Diodes used in power electronics applications are generally required to have special characteristics, these are:
Unfortunately, it is not possible to achieve all of these criterion with one single style of diode and thus a number of different types of power diode are available for various applications. It is up to the circuit designer to judge which component is best suited for a particular application This will often result in a conflict between what is required and what is available and it is here the circuit design can be very important.
The most common diodes used in rectifier circuits, switching and inverter and converter circuits are:
Table 1: Diode ratings
| Type | Maximum Breakdown Voltage | Maximum Current Rating | Forward Voltage Drop | Switching Speed | Applications |
| High Voltage Rectifier Diodes | 30kV | ~500mA | ~10V | ~100nS | HV circuits |
| General Purpose diodes | ~5kV | ~10kA | 0.7 - 2.5 V | ~25mS | 50 Hz Rectifiers |
| Fast Recovery | ~3kV | ~2kA | 0.7 - 1.5 V | <5uS | SMPS. Inverters, Resonant ckts. |
| Schottky Diodes | ~100V | ~300A | 0.2 - 0.9 V | ~30nS | LV HF Rectification |
| Power Zener Diodes | Operates in break down ~300 V | ~75 W | - | - | References, Voltage Clamps |
From Table 1 general trends can be seen. If high voltage and high current ratings are needed then general purpose diodes can be used; this is as long as switching speeds are not too important. The faster switching diodes have restricted voltage and current ratings and if they are used in high stress application they must be placed in parallel and series to avoid damage.
The physical construction of a diode with a diffusion junction is shown in figure 1. When a diode is reverse biased ie. a positive voltage is applied to the cathode with respect to the anode, an electric field is formed between the cathode and anode specifically across the depletion region. The diode is 'reverse biased' and cannot conduct except for small leakage currents. However, if the electric field becomes too strong 'avalanche breakdown' occurs and the diode will become a short circuit and often be damaged. To counteract this the physical distance between the anode and cathode is increased by increasing the size of the bulk region and changing impurity atom doping levels.
Figure 1: Diffusion junction diode
Construction process: N type silicon substrate heated to ~1000oC in presence of vapour containing positive charged impurity atoms. P region diffused into N region. The resultant effect is to cause more charge carriers to be present within the diode when it is conducting. For the diode to switch OFF, the charge carriers must either recombine (minority) or be removed, the latter mechanism appearing as a reverse current (reverse recovery) flowing in the diode as it turns OFF. Put simply, diodes with higher voltage ratings have larger bulk regions, require more time to remove internal charges at turn OFF and are thus slower switching.
To achieve very fast switching, Schottky diodes (Fig. 2) can be used although their current and voltage ratings are restricted. Rectifying action dependant solely on majority carriers therefore no minority carrier recombination. Recovery is only dependant on the capacitance of metal-silicon junction. Polished pre-doped N+ epitaxial substrate with thin N layer barrier metal deposit. Interface between metal and N layer creates a barrier potential.
Figure 2: Schottky diode construction
Figure 3a and b
show typical styles of reverse recovery. The area within the negative portion
of each curve, 
,
is the total reverse recovery charge Qrr and represents the charge removal from
the junction and the bulk regions of the diode and is effectively independent
of the forward current in the diode. The recovery time t2 - t1
is dependant on the size of the bulk region thus high di/dt currents can be
obtained when using fast diodes. If the di/dt of the snap recovery is too high
and stray inductance exists in the circuit then extremely high and possibly
damaging voltage spikes can be induced.
(Note: 
).
Qrr can be found from manufacturers specifications thus the maximum reverse
recovery current Irr is given by:
If ta is very small compared to ta then ta trr and knowing the rate of decrease of current di/dt = Irr/ta Irr/trr leads to:
.
Figure 3: (a) Reverse recovery of a general purpose diode, (b) fast diode. Reverse recovery time trr = t2 - t0.
The effect of reverse recovery on the output voltage of a rectifier feeding a resistive load is shown in figure 4.
