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| (R) AN1357 - APPLICATION NOTE VIPower: LOW COST POWER SUPPLIES USING VIPer12A IN NON ISOLATED APPLICATIONS N. Aiello - F. Gennaro - R. Santori INTRODUCTION The industrial field is asking for an increasing number of low power low voltage applications both in isolated and non isolated topology. The main specification is related to the cost of the power converter, which has to be operated in off-line mode. VIPer12A is a low cost monolithic smart power device with an integrated PWM controller that is suitable for such applications. In this paper a detailed analysis of VIPer12A in non isolated applications is performed, exploiting the main features of such a device, in Buck and Buck-boost configuration. 1. OFF-LINE NON ISOLATED CONVERTERS Off-line non isolated topologies convert the mains to regulated DC voltage with no need of any isolation transformer, adjusting the duty cycle of a power switch. The energy is transferred from input to output by a low cost inductor. Two main topologies are used to generate either positive or negative voltage with respect to the common terminal of the input voltage, "step down" (or "buck") and "step-down/up" (or "buck-boost") topology respectively, shown in figure 1. Both topologies use an inductor for energy storage, charged during the turn-on of the power switch. The inductor current waveform defines the mode of operation of the converter, i.e. "continuous" and "discontinuous". In the continuous mode, the inductor current never goes to zero, while in the discontinuous mode such a current is zero for a given time period. A "boundary mode" is also defined, referring to a single zero point of the inductor current. In continuous mode, both converters operate like a transformer with a voltage ratio continuously controlled electronically by means of duty cycle of the switch, D, defined by the ratio between the turn on time, ton, and the switching period Ts, according to (1) and (2): Vo ------ = D Vi Vo D ------ = ------------Vi 1-D buck converter (1) buck-boost converter (2) When the switch is on, there is a constant voltage across the output inductor (Vi-Vo), making the current in it rising linearly at a rate, di/dt, given by (3). Vi - Vo di ---- = ----------------dt L (3) May 2001 1/26 AN1357 - APPLICATION NOTE As the switch is turned off, the voltage polarity across the inductor immediately reverses trying to maintain the same current as just before turn off. The diode turns on and clamps the inductor voltage to ideally zero. The voltage across L is Vo and its current ramps down linearly at a rate given by (4). Vo di ---- = - -----L dt (4) Once the switch turns on again the current in L is diverted from D to the switch S, back biasing the diode. The current in L consists in the switch current when it is on and in the diode current when the switch is off. The inductor current ramps up and down around the DC value Io with a ripple determined by the L value. Figure 1: Not isolated power converters S L Io + D Vin C Vo _ Buck configuration S D Io _ C Vin L Vo + Buck-boost configuration 2/26 AN1357 - APPLICATION NOTE 2. FRONT-END RECTIFIER Single wave or bridge rectifiers could be used for mains rectification. However, if the neutral connection is required as ground, the single wave rectifier has to be used. Such a choice determines the minimum input filter capacitance value. The rectifier diode is rated for 800V to 1000V of reverse voltage with a 1A current since the power level of the considered application is low. The input bulk capacitor is determined on the basis of the minimum input voltage (that depends on the power supply specifications, i.e. single or wide range) and on the maximum voltage ripple acceptable in the input voltage. The rectifier topology, i.e. single wave or full bridge, affects the value of the minimum input capacitance as well. A basic consideration has to be taken about the value of the bulk capacitor: although larger values