ST AN2495 APPLICATION NOTE

AN2495

Application note

3-phase 80 W SMPS with very wide-range input voltage based on the L6565 and ESBT® STC04IE170HV

1 Introduction

The purpose of this application note is to explain the design of an 80 W 3-phase auxiliary power supply for motor drives and welding applications. To reach a high level system in terms of both efficiency and cost, the L6565 PWM controller has been selected as well as the STC04IE170HV as the main switch. The combination of these STMicroelectronics™ parts provides a highly efficient solution for high DC input voltage, a typical requirement of any three-phase application. The L6565 driver is a variable frequency PWM driver suitable for a design flyback converter working in quasi-resonant mode. It also includes some very useful additional features.

The frequency response study reported in the this document is carried out using MATLAB.

All the design choices are thoroughly discussed to allow the user to adapt the project to specific needs. The input voltage can also be extended up to 1000 VDC as enough margin exists to do so. Finally, the experimental results are analyzed to better understand the benefits offered by the use of ESBT® in this application.

The document is associated with demonstration boards STEVAL-ISA019V1, STEVALISA019V2 and STEVAL-ISA019V3 (Figure 1).

Figure 1. 80 W 3-phase SMPS (working prototype)

April 2011

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Contents	AN2495

Contents

1	Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		1
2	Design specifications and L6565 brief description . . . . . . . . . . . . . . . .		5
3	Flyback stage design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		6
	3.1	Transformer design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8

3.1.1 Core size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1.2 Transformer losses and air gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1.3 Wire size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4	Base driving circuit design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
5	Output circuit design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16
6	Startup network design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
7	Frequency response and loop compensation . . . . . . . . . . . . . . . . . . .	18
8	Efficiency, waveforms and experimental results . . . . . . . . . . . . . . . . .	22
9	Board modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	26
10	References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	34
11	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	35

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AN2495	List of tables

List of tables

Table 1. Converter specification data and fixed parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Table 2. Skin effect AC-DC resistance ratios for square-wave currents. . . . . . . . . . . . . . . . . . . . . . 11 Table 3. Transfer function main parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Table 4. Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Table 5. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

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List of figures	AN2495

List of figures

Figure 1. 80 W 3-phase SMPS (working prototype) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 2. Flyback topology basic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 3. Demonstration board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 4. Dynamic magnetization curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 5. Relative core losses versus frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 6. Proportional driving schematic and equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 7. Converter feedback network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 8. Stabilized open loop transfer function G(s) = G1(s) · G2(s) (Bode plots) . . . . . . . . . . . . . . 21 Figure 9. Overall efficiency versus output power for two different values of input voltage. . . . . . . . . 22 Figure 10. Minimum input voltage-maximum load (250 V - 80 W) in steady state. . . . . . . . . . . . . . . . 23 Figure 11. Medium input voltage-maximum load (500 V - 80 W) in steady state . . . . . . . . . . . . . . . . 23 Figure 12. Maximum input voltage-maximum load (850 V - 80 W) in steady state . . . . . . . . . . . . . . . 24 Figure 13. Very low load condition (750 V - 5 W) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Figure 14. Low load condition (750 V - 24 W) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Figure 15. High load condition (750 V - 80 W) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Figure 16. Proportional base driving circuit relevant waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Figure 17. STEVAL-ISA019V3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Figure 18. STEVAL-ISA019V3 schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Figure 19. Power transformer EGSTON 45371 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Figure 20. Silk screen (top side) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Figure 21. Silk screen (bottom side) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Figure 22. Copper tracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

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AN2495	Design specifications and L6565 brief description

2 Design specifications and L6565 brief description

Table 1 lists the converter specification data and the main parameters set for the demonstration board.

Table 1.	Converter specification data and fixed parameters
Symbol		Description	Values
	Vinmin	Rectified minimum input voltage	250
	Vinmax	Rectified maximum input voltage	850
	Vout	Output voltage 1	24 V/3.33 A
	Vaux	Auxiliary output voltage	15 V/0.1 A
	Pout	Maximum output power	80 W
	h	Converter efficiency	> 80%
	F	Minimum switching frequency	50 kHz
	Vspike	Max. overvoltage limited by clamping circuit	200 V

Figure 2 shows a simplified schematic diagram of a flyback converter.

