The UCD3138 is the latest generation of digital power controllers from Texas Instruments and was officially released in the first quarter of 2012. Compared with the previous generation digital power controller UCD30xx, it has important improvements in many aspects, more features and more powerful performance. This article is based on a switching power supply with a hard-switched full-bridge (sub-side full-wave synchronous rectification) topology, detailing the UCD3138's Single Frame function and input voltage feedforward function. While understanding the above functions, you can clearly understand the advantages of UCD3138. References are given at the end of this article.
1 Single Frame function design and implementationSingle Frame is a new way to complete the input voltage detection on the primary side. This method can directly detect the current input voltage on the secondary side and realize the undervoltage protection function, so no additional input undervoltage protection circuit and optocoupler or digital isolator are needed.
1.1 Single Frame Function Introduction
The UCD3138 chip periodically issues a single or several drive pulses before the power system is officially operational, allowing the hard-switch full-bridge to operate for a single or multiple cycles. At this time, a voltage can be generated at the center tap of the secondary side of the transformer, and the voltage is linear with the input voltage. Therefore, the voltage is detected by the UCD3138 and calculated by software to obtain the input voltage. Based on the calculation result, the system can determine whether the input voltage reaches the turn-on point to determine whether to exit the undervoltage protection and start normal operation. In practical applications, in order to ensure that the upper tube on the primary side of the full bridge (the driving ground is floating) can be smoothly turned on, it is usually necessary to continuously transmit driving pulses for several cycles. In the hard-switched full bridge involved in this article, two drive pulses are actually sent to ensure that the UCD3138 successfully detects the correct input voltage.
1.2 hardware circuit design
The hardware circuit that implements the Single Frame function is relatively simple, as shown in Figure 1. The VIN_CT network is connected to the center tap of the secondary side of the transformer, then filtered by RC and connected to the EAP2 pin of the UCD3138. The main function of this circuit is to achieve the filtering and scaling of the voltage at the center tap of the transformer.
Figure 1: Single Frame hardware circuit
The turns ratio of the transformer is 5:2:2. The relationship between the voltage at EAP2 and the input voltage is:
1.3 Software Design
The software design that implements the Single Frame function includes two parts: the configuration of the drive signal and the voltage acquisition and conversion. The first part is the core configuration. The following section focuses on the first part.
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