mean low peak and rms current through the power device and less duty cycle adjustment for regulation, a higher peak current is drawn from the mains, leading to higher power dissipation in the rectifier and poorer harmonic behavior. For low power applications a good compromise is given by the peak-to-peak voltage ripple of 25-30% related to the peak mains voltage. If hold-up capability is required, a larger capacitor is needed. When a VIPer12A is used as a power switch, a minimum DC input voltage (i.e. 50V) is needed to perform a proper start-up operation, thus a higher voltage ripple is allowed. In conclusion, the input bulk capacitor is selected to provide the minimum DC voltage with the lower input AC voltage and the maximum load, in order to enable VIPer12A control circuit. In such a way, a better power quality performance is obtained with no effect on the output voltage regulation, thanks to the high switching frequency and the current mode control of VIPer12A. Single wave rectification has been chosen to have the output ground connected to one of the mains lines, as is required in most of non isolated applications. High-grade electrolytic capacitors with high ripple current capacity and low ESR are usually used with a rated voltage of 400Vdc minimum. Referring to Fig. 2, for a given output power Pout and voltage ripple V(=Vinpeak - Vinlow), the input capacitor Cin can be calculated using eq. (5). C in Pout --------------------- t V indc It = ------- = --------------------------V V (5) where "I" is the discharging current, is the efficiency, V indc is the DC input voltage and "t" is the time during which the capacitor must supply current, given by (6). V inlow T t = 0.75 T + T = 0.75 T + ------ arc sin ----------- --- 2 V peak The DC input voltage is given by: (6) V indc = 2 V inac (7) 3/26 AN1357 - APPLICATION NOTE Figure 2: DC bus voltage Vpeak Vmin t T 3. EMI INPUT FILTER EMI is an issue for SMPS since it is a requirement for CE certification. Some applications may require a decoupling input filter to limit the conducted emission to the mains. The limits established by EU depend on the classification of the power supply, whether industrial or domestic appliances. In order to comply with standard requirements, simple filters can be used in low cost applications, as shown in figure 3, reducing the common-mode emissions. The most common filter topology is the CLC filter, however for very noisy high performance circuits the multistage approach is a mandatory solution. Figure 3: Basic EMI filter L L C C1 C2 LC filter CLC filter L1 Ln L2 C1 C2 Cn Multistage filter 4/26 AN1357 - APPLICATION NOTE Considering a CLC filter, a simplified design method is given, based on the corner frequency of the 2nd order filter, providing a dumping factor higher than 0.5 and a corner frequency much lower than the switching frequency. 4. OUTPUT INDUCTOR The design of the output inductor depends on the converter topology as described hereafter for the Buck and Buck-boost converter respectively. 4.1. BUCK CONVERTER As described in Section 1, the energy is stored in the inductor L during the on-time and then released to the load during the off-time of the power device. Thus the load is fed both during the on-time and off-time of the power switch. Since the energy related to the free-wheeling period is negligible compared to the energy transferred during the on-time -considering only the power transferred to the load during on-timethe inductor value is given by (8) with a good approximation. 2 Po ut L ---------------------------2 I dpeak f s w (8) where: "Idpeak" is the maximum allowable peak drain current and "fsw" is the switching frequency. The value of L determines whether the circuit operates in continuous mode or discontinuous mode for a given output current. Regulators operating in the discontinuous mode are very stable and have a very good closed loop response; therefore, the discontinuous mode is preferred. However, in discontinuous mode the current stress is higher for a given output power, increasing the ratings of power devices and decreasing the reliability of the system. The boundary condition between continuous and discontinuous modes of operation is given by (9), thus to operate in discontinuous mode, the inductor value has to be lower than such a value. 