AND8098/D Low−Cost 100 mA High−Voltage Buck and Buck−Boost Using NCP1052 Prepared by: Kahou Wong ON Semiconductor http://onsemi.com APPLICATION NOTE INTRODUCTION PRINCIPLE OF OPERATION This application note presents low-cost high-voltage 100 mA non-isolated power supply using NCP1052 by buck and buck-boost topology. The NCP1052 is one of the latest low-cost switching controllers with integrated 700 V/ 300 mA power switch from ON Semiconductor. It is primarily designed for isolated 10 W-range flyback converter. If isolation is not needed, the IC can also be used as stepping-down buck and buck-boost converter for further cost saving by removing optocoupler and replacing the transformer by an inductor. The output current capability is 100 mA. The possible operating range is from input range between 20 Vdc and 700 Vdc to output range of 5.0 V or above with 100 mA. Typical efficiency around 65% is obtained in the 12 V buck demo board. Advantages of the proposed circuits include: • Comparing to flyback, buck and buck-boost eliminates optocoupler and replaces transformer by an inductor for cost saving. • Buck and buck-boost offers smaller voltage stress in switches comparing to flyback. It minimizes the switching loss and increases efficiency. • NCP105x can power up itself from the high input voltage with wide range between 20 Vdc and 700 Vdc. It needs no extra supply circuit. • NCP105x operates at 44, 100, or 136 kHz and accommodates low-cost components such as aluminum electrolytic capacitors and powered-iron core magnetic. • NCP105x offers frequency jittering for reduced electromagnetic inference (EMI). • NCP105x offers thermal and short circuit fault protection. • Simple design as no control-loop compensation is concerned. The proposed buck and buck-boost converters are very similar to each other. Their major difference is that buck provides a positive output voltage but buck-boost provides a negative output voltage referring to the input ground. Figure 1 shows the proposed buck and buck-boost converters. The rectifier circuit, which consists of capacitor C3 and diode D3, is in the front end for AC or DC input voltage. Then, the NCP1052 is self-powered up from the rectified input voltage directly with a VCC capacitor C2. When the switch inside the IC is opened, there is a voltage across Drain (D) and Source (S) pins of the IC. If this voltage is greater than 20 V, an internal current source Istart = 6.3 mA (typ.) inside the IC charges up C2 and a voltage in C2 is built up for the operation of the IC. Comparing to the switching frequency, the VCC voltage level is in a lower-frequency 7.5-8.5 V hysteresis loop. This VCC hysteresis loop is for frequency jittering features to minimize EMI and short-circuit fault timing function. Semiconductor Components Industries, LLC, 2003 June, 2003 - Rev. 1 D2 Z2 FB D3 Input D1 D C1 S VCC C3 R1 L D C Z1 Output C Z1 Output C2 (a) Buck D2 Z2 D3 Input FB D S C1 D VCC C3 D1 L R1 C2 (b) Buck-boost Figure 1. Proposed Circuit Using NCP1052 In Figure 2a it is noted that in the buck topology the input voltage powers up the IC through the path across the inductor L and capacitor C. This charging path passes 1 Publication Order Number: AND8098/D AND8098/D through the output and a low-frequency ripple will be found in the output voltage. Hence, the value of C2 is needed to be small enough to increase this charging frequency fVCC in order to reduce output voltage ripple because some efficiency is lost due to this low-frequency ripple. D2 Z2 D1 R1 Istart FB D3 Input D C1 S VCC C3 The function of diode D1, capacitor C1 and resistor R1 are to transfer the magnitude of output voltage to a voltage across C1 so that the IC can regulate the output voltage. In Figure 3, when the main switch inside the IC is opened and the diode D is closed. In buck, the potential of the IC reference ground (pin S) becomes almost 0 V in this moment. In buck-boost, the potential of the IC reference ground (pin S) becomes -Vout in this moment. The voltage in C1 will be charged to the output voltage. On the other hand, when main switch is closed and the diode D is opened, diode D1 is reverse biased by a voltage with magnitude Vin and Vin+Vout respectively. Hence, D1 does not affect the normal operation of the buck and buck-boost converter. It is noted that the instantaneous voltage in C1 can be possibly greater than the output voltage especially when output current or output ripple is too large. It directly affects the load regulation of the circuit since the IC regulates the output voltage based on the voltage in C1. In order to solve it, larger values of L and R1 can help to slow down the charging speed of C1. It reduces the maximum instantaneous voltage in C1 so that output voltage at high output current can be pulled up and a good regulation is made. Larger value of L can help the load regulation