Components

Ideal Diodes OR Else! Part One: The Ideal Diode in the Small-Signal Domain

The first part of our series to explain “ideal diodes” in the small-signal domain and to contrast them with those found in the power domain.

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The intent of this series is to illustrate the “ideal” diode in the small-signal domain and contrast it with the ideal diode controllers found in the power domain. In the small-signal domain the descriptor “precision” is favored over “ideal diode.” In the power domain “ideal diode” is unabashedly found throughout—notably applied to special-purpose power management integrated circuits called Ideal Diode Controllers.

This series is provided in three parts:

The Ideal Diode from that First Electronics Class

Typically, in a first electronics class, the students are introduced to basic solid-state physics, the p-n junction and then the diode. When the task of analyzing circuits that incorporate diodes begins, the notion of using diode equivalent circuits or models is presented. The most basic diode equivalent circuit is called the ideal diode model. The ideal diode model is defined in Figure 1.

Figure 1. Used with author’s permission from Discrete and Integrated Electronics Analysis and Design for Engineers and Engineering Technologists.

Figure 1. Used with author’s permission from Discrete and Integrated Electronics Analysis and Design for Engineers and Engineering Technologists.

In Figure 1(a) we see the p-n junction, its correlation to a JEDEC (Joint Electron Device Engineering Council) DO-7 (also known as DO-204-AA) through-hole axial lead package, and its schematic symbol. We also observe the diode has two leads called the “anode” and “cathode,” respectively. Figure 1(b) illustrates the ideal diode model. When a diode is forward biased, its anode is more positive than its cathode and it is treated as a closed switch. Since a closed switch has zero ohms of resistance, it drops zero volts regardless of the size of the current through it. When a diode’s cathode is more positive than its anode, the diode is reverse biased. A reverse-biased diode behaves like an open switch. No current can flow through it. The results of the analyses are also provided in Figure 1(b).

In the case of forward bias, the diode voltage (VD) is 0 V. By Kirchhoff’s Voltage Law, the voltage across resistor R1 is 15 V. Ohm’s Law indicates the current through the diode (ID) is 7.5 mA. When the diode is reverse biased, the diode current (ID) is zero. By Ohm’s law, the voltage across the resistor R1 is zero and by Kirchhoff’s Voltage Law the diode voltage drop must be equal to the source voltage of 15 V.

When the diode it is forward biased and ID flows, its voltage drop is zero. Consequently, its power dissipation (PD) must be zero since PD = VDID. Similarly, when the diode is reverse biased, the current is zero. Even though, there is a voltage across it, the power loss will again be zero PD  = VRIR = 0.

A comparison between a 1N4004 silicon rectifier diode and the ideal diode model is shown in Figure 2. The silicon rectifier has a forward voltage drop that increases with increasing forward current. When reverse biased, the silicon rectifier diode passes a small (nA) reverse current. The ideal diode has a forward voltage drop of zero regardless of the size of its forward current. When reverse biased, the ideal diode has a reverse current of zero.

Figure 2. Used with author’s permission from Discrete and Integrated Electronics Analysis and Design for Engineers and Engineering Technologists.

Figure 2. Used with author’s permission from Discrete and Integrated Electronics Analysis and Design for Engineers and Engineering Technologists.

Real Diode Losses

A 1N5404 silicon rectifier is being used in the test circuit given in Figure 3. The knee voltage of a silicon diode is held to be between 0.6 to 0.7 V. However, the bulk resistance causes the forward voltage drops to increase beyond the knee voltage as the forward current increases (see Figure 2). At 3 A of forward current the 1N5404 rectifier is dropping 0.9 V. In low voltage applications, losing 0.9 V could result in poor performance. As can be seen, the diode dissipates 2.7 W and the diode’s temperature is 68oC (154oF) as measured by using a thermal camera.

Figure 3. A Silicon 1N5404 Rectifier Test Circuit.

Figure 3. A Silicon 1N5404 Rectifier Test Circuit.
Figure 4. A Schottky 1N5822 Rectifier Test Circuit.

Figure 4. A Schottky 1N5822 Rectifier Test Circuit.

A Schottky diode has a knee voltage that ranges from 0.2 to 0.3 V. Its bulk resistance tends to be smaller than that associated with a silicon rectifier. In Figure 4 we see that a forward current of 3 A, the 1N5822 Schottky diode has a forward voltage drop of 0.42 V. The power dissipation in this case is 1.26 W as illustrated in Figure 4. A thermal camera reveals the corresponding diode temperature is 44oC (112oF).

When a diode is required to operate at very large currents (e.g., 100 A or more) the power dissipation in a Schottky diode may become more than 50 W.

Silicon Versus Schottky

A simple performance comparison between the silicon diode rectifier and the Schottky diode is provided in Table 1. The Schottky diode has a low forward voltage drop as compared to the silicon rectifier diode. However, the Schottky diode has a much larger reverse leakage current. The available maximum reverse voltage ratings of Schottky diodes are much smaller than those of silicon rectifiers. The bipolar silicon rectifier uses both holes and electrons. When it goes from forward conduction to reverse bias, minority carriers must be cleared at the p-n junction, and this means significant reverse current will flow for a while. The time it takes from the application of the reverse bias until the reverse current has decayed to 10 percent of its maximum reverse-current value is called the reverse recovery time (tRR). The Schottky diode is a unipolar device. That means either electrons or holes are involved in its operation. There are no minority carriers and no minority carrier storage. Therefore, ideally its reverse recovery time (tRR) is zero. A Schottky diode is a splendid choice for reverse-polarity protection—particularly when transient events can occur.