1 L b = -- R T ( 1 - D ) 2 where "R" is the load resistance and "T" is the switching period. Finally, it is better to avoid the continuous mode because of several reasons: (9) - Higher switching losses in the switch and free-wheeling diode; - The inductor size and price would increase - Regulators operating in the discontinuous mode are very stable and have a very good closed loop response. 4.2. BUCK-BOOST CONVERTER In the Buck-boost converter the load is supplied only during off-time. Neglecting the power consumption of VIPer12, the inductor value is still given by (8). The boundary condition between continuous and discontinuous mode of operation in a buck-boost regulator is given by (10). 1 L = -- R T D ( 1 - D ) 2 2 (10) The considerations taken about the inductor value in Buck topology apply to Buck-boost topology as well. 5/26 AN1357 - APPLICATION NOTE 5. OUTPUT CAPACITOR 5.1. BUCK CONVERTER The filter capacitor is not an ideal capacitor, but it has a series resistance (ESR-Equivalent Series Resistance) and a series inductance (ESL-Equivalent series inductance). Below about 300KHz, ESL can be neglected and the output ripple is only determined by ESR and the capacitor value. The voltage ripple consists in two components related to ESR and the capacitance value: the first one is proportional to the slope of the inductor current; the second one is proportional to the integral of that current. For a worst-case comparison it can be assumed that they are in phase. The output capacitor filter value is chosen to minimize the output ripple voltage component, according to (11). T 2 V out C out = ------------------------------ ( 1 - D ) 8 V out L (11) where: "Vout" is the required voltage ripple, "D" is the duty-cycle and "L" is the output inductor value. In the frequency domain, the output voltage ripple due to the switching frequency is controlled by the second order LC low-pass filter, which corner frequency fc is given by (12). 1 f c = --------------------------2 L C To minimize the voltage ripple a proper corner frequency has to be chosen, according to (13). (12) fc fs wi tc hing The peak-to-peak ripple voltage ESR component is given by (14). (13) V out = I ESR where: (14) t on ( V i - V out ) I = -------------------------------------L (15) Thus, it is important to choose a capacitor with a low ESR since it is the most important parameter affecting the voltage ripple. A figure of merit for capacitors is the product "Ro * Co", with a typical value for aluminum electrolytic capacitor about 50-80F. 5.2. BUCK-BOOST CONVERTER In the Buck-boost regulator the output capacitor supplies the load only during the on time ton of the switch, thus the capacitor value can be determined by (16): D T Iout C out = ------------------------Vout (16) 6/26 AN1357 - APPLICATION NOTE As in the Buck regulator, an important parameter affecting the output voltage ripple is the capacitor ESR. Moreover, since the capacitor in the Buck-boost regulator supplies the current during the ton of the power switch, the capacitor voltage has a much higher current ripple, making the ESR requirement more stringent. 6. DESIGNING A STEP DOWN CONVERTER USING A VIPer12A The design of a Buck converter using a VIPer12A is straightforward thanks to the device features, e.g. start-up capability, integrated PWM controller and thermal and overcurrent protection. Moreover the PWM controller is very simple and does not require any external feed-back compensation network. The circuit schematic is shown in figure 4 for single wave rectification. The power circuit consists in input rectifier Dr, bulk capacitor for DC bus filtering, Cbulk, power switch VIPer12A, free-wheeling diode D, and output LC filter. 