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 STEVALIP001Vxx 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 output 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 for the small signal power switch model with all parasitic components.

Figure 1. STEVAL-ISA030V1

July 2007

Rev 1

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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

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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 Vac input voltage overall1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 8. 110 Vac input voltage overall2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 9. 110 Vac input voltagestorage highlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 10. 110 Vac input voltage turn-off highlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 11. 380 Vac input voltage overall1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 12. 380 Vac input voltage overall2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 13. 380 Vac input voltage storage time highlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 14. 380 Vac input voltage - turn-on highlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 15. 600 Vac input voltage overall1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 16. 600 Vac input voltage overall2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 17. 600 Vac input voltage turn-off highlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 18. 600 Vac input voltage turn-on highlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 19. Vcomp vs TBlank (minimum OFF-time) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 20. 110 Vac input voltage, max load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 21. 380 Vac input, max load: frequency reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 22. 600 Vac input, max load: further frequency reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 23. 600 Vac input, max load: increased OFFtime highlight . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 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

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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 preliminary choices
Symbol		Description	Values
Vinmin		Rectified minimum input voltage	150
Vin		Rectified maximum input voltage	850
Vout		Output voltage	14 V/430 mA
Pout		Maximum output power	6 W
η		Converter efficiency @ max load	> 80%
F		Minimum switching frequency	30 kHz
Vfl		Reflected flyback voltage	250 V
Vspike		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

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	F1	L1	D1	STTH112U
1		2	A	C

	TR5FUSE M600X	C	D2 STTH112U
	J1		A
	3		D4 STTH112U	R1
	2
		A	C	100K /1206
	1
	Phoenix 3 pin		D5

	C	A	33uF/450V	+
		STTH112U	C2	R2
				100K/ 1206
						D6
						LL4148
R4	R5		33uF/450V+	R3	C	A
1M	1M		C3	100K/ 1206

C4			R7
			100K/ 1206
+
33uF/450V	R9	R10	D7

C A

		100K/ 1206		100K/1206	LL4148
		U1			R24
		L6565			2.2K/ 1206
	1			8	R13
		INV	Vcc
	2			7	22/ 1206
		COMP	GD
	3	Vff	GND	6
R18	4	CS	ZCD	5
22K
			R22	330
	C8	C9		+ C10	+ C11
	3.3nF	4.7nF		47uF/25V	22uF/25V

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		T1
		CSM 2010-104				STPS3L60U
	1	1	7	7	A	D3	C
					35T		+ C1
	48T		6 6				330uF/25V
	2	2
R6	3
		3
1/ 1/8W	27T				9T
	4
		4
R8
47k/ 1/8W
		C5
		.0022uF
		Y1 cap
R12	1			ISO1			R11
	Q1			H11A817			1.5K/ 1/8W
10/ 1206
	4
	STC04IE170HP
	2
					4	1	R15
C6	3						1.2K/ 1/8W

10nF	3	2

R17

C7 10nF

C			4.7K/ 1/8W
		R
A		U2
		TL431_ARC

R23 4.7/ 1/4W

14V @ 0.43A

J2

1

2

Phoenix 2 pin

R14

11K/ 1/8W

R21 2.4K/ 1/8W

	AN2528
.2 Figure
diagram schematic Complete	Design
	diagram schematic and specifications

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

Voff = Vinmax – Vfl – Vspike

where Vfl = flyback voltage = (Vout + VF, diode) * Np/Ns and Vspike is the over-voltage on the 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. Np is the number of turns on the 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

Vfl = BV – Vinmax – Vspike – Vm argin= 1700 – 850 – 200 – 300= 350V

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:

Equation 3

	Np	=	Vfl	=	350		=	23.3
	------Ns		V----------------------------------------out + VF, diode		14---------------	+ 1

As a first approximation, since the 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

Vdcmin • Tonmax = Vfl • Treset

and

Equation 5

Tonmax – Treset = TS

Where Tonmax is the maximum on time, Treset is the time needed to demagnetize the transformer inductance and TS is the switching time. Combining the two previous formulas

Tonmax results in:

Equation 6

	Tonmax	=	Vfl	• TS	14	s
			V------------------------------dcmin + Vfl

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

1

• LP • I

2

1

• V

2

2

= 1.25 • P

--

P

--

dcmin • T

onmax

P

IN

OUT

= -----------------------------2

Ts

-=

----------------------------------------------------------2

LP • TS

Hence

Equation 8

LP =

V2dcmin • T2onmax

=

14.7mH

------------------------------------------------2.5

• TS • POUT

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

Equation 9

Vdcmin	• Tonmax	≈ 143mA
IP = ------------------------------------------	LP -

To keep the transformer size very small and to get a very effective cost solution, we prefer to slightly increase the minimum working frequency in order to decrease the primary inductance.

In order to have a 15 mH inductance and to keep an EF20 core, a lot 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. However the maximum 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

LP = 7.5mH

Equation 11

Np	Np
------= 23.8	------------= 18.87
Ns	Naux

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.

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