Summary
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 paper is based on a switching power supply with a hard-switched full-bridge (full-wave rectification on the secondary side) topology. The hardware design, software configuration, and measured data of the cycle-by-cycle protection function (cycle by cycle limitaTIon) of the UCD3138 are described in detail. While understanding the above functions, you can clearly understand the advantages of UCD3138. References are given at the end of this article.
1. Design and implementation of cycle-by-cycle protection
The cycle by cycle protection function is an important improvement of the UCD3138 compared to the UCD30xx. It is designed to achieve cycle-by-cycle protection on the primary side, enabling fast response and protection in situations such as input voltage surges and output short circuits.
1.1 Description of the cycle-by-cycle function
Each DPWM module inside the UCD3138 chip has one and only one cycle by cycle (CBC) hardware module, as shown in Figure 3 below. When the CBC module receives the trigger signal (FAULT), the CBC module responds immediately to limit the duty cycle of the current DPWM A and DPWM B, which implements the cycle by cycle protection.
Figure 1: CBC module inside UCD3138
1.2 Hardware design of cycle-by-cycle function
As shown in Figure 2 below, this circuit is the primary current detection circuit. With a current transformer (ratio 100:1), the voltage obtained at the AD04 network reflects the magnitude of the primary current. The relationship is: 
. The voltage at this network will be transferred to the UCD3138 chip via an analog comparator.
Figure 2: Primary current detection circuit
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