6.1. VIPer12A SUPPLY CIRCUIT The supply voltage for VIPer12A can be easily obtained through the output of the converter by means of a diode and a capacitor on Vdd pin, D1 and C1 in figure 4. The diode voltage rating depends on the input voltage, while the current rating is not an issue. For example, for the European voltage range 185265Vac a 400V diode can be used. The capacitor value is chosen according to the start-up time and the short circuit behavior of the converter. In fact, during a short circuit the Vdd voltage falls down below the minimum required value, enabling the internal high voltage current generator for a new start-up sequence. The charging and discharging time of the capacitor depends on its value, thus the time period during which the power switch is properly supplied changes accordingly. Moreover, if current pulses heat considerably the device, the over temperature protection shuts off the circuit. Typical values range from 300nF to 10uF, while the voltage rating is at least 40-50V. 6.2. VIPer12A REGULATION CIRCUIT The output voltage regulation circuit simply consists of a zener diode Dz across FB and Vdd pins, as shown in figure 4. To perform a better regulation, a filter capacitor C2 can be connected across FB and S pins. The C2 value is in the order of some tens of nF. If the output voltage is lower than 15V the circuit has to be slightly modified in order to allow the start up of VIPer12A. In such a case the regulation circuit has to be decoupled from the supply circuit, using a separate diode and capacitor to supply the zener diode on FB pin, D2 and C3 in figure 5. The diode D2 is a low voltage diode, e.g. 1N4148, and allows the voltage on Vdd to reach the start-up value. Since D2 and C3 realize a peak detector of the output voltage, C3 value influences the precision of the regulation circuit. If high precision is not an issue it can be as low as 100nF. To prevent disturbance resulting in possible output overvoltage or incorrect start-up, a zener diode Do is connected across the output. The value must be 2 or 3 volts higher than the output voltage. For further details please refer to the A.N. 1317 (NON ISOLATED POWER SUPPLIES IN BUCK AND INVERTER CONFIGURATION USING VIPer20 DEVICE -A. Bailly - S. Luciano). 7/26 AN1357 - APPLICATION NOTE Figure 4: Buck converter with Vout>15V D1 Dz C1 C2 Dr Vdd FB VIPer 12 S D L + Vin(ac) Cbulk D C Do Vout(dc) GND _ Figure 5: Buck converter with Vout<15V Dz D2 C1 Vdd FB VIPer 12 S D C3 D1 Dr C2 L + Vin(ac) Cbulk D C Vout(dc) _ GND 8/26 AN1357 - APPLICATION NOTE If the required output voltage is lower than 8V, a different solution has to be used. In fact, although the control feedback path and supply circuit are separated, since the output voltage is lower than the minimum voltage on Vdd pin, the device will not start anyway and will work in start up mode with no voltage regulation and high peak current. A patented approach is introduced that allows supplying the power IC. As shown in figure 6 the proposed circuit needs one more inductor compared to the standard Buck topology to generate the supply voltage. It stores the required energy in an auxiliary capacitor through a low voltage diode by means of a voltage divider. The capacitor C4 is charged during the turn-on time of the power switch through D3 and then is discharged during the turn-off time through D1 transferring energy to C1. D3 is a low voltage diode, i.e. 1N4148, and C4 is a low voltage capacitor which value ranges from 10nF to 1uF. In particular, such a capacitor has to be calculated in order to store the charge required by the VIPer12A and supply the proper voltage depending on output inductors ratio L1/ L2 and AC input voltage. Due to inductor ESR, the voltage across C4 may depend on the output current too. The capacitor C1 is calculated following the standard procedure. Figure 6: Buck converter with Vout<8V D1 D2 C3 C1 Dz Dr Vdd FB VIPer 12 S D C2 L1 