The L6565 features a current mode control and is designed for flyback converters working in quasi-resonant mode and ZVS (zero voltage switching) at turn-on, or at least quasi ZVS, which means valley switching during turn-on. This condition allows the designer to reduce the power losses at turn-on as much as possible.

Since the input range is from 250 V up to 850 V, the ZVS is obtained only when Vin = Vinmin = Vfl = 250 V.

The L6565 has 8 pins. For a detailed explanation of each pin function, please refer to the L6565 datasheet.

Figure 2. Flyback topology basic diagram

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Flyback stage design	AN2495

3 Flyback stage design

In Figure 3 the complete schematic of the 80 W SMPS is shown.

Figure 3. Demonstration board schematic

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														2													!-
																									2
			#		6 6									2 2
																			2		# N& 5
$		3403$	#													2									4,
				.S												)3/				0#
4		.P			.AUX		42/.)#			2							#	N&		#	P&
			$		.
			2													:#$		#/-0	'.$
																6##			).6
				#N& 6## # #K6		N&				2						/54		#3	6&&
												$	.
							6														P&
										TURNS						5 , . 2				# 2 2
								344(				1	2
							$ $		$ 6/'43,		4	2 34# )% (6										7 7
								344(		TURNS
										344(				$	6
																										#N&
			#						#		6##			#	N&		2			2				2		2
				$ .					$ .
				$ .					$ .				&	&53%	*		6!#		*			6!#

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AN2495	Flyback stage design

As commonly known, 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 maximum overvoltage allowed by the clamping network. It has been set at 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.

Taking into account a 200 V margin, the maximum flyback voltage that can be chosen is:

Equation 2

Vfl = BV–Vinmax – Vspike – Vm argin= 1700 – 1000 – 200 – 250= 250V

After calculating the flyback voltage, proceed with the next step in the converter design.

The turn ratio between primary and secondary side is calculated with the following equation:

Equation 3

N-----p-	=	------------------V----fl------------------	=	----250-----------	= 10
Ns		Vout + VF, diode		24 + 1

As a first approximation, since the turn-on of the device occurs immediately after the energy stored on the primary side inductance has been totally transferred to the secondary side:

Equation 4

VdcminTonmax = VflTreset

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 equations, Tonmax is:

Equation 6

Tonmax =	Vfl • TS	10μs
	------------------------------
	Vdcmin + Vfl

The next step is to calculate the peak current. According to the converter specification in Table 1, output power of 80 W and desired efficiency (at least 80%), by using a formula that does not take into account the losses on the power switch, on the input bridge, and on the rectified network, we have:

Equation 7

		1	• LPIP	2		1	2	2
		--				2--Vdcmin Tonmax
PIN	= 1.25POUT=	2			=
		-------	--T----S----------	-		-----------------	L----P---T-----S-----------------

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	Flyback stage design			AN2495
	Hence:
	Equation 8
	LP =	Vdcmin2Tonmax2	= 1.56mH
		--------2.5T-----------S----P----OUT---------------
	Now we can calculate the peak current on primary.
	Equation 9
	IP	VdcminTonmax	=	1.6A
		= -------------------------------------
		LP

3.1Transformer design

3.1.1Core size

The core size must be chosen according to the power that must be managed, to the primary inductance, and to the saturation current as well. An approximate but efficient formula could be used as a starting point. Eventually, the designer may choose a bigger core and repeat the following steps.

Equation 10

	3		LPIrms(primary)			1.316
AP = 10						4
		1-------------		------------------------------		[cm	]
		--
			T2	• Ku • Bmax

where:

●T is the maximum temperature variation with respect to the ambient temperature

●KU is the utilization factor of the window (say the portion of the window used for winding that generally ranges between 0.4 and 0.7)

●Bmax is the maximum flux in the core.

From Equation 10, we can deduct that the final best choice is an ETD34.

3.1.2Transformer losses and air gap

From Faraday's law we can define the minimum primary winding turns to avoid saturation of the core. Looking at the saturation curve of the core, we can safely work up to 200 mT:

Equation 11

Npmin	Vin, min • TO(N, max)	=	250 • 10μ		=	117
	= ------------------------------------------------------		0.200-----------------	•-----97-------μ-
	B • Ae

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AN2495	Flyback stage design

Figure 4. Dynamic magnetization curves

Concerning the gap, from the EPCOS datasheet, we can use the following approximate equation:

Equation 12

1

-----

AL K2 lg = gaplenght= -K----1-

K1 = 153, K2 = -0.713, while AL has to be calculated.

