ST AN2950 APPLICATION NOTE

AN2950

Application note

EVLVIPER28L-10W: 5 V/10 W, 60 kHz isolated flyback with extra power management

Introduction

This document describes a 5 V, 2 A application with 3.3 A peak current capability of 1.9 sec, using VIPer28, a new offline high-voltage converter from STMicroelectronics.

In some applications, an SMPS sometimes undergoes load peaks that can be two or more times as much as the power it is supposed to deliver, though only for a short time interval compared to the thermal time constants of the power components. Typical examples of such loads are printers and audio systems.

In such cases, it is more cost-effective to thermally design the system for the maximum continuous power and not for the peak power demand, which is sustained only for a limited time window.

Such a design is possible thanks to the EPT function of the VIPer28, which allows the designer to fix the maximum time window during which the converter is able to manage the peak power and still maintain output voltage regulation. If the overload lasts for longer than this time window, the converter is automatically shut down and enters auto-restart mode until the overload is removed, so as to prevent damage to the power components.

The device has many other features such as an 800 V avalanche rugged power section, PWM operation at 60 kHz with frequency jittering for lower EMI, a limiting current with adjustable setpoint, an on-board soft-start, a safe auto-restart after a fault condition, and a low standby power (< 50 mW at 265 VAC).

The available protections include adjustable and accurate overvoltage protection, thermal shutdown with hysteresis and delayed overload protection.

Figure 1. Demonstration board

April 2010

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www.st.com

Contents	AN2950

Contents

1	Adapter features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 6
	1.1	Circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	1.2	Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
2	Testing the board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		11
	2.1	Typical board waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
	2.2	Regulation precision and output voltage ripple . . . . . . . . . . . . . . . . . . . .	12
	2.3	Burst mode and output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
	2.4	Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	2.5	Light load performances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
	2.6	Overload protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	20
	2.7	Second OCP protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	22
	2.8	Output overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	23
	2.9	EPT function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	26
	2.10	Thermal measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	27
	2.11	EMI measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	28
	2.12	Board layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	30
3	Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		31

Appendix A Test equipment and measurement of efficiency and low load performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

A.1 Notes on input power measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

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AN2950	List of tables

List of tables

Table 1. Electrical specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Table 2. Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Table 3. 1338.0019 transformer characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Table 4. Transformer pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Table 5. Output voltage and VDD line-load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Table 6. Output voltage ripple. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Table 7. Burst mode related output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Table 8. Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Table 9. Active mode efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Table 10. Input voltage averaged efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Table 11. Energy efficiency criteria for standard models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Table 12. Energy efficiency criteria for low-voltage models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Table 13. No load input power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Table 14. Energy consumption criteria for no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Table 15. Low load performance - POUT = 25 mW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Table 16. Low load performance - POUT = 50 mW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Table 17. Output power when the input power is 1 W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Table 18. Output overvoltage threshold at different input/output conditions. . . . . . . . . . . . . . . . . . . . 25 Table 19. Temperature of key components at 115 VAC full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Table 20. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

