ST AN1657 Application note

AN1657

APPLICATION NOTE

SMPS FOR CRT MONITORS WITH THE L6565

by Claudio Adragna

This note shows and discusses a couple of designs of a 90W wide-range-mains SMPS for CRT monitor based on the QR controller L6565. The first design refers to a low-cost SMPS that meets current EnergyStar® requirements on OFF-mode consumption (Pin<2W). The second design is an evolution of the first one so as to be compliant with IEA's "1W initiative". Both have been realized and tested on the bench. The result of their evaluation is presented along with some significant waveforms.

Design Specification

The typical electrical specification of an SMPS of a 17" CRT monitor for PC is summarized in table 1. Two goals concerning the off-mode consumption of the SMPS have been set: the first one is to meet the present EnergyStar® requirements, which envisage less than 2W absorbed from the mains; the second more ambitious goal is to comply with IEA's "1W initiative" as well as to be eligible for GEEA label. Both voluntary standards require to achieve a power consumption below 1W.

Table 1. 90W SMPS for CRT monitor: electrical specification

Input Voltage Range (Vin)		88 to 264 Vac

Mains Frequency (fL)		50/60 Hz
Maximum Output Power (Pout)		92 W

		Vout = 200V ± 3%

	Horizontal Deflection	Iout = 0.33A

		Full load ripple = 1%

		Vout = 80V ± 5%

	Video Amplifier	Iout = 0.13A

		Full load ripple = 1%

		Vout = ± 15V ± 10%

Outputs	Vertical Deflection	Iout = 0.33 A

		Full load ripple = 1%

		Vout = 6.5V ± 10%

	Heater	Iout = 0.6A

		Full load ripple = 2%

		Vout = 5.0V ± 2%

	Micro	Iout = 0.05A

		Full load ripple < 1%

Minimum Switching Frequency in Normal Mode (fMIN, @ Vin = 100 VDC, full load)		25 kHz
Target Efficiency (Vin =88 to 264 Vac, full load) (η)		> 85%

Suspend-Mode Input Power (Vin = 88 to 264 Vac)		<15 W

OFF-Mode Input Power (@Pout = 125 mW on 5V output, Vin = 88 to 264 Vac)		< 2W (EnergyStar®)
		< 1W (IEA, GEEA)
		< 1W (IEA, GEEA)

QR approach and the L6565

The SMPS will be realized with a Quasi-resonant (QR) flyback converter based on the L6565, a control IC specifically designed to handle such kind of converters. Referring to [1] and [2] for a detailed description of the device and the topology, it is here worthwhile reminding that QR operation implies that the trans-

February 2003

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AN1657 APPLICATION NOTE

former always works close to the boundary between continuous and discontinuous conduction mode and thereby at a switching frequency that depends on the input voltage and the output current. The ripple across the input bulk capacitor modulates the switching frequency in itself. This characteristic, besides being advantageous in terms of EMI emissions (it spreads the spectrum), makes it more difficult to see the noise on the screen. Furthermore, with QR operation MOSFET's turn-on occurs with zero or minimum drain voltage, which minimizes the switching noise generated. Finally, since the converter always operates in discontinuous conduction mode the reverse recovery characteristics of the secondary rectifiers are not invoked, which goes in favor of a "quiet" operation too.

The above-mentioned characteristics, coupled with the high degree of safety under short circuit conditions inherent in its operation, make QR approach ideal for noise-sensitive applications as monitors are.

The L6565 is an excellent low-cost solution to implement reliable and energy-efficient QR flyback converters both under maximum and minimum load conditions. The internal functions of the IC (frequency foldback and burst-mode operation at light load) as well as its inherent low consumption (less than 70 µA startup current and less than 3.5 mA quiescent current) make designer's life easier when they face the challenging tasks of meeting energy-saving requirements.

Additionally, the L6565 offers a safety feature (device disable upon secondary rectifier short circuit) that can be fruitfully put to use in the present context to achieve an ultra-low consumption at light load. To protect the converter in the event of such failure, an internal comparator senses the voltage on the current sense pin of the IC and disables the gate driver if this voltage exceeds 2V. To re-enable the driver, the supply voltage of the IC must fall below the UVLO threshold and then exceed again the start-up threshold.

EnergyStar® compliant design

The first proposed schematic is shown in figure 1. Only its more significant features will be commented, please refer to [2] for the standard characteristics of an L6565-based QR flyback.

Figure 1. L6565-based, EnergyStar® compliant, 90W SMPS for CRT monitor: electrical schematic