D3 L2 + Vin(ac) Cbulk D C4 C Do Vout(dc) GND _ 6.3. INDUCTOR CONSIDERATIONS Inductor selection is done according to (8), where "Idpeak" is the VIPer12A peak drain current, i.e. 360mA, and "fsw" is the fixed 50kHz switching frequency. The maximum peak current limits the power delivered by these topologies using VIPer12A. In the discontinuous mode the maximum output current is half of the peak current, i.e 180mA. However, if a higher output current is required the continuous mode can be entered without affecting voltage regulation. For low output power an axial insulated inductor can be used. Such a kind of inductor meets low cost considerations but features a high series resistance that affects the efficiency of the converter. The current capability of this kind of inductor is determined, for a given package, by its series resistance. For example, a 1.5mH inductor has a current capability of about 100mA since its series resistance is about 30. If the inductor current is higher, a custom ferrite inductor has to be used. 9/26 AN1357 - APPLICATION NOTE 6.4. APPLICATION EXAMPLES The first application example is a 16V-100mA off-line power supply. Converter specifications are given in table 1. The schematic of the circuit is shown in figure 7. A single wave rectification is performed in order to have the ground connected to the neutral of the mains. In this case to improve the reverse voltage robustness a series connection of two high voltage diodes can be used for rectification. The bulk capacitor can be selected according to table 2, that lists the minimum input capacitor for a given input voltage range and output current. Such a minimum value results in DC voltage ripple that is beneficial in terms of power quality and efficiency. A higher voltage ripple means that the capacitor draws current for a longer period reducing the current peak and thus reducing the low frequency harmonic content of the input current. On the other hand, for a given minimum duty cycle the current in the switch is lower because of a lower input voltage, reducing power losses and thus increasing the efficiency. Of course, in the case of a bridge rectifier the values can be considerably reduced. As shown in figure 7, the input bulk capacitor is split in two capacitors and an inductor is inserted in between, realizing an EMI filter as described in Section 3. The components list is given in table 3. Table 1: Buck converter specifications AC input voltage Output power Output voltage Table 2: Input capacitor selection table Input Capacitor Cin 1F 2.2F 3.3F 4.7F Maximum Output current 90Vac Minimum input voltage 180Vac Minimum input voltage (Wide range) 15mA 40mA 60mA 80mA (Single range) 80mA 100mA 120mA 150mA 185 - 265Vac 1.5W +16V Figure 7: Application example: 16V-100mA power supply D1 Dz C1 C2 Rf Dr Lf Vdd FB VIPer 12 S D D L +16V Vac 185-265V Cin1 Cin2 C Do Vout(dc) GND _ 10/26 AN1357 - APPLICATION NOTE Table 3: Components list REFERENCE VALUE PART NUMBER Rf Cin1 Cin2 C1 C2 Cout Do D1 D2 Dr Dz L Lf IC1 10 / 1/2W 1F / 400V Electrolytic 1F / 400V Electrolytic 10F / 25V Electrolytic 22nF / 25V Ceramic 10F / 25V Electrolytic 18V Zener 1A - 600V 1A - 600V 1A - 1000V 15 V Zener 1.8mH 680H BZX55C18V STTA106 STTA106 1N4007 VIPer12ADIP The proposed power supply can be successfully applied in microcontroller-based low power motor drive systems. Such an application requires two regulated output voltage: 15V for power switch gate driver and 5V for microcontroller. In figure 8 the circuit schematic is shown, using a 5V linear regulator with reset for proper system operation. This circuit assures good performance in terms of size and performances at very low cost. Figure 8: Application example: double output converter for microcontroller supply D1 Dz C1 C2 Rf Dr Lf FB Vdd VIPer 12 S D L +16V IN Reset OUT L4949 +5V Co VAC Cin1 Cin2 D C Do Vout(dc) GND _ 11/26 AN1357 - APPLICATION NOTE 6.5. LAYOUT CONSIDERATIONS The circuit layout is shown in figure 9 for all the proposed