ST AN1948 Application note

		AN1948
®		- APPLICATION NOTE
	DVD Combo Power Supply with VIPer53
		A. Bailly - S. Luciano

Despite the strong growth of the DVD readers, the VCR ones are still present on the market. 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 standby 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	Maximum Current		Output Power			Product(s)
							MaximumPower		Board Size
		+/-5%							L x W x H
	Output 1	3.3V	1.5A			4.95W	Normal Operation:		170x 70 x 40
	Output 2	5V Stand-by	100mA			500mW	35W max utput		(mm)
	Output 3	5V Power	1.5A			7.5W		power
	Output 4				Obsolete
		12V Power	1.5A			18W	S and-by Operation:
	Output 5	12V Audio	200mA			2.4W	750mW max input
	Output 6	-12V Power	15mA			180mW		power
	Output 7	-25V	25mA			625mW	With 40mA on the
	Output 8	4.2V Display	50mA	-		210mW	5VStandby output
		Product(s)
	Obsolete

April 2004

1/17

AN1948 - APPLICATION NOTE

1. VIPer53 DESCRIPTION

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 package without a heatsink, answering the need for higher 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 pin extends from 0.5V to 4.5V, with a corresponding drain current range from 0A to 2A.

This COMP pin can be either used as an input when working in secondary feedback configuration, or as an output when the internal error amplifier connected on the VDD pin operates in primary feedback to regulate the VDD voltage to 15V.

The VDD undervoltage comparator drives a high voltage startup current source, which is switched off during the normal operation of the device. This feature together with the burst mode capability allows to

							Product(s)
reach very low level of input power in standby mode, when the converter is lightly loaded.
Figure 1: VIPer53 block diagram					Obsolete
									DRAIN
					OSC
			ON/OFF
				OSCILLATOR
				-
				PWM
		Product(s)
			OVERTEMP.	LATCH
			DETECTOR	R1	FF		SELECTION	1V
				R2
				R3	R4	R5
		UVLO						0.5V	HCOMP
	COMPARATOR
VDD						150/400ns
						BLANKING			CURRENT
8.4/
							PWM		AMPLIFIER
11.5V			STANDBY				COMPARATOR
			COMPARATOR
			0.5V			4V	8V
Obsolete
							125k
15V
		ERROR
	AMPLIFIER		4.35V
			OVERLOAD
			COMPARATOR
	OVERVOLTAGE
	COMPARATOR
18V						4.5V
							TOVL	COMP	SOURCE

2/17

AN1948 - APPLICATION NOTE

1.2 Overload protection

A threshold of 4.35V typical has been implemented on the COMP pin. This overload threshold is 150mV 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 value of 4V typical is reached, 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 threshold. The drain current that the device is able to deliver without triggering the overload threshold is called "current capability", specified as IDmax in the datasheet. This value must be used to correctly 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 capacitor connected on the TOVL pin, the device is reset, and a new restarting sequence 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:

COVL > 12.5 × 10– 6 × tss							COVL × IDDch2	Product(s)
C		– 4	×	æ	1	ö
		> 8 × 10			----------------	– 1	× -------------------------------------------
	VDD			èDRST		ø	VDDhyst
CVDD		IDD1 × tss
		> ---------------------------
		VDDhyst
Where tss and DRST are respectively the time needed for the output voltages to pass from 0V to their
value.								Obsolete

nominal values at startup, and the restart duty cycle in overload or short circuit condition. A typical 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 maximum value will be retained to define its

- 1.3 Stand-by operationProduct(s)

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, the output voltage decreases and the regulation loop rises again above the shutdown threshold, thus resuming the normal switching operation. A burst mode with pulse skipping takes place, as long as the output power is below the one corresponding 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 Obsoleteactually 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 hysteresis between normal operation and burst mode which is 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.

3/17

Obsolete

J2-1

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Schematics21.

SUPPLY POWERCOMBODVD02.

