ST AN2840 APPLICATION NOTE

AN2840

Application note

EVLVIP17-5WCHG: 5 W low standby consumption battery charger demonstration board based on the VIPER17HN

Introduction

The EVLVIP17-5WCHG demonstration board is a 5 W SMPS for use as a travel battery charger for applications such as mobile phones, PDAs and electronic games. The purpose of the board is to demonstrate the performance of the VIPER17HN off-line high voltage converter. Thanks to its low consumption and other features, good electrical performance is achieved. To obtain constant output voltage and current regulation (CV/CC), the TSM1052 CV/CC controller is used on the secondary side. This TSM1052 is well-suited for this type of application, offering very low current consumption in a very small package (SOT23-6L). Another important feature of the SMPS is the elimination of the Y1 safety capacitor between the primary and the secondary side.

Figure 1. EVLVIP17-5WCHG demonstration board (top side)

Figure 2. EVLVIP17-5WCHG demonstration board (bottom side)

December 2008

Rev 1

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www.st.com

Contents	AN2840

Contents

1	Main characteristics and circuit description . . . . . . . . . . . . . . . . . . . .		. 3
2	Operating waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		6
	2.1	No-load waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
3	Short-circuit operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		9
4	Electrical performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		11
	4.1	Efficiency and no-load measurements . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
	4.2	V-I output characteristics and cable drop compensation . . . . . . . . . . . . .	12
5	Conducted noise measurements (pre-compliance test) . . . . . . . . . . .		14
6	Thermal measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		15
7	Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		16
8	PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		17
9	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		18

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AN2840	Main characteristics and circuit description

1 Main characteristics and circuit description

The following is a list of the main characteristics of the EVLVIP17-5WCHG:

●Input mains range: 90 - 264 VRMS, f: 45 - 66 Hz

●Output parameters: 5.1 VDC ± 2%, 1 A ± 5%, cable drop compensation (0.2 V/A)

●No-load consumption: input power below 100 mW at high mains

●Short-circuit protected with auto-restart at short removal

●PCB type and size: CEM-1, single side 35 µm, 53 x 26 mm

●Safety: EN60065 compliant

●EMI: conforms to EN55022 - class B standards

The converter implements a flyback topology, which is ideal for low power, low cost isolated converters.

On the primary side, the VIPER17HN is used. This IC is a member of the VIPer+ family, and benefits from its additional features and protections.

This device, designed using a multi-chip approach, includes an advanced current mode PWM controller and an avalanche-rugged 800 V power MOSFET in a small DIP-7 package. The converter works in both continuous and discontinuous conduction mode depending on the input voltage (the circuit has a wide range input) and output load. The controller suffix “H” specifies that the switching frequency is 115 kHz, internally fixed, allowing the reduction of the power components. The application is designed to reduce overall component count and adapter cost.

The input section includes a fuse resistor for inrush current limiting and fault protection, a rectifier bridge, two electrolytic bulk capacitors and an inductor as front-end ac-dc converter and EMC filter. The transformer is a layer type, utilizing a standard EF12.6 ferrite core and is designed with approximately 75 V reflected voltage. The peculiarity of this transformer is the winding technique that eliminates the needs for the commonly-used Y1 safety capacitor between the primary and the secondary side. An RCD clamp network is used for leakage inductance demagnetization.

The startup of the circuit is managed by the internal high voltage startup generator of the VIPER17HN. This circuit sinks a typical current of 3 mA from the drain pin and charges the VDD capacitor. This current value is reduced to 0.6 mA when there is a protection intervention, in order to increase restart trial period and thus to reduce the stress on the power components in case of permanent fault. The power supply for the VIPER17HN is obtained by a self-supply winding from the transformer connected in a flyback configuration. This circuit provides a voltage that is, ideally, directly proportional to the output voltage. In practice, since this particular no Y-cap transformer has a high leakage inductance, the selfsupply voltage increases as the peak primary current increases. In any case, thanks to the wide VDD voltage range of the VIPER17HN (from 8.5 V to 23 V) the correct supply is always provided. The internal MOSFET current limit is decreased (from the nominal value of 0.4 A)

by using resistor R16 connected to the CONT pin. With this function, the IDlim is fixed at about 280 mA, allowing the use of a small-sized transformer (EF12.6 for 5 W output) without

risk of saturation.

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Main characteristics and circuit description	AN2840

The brownout voltage divider is not mounted (so this feature is disabled) to save power, especially for the no-load consumption.

The VIPER17HN has several built-in features, such as a frequency jittering to reduce EMI problems, soft-start, and burst-mode operation for low power consumption during light load and no-load conditions. Over-current, overload and over-temperature protections are also implemented internally and guarantee safe operation of the board.

On the secondary side, the TSM1052 constant voltage constant current (CV/CC) controller is used. The TSM1052 and the photodiode of the optocoupler are supplied directly from the output voltage. The wide supply voltage range of the TSM1052 (1.7 V min) allows accurate constant current regulation even with output voltages down to 1.5 V-1.6 V. This range is usually enough for all battery charger applications. When the output voltage falls below this limit the circuit loses regulation, the OLP protection is invoked and the system starts working in HICCUP mode.

If, for some reason, current regulation is required down to the zero output voltage level (i.e. short-circuit), it is enough to supply the TSM1052 and the photodiode with a voltage equal to the sum of the output voltage and the voltage from TR1C winding rectified in a forward way. For more details on this specific schematic, please see application note AN2448 (Ultra small battery charger using TSM1052).

The R7 resistor is added to provide the cable drop compensation. The higher the output current, the higher the output voltage measured on the output terminals of the PCB. In this way, the voltage drop on the cable that connects the unit to the load is compensated and the voltage supplied to the load is essentially constant. More in detail, since R7 equals R9, the output voltage increase is 0.2 mV per mA of output current. This amount is chosen based on the typical cable resistance for these applications (typically around 0.2-0.3 Ω).

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AN2840

diagram Electrical .3 Figure

description circuit and characteristics Main

Operating waveforms	AN2840

2 Operating waveforms

Figure 4 and Figure 5 show some VIPER17HN waveforms during normal operation at full load (5 W). Note that at low mains (115 VRMS) the converter operates in continuous conduction mode, while at 230 VRMS it works in discontinuous conduction mode. This converter design takes advantage of both operating modes. In the illustrations below, the VDD voltage powering the device and the feedback pin voltage are also shown.

Figure 4. VIN = 115 VAC – 60 Hz, full	Figure 5. VIN = 230 VAC - 50 Hz, full
load, normal operation	load, normal operation

!-V	!-V
CH1: VIPER17HN drain pin (black trace)	CH1: VIPER17HN drain pin (black trace)
CH2: VIPER17HN VDD pin (green trace)	CH2: VIPER17HN VDD pin (green trace)
CH3: VIPER17HN FB pin (red trace)	CH3: VIPER17HN FB pin (red trace)

In Figure 6 it is worth noting that once the circuit is working in DCM, the voltage on the FB pin has a triangular shape due to frequency jittering. The loop must adjust the FB pin voltage according to the actual switching frequency to keep the output voltage regulated. This happens because the transferred power in DCM is directly proportional to the switching frequency. Hence, the low frequency sawtooth superimposed on the FB pin has the same frequency as the modulating frequency (250 Hz typ.).

On the drain waveform in Figure 7, the modulation depth of the jittering function is visible. The waveform is captured while synchronizing the scope on the rising edge and using the Envelope acquisition. Thus the trailing edge of the following switching cycle shows frequency variation that is identical to the oscillator frequency with a variation depending on the jittering of the oscillator, in this case 115 kHz and +/- 8 kHz of modulation.

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