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

2/25

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 V

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

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

in

3/25

Basis of forward topology AN2623

R

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

D

1

V

d

Reset

Circuit

L

V

D

2

C

0

_

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

C

R

N

1

D

(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.

4/25

N

2

N

R

N

1

D

R

N

2

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 Tabl e 1 below.

Table 1. Input and output parameters

Input parameters

f

V

line

in

Input voltage 88 ÷ 290 V

Line frequency 50/60 Hz

Output parameters

V

out

I

out

P

out

Output voltage 35 V

Output current 4.5 A max continuous, 0.45 A min

Output power 160 W max

Efficiency at full load 80%

∆V

% Max tolerance on output voltage 3%

out

∆V

T

out HF

A max

Max output voltage ripple at switching frequency 350 mV

Maximum ambient temperature 70 °C

RMS

5/25

Main characteristics AN2623

e

Figure 4. Electrical schematic

1

2

J2

CON2J2CON2

R5

15 kOhm

R5

15 kOhm

+

C1

L1

390 uH-5A

L1

390 uH-5A

DN1

BYT16P-400 heatsink

16

T1

5

C1

270uF, ESR=42 mOhm, 50 V

143

1

4

8

7

R3

5.6 kOhm

TRAN_ISDN_06

0

R6

12

OPTO 1

43

R7

C8

1.2 kOhm

ISO1

Q1

20 kOhm

6 nFC86 nF

3

TL 431

......1

2 1

R9

R10

1.153 kOhm (+/- 1%)

0

<Doc> <R

A

Title

Size Document Number Rev

Title

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Title

Size Document Number Rev

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Title

Size Document Number Rev

Title

Size Document Number Rev

3

LFILTERIN1

HT3545-472Y4R0-T01

FUSE14AFUSE1

4A

D1

STTH110

D1

STTH110

0

1

600V-6A

DIODE BRIDGE1

4

0

-+

2

CB1

2 3

47 nF X2 Cap

1 4

CA1

NTC1

47 nF X2 Cap

1

2

J1

CON2J1CON2

+

C4

330 uF, 450V

+

C3

+

C2

100 uF, 450V

2.5 Ohm

R1

220 kOhm, 1/4W

R1

220 kOhm, 1/4W

100 uF, 450V

C5

2.2 nF Y1 Cap

R2

220 kOhm,1/4W

0

C6

R4

50 Ohm-1/2W

D2

33nFC633nF

D3

1N4148D21N4148

+

C7

15VD315V

10uF, 20V

STW12NK90Z-heatsink

Rg

10 OHM1

0

14

16

15

DIS

ST-BY

DC-LIM

Sync1RCT2DC3Vref4Vfb5Vcomp6SS7Vcc

U1

RA1

4.7 kOhm

RA1

4.7 kOhm

13

12

ISEN

SGND

RDOWN1

CT1

4.7 nF

R8

2.2 kOhm

11

PGND

RUP1

4.7 kOhm

3.6 kOhm

0.21 Ohm

C10

C9

9

10

Vc

Vout

8

100 pF

100 pFC9100 pF

L5991U1L5991

C12

+

C11

C13

33 nF

1 nF

22uF, 25V

0

0 0

0

0 0

6/25

AN2623 Design circuit

3 Design circuit

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

3.1 Primary 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 (R and one capacitor (C oscillator in normal operation (f established that the device must work at the unique frequency (in this case R

) which allows setting separately the operating frequency of the

T

) and in standby mode (fsb). In this application, it was

osc

B

kHz in normal and in standby operation. This frequency is calculated using R following formula:

Equation 1

f

osc

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

C

T

1

0.693 RAKT+⋅()⋅

, RB )

A

→ ∝ ) of 60

in the

A

where K T

=5%T

d

=160 Ω and CT is calculated fixing the discharge oscillator capacitor time

T

sw

Equation 2

Establishing a D

max

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

fixing (refer to Figure 4) R

3.2 Output filter

Admitting a max current ripple on the inductor ∆I necessary to select an inductor value according to Equation 4:

Equation 4

T

30 109–KtCtCt4.7= nF RA5.6 k Ω=,⇒⋅+⋅=

d

= 50%, L5991 allows obtaining this last value in two different ways. The

2D

–()

52

V

3

=4.70 kΩ, we can then immediately calculate R

up

V

– V

2minVdiode

L

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

out

∆I

out

max

V32.17 V=⇒–=

=3.60 kΩ.

down

equal to 20% of I

Lout

out

D

max

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

f

sw

342 µH=⇒⋅=

L

out

–

outMAX

, it is

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

Equation 5

I

RMS Lout–

2

I

out

2

∆

out

-------------+ I 12

RMS Lout–

4.58 A=⇒=

7/25

Design circuit AN2623

The peak current through the inductor is:

Equation 6

I

Peak Lout–

I

out∆ILoutIPeak Lout–

5.4 A==+=

According to these results, L whose inductance value is L

According to the max high frequency voltage ripple (∆ specifications, the necessary minimum capacitor value (C

was chosen as the Coil Craft's inductor PCV-1-394-05L

out

=390 µH.

out

=350 mV) from the electrical

VoutHF

in the Figure 4) and its maximum

1

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

Equation 7

C

outmin

V

out

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

∆V

outHF

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

⋅

8f

1D

sw

–

max

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

L

out

C

outmin

4.5 µF=⇒⋅⋅=

1

2

Equation 8

∆V

outHF

max

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

∆I

out

ESR

max

388 mΩ=⇒=

ESR

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

Equation 9

I

RMS Cout–

2

I

RMS Lout–

According to these requirements a C

out=C1

2

I

– I

out

RMS Cout–

860 mA=⇒=

=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

1max

V

diodeF

---------------- V n

V

---------------- V

V

diodeR

Equation 11

V

dropF

and V

are, respectively, the voltage drop in the freewheeling diode and in the

dropR

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 V

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

8/25

dropFVdiodeR

1max

n

dropRVdiodeR

328V=⇒–=

⇒–=

diodeR

= V

diodeF

.