The TL431 plays a crucial role in the feedback loop of switching power supplies (SMPS). It functions as a reference voltage combined with an open-collector error amplifier, making it cost-effective and easy to use. Despite its long-standing popularity, some designers overlook its bias current, which can negatively impact system performance.
A simplified schematic of the TL431 is shown in Figure 1. The circuit includes a reference voltage and an error amplifier that drives an NPN transistor. In this closed-loop system, a portion of the output voltage is compared to the internal reference voltage (Vref) of the TL431.
**Figure 1: TL431 Equivalent Circuit**
Another simplified DC model of the SMPS is presented in Figure 2. Here, Vout is compared to Vref via a resistor divider, and the theoretical output voltage is Vref divided by α. However, the actual output voltage is influenced by various gains and impedances in the system. This relationship is expressed in the following equations:
$$
V_{out} = \frac{V_{ref} \cdot \beta \cdot G}{1 + \alpha \cdot \beta \cdot G + \frac{R_{SOL}}{R_L}} \quad (2)
$$
$$
\text{Static Error} = \frac{V_{ref}}{\alpha} - V_{out} = \frac{V_{ref} \cdot (R_{SOL} + R_L)}{\alpha \cdot (R_{SOL} + \alpha \cdot \beta \cdot G \cdot R_L + R_L)} \quad (3)
$$
From equation (3), it's clear that increasing the gain helps reduce static error and improve output voltage accuracy. Another important factor affected by the gain loop is the system’s output impedance. By simplifying the system to its Thevenin equivalent, we can calculate the output impedance (Rth) using the no-load voltage (Vth) and the load resistance (RL).
When the load resistance equals the closed-loop output impedance (Rth), the output voltage drops to half of Vth. This allows us to compute Rth, also known as RSCL, using the following equation:
$$
RSCL = \frac{R_{SOL}}{1 + \alpha \cdot \beta \cdot G} \quad (5)
$$
This equation shows that a higher DC gain reduces the closed-loop output impedance, improving stability and dynamic response. To maintain high DC gain, most SMPS designs use a pure integrator configuration for the TL431, as shown in Figure 3.
**Figure 3: TL431 in Conventional Shunt Regulator Configuration**
In the absence of Rbias, the bridge current must be greater than the TL431’s reference pin bias current (up to 6.5 mA). For a 12V output, assuming a bridge current of 1 mA, Rlow and Rupp can be calculated accordingly. However, reducing the bridge current may help lower standby power consumption under no-load conditions.
The resistor RS must supply enough current to activate the optocoupler’s collector pin. If the optocoupler has a low current transfer ratio (CTR), RS should be chosen carefully to ensure proper feedback. For example, with a CTR of 150%, the required current through the LED is reduced, affecting the feedback loop.
To address these issues, adding a bias resistor (Rbias) ensures a stable bias current for the TL431. Calculating Rbias based on worst-case scenarios—such as heavy load and high CTR—ensures consistent performance. Experimental results show that adding a 3.3kΩ bias resistor significantly reduces the output impedance from 57mΩ to 4mΩ, as illustrated in Figure 4.
**Figure 4: Impact of Bias Current on TL431 Performance**
In conclusion, properly biasing the TL431 with an external resistor is essential for optimal performance. If minimizing no-load power is a priority, alternatives like the TLV431 or NCP100 offer lower bias currents and smaller reference voltages. Additionally, choosing a resistor value close to standard values (e.g., 1kΩ) can simplify design and improve reliability.
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