DC/DC Converter Design in Telecom and Distributed Power Systems

This article examines some of the requirements that need to be met when designing DC/DC converters used in telecom and distributed power systems (DPS). Telecom DC/DC converters have an input voltage range of 36V to 75V DC. Today, distributed power systems typically use an intermediate bus voltage of 48V. The DC/DC converters used at the output of these systems have several basic functions. The main function is voltage level conversion, which is mainly used to reduce the required level while having the necessary accuracy and good transient response time. Because these converters are located very close to the load, they are called point-of-load (POL) converters. Another important function of the POL converter is isolation. Telecom applications require an input-to-output isolation voltage of 1500V.

To meet the above requirements, medium-power POL converters usually use a forward converter topology. However, using a forward converter brings certain challenges, so the price of the POL converter is higher than that of the forward converter. The main problem is how to power the PWM controller when the input voltage range is too large. The maximum supply voltage rating of most PWM controllers is much lower than the upper input voltage limit of the POL converter. There are several ways to solve this problem. First, a

linear regulator can be used to maintain the required voltage level (Figure 1). However, this solution wastes energy and therefore increases the no-load power consumption of the converter. Of course, it also increases the price.

Another possible solution is to use an auxiliary winding connected to the transformer (Figure 2). However, the duty cycle varies with input voltage and load, so the supply voltage can be maintained over a wide range. In addition, a dummy load needs to be connected to the output so that the controller can continue to operate when the converter load is unplugged, as the duty cycle is very small in this case. This solution requires higher no-load power consumption and transformer complexity, so its low price does not offset this disadvantage.

The first two solutions share a common disadvantage: the supply voltage still exists in the overload condition. Therefore, an overload protection independent of the controller supply voltage needs to be built in during the overload condition. This requirement increases the price and complexity of the converter.
To ensure that the supply voltage has reasonable stability across the load and at the same time ensure effective overload protection, it is necessary to connect the auxiliary winding to the converter's stabilizing inductor (Figure 3). A major disadvantage of this solution is that two transformers with perfect isolation are required. Therefore, the complexity and price of the converter increase again.

With its experience in power conversion, ON Semiconductor has launched a product called NCP1216A, which is a device specifically designed to solve the above problems.
NCP1216A uses high-voltage technology and no longer has controller power supply issues, because the device can be powered directly by the intermediate bus voltage through its high-voltage pin, which has a maximum rated voltage of 500V. The controller's power supply voltage is derived from this pin through dynamic self-powering (DSS) technology. The internal layout of this technology is shown in Figure 4.

As can be seen from the figure, the internal high voltage (HV) current source turns on when the Vcc voltage is too low and charges the external capacitor. When the voltage on this capacitor is high enough, the HV current source turns off. In this way, the supply voltage is subject to a 2.2V ripple (Figure 5) and is hysteresis regulated to an average value of 11V.
The user does not need to use any passive regulators (resistors and Zener diodes) or auxiliary windings with dummy loads, so the number of components is reduced. In addition, the HV current source is off for most of the time under no-load conditions, so the no-load power consumption decreases rapidly. The cycling of the Vcc supply voltage is also used to establish an overload protection scheme, which will be explained in the next paragraph.
Built-in overload protection is another advantage of the NCP1216A controller. This protection is implemented by monitoring the feedback pin operation (see Figures 6 and 7 for a better understanding of this function).

When an overload condition occurs, the feedback pin voltage is pulled high by an internal resistor. When this voltage reaches the 4.2V level, the internal fault flag is asserted. If the fault disappears during the Vcc drop, the controller does not interrupt operation. If the fault flag is still active when the Vcc voltage reaches the Vcc on-level, the controller stops and will try to recover during another Vcc cycle: The controller operates in burst mode, which means that the power consumption in overload conditions is low.
As can be seen in Figure 6, the operation of the NCP1216A controller is simple and the number of external components required is minimal. The DC/DC converter equipped with this controller is naturally short-circuit protected and has low power consumption in no-load conditions, without the need to add a simulated load to maintain Vcc.

Applying the NCP1216A to a 36W DC/DC circuit board, providing a 12V output at a maximum output current of 3A, the device's measured performance at a 48V input voltage level is as follows: efficiency: 86.5%, no-load standby power consumption: 93mW.