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| (R) AN1489 - APPLICATION NOTE VIPower: NON ISOLATED POWER SUPPLY USING VIPer20 WITH SECONDARY REGULATION N. Aiello - F. Gennaro - R. Santori Abstract Output voltage regulation with adjustable feedback compensation loop is very simple when a VIPer device is used in low cost non-isolated applications. In this paper such a simple regulation circuit is introduced and analyzed in Buck and Buck-boost converter based on VIPer20, a monolithic smart power with an integrated PWM controller. The device is suitable for off-line applications and features integrated protection circuits such as over-temperature and over-current protection. 1.0 NON ISOLATED POWER SUPPLY 1.1 Buck converter In the basic circuit, shown in figure 1, when the switch is closed current flows through the inductor L producing an output voltage. When the switch opens the magnetic field in L changes polarity and the freewheeling diode provides a return path for the current to circulate through the load. Varying the diode voltage waveform duty-cycle can control the output voltage. Figure 1: Buck converter S L Vin + D C Feedback +Vout PWM 1.1 Buck-boost converter In this configuration the energy is stored in the inductor L during the on time of the switch, when the switch is off the voltage across L reverses as the inductor transfers the stored energy to the smoothing capacitor and to the load. The output voltage is the opposite polarity to the input. December 2001 1/11 AN1489 - APPLICATION NOTE Figure 2: Buck-boost converter S Vin + L D -Vout C Feedback PWM 2.0 VIPer20 IN NON ISOLATED SECONDARY REGULATION VIPer20 is an integrated device with: 620V breakdown voltage power MOSFET (400V VIPer20B, 700V VIPer20A), PWM controller, start up circuit and protections. The start up of the power supply is provided through the DRAIN pin by means of an integrated high voltage current source, which is switched off during normal operation. The VDD pin provides two functions. The first one is to supply the control section of the power IC; in fact if VDD goes below 8V, the start-up current source is turned on and the power MOSFET is switched off until the VDD voltage reaches 11V. During the start up the internal current consumption is reduced, the VDD pin provides a current of about 2mA and the COMP pin is shorted to ground. After that (VDD=11V), the current source is switched off, and the device starts switching. The second function is related to the control of the device since this pin is also connected to the error amplifier, in order to allow primary as well as secondary regulation configurations. In case of primary regulation, an internal 13V trimmed reference voltage is used to maintain VDD at 13V. For secondary regulation, a voltage between 8.5V and 12.5V will be put on VDD pin, in order to stick the error amplifier output transconductance to the high state. The COMP pin behaves as a constant current source, and can easily be connected to the output of an optocoupler. Note that any over-voltage due to regulation loop failure is still detected by the error amplifier through the VDD voltage, which cannot overpass 13V. The output voltage will be somewhat higher than the nominal one, but still under control. The COMP pin provides two functions: it is the output of the error amplifier, and allows the connection of a compensation network to provide the desired transfer function of the regulation loop (R3, C2). Its bandwidth can be easily adjusted using usual component values. As stated above, secondary regulation configurations are also implemented through the COMP pin. When the COMP voltage goes below 0.5V, the shutdown of the circuit occurs, with a zero duty cycle for the power MOSFET. This feature can be used to switch off the converter, and is automatically activated by the regulation loop (whatever is the configuration) to provide a burst mode operation in case of negligible output power or open load condition. The switching frequency is set by an RC network connected to the OSC pin, regardless VDD value in the range from 8V to 15V. This pin also provides a synchronization capability, when connected to an external frequency source. 