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List of figures	AN2950

List of figures

Figure 1. Demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 2. Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 3. Transformer size and pin diagram - bottom view (pin side) . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 4. Transformer size and pin diagram - electrical diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 5. Transformer size - side view 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 6. Transformer size - side view 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 7. Drain current and voltage at maximum load 115 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 8. Drain current and voltage at maximum load 230 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 9. Drain current and voltage at maximum load 90 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 10. Drain current and voltage at maximum load 265 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 11. Drain current and voltage at peak load 90 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 12. Drain current and voltage at peak load 265 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 13. Output voltage ripple 115 VIN_AC full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 14. Output voltage ripple 230 VIN_AC full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 15. Output voltage ripple at 115 VIN_AC no load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 16. Output voltage ripple at 230 VIN_AC no load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 17. Output voltage ripple at 115 VIN_AC 50 mA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 18. Output voltage ripple at 230VIN_AC 50 mA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 19. Efficiency vs VIN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 20. Efficiency vs load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 21. Active mode efficiency vs VIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 22. Input voltage averaged efficiency vs load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 23. ENERGY STAR® efficiency criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 24. Pin vs VIN at low load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 25. Pout vs VIN at Pout = 1 W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 26. OLP: output short and protection tripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 27. OLP: output short and protection tripping (zoom on FB pin voltage) . . . . . . . . . . . . . . . . . 21 Figure 28. OLP: steady state (autorestart mode). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 29. OLP: steady state (zoom on FB pin voltage) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 30. OLP: steady-state, short removal and restart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 31. OLP: restart after short removal (zoom) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 32. Second OCP protection tripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 33. Operating with secondary winding shorted. Restart mode . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 34. Operating with secondary winding shorted. Steady state . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Figure 35. Operating with secondary winding shorted. Steady state (zoom). . . . . . . . . . . . . . . . . . . . 23 Figure 36. OVP circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Figure 37. Output overvoltage protection at 115 VIN_AC 0.2 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Figure 38. Output voltage ripple at 230 VIN_AC no load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Figure 39. Auto-restart mode of the overvoltage protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Figure 40. Auto-restart mode of the overvoltage protection (zoom) . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Figure 41. Normalized output OVP threshold vs load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Figure 42. Extra power applied and removed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Figure 43. Extra power applied indefinitely . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Figure 44. EPT thresholds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Figure 45. Thermal measurements at 90 VAC full load, Tamb = 25 °C . . . . . . . . . . . . . . . . . . . . . . . 27 Figure 46. Average measurement of background noise (board disconnected from mains). . . . . . . . . 28 Figure 47. Average measurement at 115 Vac, full load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Figure 48. Average measurements at 230 Vac, full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

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Figure 49. Top layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Figure 50. Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Figure 51. Connection of the UUT to the wattmeter for power measurements . . . . . . . . . . . . . . . . . . 32 Figure 52. Suggested connection for low power measurements (switch in position 1) . . . . . . . . . . . . 33 Figure 53. Suggested connection for high power measurements (switch in position 2) . . . . . . . . . . . 33

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Adapter features	AN2950

1 Adapter features

Table 1 lists the electrical specifications of the demonstration board.

Table 1.	Electrical specification
	Parameter	Symbol	Value
Input voltage range		VIN	[90 VRMS; 265 VRMS]
Output voltage		VOUT	5 V
Maximum output current		IOUTmax	2 A
Peak output current		IOUTpk	3 A
Precision of output regulation		VOUT_LF	±5%
High-frequency output voltage		VOUT_HF	50 mV
ripple
Maximum ambient operating		TA	60°C
temperature

1.1Circuit description

The power supply is set-up in a flyback topology. Its schematic is shown in Figure 2. The input section includes the protection elements (fuse and NTC for inrush current limiting), a filter for EMC suppression (C1, T2, C13), a diode bridge (BR1) and an electrolytic bulk capacitor (C3) as the front-end AC-DC converter. The transformer uses a standard E25 ferrite core. A transil clamp network is used to demagnetize the leakage inductance.

At power-up, the DRAIN pin supplies the internal HV start-up current generator that charges the C4 capacitor up to VDDon. At this point, the power MOSFET starts switching, the generator is turned off and the IC is powered by the energy stored in C4 until the auxiliary winding voltage becomes high enough to sustain the operation through D1 and R1.

The value of the resistor R3 between the CONT and GND pins is high enough that the VIPer28’s current limit does not change with respect to the datasheet’s default value IDlim. This resistor, in conjunction with D2, R14 and R15, is used to realize the overvoltage protection and the feedforward correction function, as described further in this document.

The output rectifier D4 has been selected according to the calculated maximum reverse voltage, forward voltage drop and power dissipation, and is a power Schottky type.

