ST AN2623 APPLICATION NOTE

AN2623

Application note

Evaluation board for off-line forward converter based on L5991

Introduction

This application note gives a practical example of a 160 W, isolated, forward converter using the L5991, high frequency current mode PWM controller. Design procedures for both the power stage and controller are presented.

Generally for this power level the norm ICE61000-3-2 imposes the use of a PFC preregulator stage, but some countries do not require compliance to this norm. The forward converter presented here does not have a PFC.

Figure 1. 160 W off-line forward converter, evaluation board

October 2007

Rev 1

1/25

www.st.com

Contents	AN2623

Contents

1	Basis of forward topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 4
2	Main characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		5
3	Design circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		7
	3.1	Primary controller: L5991 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
	3.2	Output filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
	3.3	Output diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
	3.4	Power transformer design and MOSFET choice . . . . . . . . . . . . . . . . . . . .	9
	3.5	Feedback loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
4	Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		15
	4.1	High frequency ripple of output voltage and load regulation . . . . . . . . . .	16
	4.2	Dynamic load test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
	4.3	Start-up behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
	4.4	Wake-up time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	20
	4.5	Short circuit test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	21
	4.6	Thermal measurement and global efficiency . . . . . . . . . . . . . . . . . . . . . .	22
5	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		24

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AN2623	List of figures

List of figures

Figure 1. 160 W off-line forward converter, evaluation board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 2. Basic forward converter topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 3. Reset circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 4. Electrical schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 5. Vds and Ids of STW12NK90Z in full load condition at different input voltages . . . . . . . . . 15 Figure 6. High frequency ripple of output voltage in full load condition at different input voltages. . . 16

Figure 7. Output voltage behavior against the load and the Vin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 8. Behavior of system under dynamic load at different input voltages . . . . . . . . . . . . . . . . . . 18

Figure 9. Behavior of system under dynamic load at different input voltages . . . . . . . . . . . . . . . . . . 19 Figure 10. Wake-up time of the system at different input voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 11. Behavior of the system in short circuit condition at different input voltages . . . . . . . . . . . . 21 Figure 12. Efficiency of the system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

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Basis of forward topology	AN2623

1 Basis of forward topology

A forward converter is typically used in off-line applications in the 100 W - 300 W power range. A simplified schematic of the forward converter can be seen in Figure 2.

Figure 2. Basic forward converter topology

	D1	L
+			+
	D2	C	V0
	D2	C
Vd	Reset
Vd	Circuit		_
	Circuit		_
_

A natural limitation of the forward converter is the need to completely reset the transformer, cycle by cycle, before the next MOSFET switches on. Different circuits are used for this purpose with advantages and drawbacks. The two simplest and most commonly used reset schemes are: the RCD reset circuit and the reset auxiliary winding both shown in Figure 3 (a-b). In the design presented in this document, the reset winding was used. It is advantageous with respect to efficiency because the energy stored in the magnetizing inductor goes back to the input and is not lost as using an RCD snubber net. The drawback of the reset circuit is that, generally, a higher voltage Power Mosfet is needed. In the present design a 900 V MOSFET was used.

Figure 3. Reset circuits

CR	RR	N1	N2	NR	N1	N2

(a) (b)

The primary controller IC used is the L5991. It is based on a standard current mode PWM controller and includes features such as programmable soft start, adjustable duty cycle limitation and a standby function that reduces the switching frequency when the converter is lightly loaded. The standby function, in this case, is not used to prevent the transformer from saturation. The output voltage regulation is obtained through a voltage reference and an error amplifier (TL1431) placed at the secondary side. A charge pump connected to an auxiliary winding guarantees a stable supply at the controller itself.

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AN2623	Main characteristics

2 Main characteristics

The design procedure is presented in this section and we will refer to the electrical schematic in Figure 4. The power supply electrical specifications are shown in Table 1 below.

Table 1.	Input and output parameters
	Input parameters

Vin	Input voltage	88 ÷ 290 VRMS
fline	Line frequency	50/60 Hz
	Output parameters

Vout	Output voltage	35 V
Iout	Output current	4.5 A max continuous, 0.45 A min
Pout	Output power	160 W max
	Efficiency at full load	80%

∆Vout%	Max tolerance on output voltage	3%
∆Vout HF	Max output voltage ripple at switching frequency	350 mV
TA max	Maximum ambient temperature	70 °C