2/11 AN1489 - APPLICATION NOTE 2.1 Comp pin control In order to perform secondary regulation a suitable control circuit is applied to COMP pin, as shown in figure 3. Since the output voltage supplies directly the VIPer20 by means of a high voltage diode, it has to be in the range 8V / 12.5V, in order to guarantee the minimum supply voltage and to avoid the primary regulation. The control circuit consists in an npn transistor Q1, a Zener diode Dz, a high voltage decoupling diode D2 and a bias network (C1, R1 and R2). The feedback circuit (D2,Dz) works during the off state of the power switch, charging the capacitor C1. During the on state, the voltage across C1 drives Q1 base, through R1 and R2, drawing current from the COMP pin and performing the regulation. The capacitor value is selected considering both the feedback precision and stability, thus it depends on the desired output precision and the switching frequency. In steady state condition the voltage across C1 is constant. As an output voltage change is detected by the feedback network (D2, Dz), the voltage across C1 changes driving the base current of the transistor Q1 and consequently the comp pin voltage. Thus the regulation loop sets the output voltage to the proper value. Figure 3: COMP pin control scheme VDD + DRAIN VIPer20 OSC 13V COMP SOURCE D2 V out Dz R1 Q1 C1 R2 R3 C2 2.2 Inductor and frequency selection The Inductor and frequency value are based to maximum output current and mode of operation, besides the peak drain current has not to overcome 0.5A. It suitably works in discontinuous mode in which the inductor current never goes to zero, in fact it is better to avoid the continuous mode because of several reasons: - higher switching losses in the switch and in the free wheeling diode - the inductor size and price would increase - regulators operating in the discontinuous are very stable and have a very good closed loop response - higher EMI The interaction between inductor and switching frequency in order to have the maximum output current with limited drain current peak lower (for VIPer20 is 0.5A), can be calculate by means of following formula: 2 V out Iout L = -----------------------------------2 I dmax f sw 3/11 AN1489 - APPLICATION NOTE We can do two considerations - fixed inductor, the maximum output current increased with the switching frequency - fixed switching frequency, the maximum output current increased with the inductor value The following relation shows interaction between inductor value and switching frequency in order to work in discontinuous mode: V out f < ------------------------Iout 2 L 3.0 APPLICATION EXAMPLES 3.1. Buck converter The schematic of the circuit is shown in Fig. 4.The specifications of the power supplies are listed in Tab.I. Due to the basic operation of the power supply a Zener diode, Dzo, is connected across the output in order to allow voltage regulation in open load condition and to avoid voltage spike due to slow input voltage transients (see AN 1317). The maximum output current is related to the drain current limitation of the VIPer. The typical value for VIPer20 is 0.67A with a minimum guaranteed value of 0.5A. As the output current increases over the current limitation the output voltage drops down to the minimum supply voltage (8V) shutting down the device. Table 1: Buck converter specifications AC input voltage Vinac Output current Iout Output voltage Vout Switching frequency Output voltage ripple 85 - 265Vac 300mA +12V 20kHz 5% Figure 4: Buck converter using VIPer20 in secondary regulation. Rf Din Lf C3 R4 OSC 13V D3 VDD + RL DRAIN VIPer20 COMP SOURCE Vac 85-265V Cin1 Cin C4 R1 Q1 C1 R2 R3 C2 L Vout D2 D1 Dz C out Dzo GND 4/11 AN1489 - APPLICATION NOTE Table 2: Components list Reference Rf RL R1 R2 R3 R4 Cin Cin1 Cout C1 C2 C3 C4 Dr D1 D2 D3 DZ DZ0 L Lf Q1 IC1 Value 10 / 1/2W 5% 10K / 1/2W 5% 10K / 1/4W 5% 47K / 1/4W 5% 8.2K / 1/4W 5% 10K / 1/4W 5% 10F / 400V Electrolytic 3.3F / 400V Electrolytic 100F / 25V Electrolytic 470nF / 25V Ceramic 100nF / 25V Ceramic 10F / 25V Electrolytic 10nF / 25V Ceramic Diode Diode Diode Diode 9.1V Zener 13V Zener 2.1mH Inductor 470H Inductor 470H Inductor Part Number 1N4007 STTA106 1N4005 1N4005 Transistor BC547B STMicroelectronics VIPer20ADIP Figure 5: PCB layout using VIPer20 in secondary regulation (not in scale) 5/11 AN1489 - APPLICATION NOTE Figure 6: VIPer20 board in secondary regulation 3.2 Experimental results In this section some experimental waveforms are shown and a performance evaluation is carried out in terms of line and load regulation as well as efficiency. The output