but it usually unwanted because it is bulky. Hence, resistor R1 is recommended. Larger value of R1 makes higher output voltage. Hence, it is called as a “pull-up resistor” and it can help to pull up the output voltage slightly. The voltage in C1 representing the output voltage is feedback to the feedback (FB) pin of the NCP1052 through a diode D2 and zener diode Z2. When output voltage is too high, there will be a greater-than-50 A current inserting into the feedback pin of the NCP1052. The NCP1052 will stop switching when it happens. When output voltage is not high enough, the current inserting into the feedback is smaller than 50 A. The NCP1052 enables switching and power is delivered to the output until the output voltage is too high again. The purpose of the diode D2 is to ensure the current is inserting into the feedback pin because the switching of NCP1052 can also be stopped when there is a greater-than-50 A current sinking from the FB pin. The purpose of the zener diode Z2 is to set the output voltage threshold. The FB pin of NCP1052 with a condition of 50 A sourcing current is about 4.3 V. The volt-drop of the diode D2 is loosely about 0.7 V at 50 A. Hence, the output voltage can be loosely set as follows: L D C Z1 Output C Z1 Output C2 (a) Buck D2 Z2 D1 Istart D3 FB D S D VCC Input C3 C1 R1 L C2 (b) Buck-boost Figure 2. Charging Current of C2 In Figure 2b it is noted that in the buck-boost topology the charging current path is blocked by diode D and hence the charging of C2 does not affect the output voltage directly. However, it still affects the output voltage indirectly and slightly by adding some low-frequency noise on the inductor. Hence, small value of C2 is also wanted. D1 R1 C1 Vout (a) Buck D1 C1 Vout zener 4.3 V 0.7 V zener 5 V R1 (eq. 1) According to (1), the possible minimum output voltage of the circuit is 5.0 V when there is no zener diode Z2. If there is no load, the IC will automatically minimize its duty cycle to the minimum value but the output voltage is still possible to be very high because there is no passive component in the circuit try to absorb the energy. As a result, Vout (b) Buck-boost Figure 3. Output Voltage Couples to C1 with a Charging Current http://onsemi.com 2 AND8098/D output voltage will rise up dramatically and burn the output capacitor eventually. Hence, a zener diode Z1 or minimum “dummy” load resistor is needed to consume the minimum amount of energy as shown in Figure 1. It is also noted that when R1 pulls up the output voltage at a given output current condition, the output voltages at lower output current conditions are also pulled up. Hence, the clamping zener diode Z1 is needed to be with the breakdown voltage as same as the output voltage but it will reduce some of the efficiency at lower output current conditions. Because of burst-mode control, the effective maximum duty is lower and said to be 70% roughly. When a buck converter is in continuous conduction mode (CCM), the input voltage Vin and output voltage Vout are related by the duty ratio D. DESIGN CONSIDERATION Another aspect on topology is the output current. The maximum output current is always smaller than the maximum switch current in non-isolated topologies. However, in isolated topologies such as flyback the maximum output current can be increased by a transformer. Vout D 0.7 Vin (eq. 2) The relationship in buck-boost is Vout D 0.7 2.33 1 0.7 Vin 1D Topology Buck circuit is to step down a voltage. Buck-boost circuit is to step up or down a voltage. The output voltage is inverted. The maximum duty of NCP1052 is typically 77%. (eq. 3) Table 1. Summary of Topology Difference Using NCP1052 Buck Buck-boost Flyback Output voltage < 0.7 Vin Negative & < 2.33 Vin Depending on transformer ratio Output current < 300 mA << 300 mA, output current is only a portion of the inductor current < 10 W. It depends on operating condition and audible noise level Input voltage < 700 V << 700 V. It depends on transformer ratio Operating mode in nominal condition Continuous 700 Vout V 700 V Continuous Standby ability on VCC charging current Bad. The current flows through output even if there is no load Good. The current passes through inductor only Good. The current passes through primary winding only Transformer / Auxiliary winding It is only for standby improvement or additional output It is only for standby improvement or additional output It is a must for the main output. Additional auxiliary winding can improve standby performance Isolation No No Yes. Opto coupler can be eliminated if isolation is not needed Discontinuous Burst-mode Operation The NCP1052 is with a burst-mode control method. It means the MOSFET can be completely off for one or more switching cycles. The output voltage is regulated by the overall duration of dead time or non-dead time over a number of switching cycles. This