The output voltage regulation is performed by a secondary feedback with a TS431 driving an optocoupler (in this case a PC817) ensuring the required insulation between the primary and secondary. The opto-transistor drives directly the FB pin of the VIPer28, which is connected to the compensation network made up by C6, C7 and R12.

A small LC filter has been added at the output in order to filter the high-frequency ripple without increasing the size of the output capacitors, and a 100 nF capacitor has been placed very close to the solder points of the output connector to limit the spike amplitude.

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AN2950	Adapter features

Figure 2. Application schematic

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										!-V

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	Adapter features			AN2950
	Table 2.	Bill of materials
	Reference	Part	Description	Manufacturer
	BR1	DF06M	600 V 1 A diodes bridge	VISHAY
	C1, C13		100 nF X2 capacitor	EVOX RIFA
	C3		47 µF 450 V electrolytic capacitor	PANASONIC
	C4		22 µF 35 V electrolytic capacitor	PANASONIC
	C5, C9		Not mounted
	C6		3.3 nF 25 V ceramic capacitor	EPCOS
	C7		33 nF 50 V ceramic capacitor	EPCOS
	C8		2.2 nF Y1 capacitor	CERAMITE
	C14, C15	MCZ series	1000 µF 10 V ultra-low ESR electrolytic	RUBYCON
			capacitor
	C10		100 µF 10 V electrolytic capacitor	PANASONIC
	C11		4.7 nF 100 V ceramic capacitor	EPCOS
	C12		2.2 µF 50 V electrolytic capacitor	PANASONIC
	C16		2.2 nF 100 V ceramic capacitor	AVX
	D1	BAT46	Diode	STMicroelectronics
	D2	1N4148	Small-signal, high-speed diode	NXP
	D3	STTH1L06	Ultra-fast, high-voltage diode	STMicroelectronics
	D4	STPS5L40	5 A - 40 V power Schottky rectifier	STMicroelectronics
	D5	1.5KE300	Transil	STMicroelectronics
	Dz	BZX79-C18	18 V Zener diode	NXP
	F1	TR5 250 V 500 mA	Fuse	SCHURTER
	L1	RFB0807-2R2L	2.2 µH	COILCRAFT
	NTC1		2.2 Ω thermistor	EPCOS
	OPTO1	PC817D	Opto coupler	SHARP
	R1		4.7 Ω 1/4 W axial resistor
	R3		68 kΩ 1/4 W axial resistor
	R6		15 kΩ 1/4 W axial resistor
	R8		82 kΩ 1/4 W axial resistor 1% tolerance
	R9		27 kΩ 1/4 W axial resistor 1% tolerance
	R10		560 kΩ 1/4 W axial resistor
	R12		10 kΩ 1/4 W axial resistor
	R13		3.3 kΩ 1/4 W axial resistor
	R14		330 kΩ 1/4 W axial resistor
	R15		2 MΩ 1/4 W axial resistor
	Rz		68 Ω 1/4 W axial resistor

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AN2950				Adapter features
Table 2.	Bill of materials (continued)
Reference	Part		Description	Manufacturer
T2	BU16-4530R5BL		Common mode choke	COILCRAFT
VR1	TS431		Voltage reference	STMicroelectronics
U1	VIPER28LN		Offline high voltage controller	STMicroelectronics
T1	1338.0019		Switch mode power transformer	MAGNETICA

1.2Transformer

Table 3 lists the electrical characteristics of the transformer.

	Table 3.	1338.0019 transformer characteristics
		Properties	Value	Test Condition
	Primary inductance		1.5 mH ±15%	Measured at 1 kHz
	Leakage inductance		0.8% nom.	Measured at 10 kHz
	Primary to secondary turn ratio		12.85 ±5%	Measured at 10 kHz
	(4 - 5)/(6,7 – 10,9)
	Primary to auxiliary turn ratio		5.29 ±5%	Measured at 10 kHz
	(4 - 5)/(1 - 2)
	Nominal operating frequency		60 kHz
	Nominal/peak power		10 W/15 W
	Saturation current		1 A	BSAT = 0.32T
	Insulation		4 kV	Primary to secondary

The size, pinout and mechanical characteristics are given in the following figures.