voltage is almost independent of input voltage, featuring a horizontal line as the input voltage changes from 80V to 265V, confirming the superior regulation behavior. Load regulation is performed as well, ranging from 10.8V to 12 V as the load increases from no load to 300mA. The efficiency has been evaluated and it is shown in the figure as function of output current. The circuit works with a good efficiency that is always over 55% with peak over 75%. Figure 7: Buck converter: line regulation 12.8 12.7 12.6 12.5 12.4 12.3 12.2 12.1 12 11.9 11.8 Vin [V] 0 30mA 100mA 200mA 300mA Vin [Vac] 50 100 150 200 250 300 6/11 AN1489 - APPLICATION NOTE Figure 8: Buck converter: load regulation 1 2.8 1 2.7 1 2.6 1 2.5 1 2.4 1 2.3 1 2.2 1 2.1 12 1 1.9 1 1.8 Vo ut [V ] 85V 150V 200V 260V Iou t [mA] 0 100 200 3 00 40 0 Figure 9: Buck converter: efficiency 80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 0 Vout [V] 85V 150V 200V 260V Iout [mA] 50 100 150 200 250 300 350 3.3 EMI behavior In this session the circuit EMI behavior has been analyzed, both with an LC filter and without, in the following conditions: Vin = 185Vac Vout = 9.34Vdc Pin = 1.5W Rout = 100 Iout = 100mA 7/11 AN1489 - APPLICATION NOTE Figure 10: EMI behavior without filter REF 75 dBuV START 150 kHz #IF BW 9.0 kHz AVG BW 30 kHz STOP 30 MHz Figure 11: EMI behavior with filter LC REF 75.0 dBuV START 150 kHz #IF BW 9.0 AVG BW 30 kHz STOP 30 MHz The figure 11 shows a good EMI behavior for the circuit with a LC filter. 3.4 Buck-boost converter The schematic of the circuit is shown in Fig. 12. The specifications of the power supplies are listed in Tab. III. The output voltage supplies directly the VIPer20, then it has to range from 8V to 12.5V, in order to keep on the VIPer and hold the error amplifier in saturation allowing voltage regulation through the COMP pin. The control circuit consists in a BC547B transistor and a 10V Zener diode. Table 3: Buck-boost converter specifications AC input voltage Vinac Output current Iout Output voltage Vout Switching frequency Output voltage ripple 85 - 265Vac 300mA -11V 20kHz 5% 8/11 AN1489 - APPLICATION NOTE Figure 12: Buck-boost converter using VIPer20 in secondary regulation Rf Din Lf C3 R4 OSC 13V D3 VDD + RL DRAIN VIPer20 COMP SOURCE Vac 85-265V Cin1 Cin C4 R1 Q1 C1 R2 R3 C2 D1 -Vout D2 L Dz C out GND Table 4: Buck-boost converter component list Reference Rf RL R1 R2 R3 R4 Cin Cin1 Cout C1 C2 C3 C4 Dr D1 D2 D3 DZ DZ1 L Lf Q1 IC1 Value 10 / 1/2W 5% 10K / 1/2W 5% 10K / 1/4W 5% 47K / 1/4W 5% 8.2K / 1/4W 5% 10k 1/4W, 5% 3.3F, 400V Electrolytic 3.3F, 400V Electrolytic 100F, 25V Electrolytic 470nF, 25V Ceramic 100nF, 25V Ceramic 10F, 25V Electrolytic 10nF, 25V Ceramic Diode Diode Diode Diode 10V Zener 13V Zener 2.1mH Inductor 470HInductor Transistor BC547B STMicroelectronics VIPer20ADIP 9/11 Part Number 1N4007 STTA106 1N4005 1N4005 AN1489 - APPLICATION NOTE Figure 13: Buck-boost converter: line regulation Vout [V] 12 11.75 11.5 11.25 11 10.75 50 100 150 200 250 300 Vin [Vac] 0mA 50mA 100mA 200mA 300mA Figure 14: Buck-boost converter: load regulation Vout [V] 12 11.75 11.5 11.25 11 10.75 0 100 200 300 400 Iout [mA] 85V 110V 185V 220V 265V Figure 15: Buck-boost converter: efficiency 80 70 60 50 40 30 20 10 0 Vout [V] 85V 110V 185V 220V 265V Iout [mA] 0 100 200 300 400 10/11 AN1489 - APPLICATION NOTE CONCLUSION A simple way to obtain secondary regulation in an off-line power supplies based on VIPer20 family has been introduced. The circuit features high efficiency and good overall performance in terms of load and line regulation. In such a way there is no need of any linear post regulator improving efficiency and reducing the size and cost of the supply. The considered topology can be realized with other devices of the VIPer family for higher power level. For further information about SMPS PWM controller ICs please consult the VIPower web pages at: http://www.st.com/vipower. Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may results from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics (c) 2001 STMicroelectronics - Printed in ITALY- All Rights Reserved. STMicroelectronics GROUP OF COMPANIES Australia - Brazil - China - Finland - France - Germany - Hong Kong - India - Italy - Japan - Malaysia - Malta Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - U.S.A. http://www.st.com 11/11 |
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