AND8098 ONSEMI | Alldatasheet

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obtained in the 12 V buck demo board.

  • 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. PRINCIPLE OF OPERATION Figure 1 shows the proposed buck and buck-boost converters. The rectifier circuit, which consists of capacitor C 3 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 V CC 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 I start = 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 V CC 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. Input Output D 3 C 3 D LC 1 C Z1 C 2 VCC FB DS D 2 Z2 D 1 R 1 (a) Buck Input Output D 3 C 3 D L C 1 C Z1 C 2 VCC FB DS D 2 Z2 D 1 R 1 (b) Buck-boost

Figure 1. Proposed Circuit Using NCP1052

at lower output current conditions. inverted. The maximum duty of NCP1052 is typically 77%. maximum switch current in non-isolated topologies. maximum output current can be increased by a transformer. Table 1. Summary of Topology Difference Using NCP1052 It is a must for the main output. Figure 4. DCM Inductor Currents in Burst Mode

http://onsemi.com 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 V CC 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 D 1 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 V CC 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 V CC 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 C 1 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 R 1, inductor value L is chosen to be as large as possible, say 2 mH. 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. Buffering Capacitor 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 D 1 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. D 2 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. Z 2 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 R 1 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 Z 1 absorbs the charging current. It consumes main portion of energy in standby.

the efficiency of the circuit is benefited.

12 V / 100 mA NCP1052 Buck Demo Board

Figure 9. Layout of the Demo Board Table 2. Bill of Material of Buck Demo Board

Figure 12. Line Regulation of the Dual Output

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