What is a Passive Output Stage of SMPS Circuits

The passive output stage is a traditional design approach commonly found in power supply systems. It utilizes passive semiconductor rectifiers and is suitable for applications that do not rely on battery power. This method is widely adopted in scenarios where switching power supply efficiencies between 72% and 84% are acceptable, such as off-line applications where heat dissipation is not a significant concern.

The Role of Rectifiers in Passive Output Stages

A key component in the passive output stage is the rectifier, which plays a vital role in determining the overall efficiency of the power supply. The selection of rectifier technology depends largely on input and output voltage requirements. These parameters influence the maximum reverse voltage experienced by the rectifier, dictating the appropriate rectifier type.

The maximum reverse voltage is calculated using the input voltage, the transformer turns ratio, and the configuration of the secondary winding. For example:

• In full-wave rectified outputs, rectifiers experience the input voltage multiplied by the secondary-to-primary turns ratio.

  

• Center-tapped secondary windings expose rectifiers to twice the voltage seen in full-wave rectification.

  

The equations for determining the minimum reverse blocking voltage (β) in different configurations are:

1. Forward Converter:

   $$V_r > k \cdot \left(\frac{n_2}{n_1}\right) \cdot V_{\text{in(max)}}$$

2. Flyback Converter:

   $$V_r>\left(\frac{n}{n_1}\right)\cdot (V_{\text{in}}+V_{\text{out}})$$

where k is 1 for full-wave secondary, and k is 2 for a center-tapped secondary.

For a detailed guide on how to design output stages, check out this guide on designing output stages in PWM circuits.

Types of Rectifiers and Their Characteristics

1. Ultrafast Diodes

Ultrafast diodes exhibit a forward voltage drop ranging from 0.8 V to 1.1 V and a reverse recovery time between 35 ns and 85 ns. While all P-N junction diodes have a significant reverse recovery time, ultrafast diodes are designed to minimize this delay.

• Reverse Recovery Time: This is caused by the stored charge in the P-N junction that must dissipate when reverse voltage is applied. During this process, a momentary reverse current flows, leading to substantial instantaneous power loss.

• Applications: Ultrafast diodes are preferred in output stages where the reverse voltage exceeds the capability of Schottky diodes.

2. Schottky Diodes

Schottky diodes have a forward voltage drop between 0.3 V and 0.6 V and a reverse recovery time of less than 10 ns. They are generally more efficient than ultrafast diodes but are limited by their maximum reverse blocking voltage.

• Voltage Limitations: Typically, Schottky diodes can handle reverse voltages up to 40-50 V, which restricts their use to outputs of 15 VDC or less. Higher voltage Schottky diodes (up to 200 V) exist but exhibit high junction capacitance, resembling P-N diode characteristics.

• Conduction Resistance: Schottky diodes have higher conduction resistance, causing their forward voltage drop to increase with higher currents.

For more details on choosing the right components for switching power supplies, refer to this guide on  PWM switching power supplies.

Comparing Ultrafast and Schottky Diodes

• Efficiency: Schottky diodes are generally more efficient due to their lower forward voltage drop.

• Reverse Voltage Capability: Ultrafast diodes are better suited for applications with high reverse voltage requirements.

• Forward Conduction: Ultrafast diodes have a flatter forward conduction voltage characteristic, while Schottky diodes’ forward voltage increases with higher current due to their resistive nature.

Learn more about the difference between various power supply designs, including buck converters and switching power supplies.

Efficiency Considerations in Full-Wave Bridges

The use of a full-wave bridge in passive output stages introduces additional rectifier losses, as the rectifiers are placed in series with the output current. This can reduce overall efficiency. However, if Schottky diodes can be used instead of P-N diodes, they often provide advantages such as:

• Smaller Secondary Windings: Schottky diodes’ improved reverse recovery performance enables more compact transformer designs.

• Better Reverse Recovery: The faster recovery time of Schottky diodes enhances overall efficiency.

See the diode comparison graph provided below.

If you're interested in exploring magnetic component design for power supplies, check out this resource on designing magnetic components.

Conclusion

The passive output stage remains a reliable choice for applications where moderate efficiency is acceptable, and heat dissipation is manageable. The choice between ultrafast and Schottky diodes depends on specific application requirements, including reverse voltage, efficiency, and current handling. By carefully selecting rectifiers, engineers can optimize the performance and reliability of their power supply designs. To learn more about buck converters for efficient voltage regulation, see this article on 555 timer buck converter design.