Figure 3. Transformer size and pin diagram - Figure 4.	Transformer size and pin diagram -
bottom view (pin side)	electrical diagram

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Adapter features	AN2950
Figure 5. Transformer size - side view 1	Figure 6. Transformer size - side view 2

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	Table 4.	Transformer pin description
	Pin	Description	Pin	Description
	5	Primary, to the DC input voltage (400 V)	6	Secondary output
	4	Primary, to the drain of the MOSFET	7
	3	Removed	8	n.c
	2	Auxiliary GND	9	Secondary GND
	1	Auxiliary output	10

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AN2950	Testing the board

2 Testing the board

2.1Typical board waveforms

The drain voltage and current waveforms are reported for the two nominal input voltages and for the minimum and maximum voltages of the converter’s input operating range.

Figure 7 and Figure 8 show the drain current voltage waveforms at nominal input voltages (115 VAC and 230 VAC) and maximum load (2 A). Figure 9 and Figure 10 show the same waveforms for the same load conditions, but with minimum (90 VAC) and maximum

(265 VAC) input voltages.

Figure 11 and Figure 12 show the same waveforms during peak load conditions (3.3 A), which the converter is able to sustain for approximately 1.9 seconds while still keeping the output voltage under regulation.

The converter is operated in continuous conduction mode (CCM) at low input voltage during full load conditions and even at high input voltage during peak load conditions. CCM allows reducing the value of the root mean square currents, both on the primary side (in the power switch) and on the secondary side (in the output diode D4 and in the output capacitors C9 and C14), thus reducing the power dissipation and the stress on the power components.

Figure 7. Drain current and voltage at maximum load 115 VAC

Figure 8. Drain current and voltage at maximum load 230 VAC

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Figure 9. Drain current and voltage at maximum load 90 VAC

Figure 10. Drain current and voltage at maximum load 265 VAC

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Testing the board	AN2950

Figure 11. Drain current and voltage at peak load 90 VAC

Figure 12. Drain current and voltage at peak load 265 VAC

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2.2Regulation precision and output voltage ripple

The output voltage of the board has been measured with different line and load conditions. The results are reported in Table 5. The output voltage is not affected by the line condition. The VDD voltage has also been measured to verify that it is within the device’s operating range.

As confirmed by the results reported in Table 5, the VDD voltage (unregulated auxiliary output) increases as the load on the regulated output increases. To prevent the VDD voltage from exceeding its operating range, an external clamp has been used (Dz, Rz).

Table 5.	Output voltage and VDD line-load regulation
VIN_AC	No load		Half load		Full load		Peak load
(V)	VOUT (V)	VDD (V)	VOUT (V)	VDD (V)	VOUT (V)	VDD (V)	VOUT (V)	VDD (V)
90	5.02	10.8	4.99	18.8	4.99	19.6	4.98	20.5
115	5.02	10.4	4.99	18.7	4.99	19.5	4.98	20.3
230	5.02	10.3	4.99	18.6	4.99	19.4	4.98	20
265	5.02	10.1	4.99	18.5	4.99	19.2	4.98	19.8

The ripple at the switching frequency superimposed at the output voltage has also been measured and the results are reported in Table 6. The board is provided with an LC filter to better filter the voltage ripple.

	Table 6.	Output voltage ripple
	VIN_AC (V)		Half load	Full load	Peak load
			VOUT (mV)	VOUT (mV)	VOUT (mV)
		90	15	17	30
		115	14	21	28
		230	15	22	25
		265	14	20	24

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Doc ID 15494 Rev 1