ST AN1948 Application note

AN1948

Obsolete Product(s) - Obsolete Product(s)

- APPLICATION NOTE

DVD Combo Power Supply with VIPer53

A. Bailly - S. Luciano

Despite the s trong growth of t he DVD readers, the VCR ones are still present o n the marke t. A lot of equipment now includes both types of media in the same case. This paper proposes a typical solution to efficiently supply such applications and other equipment where logic, DC motor drive and LCD display are to be implemented together in the 35W power range and suite any input voltage standard (85Vac to 265Vac).

Key features for this application are high efficiency, low st andby consumption and cost effective solutions to fit the high volume consumer market needs.

The specification can be summarized as shown in the following table:

Voltage

+/-5%

Output 1 3.3V 1.5A 4.95W Output 2 Output 3 Output 4 Output 5 Output 6 Output 7 Output 8

5V Stand-by 100mA 500mW

5V Power 1.5A 7.5W

12V Power 1.5A 18W

12V Audio 200mA 2.4W

-12V Power 15mA 180mW

-25V 25mA 625mW

4.2V Display 50mA 210mW

Maximum Curr ent Outp ut Po wer MaximumPower Board S i ze

L x W x H

Normal Operation:

35W max output

power

Stand-by Operation:

750mW max input

power

With 40mA on the 5VStandby output

170x 70 x 40

(mm)

April 2004 1/17

Obsolete Product(s) - Obsolete Product(s)

AN1948 - APPLICATION NOTE

1. VIPer53 D ESCR IPTION

VIPer53, the first multichip device of the VIPer family has been chosen to fulfill the requirements. It features very low Rdson of 1Ω allowing to deliver up to 35W in wide range in a standard DIP -8 pack age without a heatsink, answering the need f or high er efficiency and reduced space thanks to a lower power dissipation.

1.1 General features

The block diagram is given in figure 1. An adjustable oscillator drives a current controlled PWM at a fixed switching frequency. The peak drain current is set for each cycle by the voltage present on the COMP pin. The useful range of the COMP pi n extends from 0.5V to 4.5V, with a corres ponding drain current range from 0A to 2A.

This COMP pin can be either used as an in put wh en working i n sec ondary f eedback conf iguration, or as an output when the internal error amplifier co nnected on the VDD pin operates in primary feedba ck to regulate the VDD voltage to 15V.

The VDD undervoltage comparator drives a high vol tage startup current source, which is switched off during the norm al o pera tion of the devic e. Th is fe ature toge ther wi th th e burs t mod e c apabilit y allo ws t o reach very low level of input power in standby mode, when the converter is lightly loaded.

Figure 1: VIPer53 block diagram

OSC DRAIN

ON/OFF

OSCILLATOR

PWM

LATCH

R3 R4 R5

4.5V

VDD

8.4/

11.5V

15V

18V

UVLO

COMPARATOR

ERROR

AMPLIFIER

OVERVOLTAGE COMPARATOR

0.5V

4.35V

OVERTEMP.

DETECTOR

STANDBY

COMPARATOR

OVERLOAD

COMPARATOR

BLANKING TIME

SELECTION

150/400ns

BLANKING

PWM

COMPARATOR

125k

TOVL COMP SOURCE

0.5V

COMP

CURRENT

AMPLIFIER

2/17

Obsolete Product(s) - Obsolete Product(s)

AN1948 - APPLICATION NOTE

1.2 Overload protection

A threshold of 4.35V typical has been i mplem ente d on t he CO MP pin . This ov erload thresho ld is 150 mV below the clamping voltage of 4.5V which corresponds to the current limitation of the device. In case of a COMP voltage exceeding the overload threshold, the pull up resistor on the TOVL pin is released and the external capacitor connected on this pin begins to charge. When a valu e of 4V typical is reac hed, the device stops switching and remains in this state until the VDD voltage reaches VDDoff, or resumes normal operation if the COMP voltage returns to a value below the overload threshol d. The drain current that the device is a ble to deliver wit hout triggering the overl oad threshold is c alled "current capabilit y", specified as IDmax in the datasheet. This value must be used to correc tly size the converter versus its maximum output power.

When an overload occurs on secondary side of the converter, the output power is first limited by the current limitation of the device. If this overload lasts for more than a time constant defined by a c apacitor connected on the TOVL pi n, the dev ice is reset , a nd a new res tarting s equence is initiated by turning on the startup current source. The capacitors on the VDD pin and on the TOVL pin will be defined together in order to insure a correct startup and a low restart duty cycle in overload or short circuit operation. Here are the typical corresponding formulas:

–

OVL

VDD

12.5 10

⋅⋅ ⋅>

810

DD1

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

DDhyst

Where tss and DRST are respect ively the time needed for the output voltages to pass from 0V to their nominal values at startup, and the restart duty cycle in overload or short circuit condition. A ty pical value of 10% is generally set for this last parameter, as it insures that the output diodes and the transformer don’t overheat. The other parameters can be found in the datasheet of the device.

As the VDD capacitor has to respect two conditions, the m aximum value will be retained to define its value.