configurations, with single and full wave rectification. The size of the board is 3.5x4 mm and could be further reduced if SMD components were used. However, in such a case the board is more expensive and does not fit the low cost requirements. The design of a PCB layout is not an easy task. Component placing and trace routing strongly affect the functionality and EMI behavior of the board. Moreover, adequate clearance is required for insulation issues between high and low voltage circuits. In doing so, power traces have to be as short and wide as possible, minimizing resistive and inductive effects in order to improve efficiency and reduce the EMI noise. Figure 9: PCB Layout (not in scale) 6.6. EXPERIMENTAL RESULTS Typical waveforms of the 16V power supply are shown in figure 10 and figure 11, with Iout=50mA and Iout=100mA respectively. In particular, as shown in figure 11, the converter is in boundary condition for Iout=100mA. In figure 12 the output voltage and the source voltage in open load condition are shown. In such a case the VIPer12A operates in burst mode, decreasing the effective switching frequency and reducing the power consumption to less the 100mW. 12/26 AN1357 - APPLICATION NOTE Figure 10: Buck converter with Vout=16V: normal operation with Iout=50mA C1 - Iout C2 - Vsource C4 - Vout Figure 11: Buck converter with Vout=16V: normal operation with Iout=100mA C1 - Iout C2 - Vsource C3 - IL C4 - Vdd 13/26 AN1357 - APPLICATION NOTE Figure 12: Buck converter with Vout=16V: open load condition Open - Load Condition C2 - Vsource C4 - Vout Figure 13: Buck converter with Vout=16V: short circuit condition Short-circuit condition C1 - Iout C2 - Vsource C4 - Vdd 14/26 AN1357 - APPLICATION NOTE In short circuit condition VIPer12A limits the short circuit current thanks to three main features: start-up sequence due to low Vdd voltage, current limit function and thermal shut down. This way the power dissipation is strongly reduced and the converter is safely protected. It is worth noting that the input resistor and the series resistance of the output inductor also limit the short circuit current . A typical short circuit operation is shown in figure 13. The performance of the circuit has been evaluated and fully characterized, as shown from figure 14 to figure 19. In particular, the circuit features a good line and load regulation, as shown in figure 14 and figure 15 respectively. The worst regulation at low input voltage and high output current is basically related to the value of the input capacitors, a trade off between performance, size and cost. The efficiency of the power supply is higher than 50%, as shown in figure 16 and figure 17 as a function of output current and input voltage respectively, and is considerably affected by the parasitic resistance of the components. High quality components could be used to improve the efficiency, but a trade off between cost and performance has to be done for such a low cost application. Figure 14: Buck converter with Vout=16V: line regulation Vout [V] 17 16.5 16 15.5 15 150 7mA 20mA 40mA 60mA 70mA 80mA 85mA 100mA 200 250 Vin [V] 300 The EMI behavior of VIPer12A-based power supply is shown in figure 18 and figure 19. Conducted emission measurements have been performed according to the standard EN55022 using a 50 LISN (Line Impedance Stabilization Network), with a peak detector, in the frequency range 0.15-30 MHz. As shown in figure 18, the power supply features low conducted emissions even with no input EMI filter, taking into account that the limit lines are related to Quasi-peak and Average detector and the experimental traces have been measured with a peak detector. In figure 19 the effect of the input EMI filter on the conducted emissions is shown. 