Today, distributed power architectures are widely used in applications such as telecommunications and networking to keep power supplies as close as possible to the load, providing power to different loads in the system and providing greater reliability, flexibility, and thermal performance. ON Semiconductor offers a wide range of distributed power solutions for these applications, including both isolated and non-isolated solutions.
This article focuses on ON Semiconductor's new isolated flyback/boost regulator NCP1032 with integrated 200V power transistors and high voltage start-up circuitry. The NCP1032 is a miniaturized PWM switching regulator for flyback, forward or boost voltage conversion circuits. It integrates a 200V power MOSEFT tube and a high voltage start-up circuit; the external adjustable switching frequency can be up to 1MHz, and the switching frequency can be externally synchronized. Other key features include +/-1% reference voltage accuracy, externally adjustable wave-by-wave current limit, adjustable input undervoltage and overvoltage protection, frequency retraction in fault conditions, integrated current sampling leading edge blanking circuitry, and overheating Protection and so on. The NCP1032 is ideal for 24V/48V telecom power applications, as well as for medical system isolated power, Power over Ethernet (PoE), isolated DC-DC converter secondary-side bias supply, stand-alone low-power DC-DC converter, Low power bias supply, low power boost converter and other applications.
NCP1032 main function
1) High voltage start circuit and dynamic self-power supply
The NCP1032 integrates a 200V current source. When the VDRAIN voltage rises above 16.3V, the current source starts to output 12.5mA, charging the capacitor on Vcc. When the Vcc capacitor is charged to 10.5V, the current source is turned off; when the Vcc voltage drops to 7.55. When V is internal, the internal current source is turned back on to recharge the Vcc capacitor. The voltage on the Vcc capacitor maintains the chip's normal operation; the high-voltage startup and dynamic self-powered circuitry eliminates the need for an external auxiliary power supply circuit, saving cost and area.
In most cases, users want to reduce the power consumption of the chip's self-power supply, which can be solved by taking power from the auxiliary winding of the transformer. When Vcc rises to 10.5V, the chip can start normally. As long as the voltage generated on the auxiliary winding can maintain Vcc above 7.55V, the internal high-voltage current source can be prevented from being turned on, thereby reducing power consumption. At this time, the chip operates normally; In the short circuit or overload state, Vcc may drop below 6.95V. At this time, the power transistor is turned off, the chip will enter the reset start mode, the high voltage current source will be turned on to charge the Vcc capacitor, and the output will restart when Vcc rises to 10.2V. When the output is overloaded, Vcc does not enter the reset start mode when it is above 6.95V. Figure 1 is a high voltage start-up circuit.
At the end of the startup, the NCP1032 will have an overshoot of 1V. If you want to reduce the overshoot voltage at the end of the soft start, the conversion time between the 4.2V and the steady state value of the COMP pin voltage should be as short as possible. To speed up the compensation response speed.
At higher frequencies, the input power will rise linearly with the input voltage, mainly because the NCP1032's current-limit leading edge blanking circuit (LEB) and propagation delay will cause the chip to have at least 100ns of on-time, at higher operating frequencies. Under the 100ns duty cycle time is a bit large, displacement will occur, the input transmission power will be relatively large, resulting in increased power of short circuit protection at high frequencies.
The current limit setting of the NCP1032 includes the leading edge blanking circuit. The peak current of the power tube is set by an external resistor. The left side of Figure 3 is the external resistor setting current value curve.
2) Soft start
The NCP1032's integrated soft-start circuit reduces the voltage stress on the power tube during startup and the peak current on the transformer. When Vcc rises to 10.5V and the undervoltage protection is released, the chip enters the soft-start process. During the soft start process, the COMP voltage is clamped at 4.2V, and the peak current of the power tube increases from 57mA to the cycle, until the current rises to the current limit set point or the COMP pin voltage drops to 3.5V, the output voltage enters. Correction phase.