feature offers advantages on saving energy in standby condition since it can reduce the effective duty cycle dramatically. In flyback topology, the circuit is mainly designed for discontinuous conduction mode (DCM) in which the inductor current reaches zero in every switching cycle. The DCM burst-mode waveform can be represented in Figure 4. It is similar to the pulse-width modulation (PWM) one. Burst mode PWM Figure 4. DCM Inductor Currents in Burst Mode and PWM Control http://onsemi.com 3 AND8098/D In non-isolated topologies such as buck or buck-boost, the circuits are mainly designed for CCM. The CCM burst-mode waveform is different to the PWM waveform in Figure 5. Because of this characteristic, burst mode requires a higher peak value of the inductor current in order to have the same level of averaged inductor current (or output current). Vout VCC FB current time Output waveforms with big enough VCC capacitor Burst mode Desired level of Vout VCC PWM Vout Figure 5. CCM Inductor Currents in Burst Mode and traditional PWM Control time Output waveforms with too small VCC capacitor As shown in Figure 4 and 5 burst-mode control produces low-frequency waveform comparing to the switching frequency. Part of the power loss in this low frequency becomes audible noise. Therefore, burst-mode control is not suitable for high power applications such as more than 20 W. Figure 6. Startup Scenarios of the Circuits with Big Enough or Too Small VCC Capacitor Practically, the NCP1052 consumes approximately 0.5 mA in normal operation. The concerned fault sampling time for feedback signal is from 8.5 V to 7.5V. Hence, VCC Capacitor -3 C I dt 0.5 10 · sampling time 1 dV 0.5 10- 3 · sampling time The VCC capacitor C2 is the key component to make the circuit operate in normal mode or fault mode. The device recognizes a fault condition when there is no feedback current in the FB pin during the time from VCC = 8.5 V to 7.5 V. The VCC capacitor directly affects this time duration. In normal mode, the VCC follows a 8.5 V-7.5 V-8.5 V hysteresis loop. When the circuit is in fault mode, the VCC follows a 8.5 V-7.5 V-4.5 V-8.5 V hysteresis loop. The device keeps its MOSFET opened except for the time from VCC = 8.5 V to 7.5 V and delivers a little amount of power to the output in fault mode. A common and extreme case to enter fault condition is the startup. The MOSFET begins switching at the VCC is firstly charged to 8.5 V and hence output voltage rises. The output voltage needs some time to build up the output voltage from 0 V to a desired value. When the desired level is reached, a feedback current flows into the device to stop its switching. If the feedback current is determined before VCC reaches 7.5V, the circuit will remain in normal mode. Otherwise, the circuit will enter the fault mode and cannot provide the output voltage at its desired level. Therefore, the VCC capacitor is needed to be big enough to ensure sufficient time for VCC going from 8.5 V to 7.5 V to sample feedback current in startup. (eq. 4) For example, if sampling time or startup transient is designed to be 20 ms, 10 µF VCC capacitor is needed. Inductor The 300 mA current limit in the NCP1052 is measured with a condition that the di/dt reaches 300 mA in 4 µs. When the buck or buck-boost circuit is designed for universal ac input voltage (85 to 265 Vac), the rectified input voltage will be possibly as high as 375 Vdc. In order to keep the 4 µs condition, the inductance value will be 5 mH by (5) and (6). For buck, di Vin Vout Vin dt L L (eq. 5) For buck-boost, di Vin dt L (eq. 6) The 5 mH is practically too high and hence not very practical. Therefore, the inductor is basically selected by market available inductor models which is with a normally smaller inductance (but not too small). It must have enough saturation current level (>300 mA). If inductance is too http://onsemi.com 4 AND8098/D Buffering Capacitor small, the di/dt becomes too high and the NCP1052 will have a very high current limit effectively because there is a propagation delay (typically 135 ns) to turn off the switch. The current flowing through the inductor L includes three parts. First, there is a VCC charging current Istart in Figure 2. It happens when VCC needs charging. Its magnitude is 6.3 mA. It is noted that the VCC discharging current does not flow through the inductor. Second, it is the main inductor current to deliver the output current. It is noted that the peak of burst-mode inductor current is higher than PWM one as in Figure 5 for the same level of averaged inductor current (or output current). Finally, there is a current flowing through diode D1 to charge up C1. It also flows through the inductor as shown in Figure 3. Its magnitude is a greater-than-50 µA current and practically it is about 1 mA. Hence, the saturation current of the inductor L is needed to be bigger than