ST AN2528 APPLICATION NOTE

AN2528

Application note

Very wide input voltage range 6 W SMPS for metering

Introduction

This document presents the design of a universal input power supply for metering applications. The design is mainly based on the following ST parts: an L6565 PWM driver and STC04IE170HP as the main switch. It is linked with the release of the STEVA LIP001Vxx demo board (see Figure 1 below). The design is a complete solution for a 5 W single output SMPS, which is widely used as a power supply in metering applications. However the design method can be applied to an SMPS suitable for other applications working on a three-phase mains and it can easily be upgraded for higher ou tput power.

The ESBT base driving circuit as well as guidelines for the optimization of the power dissipation are given.

The influence of parasitic capacitances of the transformer on the ESBT is also explained in detail.

Finally, the most important waveforms and thermal results are given in Section 5 and

Section 6. They demonstrate the benefits of using a QR flyback with ESBT.

Refer to AN1889 and AN2254 for the overall design of an auxiliary power supply using ESBT in flyback QR with L6565, while refer to AN2454 fo r the small signal power switch model with all parasitic components.

Figure 1. STEVAL-ISA030V1

July 2007 Rev 1 1/21

www.st.com

Contents AN2528

Contents

1 Design specifications and schematic diagram . . . . . . . . . . . . . . . . . . . 4

2 Flyback stage design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Parasitic capacitances and related issues . . . . . . . . . . . . . . . . . . . . . . . 8

4 Base drive circuit design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5 Experimental results: waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6 Experimental results: efficiency and further considerations . . . . . . . 15

7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

8 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2/21

AN2528 List of figures

List of figures

Figure 1. STEVAL-ISA030V1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 2. Complete schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 3. The small signal equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 4. ESBT base driving network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 5. DC current gain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 6. Dynamic collector-source saturation voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 7. 110 Figure 8. 110 Figure 9. 110 Figure 10. 110 Figure 11. 380 Figure 12. 380 Figure 13. 380 Figure 14. 380 Figure 15. 600 Figure 16. 600 Figure 17. 600 Figure 18. 600 Figure 19. V Figure 20. 110 Figure 21. 380 Figure 22. 600 Figure 23. 600

Figure 24. PCB picture top view (components and copper) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 25. PCB picture top view components and bottom layer copper . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 26. PCB picture top view components and bottom layer copper . . . . . . . . . . . . . . . . . . . . . . . 20

input voltage overall1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

input voltage overall2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

input voltage- storage highlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

input voltage turn-off highlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

input voltage overall1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

input voltage overall2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

input voltage storage time highlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

input voltage - turn-on highlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

input voltage overall1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

input voltage overall2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

input voltage turn-off highlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

input voltage turn-on highlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

vs T

comp

input voltage, max load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

input, max load: frequency reduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

input, max load: further frequency reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

input, max load: increased OFF- time highlight . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

(minimum OFF-time) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Blank

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Design specifications and schematic diagram AN2528

1 Design specifications and schematic diagram

The table below lists the converter specification data and the main parameters fixed for the demo board.

Table 1. Converter specification and p r eliminary choices

Symbol Description Values

inmin

out

η Converter efficiency @ max load > 80%

F Minimum switching frequency ≅

spike

Rectified minimum input voltage 150

Rectified maximum input voltage 850

Output voltage 14 V/430 mA

Maximum output power 6 W

30 kHz

Reflected flyback voltage 250 V

Max over voltage limited by clamping circuit 150 V

A schematic diagram of the SMPS is given in Figure 2. The most relevant components are:

1. HV ESBT main switch and simple driving circuit

2. L6565 QR PWM driver to get the best efficiency

3. Special transformer construction with very low parasitic capacitance

4/21

AN2528 Design specifications and schematic diagram

Figure 2. Complete schematic diagram

Phoenix 2 pin12

R14

14V @ 0.43A

330uF/25V

.0022uF

Y1 cap

LL4148

R11

ISO1

H11A817

R12

R24

STPS3L60U

A C

35T

CSM 2010-104

48T

334

27T

1/ 1/8W

LL4148

47k/ 1/ 8W

11K/ 1/ 8W

R21

2.4K/ 1/8W

10nF

R17

R15

1.2K/ 1/8W

1.5K/ 1/8W

STC04IE170HP

10/ 1206

R13

2.2K/ 1206

22/ 1206

10nF

4.7K/ 1/8W U2

TL431_ARC

A C

R23

4.7/ 1/4W

C11

22uF/25V

STTH112U

D4 STTH112U

A C

M600X

TR5FU S E

1 2

100K/ 1206

100K /1206

33uF/450V

STTH112U

A C

123

Phoenix 3 pin

100K/ 1206

33uF/450V

100K/1206

R10

100K/ 1206

33uF/450V

Vcc

INV

L6565

ZCD

GND

Vff

COMP

R18

22K

C10

47uF/25V

R22 330

4.7nF

3.3nF

5/21

Flyback stage design AN2528

2 Flyback stage design

Well known to all SMPS designers, the voltage stress on the device (power switch) is given by:

Equation 1

off

inmaxVflVspike

where V

= flyback voltage = (V

out

+ V

F, diode

collector due caused by leakage inductance. This over-voltage is not limited by any clamping network in order to minimize as much as possible the solution cost using also the very large margin available which has been fixed to 200 V. N primary side while Ns is the number of turns on the main output secondary winding.

Now, taking into account a 300 V margin, the maximum flyback voltage that can be chosen is:

Equation 2

BV V

– V

inmax

– V

spike

minarg

After the calculation of the flyback voltage, we can proceed with the next step in the converter design. The turns ratio between primary and secondary side is calculated with the following formula:

––=

) * Np/Ns and V

1700 850– 200– 300 350V=–=–=

is the over-voltage on the

spike

is the number of turns on the

Equation 3

------ -

p s

outVF diode,

---------------------------------------- -

350

--------------- -

14 1+

23.3===

As a first approximation, since t he turn-on of the device occurs immediately after the energy stored on the primary side, inductance is completely transferred to the secondary side:

Equation 4

• VflT

dcminTonmax

•=

reset

and

Equation 5

Where T

is the maximum on time, T

onmax

transformer inductance and T T

results in:

onmax

is the switching time. Combining the two previous formulas

– TS=

onmaxTreset

is the time needed to demagnetize the

reset

Equation 6

VflT

•

onmax

------------------------------ -

dcminVfl

14µs≅=

The next step is to calculate the peak current. The output power is set to 6 W and the desired transformer efficiency must be set by the designer (at least 80% in this case). Excluding the energy losses on the input diode bridge, on the power switch and on the secondary side rectifier, the following approximate formula can be used:

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AN2528 Flyback stage design

Equation 7

-- -

LPI

1.25 P

•

OUT

------------------------------=

••

-- -

• T

dcmin

---------------------------------------------------------- -==

PTS

•

onmax

•

Hence

Equation 8

dcmin

------------------------------------------------ -

2.5 T

•

• P

•

onmax

OUT

14.7mH==

From here we can now calculate the peak current on primary.

Equation 9

•

dcminTonmax

-------------------------------------------

143mA≈=

To keep the transformer size v ery small and to get a very effe ctiv e cost solution , we pref er to slightly increase the minimum working frequency in order to decrease the primary inductance.

In order to hav e a 15 mH indu ctance and to k e ep an EF20 co re, a lo t of turns are needed on the primary side. This can generate either not enough space on the EF20 core to accomodate such a high number of windings or the remaining space is not large enough to ensure good design. These considerations might induce designing a smaller primary inductance value accepting a higher switching frequency.

There is no contraindication in using a smaller primary inductance which leads to a higher minimum switching frequency and theoretically also to a higher maximum frequency. Howev er the maxim um switching frequency is then limited not only by the inductance value, but also by the L6565 PWM driver. When using an L6565, the internal blanking time limits the minimum off-time and, in turn, the maximum switching frequency. To better understand this phenomenon, please refer to the L6565 datasheet and to the next paragraphs.

After bench tests and fine tuning we used a transformer with the following specs:

Equation 10

7.5mH=

Equation 11

------ -

23.8=

------------

aux

18.87=

The part number of the transformer is CSM 2010-104 from Cramer. In the next Section 3, we see from bench verification that the real minimum working

frequency is 50 kHz even if the inductance is 7.5 mH but with a peak current of about 250 mA.

7/21

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