Table 1. Comparison Between Silicon and Schottky Rectifier Diodes

Parameter

Silicon

Schottky

Low Forward Voltage Drop

 

X

Low Reverse Leakage Current

X

 

High Maximum Reverse Voltage

X

 

Short Reverse Recovery Time

 

X

 

The Ideal Diode in the Small-Signal Domain

To illustrate circuit performance, we’ll use NI Multisim (a free version of the software is available from its manufacturer National Instruments). Any simulation software at your disposal (like PSpice or LTspice) should produce similar results. In Figure 5 we have a half-wave rectifier. The generator produces a 30 V peak sine wave at a frequency of 100 Hz. The half-wave rectified waveform is displayed below the sine wave. The oscilloscope offset (position) controls are used to separate the two waveforms as shown in Figure 5.

Figure 5. A half-wave rectifier test circuit.

Figure 5. A half-wave rectifier test circuit.

Cursor 1 is at the peak of the sinusoidal input. While the peak is 30 V, the cursor is at the nearest quantized point at 29.967 V. The ideal half-wave rectified peak should be the peak sinusoidal value minus the knee voltage of 0.7 V for the 1N4148 silicon diode (29.3 V). Cursor 2 indicates 29.239 V. Because we are dealing with low-power signal levels, the voltage drop threatens precision rather than efficiency.

If we are required to rectify a low-level signal like 30 mV peak, that poses another dilemma. The signal is not large enough to overcome the diode’s barrier potential (e.g., about 0.7 V) to cause conduction. The diode will behave like an open circuit. Refer to Figure 6.

Figure 6. A half-wave rectifier display.

Figure 6. A half-wave rectifier display.

Using an Op Amp to Produce an Ideal (Precision) Diode

To circumvent the precision and conduction issues, we need an ideal diode circuit. Basically, we place the diode inside the feedback loop of an op amp as shown in Figure 7. An op amp in its linear mode of operation will do whatever is necessary to make its differential input voltage zero. This means when the non-inverting input terminal becomes slightly positive, the output of the op amp will go to a positive voltage level as necessary to cause the diode to conduct. Ideally, the diode will conduct to drive the inverting input terminal to the same potential as the non-inverting input terminal. The effect is equivalent to taking the op amp’s open-loop voltage gain (e.g., 100,000) and dividing the diode’s forward voltage drop (e.g., 0.7 V) to produce an effective forward voltage drop of 0.7V/100,000 = 7 µV. Additionally, temperature dependence is also minimized. The standard temperature coefficient of the silicon diode is reduced from -2.2 mV/oC to -2.2 mV/100,000 or -22 nV/oC.

In Figure 7 we see the input voltage is a 1V-peak sinusoid. The oscilloscope traces marked by cursor 1 indicate the peak input voltage is 996.9 mV and the corresponding peak rectifier voltage is 994.9 mV. They are essentially the same.

Figure 7. An “ideal” diode half-wave rectifier display.

Figure 7. An “ideal” diode half-wave rectifier display.

A “snapshot” of the circuit action is provided in Figure 8. The op amp’s non-inverting input is 1 V, the output of the op amp goes to 1.7 V, the diode drops 0.7 V, and the output voltage across RL is 1 V. The corresponding voltage at the inverting input is 1 V. The differential input voltage is zero as indicated in Figure 8.

Figure 8. “Ideal” diode half-wave rectifier operation.

Figure 8. “Ideal” diode half-wave rectifier operation.

The circuit provided in Figure 7 has frequency and amplitude limitations. If the input frequency is too high or the input signal is too small, distortion in the output signal will begin to appear.

Review and Conclusions

The simple ideal diode model taught in a first electronics class instructs us to treat a forward-biased diode as a closed switch and a reverse-biased diode as an open switch. Since the ideal diode is either a closed or open switch, it dissipates no power. Real diodes exhibit losses. Silicon rectifiers suffer greater losses than Schottky rectifiers because silicon rectifiers have greater forward voltage drops. Power losses not only impact efficiency, but also increase concerns about thermal management.

In terms of performance, silicon rectifiers offer larger forward voltage drops, larger available reverse voltage ratings, smaller leakage current, and a non-zero reverse recovery time. In contrast, Schottky rectifiers have smaller forward voltage drops and a reverse recovery time that is zero, ideally. However, Schottky diodes have smaller reverse voltage ratings and larger reverse currents. In a half-wave rectifier application, the peak load voltage will be one diode voltage drop less than the peak input voltage. Further, if the peak input voltage is too small, the diode may not even conduct. One solution is to place the diode inside the feedback loop of an op amp. This eliminates its forward voltage drop and the diode’s temperature dependence.

In part two we will see why the precision diode rectifier using an op amp is so “sluggish.” We shall see how to increase its operating frequency. We shall apply that knowledge to produce full-wave rectification. We shall also see how the technique can be applied to other applications like voltage limiters and breakpoint amplifiers.