their sum. Another consideration on the inductor is the low-pass filtering capability for the VCC hysteresis low frequency (and the 50/ 60 Hz rectified AC line voltage ripple). As shown in Figure 2, there is a low-frequency charging current with magnitude 6.3 mA flowing through the inductor and causes low-frequency ripple in the output voltage. A higher value of the inductance can help to reduce the output ripple. It is noted that when the output power is higher, the startup time becomes longer. It needs bigger VCC capacitor and makes lower VCC charging frequency. As a result, a bigger inductance is needed. The last consideration is the effect of load regulation. Large inductor can limit the inrush current flowing into capacitor C1 as shown in Figure 3. High inrush current is not desirable because it can make the C1 voltage higher than the output voltage. It makes load regulation poor. If there is no pull-up resistor R1, inductor value L is chosen to be as large as possible, say 2 mH. Buffering capacitor C2 is to provide a greater-than-50 µA to the feedback pin of NCP1052. It is relatively much smaller than the output capacitor because the current consumption in this capacitor is much smaller and the output voltage cannot copy to this buffering capacitor if the buffering capacitor voltage is higher than the output voltage. Diodes D and D1 are recommended to be the same part for compatibility in speed and voltage drop. It helps the voltage in the capacitor C1 to be similar to the output voltage. The reverse blocking voltage of D and D1 is needed to be large enough to withstand the input voltage in buck and input voltage plus output voltage in buck-boost respectively. D2 is not a critical component. Its function is to make sure that feedback current is only in one direction. The accuracy of its voltage drop used in (1) is not important since the 4.3V reference voltage in the NCP1052 is loosely set. Zener Diodes Z1 is to clamp the output voltage when there is light load or no load. Hence, the accuracy of Z1 helps the regulation accuracy in the light load or no load condition. It is also the main component to consume energy when the circuit is in no load condition. The output voltage is clamped and hence the output capacitor is protected. Z2 and R1 are to set the output voltage at the nominal load current. Hence, their accuracy affects the regulation accuracy at the nominal load condition. The relationship between zener voltage and output voltage is shown in (1). Higher value of R1 helps to pull up the output voltage higher by reducing the charging rate of the buffering capacitor C1. Standby Condition The standby ability of the proposed buck converter is not good. It is because there is a VCC charging current Istart flows through the output capacitor in Figure 2(a). This charging current is a low-frequency pulsating signal. As a result, the voltage in the output capacitor continuously rises up by the charging current pulses. In order to prevent over voltage in the output capacitor, the zener Z1 absorbs the charging current. It consumes main portion of energy in standby. Output Capacitor Because of the burst-mode characteristic and the low-frequency VCC charging current, the output ripple is larger than those in PWM. Hence, a relatively bigger output capacitor is needed to keep output ripple small. However, big output capacitor needs a long time to build up the output voltage initially and hence the circuit may enter into fault mode in the startup in Figure 6. http://onsemi.com 5 AND8098/D The proposed buck-boost is better in term of the standby ability. It is because the VCC charging current in Figure 2(b) only passes through the inductor. The charging current pulses become an averaged energy stored in the inductor and consume smaller amount of power comparing to the buck case. fault mode with the 4.5 V-8.5 V-7.5 V-4.5 V hysteresis loop. Another method to supply the VCC voltage is coupling capacitor technique in Figure 8. The output voltage is coupled to the inserted capacitor when the diodes are closed. The voltage drop of the diodes compensate each other. Hence, the diode voltage drop effect can be neglected. The NCP1052 needs a nominal VCC voltage of 8V. The inserted resistor consumes some voltage from the output voltage Vout to make a 8V to the VCC pin. Based on the 0.5mA typical current consumption of VCC pin. The inserted resistance value is (Vout - 8) / 0.5 k. (a) Buck (a) Buck (b) Buck-boost Figure 7. Auxiliary Winding to improve standby Abillity (b) Buck-boost The auxiliary winding to supply the VCC voltage in Figure 7 is a method to improve the standby ability. The auxiliary winding keeps the VCC voltage above 7.5 V and disable the VCC charging current and hence its standby loss. The auxiliary winding is coupled from the inductor L with polarity same as the regulated output voltage. The