ST AN2454 APPLICATION NOTE

AN2454

Application note

Universal input voltage power supply for ESBT based

breaker and metering applications

Introduction

This document describes how to design a 3-phase power supply with the UC3845B PWM driver and the new STC04IE170 ESBT as main switch. It is associated with the release of the STEVAL-IPB001V1 demo board (see figure below). The design is a complete solution for the 2 W single output SMPS, which is widely used as a power supply in breaker applications. However, the design method can also be applied to an SMPS suitable for 3phase power metering applications, as it can easily be upgraded for higher output power.

In this report particular attention has been paid to the ESBT base driving circuit, where some useful methods have been investigated to better optimize power dissipation (see

Section 6: Base driving circuit design).

The influence of the parasitic capacitances of the transformer on the ESBT is also explained in detail (see Section 3: Parasitic capacitances and related issues). In addition, an active start-up circuit has been implemented on the demo board to optimize the converter efficiency, is also described (see Section 7: Active start-up circuit). A dedicated active component (the Darlington Q3) has been developed to support the very high voltage required (see Figure 1).

Finally, the most important waveforms and thermal results are given in Section 8:

Experimental results: waveforms and Section 9: Experimental results: efficiency and special considerations. They demonstrate the benefits of using this solution with the start-up circuit.

Refer to AN1889 for the overall design of an auxiliary power supply using an ESBT.

STEVAL-IPB001V1 demo board

December 2006 Rev 1 1/24

www.st.com

Contents AN2454

Contents

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

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

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

4 Fine tuning of the application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5 Transformer design characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6 Base driving circuit design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

7 Active start-up circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

8 Experimental results: waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

9 Experimental results: efficiency and special considerations . . . . . . 19

10 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

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AN2454 List of figures

List of figures

Figure 1. Schematic diagram of the SMPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 2. Small signal equivalent circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 3. Current sense circuit (a) and waveform of sense resistor ( b) . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 4. The normal operation waveforms of output pulse and current spike . . . . . . . . . . . . . . . . . 10

Figure 5. ESBT driving circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 6. h

Figure 7. Start-up circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 8. Minimum input voltage: storage highlighted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 9. Minimum input voltage: switch-on highlighted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 10. Minimum input voltage 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 11. Minimum input voltage 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 12. Minimum input voltage: switch-off highlighted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 13. 560V input voltage 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 14. 560V input voltage 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 15. 560V input voltage: switch-on highlighted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 16. 560V input voltage: switch-off highlighted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 17. 1050V input voltage 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 18. 1050V input voltage 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 19. 1050V input voltage: switch-off highlighted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 20. 1050V input voltage: switch-on highlighted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 21. 110 Vac envelope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 22. 220 Vac envelope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 23. 420 Vac envelope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 24. 480 Vac envelope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 25. 600 Vac envelope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 26. 760 Vac envelope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 27. Top view of the STEVAL-IPB001V1 demo board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 28. Bottom view of the STEVAL-IPB001V1 demo board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

curve from datasheet STC04IE170HP, section 2.1, figure 3 . . . . . . . . . . . . . . . . . . . . 12

3/24

Design specifications and schematic diagram AN2454

1 Design specifications and schematic diagram

Ta bl e 1 lists the converter specifications and main parameters of the STEVAL-IPB001V1

demo board.

Table 1. Converter specifications and main parameters of the STEVAL-IPB001V1

demo board

Symbol Description Values

inmin

inmax

out max

out min

out

Rectified minimum input voltage 150

Rectified maximum input voltage 1200

Output voltage 24V/83mA

Maximum output power 2W

Minimum output power 0.2W

η Converter efficiency 60%

F Switching frequency ≅ 50kHz

spike

Max over voltage limited by clamping circuit 150V

Reflected flyback voltage 150V

A schematic diagram of the SMPS is given in Figure 1 The most relevant components are:

● HV ESBT main switch and simple driving circuit (see Section 6: Base driving circuit

design).

● Active start-up circuit with HV bipolar Darlington (see Section 7: Active start-up circuit) .

● A specially constructed transformer, with very low parasitic capacitance.

4/24

AN2454 Design specifications and schematic diagram

Figure 1. Schematic diagram of the SMPS

CON3

8 2

R22 10k

STTH110

R26 10

C7 10nF

R3 1

4 2

R17 22 R19 1k

C9 820pF

R15 3.3k

STTH110

1 2 3

R14 18k

R20 100k

33uF450V

33uF 450V

C8 100p

R16 150k

R21

3.9K

R1 1M 1/8W

R4 1M 1/8W

R7 1M 1/8W

R9 1M 1/8W

R18 2.2k

R11

1M 1/8W

C10 12nF

R12

1M 1/8W

R13 680k

PN2222A

VFB

COMP

VREF

RT/CT

UC3845B

VCC

SENSE

GND

OUT

100nF

C11

6 3

R2 10k 1/4W

R5 10k 1/4W

R8 10k 1/4W

R10 10k 1/4W

R24 10k 1/4W

R25 10k 1/4W

Q3 STP03D200

C1 220uF 35V

R6 10k

STC04IE170HP

R23

6.8

CON2

2 1

47uF 25V

1N4148

C3 330uF 25V

D5 20V

5/24

Flyback stage AN2454

2 Flyback stage

In this section, only the main steps of the flyback stage are given. For more detailed guidelines on Discontinuous Conduction Mode (DCM) flyback converter design, refer to AN1889.