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6/25

Main

LFILTERIN1

.4 Figure

characteristics

FUSE1

HT3545-472Y4R0-T01

DIODE BRIDGE1

DN1

1 4

600V-6A

CA1

CB1

BYT16P-400 heatsink

Electrical

+ 1

47 nF X2 Cap

390 uH-5A

47 nF X2 Cap

CON2

NTC1

STTH110

2.5 Ohm

CON2

100 uF,+ 450V

330 uF, 450V

270uF, ESR=42 mOhm, 50 V

schematic

100 uF, 450V

220 kOhm, 1/4W

2.2 nF Y1 Cap

TRAN_ISDN_06

220 kOhm,1/4W

5.6 kOhm

50 Ohm-1/2W

1N4148

33nF

10uF, 20V

15V

OPTO 1

15 kOhm

RA1

4.7 kOhm

10 OHM1

ISO1

1.2 kOhm

Sync

ST-BY

STW12NK90Z-heatsink

RCT

DC-LIM

DIS

CT1

6 nF

20 kOhm

Vref

ISEN

4.7 nF

RDOWN1

Vfb

SGND

3.6 kOhm

TL 431

RUP1

Vcomp PGND

2.2 kOhm

......1

4.7 kOhm

Vout

C10

0.21 Ohm

R10

Vcc

100 pF

1.153 kOhm (+/- 1%)

L5991

C11

C12

C13

Title

AN2623

22uF, 25V

1 nF

33 nF

Size

Document Number

Rev

<Doc>

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AN2623	Design circuit

3 Design circuit

This section describes the design of the major parts of the circuit.

3.1Primary controller: L5991

As previously stated, the L5991 is used as the primary controller and its components must first be selected. Refer to the L5991 datasheet for the choice of the two resistors (RA, RB ) and one capacitor (CT) which allows setting separately the operating frequency of the oscillator in normal operation (fosc) and in standby mode (fsb). In this application, it was established that the device must work at the unique frequency (in this case RB → ) of 60 kHz in normal and in standby operation. This frequency is calculated using RA in the following formula:

Equation 1

f = --------------------------------------------------------------

osc CT ( 0.693 RA + KT)

where KT=160 Ω and CT is calculated fixing the discharge oscillator capacitor time Td=5%Tsw

Equation 2

Td = 30 10–9 + Kt Ct Ct= 4.7 nF, RA= 5.6 kΩ

Establishing a Dmax = 50%, L5991 allows obtaining this last value in two different ways. The method that allows implementing the slope compensation, if needed, was used.

The duty cycle limitation is obtained by applying the following voltage to pin3 :

Equation 3

(2 – Dmax)

V3 = 5 – 2 V3= 2.17 V

fixing (refer to Figure 4) Rup=4.70 kΩ, we can then immediately calculate Rdown=3.60 kΩ.

3.2Output filter

Admitting a max current ripple on the inductor ∆ILout equal to 20% of IoutMAX, it is necessary to select an inductor value according to Equation 4:

Equation 4

Lout	=	V2min – Vdiode – Vout	Dmax	Lout=	342 µH
		-------------------------------------------------------∆Iout	-------------
			fsw

The RMS (root mean square) current through the inductor is given by Equation 5:

Equation 5

I I2 ∆2out I 4.58 A

RMS – Lout = out + ------------- RMS – Lout=

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Design circuit	AN2623

The peak current through the inductor is:

Equation 6

IPeak – Lout = Iout + ∆ILout= IPeak – Lout= 5.4 A

According to these results, Lout was chosen as the Coil Craft's inductor PCV-1-394-05L whose inductance value is Lout=390 µH.

According to the max high frequency voltage ripple (∆VoutHF=350 mV) from the electrical specifications, the necessary minimum capacitor value (C1 in the Figure 4) and its maximum

admitted ESR (Equivalent Series Resistance) are calculated as follows:

Equation 7

Coutmin =	Vout	1	1 – Dmax	Coutmin	= 4.5	F
	∆---------------------VoutHF	8--------------------f2sw	----------------------Lout

Equation 8

ESRmax	∆VoutHF	ESRmax= 388 mΩ
ESRmax	= ---------------------	ESRmax= 388 mΩ
	∆Iout

The RMS current through the output capacitor must not exceed the current rate of the selected capacitor and is calculated as:

Equation 9

IRMS – Cout = I2RMS – Lout – I2out IRMS – Cout= 860 mA

According to these requirements a Cout=C1=270 µF (capacitance value) 63 V (Voltage rate) ZL series Rubycon electrolytic capacitor was selected with an ESR of 42 mΩ and max current capability of 1495 mA.

3.3 Output diodes

The maximum reverse voltages across the rectifier diode and the free wheeling diode (D1D2 in the Figure 2) can be calculated as:

Equation 10

V V1max V V 328V diodeR = ---------------- – dropF diodeR=

Equation 11

V V1max V V diodeF = ---------------- – dropR diodeR

VdropF and VdropR are, respectively, the voltage drop in the freewheeling diode and in the rectifier diode, when they are forward biased, and n=1.25 is the turn ratio between the

primary and the secondary winding of the transformer. Considering that the voltage drops in the two diodes are the same, we can conclude from Equation 10 that VdiodeR = VdiodeF .

The maximum RMS and the average currents through the rectifier diode are calculated as:

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