Knowing that Lp = 1.56mH and Np = 120,

Equation 13

	AL	=	LP	=	1.56m	=	108nH
			N-----2----P		----120---------2---
	1
	-----------------
	108 –0.713
Hence: lg = gaplenght=	---------	=	1.63mm
	153

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Flyback stage design	AN2495

Figure 5. Relative core losses versus frequency

From Figure 5, operating at 50 kHz with 220 mT flux excursion, the power dissipation density is about 300 mW/cm3. Once again, referring to the EPCOS datasheet, the total volume of ETD 34 is 7.63 cm3, therefore:

Equation 14

Pcore = 0.3 • 7.6= 2.29W

Assuming a 95% efficiency for the transformer, only 4 W can be lost on it, of which about 2.3 is lost on the core while the residual 1.7 W is dissipated on the copper. Achieving this efficiency is detailed in the following Section 3.1.3: Wire size.

3.1.3Wire size

To chose the right wire size we must know the rms current on both the primary and secondary sides. Since Ipeak, primary = 1.6 A and I peak, secondary = 16 A

Equation 15

Irms, primary = 0.65A and Irms, secondary = 6.53A

By imposing a 1 W loss on the primary side wire, the maximum series resistance can be calculated as follows:

From Joule’s law we can calculate the resistance of both the primary and secondary windings.

Equation 16

R

=

PCU, pri

R

= 2.36Ω

R

=

PCU, sec

= 0.016Ω

P

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

P

S

------------------------ R

S

2

2

IPRMS

ISRMS

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		From that, knowing the copper resistivity at 100 °C (ρ 100 = 2.303 10-6 Ω cm), and the
		average wind length Lt (Lt = 5.6 cm), we can easily calculate the wire sections (in cm2).
		Equation 17
				APCU		=	ρ 100NpLt					–4	[ cm		2		] dp=		0.028[ cm]
							-----------------------= 6.54 • 10
								RP
		Equation 18
				ASCU		=	ρ	100NSLt		0.0096			[ cm	2	] ds=				0.011[ cm]
							------------------------=
									RS
		Table 2 provides the skin effect resistance ratios due to Eddy currents for different
		frequencies.
Table 2.		Skin effect AC-DC resistance ratios for square-wave currents
			25 kHz						50 kHz				100 kHz								200 kHz
Wire	Diameter		Skin			Rac/			Skin			Rac/	Skin						Rac/		Skin		Rac/
			depth	d/S					depth		d/S		depth				d/S				depth	d/S
no.	d, mils		S, mils			Rdc			S, mils			Rdc	S, mils						Rdc		S, mils		Rdc
12		81.6	17.9	4.56		1.45			12.7		6.43	1.55	8.97				9.10		2.55		6.34	12.87	3.50
14		64.7	17.9	3.61		1.30			12.7		5.08	1.54	8.97				7.21		2.00		6.34	10.21	2.90
16		51.3	17.9	2.87		1.10			12.7		4.04	1.25	8.97				5.72		1.70		6.34	8.09	2.30
18		40.7	17.9	2.27		1.05			12.7		3.20	1.15	8.97				4.54		1.40		6.34	6.42	1.85
20		32.3	17.9	1.80		1.00			12.7		2.54	1.05	8.97				3.60		1.25		6.34	5.09	1.54
22		25.6	17.9	1.43		1.00			12.7		2.02	1.00	8.97				2.85		1.10		6.34	4.04	1.30
24		20.3	17.9	1.13		1.00			12.7		1.60	1.00	8.97				2.26		1.04		6.34	3.20	1.15
26		16.1	17.9	0.90		1.00			12.7		1.27	1.00	8.97				1.79		1.00		6.34	2.54	1.05
28		12.7	17.9	0.71		1.00			12.7		1.00	1.00	8.97				1.42		1.00		6.34	2.00	1.00
30		10.1	17.9	0.56		1.00			12.7		0.80	1.00	8.97				1.13		1.00		6.34	1.59	1.00
32		8.1	17.9	0.45		1.00			12.7		0.84	1.00	8.97				0.90		1.00		6.34	1.28	1.00
34		6.4	17.9	0.36		1.00			12.7		0.50	1.00	8.97				0.71		1.00		6.34	1.01	1.00
Note:		To completely avoid the skin effect, the maximum diameter allowed is 20.3 mils, which is

equal to 0.5 mm.

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