Clamping Diodes And Their Application
In just about any circuit, you are sure to find diodes. As simple as they are, these devices perform important functions that can be the difference between a circuit functioning as they should or failing. The nature of diodes lends them their importance. They are made from doped silicon or germanium semiconductors, and the level of doping matters to their final application.
Diodes conduct current in one direction with an associated drop in voltage. This characteristic finds use in signal rectification. In addition, there’s a minimum threshold voltage required in the forward direction before current begins to flow in the diode.
The I-V behavior of an ordinary diode
The voltage necessary for a current to flow in the forward direction depends on the type of semiconductor material used for its construction – about 0.6-0.8V for a silicon diode. If a positive voltage less than 0.6 is applied to its terminals (the positive terminal, anode is at a higher voltage than the negative cathode), no current flows through the silicon diode.
When the voltage applied is equivalent to the threshold, current begins to flow in the positive direction. Beyond the threshold, a small increase in voltage leads to a more significant increase in the amount of current flowing. This exponential operating region of a diode is exploited in power applications.
If a negative voltage is applied to a diode (the cathode is at a higher voltage than the anode), a similar scenario occurs – except we are dealing with negative currents here. The breakdown voltage is the reverse voltage that if exceeded, the current through the diode increases significantly with a small increase in voltage. For ordinary diodes, the goal is to operate far away from the breakdown voltage. However, the breakdown voltage is a property of specialized diodes useful in circuit protection against overvoltage.
Different applications for diodes
There are many kinds of diodes, with a wide range of functions.
A few common applications are:
- Rectifying an AC voltage to a DC voltage
- Controlling the magnitude of a signal/waveform
- Isolating signals from an input
- Mixing signals
- Protection from reverse current supply
- Protection from voltage spikes
For today, we’ll be discussing diodes used as clamps (clamping diodes). In the most basic sense, a clamping diode circuit is one consisting of a diode, capacitor and a resistor to limit the output voltage to a specified range. The diode is connected parallel to the load. These circuits are often used when dealing with sensitive inputs to prevent damage from static discharge, e.g., CMOS logic circuits.
Clamping circuit (clamp circuit, clamper, DC restorer, AC level shifter, clamping diode)
In the circuit, the diode conducts current in one direction only and limits the signal to a reference voltage. The output waveform replicates the shape of the input waveform exactly, except it has one edge clamped to zero voltage or a bias voltage.
The capacitor provides a direct current from the stored charge. The resistive load, together with the capacitor, determine the magnitude of the time constant (RC), ensuring the capacitor does not discharge voltage excessively when the diode is not conducting current. RC decides the range of frequencies over which the circuit will be effective.
Operational categories of clamping circuits
The behavior of a clamp circuit is a function of the output signal they deliver. The categories of operation are positive or negative, and biased or unbiased.
Positive clamping diode circuit unbiased
The clamping circuit fixes the voltage lower limit to zero, that is, the start of the signal is 0 V. The positive clamping circuit blocks the input signal when the diode is forward biased. During the negative half cycle of an AC signal, the diode is forward biased and allows electric current through it. There’s no output signal. The flowing current charges the capacitor to the input signal’s peak value. The capacitor charges in inverse polarity with the input.
During the positive half cycle of the AC signal input, the diode is reverse biased and does not allow electric current through it. The capacitor discharges, and the output voltage is the sum of the input voltage and the voltage discharged by the capacitor. Hence, the signal shifts upwards.
Positive clamping diode circuit positive biased
To shift the level of a DC signal, a biased clamping circuit is used. A DC voltage source (e.g., DC battery) is an additional component in this circuit.
During the positive half cycle of an AC signal, the battery voltage forward biases the diode while the input voltage is less than the battery voltage. Current flows and charges the capacitor. Once the input voltage exceeds than the battery voltage, the diode becomes reverse biased and stops allowing current through it. The output voltage is the sum of the input voltage, the voltage discharged by the capacitor and the positive voltage bias due to the battery. The output signal shifts upwards with a non-zero starting voltage.
During the negative half cycle, both the input signal voltage and the battery voltage forward bias the diode. The diode allows electric current through it and the capacitor is charged.
Positive clamping diode circuit negative biased
During the negative half cycle of the AC input signal, the battery voltage reverse biases the diode while the input voltage is less than the battery voltage. No current flows in the diode, and the signal appears in the output. Once the input voltage exceeds the battery voltage, the diode is forward biases, allows current to flow through it which charges the capacitor. No output signal appears.
During the positive half cycle, both the input signal voltage and the battery voltage reverse bias the diode. A signal appears in the output. The output voltage is the sum of the input voltage, the voltage discharged by the capacitor and the negative voltage bias due to the battery. The output signal shifts upwards with a non-zero starting voltage.
The operation of a negative clamping circuit is similar and can be inferred from the discussion above.
Diodes (clamping diodes) for protection against high voltages
In an informal sense, clamping diodes refer to diodes used as voltage clamps to protect sensitive components against transients and overvoltages. Transients are steep voltage spikes lasting in the region of 10 -100 microseconds that can occur due to lightning strikes, inductive load switching and electrostatic discharge. Their origins and magnitudes are not easy to predict and do not always start from zero voltage.
Specialized clamping diodes have been designed to handle such large voltage and energy spikes. They are transient suppression devices installed parallel to the load they are to protect. They work by diverting the transient away from the load and clamping the residual voltage.
Selecting the best transient suppression device requires investigating the trade-off between current handling capacity, leakage current, failure mode, voltage overshoot, capacitance, surface-mount capability, physical size, and price.
TVS diodes for protecting sensitive circuits
These voltage clamping diodes respond faster than many other classes of transient suppression devices and are available in a variety of surface mounting packages. TVS diodes have p-n junctions with larger cross-sectional areas than are found in regular diodes.
During normal operation, a TVS diode is invisible to the circuit. In the presence of a transient voltage, they clamp the voltage across the protected load to a given level without being damaged. The avalanche breakdown effect makes this possible whereby the diode which previously was not conducting electricity (apart from leakage current) begins to conduct and the spike in voltage results in electrons being knocked free in the diode’s microstructure. The free and energetic electrons in turn knock other electrons free, creating an avalanche. Once the transient event ends, the TVS diode reurns to normal.
TVS diodes can handle kilowatts of power due to transients and are useful protection devices in general electronics and telecommunication devices. They are also used to protect input signals from electrostatic discharges, e.g., USB ports.
Another clamping diode that functions as a transient suppression device is the metal oxide varistor. Below is a summary of how they compare:
TVS diodes
- Clamp at lower voltages
- Do not degrade with time
- Have low capacitance, suited wherever signal sensitivity is a high priority, e.g., USB ports
- More expensive
Varistors (MOVs)
- Clamping voltage is higher
- Degrade over time even when used within specification and become more conducting
- More effective at protecting circuits that requiring high capacitance
- Have greater tolerance for high energy/temperatures used on high voltage environments, e.g., power mains
- More cost effective