15/26 AN1357 - APPLICATION NOTE Figure 15: Buck converter with Vout=16V: load regulation 17 16,5 16 15,5 15 Vout [V] 185Vac 220Vac 270Vac 0 20 40 60 80 100 Iout [mA] 120 Figure 16: Buck converter with Vout=16V: efficiency Vs. output current Eff% 64,0 54,0 44,0 34,0 24,0 14,0 0 20 40 60 80 100 Iout [mA] 120 185Vac 220Vac 270Vac 16/26 AN1357 - APPLICATION NOTE Figure 17: Buck converter with Vout=16V: efficiency Vs. input ac voltage Eff% 60,0 55,0 50,0 45,0 40,0 35,0 30,0 25,0 20,0 150 7mA 20mA 40mA 60mA 70mA 80mA 85mA 100mA 200 250 Vin [V] 300 Figure 18: Buck converter with Vout =16V: EMI behavior with no input filter dBuV 100 90 80 70 60 50 40 30 20 10 0 (Start = 0.15, Stop = 30.00) MHz A Iload=100mA B no load Noise 1 A: EN 55022: Class A Conducted, Quasi-Peak B: EN 55022: Class A, Average 10 17/26 AN1357 - APPLICATION NOTE Figure 19: Buck converter with Vout =16V: EMI behavior with EMI input filter dBuV 100 90 80 70 60 50 40 30 20 10 0 Noise (Start = 0.15, Stop = 30.00) MHz A B Iload=100mA no load 1 10 A: EN 55022: Class A Conducted, Quasi-Peak B: EN 55022: Class A, Average 6.7. 5V-60mA BUCK CONVERTER The second application example is related to a 5V-60mA power supply. The schematic of the circuit is shown in figure 20. In this application a suitable circuit has been developed to properly supply the device since the output voltage is lower than the minimum Vdd voltage, i.e. 8V. Such a circuit consists in standard low cost components L2, D3 and C4, as described in table 4. The selection of L2 value depends on the input ac voltage range, since the voltage provided to Vdd is function of L1/L2 ratio. For the European voltage range, i.e. 185-265V, a ratio of 3-4 is the right choice. If low cost inductors with high series resistance were used, the voltage across C4 would be influenced by the output current as well. The C4 value is about 80-100nF with a voltage rating given by the inductor ratio. Basically, C4 has to store the energy needed by the VIPer12A and then transfer it to C1. 18/26 AN1357 - APPLICATION NOTE Figure 20: Buck converter with Vout= 5V - Iout = 60mA D1 D2 C3 C1 Dz Rf Dr Lf FB Vdd VIPer 12 S D C2 L1 L2 D3 +5V Vin(ac) Cin1 Cin2 D C4 C Do Vout(dc) GND _ Table 4: Components list REFERENCE VALUE PART NUMBER Rf Cin1 Cin2 C C1 C2 C3 C4 Lf L1 L2 Dr Dz D Do D1 D2 D3 IC1 10 / 1/2W 1F / 385V Electrolytic 1F / 385V Electrolytic 100F / 16V Electrolytic 470nF Ceramic 100nF Ceramic 10F Electrolytic 100nF Ceramic 470uH 1.5mH 470H 1A - 1000V 5.1V Zener 1A - 600V 7.5V Zener 1A - 600V 1A - 600V 1N4007 STTA106 STTA106 STTA106 1N4148 VIPer12ADIP The power supply features a good line and load regulation up to 120mA, as shown in figure 21 and figure 22. The efficiency is about 40% and is strongly affected by the series resistance of the output inductors. The EMI behavior is shown in figure 23. A small input CLC EMI filter has been realized with two 1uF capacitors and a 470H inductor. The measurements have been performed using a peak detector. The procedure is described in Section 6.6. 19/26 AN1357 - APPLICATION NOTE Figure 21: Buck converter with Vout = 5V: line regulation Vout [V] 6 5.8 5.6 5.4 5.2 5 4.8 4.6 4.4 4.2 4 150 0mA 20mA 40mA 60mA 90mA 120mA Vin [V] 200 250 300 Figure 22: Buck converter with Vout = 5V: load regulation Vout [V] 6 5.5 5 4.5 4 0 50 100 Iout [mA] 150 185Vac 220Vac 260Vac 20/26 AN1357 - APPLICATION NOTE Figure 23: Buck converter with Vout = 5V: EMI behavior with CLC input filter dBuV 100 90 80 70 60 50 40 30 20 10 0 Noise (Start = 0.15, Stop = 30.00) MHz EN 55022; Class A Conducted, Quasi-Peak EN 55022; Class A Conducted, Average 1 10 7. DESIGNING A BUCK-BOOST CONVERTER USING A VIPer12A The design of a Buck-boost converter for negative output voltage is as easy as described in Section 6 for the Buck topology. The circuit schematic with output voltage lower than (-15V) is shown in figure 24 for single wave rectification. In figure 25 and figure 26 the circuit schematics for -15V< Vout < -8V and -8V < Vout are respectively shown. 