First, the transformer turn ratio, N

, must be calculated. NP and NS are the respective

P/NS

number of primary and secondary windings. Calculation of the turn ratio is correlated to the maximum voltage rating of the transistor which is used as the primary switch. The voltage of the power switch collector, V

, for flyback operation is given by:

Equation 1

-------

+()• Vfl==

oVFdiode,

+()V

oVF diode,

where

minarg+++=

spike

the flyback voltage

where V

VTV

is the over voltage limited by the clamp network. It must be chosen so that the

spike

-------

dcmax

total voltage across the power switch does not exceed the maximum breakdown voltage of the power switch device (see Equation 1).

Once the V

voltage is fixed, the designer must choose the flyback voltage taking

spike

account of various voltage capabilities available from standard transistors. The higher the flyback voltage the higher the exploitable maximum duty cycle i.e. a higher duty cycle at fixed output power leads to a lower I

current. This improves overall efficiency of the

RMS

primary side, leading to easier design of wide input range voltage converters.

ESBTs, which have breakdown voltage capabilities as high as 2200V, offer designers a valuable tool to simplify projects from an early stage.

For the STEVAL-IPB001V1 demo board using the STC04IE170HP switch, the following parameters must be set:

● Margin = 200V.

● V

From Equation 1, the flyback vlotage (V

spike

= 150V.

) gives a result of 150 V. The transformer turn ratio

may then be calculated using Equation 2.

Equation 2

– V

BV V

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

-------

6/24

dcmax

oVFdiode,

– minarg–

spike

1700 1200– 150– 200–

---------------------------------------------------------------- 6=== 24 1+

AN2454 Flyback stage

Once the turn ratio is calculated, the system must be stabilized to ensure that the converter operates in discontinuous mode. Equation 3 guarantees that the energy on the primary coil will be completely transferred to the secondary coil before the next cycle occurs.

Equation 3

dcminTonmax

-------

+()T

oVF diode,

resetVflTreset

A safety margin of 20% is recommended to guarantee the complete demagnetization of the primary side (see Equation 4).

Equation 4

– 0.8T

onmaxTreset

where T transformer inductance, and T

Combining Equation 3 and Equation 4 , T

is the maximum power-on time, T

onmax

the switching time.

the time needed to demagnetize the

reset

, may be calculated using Equation 5:

onmax

Equation 5

Vfl0.8T

onmax

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

dcminVfl

Once output power has been set to 2 W and the desired efficiency to 60%, the operating switching frequency must be chosen. To do this, a value of 50 kHz should be selected. It is then necessary to calculate the primary inductance (L

6, input power (P

) may be calculated to give an approximate value which does not account

) of the transformer. Using Equation

for losses due to the power switch, the input bridge and the rectified network.

Equation 6

Using Equation 7, L

-- -

1.66P

may be calculated as follows:

OUT

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

LPI

•

-- -

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

onmax

LPT

Equation 7

dcmin

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

3.33TSP

onmax

11m H≈=

OUT

Peak current, (I

) on the primary side may be calculated using Equation 8.

Equation 8

dcminTonmax

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

110mA≈=

It is also important to determine the maximum primary current, I and maximum secondary current, I

rms(secondary)

, (see Equation 10) to obtain correct

dimensions for the wire size of the primary windings.

rms(primary)

, (see Equation 9)

7/24

Parasitic capacitances and related issues AN2454

)

Equation 9

onmax

rms primary()

-------

------------------ - 40m A≈=

Equation 10

reset

rms ondarysec()

-------

--------------- 240mA≈=

3 Parasitic capacitances and related issues

In a flyback converter stage it is important to take into account the parasitic capacitances since their influence may affect the correct operation of the converter itself. Figure 1 shows the main schematic diagram of a flyback converter and Figure 2 shows the small signal equivalent model.

The parasitic capacitances between the ESBT collector and ground are mainly due to three components (see Figure 2):

● C

Usually transistors are mounted on a heat-sink by interposing an insulation layer. The heatsink has to be grounded either for safety reasons, or to minimize the RFI so that C are in the same range as C C power dissipation. Large parasitic capacitances may produce noise problems (origin ringing). Parasitic capacitance are worse at higher input voltages, like those observed in 3phase power supply.

, the primary inter-winding capacitance;

, the intrinsic capacitance of the ESBT between its collector and source;

, the parasitic capacitance between the collector of the ESBT and the heat-sink.

results

and C2. The resulting total parasitic capacitance (C) is equal to

+ C2 + C3. C may be large enough to produce additional and non-negligible switch-on

Figure 2. Small signal equivalent circuit

I nsula ion Pad

Heatsink

ESBT

Cbus

ESBT

The flyback converter of the demo is operated in DCM, thus, before the end of the off-time the secondary of the transformer has discharged all energy stored in the primary inductance during the previous cycle.

8/24

Ic1

Heatsink

ESBT

Ic2 Ic3

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