VCC voltage in the auxiliary winding is designed to be between the normal VCC limits of 7.5 and 8.5 V typically. The frequency jittering feature loses when the VCC voltage is fixed. When output is shorted, there will be no voltage coming from the auxiliary winding and the circuit will enter Figure 8. Coupling Capacitor Technique to Improve Standby Abillity Temperature Rise The NCP1052 is a very compact package with the control circuit and high-voltage power switch. Its typical on resistance is 22 Ω. Temperature rise exists. It is recommended to design the PCB board with a large copper area next to the device as a heatsink. This heatsink decreases http://onsemi.com 6 AND8098/D the temperature rise and reduces the on resistance. Finally, the efficiency of the circuit is benefited. As shown in Figure 9, a 2 inch by 1.5 inch small surface-mount demo board of 12 V / 100 mA buck is presented. The design is made on a single-sided board. The bill of material is shown in Table 2. The component symbols are those in Figure 1. In order to have sufficient startup ability, the VCC capacitor is 6.8 µF which gives a 3.4 ms fault sampling time. Because of this feature, the circuit enters fault mode when output current exceeds 200mA after startup as shown in Figure 10(b). The efficiency of the circuit is typically 65% at 100 mA. EXAMPLES 12 V / 100 mA NCP1052 Buck Demo Board Figure 9. Layout of the Demo Board Table 2. Bill of Material of Buck Demo Board Part No Description Manufacturer IC NCP1052ST136 Switching Regulator ON Semiconductor D, D1 MURS160T3 1A 600V ultrafast ON Semiconductor D2 MMSD914T1 General diode ON Semiconductor D3 MRA4005T1 1A 600V standard recovery ON Semiconductor Z1 MMSZ12T1 12V 5% zener ON Semiconductor Z2 MMSZ6V8T1 6.8V 5% zener ON Semiconductor R1 CRCW08052001FRT1 2 kΩ Vishay C 594D227X9016R2T 220 µF, 16 V, tantalum Vishay C1 VJ1206Y224KXXAT 0.22 µF, 25 V, ceramic Vishay C2 595D685X9016A2T 6.8µF, 16V, tantalum Vishay C3 400WA10M12.5X16 400V 10µF Rubycon L UP2B-681 680µH Cooper http://onsemi.com 7 OUTPUT VOLTAGE (V) AND8098/D 14 Dual Output Buck-boost with Increased Output Current Capability 12 Replacing NCP1052 by NCP1055, which is with a current limit of 680 mA, the output current capability is increased. Larger value of inductor L is selected for high current. On the other hand, the current consumption of NCP1055 is higher than NCP1052 and the startup transient time is longer in a higher power application. Hence, the VCC capacitor is increased. When the VCC capacitor increased, its charging frequency is decreased. Output capacitor is also needed to be increased to reduce this lower-frequency charging current/ ripple. In addition, by adding one more auxiliary winding to the inductor a secondary output is made. A typical example is shown in Figure 11. In higher output current application, the load regulation is the major problem. The 5.1kΩ resistor plays an important role for the load regulation. The primary output voltage is higher than the secondary because it can increase the output current ability by stepping up the current in the transformer. The line regulation is shown in Figure 12 when the output currents are constant. 10 8 6 VIN = 300 Vdc VIN = 100 Vdc 4 2 VIN = 200 Vdc 0 0 50 100 200 150 250 300 350 OUTPUT CURRENT (mA) (a) Load Regulation 80 70 VIN = 100 Vdc EFFICIENCY (%) 60 VIN = 200 Vdc 50 VIN = 300 Vdc 40 30 20 10 0 0 50 100 150 200 250 300 OUTPUT CURRENT (mA) (b) Efficiency Figure 10. 12V / 100mA Buck Performance 1N4005 1N4746 MUR160 5.1k 1µF NCP1055P100 MUR160 1N4005 -24V / 200mA 22µF Universal AC Input 10µF 150µF GND 1.2mH / 92.3µH 220µF MUR160 Figure 11. Dual Output Buck-boost http://onsemi.com 8 -5V / 150mA AND8098/D 0 CONCLUSION 100 mA high-voltage low-cost buck and buck-boost circuits using NCP1052 are presented. These circuits are designed for cost-saving non-isolated application so that optocoupler and transformer are saved. The possible input voltage range is from 20 Vdc to 700 Vdc so that it is suitable for general AC/DC and DC/DC applications with positive or negative output voltages. It is noted that the standby ability of the circuits is not good because of the VCC capacitor charging current. However, it can be improved by adding an auxiliary winding to the VCC. The design consideration of each component in the circuits is explained. By replacing the NCP1052 with NCP1055, the output current can be increased. By adding an auxiliary winding, multi-output can be obtained. A 12 V / 100 mA demo board is presented with typical 65% efficiency. OUTPUT VOLTAGE (V) Output 2 with 150 mA -5 -10 -15 -20 Output 1 with 200 mA -25 50 100 150 200 250 300 INPUT VOLTAGE (Vac) Figure 12. Line Regulation of the Dual Output Buck-boost http://onsemi.com 9 AND8098/D ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. 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