7.1 VIPer12A SUPPLY CIRCUIT In this topology the supply circuit for VIPer12A is implemented connecting a diode D1 between the ground of the converter and a capacitor C1 on Vdd pin, as shown in figure 24. In such a way the voltage on Vdd is positive and the VIPer12A can be properly fed. In particular, the capacitor C1 receives energy during the off time toff and supplies the VIPer12A during the on time ton. The diode selection is based on the input voltage while the capacitor value is determined by the start-up time, as discussed in section 6.1. 7.2 VIPer12A REGULATION CIRCUIT The output voltage regulation circuit consists of a zener diode across FB and Vdd pins, as described in Section 6.2 for Buck topology. If the required output voltage is higher than (-8V), i.e. (-5V), a different solution has to be used, as shown in figure 26. The diode D4 is a low voltage diode, e.g. 1N4148, and the value of C4 is about 100nF in order to store the energy required by the VIPer12A. The capacitor C1 is calculated following the standard procedure. The capacitor C2 is connected between FB and S pins to improve the regulation. 21/26 AN1357 - APPLICATION NOTE 7.3 INDUCTOR CONSIDERATIONS The considerations done about the inductor value in Buck topology apply to Buck-boost topology as well. 7.4. APPLICATION EXAMPLE The considered circuit provides regulated 16V negative voltage output using the buck-boost non isolated configuration. Converter specifications are listed in table 5. Table 6 shows the components list of the power supply shown in figure 24. As shown in figure 27 and figure 28, the circuit performs good voltage and line regulation. The efficiency of the circuit is as higher as more than 60%, as shown in figure 29 and figure 30. This power supply features higher efficiency than the positive counterpart only because of the lower series resistance of the inductor. Table 5: Buck-boost converter specifications AC input voltage Output power DC Output voltage 185 - 265VAC 3.5W -16V Figure 24: Buck-boost converter with Vout< -15V D1 Dz C1 C2 Dr Vdd FB VIPer 12 S D D _ C Vin(ac) Cbulk L Vout(dc) + GND 22/26 AN1357 - APPLICATION NOTE Figure 25: Buck-boost converter with -15V D1 Dr _ C Vout(dc) + GND Vin(ac) Cbulk L Figure 26: Buck-boost converter with -8V Dz C3 Dr Vdd FB VIPer 12 S D C2 D _ Vin(ac) Cbulk L1 D4 C Vout(dc) C4 L2 + GND 23/26 AN1357 - APPLICATION NOTE Table 6: Components list REFERENCE VALUE PART NUMBER R1 C1 C2 Cbulk C D1 D2 Dr Dz L IC1 2K / 1/4W 10F / 25V Electrolytic 22nF / 25V Ceramic 4.7F / 400V Electrolytic 100F / 25V Electrolytic 1A - 600V 1A - 600V 1A -1000V 15V Zener 1.6mH STTA106 STTA106 1N4007 VIPer12ADIP Figure 27: Buck-boost converter with Vout= -16V: line regulation Vout [V] 17 16.5 16 15.5 15 14.5 14 150 200 negative output 0mA 40mA 80mA 100mA 150mA 200mA 230mA Vin [V] 250 300 24/26 AN1357 - APPLICATION NOTE Figure 28: Buck-boost converter with Vout= -16V: load regulation 17 16.5 16 15.5 15 14.5 14 Vout [V] negative output 185Vac 220Vac 260Vac Iout [mA] 0 50 100 150 200 250 Figure 29: Buck-boost converter with Vout= -16V: efficiency Vs. output current 70.0 65.0 60.0 55.0 50.0 45.0 40.0 35.0 30.0 Eff% negative output 185Vac 220Vac 260Vac Iout [mA] 0 40 80 120 160 200 240 25/26 AN1357 - APPLICATION NOTE Figure 30: Buck-boost converter with Vout= -16V: efficiency Vs input voltage 70.0 65.0 60.0 55.0 50.0 45.0 40.0 Eff% negative output 20mA 40mA 60mA 100mA 150mA 250mA 35.0 150 Vin [V] 200 250 300 CONCLUSION Several configurations have been introduced in Buck and Buck-boost configuration exploiting the main features of VIPer12A so as to realize low power and low cost non isolated power supplies for industrial applications. The inherent advantages of high frequency switched mode power conversion, allow off-line power supplies to operate with high efficiency and reduced cost and size. Viper12A, integrating a PWM controller and a power device in a single chip, maximizes such advantages in the power range up to few watts. For further information about VIPer12A please consult the official datasheet on the VIPower web page: 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 26/26 |
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