NOTE APPLICATIONAN1948-

D4

R7

C13

-4.2V / 50mA

BAT43

C9

DZ2

150

100nF

BZX79C5.1

J1-1

2.2nF

T1

C11

1kV

25707870P1

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OREGA

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10V

R9

33k

DB1

C14

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150

DF08M

100nF

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D5

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0.5A

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68µF

Not

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R10

R11

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fitted

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910

910

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C16

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39µF

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35V

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C17

1A

1k

39µF

1N4947GP

250V

35V

D6

J1-2

STPS8H100D

L3

J2-9

+12V audio / 0.2A

47µH

L4

J2-8

D8

+12V power / 1.5A

10µH

STPS5L60

Product(s)

L6

J2-7

D2

-

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+5V / 1.5A

1N4148

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STPS5L40

L5

J2-5

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R3

C21

C23

C18

C22

C25

C19

C20

47

1200µF

1200µF

820µF

180µF

180µF

120µF

120µF

6.3V

6.3V

25V

R4

D3

10V

10V

25V

25V

0

BYT01-400

Q1

IC1

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VIPer53

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1N5818

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DRAIN

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5.6k

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SOURCE

Obsolete

C30

6.8nF

C28

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180µF

10V

4.99k

R15

D12

R19

R20

C29

1%

C26

1N4148

1k

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4.7k

10nF

100nF

R6

IC2

2.2k

PC817

R23

C3

C4

C5

C6

DZ1

4.7k

47µF

BZX79C

C10

C27

2.2nF

100nF

10nF

16V

10

4.7µF

100nF

4/17

C7

250V

1µF

IC3

63V

Q4

R16

R17

TL431LP

PN2222A

R22

J2-6

18k

18k

4.7k

GND

1%

Product(s)

J2-11

R24

Standby

18k

J2-12

would be difficult to

The regulation sets

2.1.3 Regulation

AN1948 - APPLICATION NOTE

2.1.1 Normal operation mode

In this mode, the Standby input is driven low, Q4 and Q2 are blocked and Q3 conductis. All the output 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 standard auxiliary winding though D2. The corresponding voltage is higher than the one developed by the zener diode DZ1 on the base of Q1, and this transistor is blocked. In the mean 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 observed across C10. As a consequence, 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. Product(s)

As Q2 is conducting, the 5VStandby output is supplied through D9 and the corresponding winding of 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 5VStandby 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 by 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

Obsolete

minimum consumption in this mode to insure -a suitable voltage for Q1.

The transition betweenProduct(s)the normal mode and the standby mode has been slowed down by C26. This is

mandatory to avoid any underor overvoltage on the outputs during this event. See also the following section.

the DC operating point from the 5VStandby output through R21 and R22. Note that it implement a split regulation, as the other outputs operate with a different value when

Obsoletein standby mode.

Some AC signals are also introduced into the regulation loop to insure stability. The conventional path is done through R18 connected to the 5VStandby output, but another AC component has been added thanks to C30 and R23 connected 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.

5/17

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, all these outputs are protected against a permanent short circuit. When the short circuit is applied on the 5VStandby, the short circuit current flows into D10 which bypasses Q3, thus avoiding its destruction. The protection is done through the overload feature of the VIPer53 device, which leads to hiccup mode when the COMP voltage remains high for a too long time. This time is adjusted 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 outputs 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 due to the high turn ratio existing between the 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
				Product(s)
presented by the blown diode on secondary side of the transformer.
- additional 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 the equipment: Negative
		Obsolete
25V and 12V outputs, together with a symmetrical-			+/-4.2V	centered 5V higher 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
	Product(s)
	Ch1 : 12V out
Ch2 : 5Vstby out				Ch2 : 5Vstby out
Obsolete
	Ch3 : 5V out
	Ch4 : 3.3V out			Ch1 : 12V out
				Ch3 : 5V out
				Ch4 : 3.3V out
1			1
2			2
3			3
4			4

6/17