450V

1 R881R/1W

R86470R

Q82

PM3906

R9122R

R8747R

L90FB

1

3

CN1

AC INPUT

14

23

LF1

*

1 2

F1T5A-L250V

T

NTC15D-11/13

CY1470pF/400VAC-X1Y1

CY2470pF/400VAC-X1Y1

-4

AC

3

+1

AC

2

BD1

GBJ1006CX1

0.47uFK/275V-X2

R3

3.3M

R4

3.3M

Text

L

N

R1

3.3M

R2

3.3M

14

23

LF2

*

ZNR114D561K

CX20.22uFK/275V-X2

ZNR2*14D561K

GN

D4

GN

D1

GN

D2

GN

D3

HS2KYX-TXT-022A

HV

VCC

HV1HV1

HV

VCC

Fig.8. Input circuit and APFC module

HVGND

6

FB7

HV2

MOS4

5V8

NC9

O

O

O

GND10

T1SKY-BCK-TD2215

GND1

COMP2

RF3

CS4

VDD5

GATE6

U1

LD7535M

1

R18100K/1W

C18

4.7nF400V

D14

HER207

R2610K

R24470R

nc D11

LMBD914LT1

C16

0.1(0.01)uF50V

R21100K

R23

1K

R274R7

Q3S8550LT1

Q2PM3904D

R2520R

R22150R

C1310uF50V

nc

ZD11LBZX84C16LT1

C15100pF50V

R141K(3K)

D13FR107

R11510K

R13510K

R12510K

C12

100(47)uF50V

L11FB

R284R7

R294R7

R303R3

R152R2

Q4

*PM3904D

C100.1uF50V

G1

D2

S3

Q1

SVD4N65F

VDD

VDD

VDD1

nc

ZD15*LBZX84C16LT1

nc

D10LMBD914LT1

C1122uF50V

Q5*PM3904D

C17

*1uF50V

R16*120K

C140.1uF50V

R17*39K

C9

*22uF16V

VDD1

R40*750K

R42*750K

R41*750K

C19

100pF1KV

C43

1000uF10V

D41

STPS20LCD45CFP

R4420R

C411nF50V

C48

470uF10V

R4320R

C470.1uF50V

+12V

C46

0.1uF50V

C450.22uF50V

nc

D42

LMBD914LT1

R45100K

G

D S

Q41

AOD452A

C42

1000uF10V

L412uH

GND

C44

470uF10V

+5VSB

GNDGND

+5V+5V1

GNDGND

GND

D41

D41

GND GND

A1

K2

C4

E3

U2

PC817B

R3310K

C310.47uF50V

R344.7K-F

R1

A2

K3

U3

TL431

R321K

R31220R

R0*0R

+5V1

+5VSB

R374.99K-F

C202.2nF400VAC

HV GND

1R20*4M/1W

GN

D4

GN

D1

GN

D2

GN

D3

HS1KYX-TXT-023A

+5VSB

HV

GND1

2

3

4

5

6

7

8

9

10

11

12

13

CN3

TJC3-13A-W

PSON

GND

+12V

+12V

+5VSB

GND

GND

GND

+5V

+5V

+5V

+5V

+12V

PSON

VDD1

HV1HV1

HV

+12V

+5VSB

VDD1

Fig.9. Flyback typology

199

C701uF50V

R650R(1K)

C60

0.1uF50VCSS

1

DELAY2

CF3

RFMIN4

LVG11

STBY5

ISEN6

DIS8

LINE7

PFC_STOP9

NC13

OUT14

HVG15

VBOOT16

VCC12

GND10

U61

L6599A

R6712K

C68330pF(100pF)1KV

D66BAV99

HV

R71A0R(24K)

L61FB

R7118K(24K)

G1

D2

S3

Q61

SVD7N60F

R7710K

G1

D2

S3

Q62

SVD7N60F

nc

ZD61LBZX84C16LT1

nc

D63LMBD914LT1

VCC

C670.01uF50V

R63

2K4+330R(4K7)

HV

R6415K(24K)

C69

CBB21-0.022uF/630V

C63470pF50V

R681M

R701M

R691M

R66220R(820R)

C640.1uF50V

C65NC(100uF35V)

R61

4K7(12K)

C611uF50V

R62750K(1M)

C620.47uF25V

R60

NC(100K)

R78A

NC(1K2)

Q64NC(S8550LT1)

R7847R(150R)

R75

10KR76A

NC(1K2)

Q63NC(S8550LT1)

R75ANC(20R)

R7647R(150R)

R77ANC(20R)

R720R(20R)

C71

NC

(10

0p

F1

KV

)

C661nF50V(NC)

MOS6

CAP7

GND3

GND5

GND14

12V8

GND4

12V9

GND13

O

OO

24V11

GND2

O

24V10

GND12

O

T61SKY-BCK-TGW4210

GND

D51STPS20LCD45CFP

GND

D71MBRF20100CT

GND

R513K9

A1

K2

C4

E3

U62

PC817B

R534K7

C510.01uF50V

R566K8-F

GND GND

R1

A2

K3

U63

TL431

R521K

+12V +24V

R5420K-F

R576K8-F

GND

R5582K-F

GND GND GND

GND GND

+12V

GND GND

GND GND GND

+24V

GND

C53

1000uF16V

L512uH

C760.1uF50V

C55

470uF16V

C570.1uF50V

C54

NC(1000uF16V)

C73

1000(470)uF35V

L712uH

R504K7

C75

470uF35V

C74

1000(470)uF35V

R7310K

R7410K

L52 FB

L72*FB

D51

D71

D71

D51

GN

D4

GN

D1

GN

D2

GN

D3

HS3KYX-TXT-023A

GNDGND GNDGND

1

2

3

4

CN4TJC3-4A-W

+24V

+24V

GND

GND

HV

+12V

+24V

VCC

GND

GND

GND

GND

1

2

3

4

5

6

7

8

CN5TJC3-8A-W

+24V

HV

+12V

+24V

VCC

Fig.10. Half-bridge resonant typology

Fig.11. PCB design

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5. Experiment and discussion

The performance test of switching power supply is important for performance evaluation.