1.3 Stand-by operation

On the opposite load configuration, the converter is lightly loaded and the COMP voltage decreases until it reaches the shutdown threshold typically at 0.5V. At this point, the switching is disabled and no more energy is passed on secondary side. So, t he output voltage decreases and t he regulation loop rises again above the shutdown threshold, thus resuming the norm al switching operation. A burst mode with pulse skipping takes plac e, as lo ng as the output power i s below the one correspond ing to the minimum turn on of the device. As the COMP voltage works around 0.5V, the peak drain current is very low (it is actually defined by the minimum turn on time of the device, and by the primary inductance of the transformer) and no audible noise is generated.

In addition, the minimum turn on time depends on the COMP voltage. Below 1V (VCOMPbl), the blanking time increases to 400ns, whereas it is 150ns for higher voltages. The minimum turn on times resulting from these values are respectively 600ns and 350ns, when taking into account the internal propagation time. This feature brings the following benefits:

- this brutal change induces an hyste resis between normal operation and burst mode which i s reached sooner when the output power is decreased.

- a short value in normal operation insures a good drain current control in case of short circuit on

secondary side.

- long value in standby operation reinforces the burst mode by skipping more switching cycles, thus decreasing switching losses.

More details regarding the standby operation can be found in the datasheet.

tss⋅⋅>

–



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

–



tss⋅

RST

⋅

OVLIDDch2

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

DDhyst

3/17

Obsolete Product(s) - Obsolete Product(s)

AN1948 - APPLICATION NOTE

2.0 DVD COMBO POWER SUPPLY

2.1 Schematics

The power topology is an off line flyback work ing at a fixed switching frequency of 70kHz. The board offers two operating modes. The normal one delivers 8 different output voltages and t he st andby one only provides a 5V outpu t. This is achieved by a s mart standby management feature, based on a specific transformer design which allows to dropout most of the output voltages, providing a low power consumption in standby mode. The overall schematics is presented in figure 2.

Figure 2: schematics

J2-7

J2-8

J2-3

100nF

DZ2

J2-2

C14

100nF

33k

150

BZX79C5.1

C12

100nF

C11

10V

180µF

J2-4

-25V / 25mA

+4.2V / 50mA

910

R11

910

R10

C16

39µF

100µH

C15

39µF

BAV21

J2-1

-4.2V / 50mA

C13

150

BAT43

J2-9

-12V / 15mA

+12V audio / 0.2A

+12V power / 1. 5A

C17

35V

39µF

47µHL410µH

R12

35V

STPS5L60

STPS8H100D

J2-5

+5V / 1.5A

+3.3V / 1.5A

C20

25V

120µF

25V

C19

120µF

C25

10V

180µF

C22

10V

180µF

10µH

25V

C18

820µF

C23

6.3V

1200µF

C21

6.3V

1200µF

STPS5L40

J2-10

D12

+5V standby / 100mA

10µH

100

R18

D10

1N5818

R14

MPS2907A

D11

1N4148

PN2222A

R13

5.6k 35V

C24

39µF

BAV21

1N4148

10V

C28

180µF

C30

6.8nF

R21

4.99k

C29

10nF

R20

4.7k

R19

IC2

R15

C26

100nF

J2-6

J2-11

J2-12

GND

Standby

18k

R24

R23

4.7k

R22

4.7k

C27

100nF

IC3

PC817

TL431LP

PN2222A

18k

R17

18k

R16

OREGA

25707870P1

1kV

2.2nF

DB1

DF08M

J1-1

100nF

200

DZ3

BZT03C

0.5A

18mH

400V

Not

68µF

1N4947GP

fitted

450V

VR1

250V

CT<0

J1-2

1N4148

JP1

BYT01-400

C10

250V

18k

0.66W

2N5551

DRAIN

VDD

IC1

VIPer53

SOURCECOMPTOVL

15V

OSC

10k

4.7µF

DZ1

BZX79C

1µF

63V

2.2k

10nF

100nF

2.2nF

16V

47µF

4/17

Obsolete Product(s) - Obsolete Product(s)

AN1948 - APPLICATION NOTE

2.1.1 Normal operation mode

In this mode, the Standby input is dri ven low, Q4 and Q2 are blocked and Q3 c onductis. All the ou tput voltages are delivered to the loads and both 5V and 5VStandby outputs are provided.

The transformer turn ratio leads to a voltage of about 30V across C24. This voltage is blocked by Q2, and the 5VStandby output is derived from the main 5V output through Q3. In the case where no load is connected on the 5VStandby output, R14 allows to absorb the Q3 base current delivered by R13.

The same applies at primary side for the auxiliary supply of IC1. In normal operation, the VDD pin energy is delivered by the stan dard auxiliary winding t hough D2. The c orresponding voltage is higher than the one developed by the ze ner diode DZ1 on the base of Q1, and this transistor is blocked. In the m ean time, the second auxiliary winding delivers five times more voltage than the one needed in standby mode (see section 2.1.2), and values as high as 130V can be o bserved across C10. As a consequen ce, C10, D3 and Q1 are high voltage type, and R5 may dissipate up to 1W.