During the soft start process, if the output voltage rises to a stable value before the power tube current rises to the current limit, the COMP pin voltage drops below 3.5V, and the power tube current does not rise to the set value. If the output voltage has not risen to the set value after the power tube current rises to the current limit, the power tube current will be limited to the current limit setting and will not increase. The soft-start time is related to the input voltage, load size, and output capacitor capacity.
3) Overvoltage (OV) and undervoltage (UV) protection
The NCP1032 has overvoltage/undervoltage pins for overvoltage/undervoltage protection of the input voltage. When the voltage of pin 6 is lower than 1V or higher than 2.4V, the NCP1032 power transistor will be turned off, and the chip will be powered by an internal high voltage current source. Dynamic self-powered until over/under voltage release. Undervoltage protection and overvoltage protection have hysteresis of 70mV and 158mV, respectively. In both versions of the NPC1032, the NCP1032B has only undervoltage and no overvoltage protection. Figure 5 shows the setting method and working mode of overvoltage protection and undervoltage protection.
4) Maximum duty cycle and frequency out-of-synchronization
The NCP1032 internal oscillator is designed to support operating frequencies up to 1MHz. The operating frequency setting is synchronized with the external capacitor CT setting. The chip generates a discharge current source for capacitor charging. The charging current is 172Î¼A, the discharge current is 512Î¼A, and the charge-discharge time ratio is 1:3, the peak value of the charging voltage is 3.5V, and the discharge voltage valley is 3V.
During the discharge process, the power tube is turned off, so the maximum duty cycle supported by the device is limited to less than 75%. The NCP1032 supports external synchronization of the frequency. The operating frequency set by the CT is 25% lower than the synchronous frequency, as shown in Figure 6.
5) Input voltage feedforward
The input voltage feedforward allows the converter to respond quickly to changes in the input voltage. The NCP1032 can also support the input voltage feedforward function via the CT pin, as shown in Figure 7. The presence of a feedforward resistor changes the maximum duty cycle and operating frequency. If you want to set the maximum duty cycle to a fixed value, the RFF can be connected to a fixed voltage.
6) Minimum duty cycle hop period
The PWM comparator and latch internal delay time of the NCP1032 is within 200ns. If the duty cycle is less than 200ns, the chip will enter the skip cycle mode to ensure the output voltage is stable, but the output voltage ripple may increase.
Typical application of the NCP1032
Shown is a 48V to isolated 12V/3W bias supply circuit based on the NCP1032. This circuit is powered by the auxiliary winding while voltage sampling compensation is applied to the auxiliary winding. The NCP1032 is configured in a flyback topology and operates in discontinuous conduction mode (DCM), providing a low cost, high efficiency solution. Transformer T1 can use CoilCraftB0226EL, the added winding can support multiple isolated voltage output; CCT sets the switching frequency to about 300kHz. The specific design process can refer to the application guide AND8119 of ON Semiconductor. A resistor divider consisting of R3 and R4 sets the undervoltage lockout threshold to approximately 32V. As shown in Figure 9, in a 12V application, the efficiency of the output of the NCP1032 is different at different input voltages of 300kHz.
In terms of layout recommendations, in order to prevent EMI problems, high-current copper wires of high-frequency switches should be optimized. Therefore, short and wide leads are used for the power current path and the power supply ground, especially the transformer connection (on the secondary side and the secondary side). Figure 10 is an example of an optimized PCB layout.
To help users get the most out of the NCP1032, ON Semiconductor also offers support for other design tools, including the NCP1032 evaluation board, the NCP103x design spreadsheet, the application guide AND8119, and the Pspice simulation model. With the support of these tools, engineers can streamline the design process and speed time to market for various auxiliary power supplies.
to sum up
In order to solve the problem that the secondary side control scheme needs the primary side startup IC, ON Semiconductor has introduced a flyback/boost regulator NCP1032 with integrated 200V power tube and high voltage start-up circuit, which can realize stable and reliable secondary IC power supply, widely Used in applications such as PoE, -48V communication systems and solar inverters.
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