C8A,B 4.7 nF Y2

F1 250VAC 5A

BD01

R13A,B 4.7 MΩ

88 to 264

EMI

STBR606

D6 UF4006

200V

VAC

filter

R14

0.33A

220μF

47 nF

100 kΩ

47 kΩ

D7 STTH1L06

L1 1µH

400V

250V

80V

C10

C11

0.13A

1N4148

220 µF

100 µF

22 µF

STTH1L06

100V

250V

100V

GND

R1A

R1B

68 kΩ

D3 1N4148

D8 UF4002

6.3V

C12

0.6A

47 µF

22 Ω

1 kΩ

1000 µF

25V

47 kΩ

C4 330 pF

16V

100V

D9 UF4002

1N4148

+15V

0.33A

DZ1

1N4148

C14

R6A

18V

470 µF

1.5 MΩ

0.5W

25V

C13

0.05A

33 Ω

C15

IC4

R15

2.2μF

R6B

470 µF

33 kΩ

L78L05CZ

10V

25V

-15V

1.5 MΩ

R10

STP6NK60ZFP

R16

0.33A

1 kΩ

D10 UF4002

47 Ω

R11A,B

IC1

L6565

0.56 Ω

100 pF

C16

R17

47 µF

2.7 kΩ

25V

10 nF

15 kΩ

R12

IC2

R18

4.7 kΩ

PC817A

10 kΩ

TR1

C18

15 nF

6.2 kΩ

4.7 µF

100 kΩ

250V

R20

R19

C17

330 kΩ

1.8 kΩ

IC3

22 nF

TL431

R21

4.7 kΩ

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AN1657 APPLICATION NOTE

The converter is started up by R1A, R1B and the diode D1 that draw some current from the AC side of the bridge rectifier. This inexpensive circuit wakes up the system in less then 3s @ 88 VAC and contributes to light load losses with 240 mW @ 264 VAC. Despite this dummy consumption it is anyway possible to meet the target of less than 2W input consumption thanks to the favorable features of the L6565. Supplying the IC from the AC side of the bridge helps reduce the power consumption on the start-up resistors and eliminates any chance of spurious restarts at converter's power down.

R6A and B along with R7 correct the overcurrent setpoint so as to minimize the power capability change of the converter over the entire input voltage range. C7 filters out any noise that might be coupled to the pin.

R8 and C6 provide soft-start. At start-up C6 is charged by the output of the L6565 E/A (pin 2) with a current defined by 2.5 / R8 and the E/A works temporarily closed-loop. As the E/A saturates high there is no more current through C6, the loop opens, the voltage on pin 1 (E/A input) goes to zero and pin 2 stays high at about 6V. When the L6565 turns off (because its supply voltage Vcc goes below the UVLO threshold) the capacitor is discharged internally in few milliseconds - because the impedance of the pins becomes low - in this way ensuring a correct soft-start even when the L6565 is continuously restarted (e.g. in case of overload or short circuit).

Output voltage regulation is done with a TL431+optocoupler arrangement on the secondary side and the information is fed back to the current sense pin (#4) of the L6565. Regulation is thus performed by modulating the voltage offset generated by the phototransistor current on R10. C5 adds a small filtering effect to increase noise immunity. This feedback arrangement helps reduce the load of the self-supply system (winding 7-8, D3, R3, C3). In fact with the usual arrangement, where the phototransistor sinks current from pin 2 (with pin 1 grounded), the regulation current, typically 3 mA at light load, adds up to the operating current of the IC. With this circuit, to create about 1V offset, which is required at light load, the phototransistor needs to draw only 1 mA. This load reduction will counteract the natural decay of the self-supply voltage when the converter is lightly loaded. Please note that with this technique the ZCD masking time of the L6565 (refer to [2] for details) is fixed at 3.5 µs.

The circuit made up of R4, C4, D4 and DZ1 provides overvoltage protection in case of failure of the feedback loop. R4 and C4 smooth the waveform generated by the self-supply winding to suppress the leading edge spike that could mislead the circuit. During MOSFET's off-time the winding generates a voltage proportional to the output voltage. Thus, if the feedback loop opens (e.g. the optocoupler fails), which causes the output voltage to rise above the regulated value, the voltage provided by R4, C4 will increase as well. DZ1 will be turned on and inject an additional offset on the current sense pin after MOSFET's turn off. As the voltage on the pin reaches 2V an internal comparator will be triggered, the L6565 will shut down and the converter will be stopped until L6565's Vcc voltage, after falling below the UVLO threshold, goes again above the start-up threshold. This may take some hundreds milliseconds, then the system will work in a continuous restart mode, the energy throughput will be very low and the output voltage will not reach dangerous values.

Table 2. L6565-based 90W SMPS for CRT monitor: transformer specification

Core		Philips ETD44, 3C85 Material

Bobbin		Horizontal mounting, 18 pins

Air gap		≈ 1 mm for an inductance 1-4 of 380 µH

Leakage inductance			< 10 µH

	Winding	Wire	S-F	Turns	Notes

	Pri1	4xAWG29	2-4	19	Pin 4 is cut for safety

	Sec1 (200V)	AWG25	17-18	48

Windings	Sec2 (80V)	AWG25	15-16	32

	Sec3 (6.5V)	AWG25	13-14	3	Evenly spaced
Spec & Build	Sec3 (6.5V)	AWG25	13-14	3	Evenly spaced

	Sec4 (+15V)	AWG25	11-12	6	Bifilar with Sec5
	Sec4 (+15V)	AWG25	11-12	6	Bifilar with Sec5

	Sec5 (-15V)	AWG26	10-11	6	Bifilar with Sec4

	Pri2	4xAWG29	1-2	19

	Aux (+15V)	AWG29	8-7	7	Evenly spaced

The linear regulator that supplies the 5V line for the µP takes its input from the +15V line. Using the 6.5V line would improve efficiency (especially in OFF-mode) even further. To do so, however, an LDO (low

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