How to design diode voltage clamp circuit in communication power supply system?

1. The working principle of diode voltage clamp circuit
(1) Basic principles
The diode voltage clamp circuit is mainly composed of one or more diodes, which utilize the unidirectional conductivity of the diodes to achieve voltage clamp function. When the voltage in the circuit exceeds the conduction voltage of the diode, the diode conducts, clamping the voltage near the conduction voltage of the diode; When the voltage is lower than the conduction voltage of the diode, the diode cuts off and has no effect on the normal operation of the circuit.
(2) Working characteristics of different types of diodes
Ordinary diode: has a fixed conduction voltage, usually around 0.6-0.7V (silicon tube) or 0.2-0.3V (germanium tube). Its conduction speed is fast, but the clamping voltage accuracy is relatively low.
Zener diode: It can maintain a stable reverse breakdown voltage within a certain current range in the reverse breakdown state. By selecting the appropriate breakdown voltage of the voltage regulator diode, the voltage can be precisely clamped at the desired value.
Schottky diode: It has a low forward conduction voltage (usually 0.2-0.4V) and fast switching speed, suitable for applications that require high clamping voltage and response speed.
2. Design points of diode voltage clamp circuit
(1) Diode selection
Choose according to the clamping voltage requirements: If precise clamping voltage is required, a voltage regulator diode should be selected, and its breakdown voltage should be determined according to the system requirements. If the precision requirement for clamping voltage is not high, ordinary diodes or Schottky diodes can also meet the demand.
Consider current carrying capacity: A diode will flow a certain amount of current when conducting, so it is necessary to choose a diode with sufficient current carrying capacity to ensure that the diode will not be damaged due to overheating in the event of overvoltage.
Pay attention to reverse recovery time: Reverse recovery time refers to the time required for a diode to transition from a forward conducting state to a reverse cutoff state. In high-frequency communication power systems, diodes with short reverse recovery time should be selected to reduce switching losses and electromagnetic interference.
(2) Selection of Circuit Topology Structure
Single diode clamp: suitable for situations where the clamp voltage is not high and the overvoltage amplitude is small. The structure is simple and the cost is low, but the clamping accuracy is limited.
Double diode clamping: composed of two diodes connected in reverse series, it can improve the accuracy and reliability of clamping voltage. When one diode fails, the other diode can still play a certain clamping role.
Multi level clamping circuit: By cascading multiple diodes, it can achieve graded clamping of different amplitudes of overvoltage, improving the circuit's ability to withstand overvoltage.
(3) Parameter calculation
Clamp voltage calculation: For a voltage regulator diode clamp circuit, the clamp voltage is equal to the breakdown voltage of the voltage regulator diode. For ordinary diode or Schottky diode clamp circuits, the clamp voltage is approximately the forward conduction voltage of the diode plus other voltage drops in the circuit.
Current calculation: Calculate the current flowing through a diode when it is conducting based on the amplitude and duration of the overvoltage. At the same time, it is necessary to consider the load current in the circuit to ensure that the total current of the diode does not exceed its rated current.
Power calculation: Calculate the power loss of the diode based on its conduction current and clamp voltage. Choose diodes with sufficient power capacity to ensure that they will not be damaged due to overheating during long-term operation.
3. Possible problems and solutions encountered during the design process
(1) The clamp voltage is unstable
Reason for the problem: It may be caused by the large parameter variability of the diode, temperature changes, or changes in the parameters of other components in the circuit.
Solution: Adopt temperature compensation circuit to reduce the impact of temperature on clamp voltage; Choose diodes with good parameter consistency; Add feedback control circuit in the circuit to monitor and adjust the clamp voltage in real-time.
(2) Diode overheating and damage
Reason for the problem: It is usually caused by the current or power of the diode exceeding its rated value, or poor heat dissipation.
Solution: Reasonably select the model and specifications of the diode to ensure that its current and power capacity meet the requirements; Optimize circuit design to reduce diode current and power losses; Add heat dissipation devices such as heat sinks, fans, etc. to improve the heat dissipation capacity of diodes.
(3) Electromagnetic interference problem
Reason for the problem: The diode generates rapid current changes during conduction and cutoff moments, resulting in electromagnetic interference.
Solution: Connect capacitors or inductors in parallel across the diode to form a filtering circuit that suppresses electromagnetic interference; Adopt shielding measures to shield the diode and clamp circuit, reducing electromagnetic radiation.
4. Design Example
Taking a DC-DC converter in a communication power system as an example, design a diode voltage clamp circuit to protect the sensitive components at its output end. Assuming the output voltage is 5V, it is required to clamp the overvoltage below 6V.
Diode selection: Choose a voltage regulator diode with a breakdown voltage of 6V, rated current of 1A, and power capacity of 1W.
Circuit topology: Adopting a single voltage regulator diode clamp circuit, the voltage regulator diode is connected in reverse parallel between the output terminal and ground.
Parameter verification: Through simulation and experimental verification, the voltage regulator diode can conduct normally under overvoltage conditions, clamp the output voltage below 6V, and ensure that the current and power loss of the voltage regulator diode are within the rated range.

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