The test items include overall efficiency, power factor, cross loading, ripple and noise, output

voltage overshoot and temperature rise. Here the equipment tested is composed of 3KW AC auto

transformer, CHROMA 650 electronic load, TDS3032B oscilloscope and P6021 current probe.

5.1 Overall efficiency and power factor

Overall efficiency and power factor are tested to see if they satisfy the requirement. Under

AC220V/50HZ, the overall efficiency should be no less than 85% and the power factor no less

than 0.92 when the overall power of the power supply is smaller than 1500W.

Tab. 3 Test result of overall efficiency and power factor of the power supply

Full Load（%） 80% Load（%） Half Load（%）

PF Efficiency PF Efficiency PF Efficiency

90V 99.7 86.01 99.7 86.90 99.6 86.87

120V 99.5 88.24 99.6 88.45 99.3 88.10

160V 99.5 89.22 99.2 89.40 98.3 88.68

220V 98.3 89.98 97.7 89.99 96.8 89.03

264V 96.7 90.16 95.2 90.21 94.3 89.03

As shown in Tab. 3, the minimum efficiency is 86.01% and the maximum is 90.21% under

different loads; the minimum power factor is 94.3% and the maximum power factor is 99.7%.

The performance is excellent and satisfies the requirement.

5.2 Cross loading test

Cross loading test is to evaluate the voltage regulation capacity under unbalanced load. It is

checked whether the output voltage varies with the load and whether the variation range of the

output voltage exceeds the specified value.

As shown in Tab. 4, when output voltage is 5V and 12V (heavy load) and the output voltage

is 24V (light load), the actual output voltage is 24.64V under the load of 24V. Thus, under

unbalanced load with multi-route output, the output voltage fluctuates greatly. We use the TL431

precision shunt regulator for feedback regulation and achieve satisfactory effect.

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5.3 Ripple and Noise

Ripple and noise are tested as well. The high-frequency and low-frequency ripple and noise

are displayed on the oscilloscope. For this test, a 47uF capacity together with a 0.1uF capacitor is

connected to the voltage probe of the oscilloscope. The test is performed at broadband 20MHz

AC mode. Under input voltage of 240V and full load condition, the ripple and noise of the three

output routes are shown as follow.

Tab. 4 Result of cross loading test

5V 12V 24V 5V 12V 24V

0.1A 0.3A 0.2A 5.16V 11.97V 24.22V

0.1A 0.3A 4.0A 5.16V 12.04V 23.91V

0.1A 2.0A 0.2A 5.15V 11.88V 24.63V

3.0A 0.3A 0.2A 5.12V 11.97V 24.24V

0.1A 2.0A 4.0A 5.15V 11.95V 24.30V

3.0A 0.3A 4.0A 5.12V 12.04V 23.93V

3.0A 2.0A 0.2A 5.11V 11.88V 24.64V

3.0A 2.0A 4.0A 5.11V 11.95V 24.32V

Tab. 5 Test result of ripple and noise

Load Ripple Ripple and Noise

5V 3A 09.6mv 15.6mv

12V 2A 37.8mv 52.8mv

24V 4A 117.0mv 181.0mv

Fig. 12 5 V ripple Fig. 13 12V ripple Fig. 14 24V ripple

Ripple is a component synchronized with the input frequency and switching frequency

between the output terminals. Expressed as the peak-to-peak value, ripple is usually below 0.5%

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of the output voltage. Noise is a high-frequency component between the output terminals and its

value is about 1% of the output voltage. Ripple noise is the synthesis of the two and generally

below 2% of the output voltage. As shown in Tab. 5, the ripple and noise in all three output

routes satisfy the requirement.

5.4 Output voltage overshoot and time-to-climb

Output voltage overshoot and time-to-climb are the peak values causing the changes of DC

voltage when the power supply is turned on or off. When the power supply is turned on, the

voltage overshoot and time-to-climb are recorded with the oscilloscope. Under the input voltage

of 220V and full load conditions, the output voltage overshoot and time-to-climb are measured as

follows.

Tab. 6 Output voltage overshoot and time-to-climb

Output Voltage Load Current Overshoot Rise Time

+5V MAX 0.000% 01.6ms

+12V MAX 3.840% 06.8ms

+24V MAX 2.789% 05.6ms

Fig. 15 5V overshoot Fig. 16 12V overshoot Fig. 17 24V overshoot

According to relevant standards, the overshoot of the power supply should not exceed ±10%

of the output voltage. In Tab. 6, the maximum overshoot is 3.84%, which satisfies the

requirement.

Conclusion

Switching power supply is now considered a substitute for linear power supply due to its

various advantages. We design a novel type of switching power supply with three outputs (5V,

12V, 24V) by combining flyback circuit and half-bridge resonant typology and using TL431

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circuit and PC817 optical coupler. The experiment shows that this switching power supply has

good voltage stabilizing performance, with small ripple, high power factor, high voltage

regulation and high load regulation. Compared with ordinary switching power supply, the

proposed power supply has higher precision of output voltage, higher power factor, smaller

ripple, lower load regulation and voltage regulation. Moreover, the power is larger and the load-

carrying capacity is improved.

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