ST AN1895 Application note

AN1895

Application note

EVAL6562-375W Evaluation Board

L6562-based 375W FOT-controlled PFC Pre-regulator

Introduction

This application note describes a 375W evaluation board based on the L6562 Transitionmode Power Factor Correction (PFC) controller (order code: EVAL6562-375W).

The board implements a 375W, wide-range mains input, PFC pre-regulator that is suitable for a 300/350W ATX12V power supply unit (PSU).

To enable the use of a low-cost device like the L6562 at a power level that is usually prohibitive for this device, the chip operates with a Fixed-Off-Time (FOT) control system. This allows Continuous Conduction Mode operation, normally achievable with more expensive control chips and more complex control architectures.

EVAL6562-375W evaluation board

August 2006

Rev 3

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www.st.com

Contents	AN1895

Contents

1	Board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 3
2	Power stage design procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
3	Setting up FOT control with the L6562 . . . . . . . . . . . . . . . . . . . . . . . . . .	7
4	Getting started with the evaluation board . . . . . . . . . . . . . . . . . . . . . . .	9
	4.1 Testbench results and significant waveforms . . . . . . . . . . . . . . . . . . . . . . .	9
5	References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12
Appendix A Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		13
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		15

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AN1895	Board description

1 Board description

The EVAL6562-375W evaluation board includes a Power Factor Correction (PFC) preregulator for a 300W ATX 12V power supply unit (PSU). It is able to deliver 375W continuous power on a regulated 400V rail from a wide range of mains voltage. This rail will be the input for the cascaded isolated DC-DC converter (typically a forward converter) that will provide the output rails of the silver box. Although the ATX specification envisages air cooling, typically realized with a fan capable of an airflow in the range of 25-35 CFM, this is not allowed for in the design of this evaluation board. Enough heat sinking will be provided to allow full-load operation in still air. With an appropriate airflow and without any change in the circuit, the evaluation board can easily deliver up to 400-420W.

The L6562 controller chip is designed for Transition-Mode (TM) operation where the boost inductor works next to the boundary between Continuous (CCM) and Discontinuous Conduction Mode (DCM). However, with a slightly different usage, the chip can operate so that the boost inductor works in CCM, hence surpassing the limitations of TM operation in terms of power handling capability. The gate-drive capability of the L6562 (±0.8A min.) is also adequate to drive the MOSFETs used at higher power levels.

This approach, which couples the simplicity and cost-effectiveness of TM operation with the high-current capability of CCM operation, is the Fixed-Off-Time (FOT) control. The control modulates the ON-time of the power switch, while its OFF-time is kept constant. More precisely, it will be used the Line-Modulated FOT (LM-FOT) where the OFF-time of the power switch is not rigorously constant but is modulated by the instantaneous mains voltage. Please refer to [2] for a detailed description of this technique.

Table 1 summarizes the electrical specification of the application and Table 3 lists transformer specifications.

The electrical schematic is shown in Figure 1 and the PCB layout in Figure 2.

Appendix A lists the bill of materials.

Table 1.	Electrical specifications
	Parameter	Value
Line voltage range		90 to 265 VAC
Minimum line frequency (fL)		47 Hz
Regulated output voltage		400 V
Rated output power		375 W
Maximum 2fL output voltage ripple		20V pk-pk
Hold-up time		17 ms
Maximum switching frequency (@ VIN = 90 VAC, POUT = 375 W)		100 kHz
Minimum estimated efficiency (@ VIN = 90 VAC, POUT = 375W)		90%
Maximum ambient temperature		50° C

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Board description	AN1895
Figure 1. Electrical schematic diagram

Figure 2. PCB layout, silk + bottom layer (top view) (150 x 81.5 mm)

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AN1895	Power stage design procedure

2 Power stage design procedure

The step-by-step procedure of an LM-FOT controlled PFC pre-regulator outlined in [2] will be followed. The design will be done on the basis of a ripple factor (the ratio of the maximum current ripple amplitude to the inductor peak current at minimum line voltage) Kr=0.3.

1.The range of k (kmin ÷ kmax) associated to the line voltage range is:

kmin = 2	Vin(RMS )min	= 2	90	= 0.318, kmax =	2	Vin(RMS )max	= 2	265	= 0.937 .
	-----------Vout-----------------------		400---------			------------Vout-----------------------		400---------

2.The required tOFFmin is derived from the specification on the maximum switching frequency (on the top of the line voltage sinusoid) fswmax at minimum line voltage:

	tOFFmin =	kmin	=	0.318	=	3.18µs
		f--sw---------max-------		100---------------10-------3-

3.The maximum expected input power Pin0 = Pout0/η and the maximum line peak current, Ipkmax are:

	Pin0 =	375	=	417W; Ipkmax	=	2Pin0	=	2 417	=	6.56A	.
		-0.9--------				k----min---------Vout------------		0.318--------------------400--------

4.The ripple amplitude on the top of the sinusoid at minimum line voltage, assuming it is 75% of the maximum specified, will be:

∆ILpk =	6Kr		6	0.3	6.56 =
	8-----–-----3Kr--------Ipkmax	=	8-----–-----	3--------0.3------		1.66A

5.The required inductance L of the boost inductor is:

L	= (1	– kmin)	Vout		tOFFmin = (1 – 0.318)	400	3.18 10	–6	=	523µH
			∆----I--	Lpk-------		1.66-----------

This value will be rounded up to 550 µH; the resulting value of Kr will be slightly smaller than 0.3, but we will go on using the target value, this will give some additional margin.

6.The maximum inductor peak current, ILpkmax, is calculated:

ILpkmax =	8		8	6.56 =
	8-----–-----3Kr--------lpkmax	=	8-----–-----3--------0.3------		7.39A

7.The maximum sense resistor Rsensemax is:

Rsense max	= -------1.6-------------	= --1.6--------- = 0.216Ω
	ILpkmax	7.39

(1.6V is the minimum value of the pulse-by-pulse current limiting threshold on the current sense pin of the L6562). It will be realized with four 0.68Ω, 1W-rated paralleled resistors, for a total resistance of 0.17Ω. This provides some extra power capability. The inductor peak current that the inductor must be able to carry without saturating will be:

1.8

ILpksat = ----------- = 10.6A 0.17

8.From the formulae in [2], table 4, the MOSFET RMS current is:

IQ(rms)

Pin0

16kmin

=

417

2

–

16 0.318

=

3.96A

;

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

2 – ---------

---π------

0.318-----------------

---400--------

--------3-----

---π---------

kminVout

3

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