2.1.2 Stand-by mode

All the output voltages are dropped down, except the 5VStandby one which is typically used to supply an infrared receiver and its decoding circuit, and also to maintain the Standby input in the high state, which makes Q4 and Q2 conducting, and disables Q3 thanks to D11.

As Q2 is conducting, the 5VStandby output is supplied through D9 and the corresponding winding o f the main transformer. This winding therefore reduces its voltage by a ratio of about 5, because the regulation loop still maintains the value of 5V on this output. Since all the outputs are coupled together on the same transformer core, they are all divided by a ratio of about 5. This is sufficient to insure a reduced consumption mode, as the loads are now supplied with a much lower voltage.

D11 is needed in this mode to efficiently turn off Q3. Otherwise, its base remains high as it is supplied by R13 connected to a voltage of about 5V, and some reverse current flows from the 5V Standby output to the main 5V one.

On primary side, the standard auxiliary winding doesn’t provide a sufficient voltage, and Q1 acts as a serial regulator with the voltage delivered b y the second auxiliary winding, maintaining the VDD pin of IC1 at higher level than the disabling threshold VDDoff. R14 on the 5VStandby output provides a minimum consumption in this mode to insure a suitable voltage for Q1.

The transition between the normal mode and the standby mode has been slowed down by C26. Th is is mandatory to avoid any under- or overvoltage on the out puts during this event. See also th e following sect ion.

2.1.3 Regulation

The regulation sets the DC operating point from the 5VStandby output through R21 and R22. Note that it would be difficult to implement a split regulation, as the other outputs operate with a different value when in standby mode.

Some AC signals are also introdu ced int o t he r egu lat ion lo op t o ins ure stabilit y. The conventional pat h is done through R18 connected to the 5VStandby output, but another AC component has been added thanks to C30 and R23 conne cted on the 12V power output . This is needed to prevent any instability in situations where the 5VStandby output is lightly loaded versus the 12V one. In order to transmit this signal, a resistance R20 has also been added in series with the conventional capacitive feedback C27 at the level of IC3.

The bandwidth of this regulation loop has been set at a few kHz in order to insure a good dynamic response when submitted to load variations, or during the transitions between the normal mode and the standby mode.

C29 on secondary side and C6 on primary side cancel any switching noise which may produce subharmonic operation.

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Obsolete Product(s) - Obsolete Product(s)

AN1948 - APPLICATION NOTE

2.1.4 Drain voltage clamping

The board comes with a zener (DZ3) clamp type on the drain of the VIPer53 device. Provision is made to also use an R-C type clamping network to replace this zener. The corresponding components R2 and C8 are to be populated according to the bill of material. See par. 2.3.1.

2.1.5 Short circuit protection

This paragraph only deals with the main outputs (i.e. all the 5V, 12V and 3.3V outputs). The following section deals with the plasma ones.

When in normal mode, al l these out puts are protecte d agai ns t a perman ent short circuit. When the sho rt circuit is applied on the 5V Standby, the short circuit current flows in to D10 which bypas ses Q3, thus avoiding its destruction. The protection is done through the overload feature of the VIPer53 device, which leads to hiccup mode when the COM P voltage remai ns high for a too long time. This time i s adjust ed by the capacitor on the TOVL pin, and is needed at startup for authorizing a temporary overload during the charge of the output capacitors.

In standby mode, only the 5VStandby output is protected thanks to D12 which forces the standby signal in the low state, and the converter returns to the normal mode where Q2 is off. This is mandatory to avoid the destruction of Q2 and D9 in this condition. The other out puts are not protected against a permanent short circuit, because the converter can still regulate correctly the 5VStandby output even if one of the others is short circuited. This is d ue to the high turn ratio existing between t he 5VStandby winding and the other ones, and to the low consumption on this output . Nevertheless, the user will adopt one of the following options:

- these outputs can withstand a short circuit for a few seconds. If this time is too long, the corresponding

rectifying diode may blow up, and the converter will enter into hiccup mode because of the short circuit presented by the blown diode on secondary side of the transformer.

- addi tional diodes similar to D12 can be implemented on the other outputs to force the converter to the

normal mode, where it can withstand permanent short circuit on the main outputs.

2.1.6 Plasma display outputs

A whole set of outputs are dedicated to the driving of a plasma display in front of t he equipment: Negative 25V and 12V outputs, togethe r with a symmetrical +/-4.2V centered 5V high er than the -25V output voltage.

Please note that these outputs are not protected against short circuits or overloads. For instance, the short circuit of the 4.2V outputs to ground leads to the destruction of R7 or R9. Also, the rectifying diodes chosen for these outputs don’t withstand a permanent short circuit.

Figure 3: startup in normal mode Figure 4: startup in standby mode

Ch1 : 12V out

Ch2 : 5Vstby out

Ch3 : 5V out

Ch4 : 3.3V out

1 2 3 4

6/17

1 2

2 3

3 4

Ch2 : 5Vstby out

Ch1 : 12V out

Ch3 : 5V out

Ch4 : 3.3V out

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