Figure 4: Bridge rectifier output voltage showing diode reverse recovery effects
Avalanche breakdown occurs when a high reverse voltage is applied to a diode and large electric field is created across the depletion region. The effect is dependant on the doping levels in the region of the depletion layer. Minority carriers in the depletion region associated with small leakage currents are accelerated by the field to high enough energies so that they ionise silicon atoms when they collide with them. A new hole-electron pair are created which accelerate in opposite directions causing further collisions and ionisation and avalanche breakdown.
Figure 5: Forward and reverse biased diode showing changing size of depletion region
Figure 6: Typical diode characteristics
Zener breakdown occurs with heavily doped junction regions (ie. highly doped regions are better conductors). If a reverse voltage is applied and the depletion region is too narrow for avalanche breakdown (minority carriers cannot reach high enough energies over the distance travelled) the electric field will grow. However, electrons are pulled directly from the valence band on the P side to the conduction band on the N side. This type of breakdown is not destructive if the reverse current is limited.
Figure 7: Operating range of a zener diode
Diodes are not only supplied as single components but can be purchased in modules such as single and three phase rectifiers. Often the diodes are included in transistor modules.
Figure 8: Typical diode module package
6.1 General Purpose Diodes
- Relatively high reverse recovery time (25S)
- Used in low frequency/speed applications (<1kHz)
- High current ratings (1 - 10,000A)
- High voltage ratings (50 - 5,000V)
- Generally manufactured using diffusion process
6.2 Fast Diodes
- Have low recovery times (<5S)
- Used in switched mode power supply and inverter circuits
- Reasonably high current ratings (1 - 3,000A)
- Reasonably high voltage ratings (50 - 2,000V)
- For voltage ratings above ~400V diodes made using diffusion process with reverse recovery time decreased by gold or platinum diffusion to increase conductivity
- For voltage ratings below ~400V diodes constructed using an epitaxial substrate provide faster switching giving reverse recovery times trr 50nS.
6.3 Schottky Diodes
- Barrier potential created at anode silicon-metal interface eliminates charge storage problems
- Metal layer deposited on thin epitaxial layer of N types silicon
- There are no minority carriers so recombination time does not exist and rectification only depends on majority carriers
- Recovery time only influenced by capacitance of silicon-metal interface
- Have relatively low forward voltage drops dependant on doping levels and thus barrier potential (0.2 - 0.9V)
- Leakage currents are higher ( >100mA)
- Low current ratings (1 - 300A)
- Low voltage ratings ( <100V)
6.4 Power Zener Diodes
- Zener diode more highly doped and designed to operate in breakdown region
- Breakdown above ~5V is avalanche rather than zener (limit current and no problem)
- Zener voltage levels over a wide range ( < 300V)
- Continuous power rating over moderate range (250mW - 75W)
- Can be used in transient suppression as a clamp and absorb up to 50kW for limited times
7.1 Maximum Average Forward Current, IFAV {IFAVM}
IFAV is the maximum average forward current usually specified at a given case temperature.
Examples:
7.2 Maximum RMS Forward Current, IFRMS {IFRMSM}
IFRMS is the maximum RMS value of the forward current which indicates the I2R heat dissipation capability of the diode.
Examples:
7.3 Maximum Peak Repetitive Forward Current, IRM
IRM is the maximum repetitive peak current usually specified for a 10mS half sine wave followed by zero amps for 10mS.
7.4 Maximum Peak Non-Repetitive Forward Current, IFSM
IFSM is the maximum peak non-repetitive 10mS half sine wave current at a specified temperature. These large currents can in fact be repeated as long as the maximum junction temperatures is not exceeded. Either delays between pulses will be stated or a graph such as shown in figure 10 may be given.
Figure 10: Non-repetitive surge current ratings showing derating as number of pulses are increased.
7.5 Maximum Repetitive Peak Reverse Voltage, VRRM
VRRM gives the maximum instantaneous and continuous reverse voltage that can be applied across the diode without causing avalanche breakdown.
7.6 Maximum Peak Non-Repetitive Reverse Voltage, VRSM
VRSM is the maximum instantaneous peak reverse voltage that can be applied across the diode usually over a 10mS period without causing avalanche breakdown.
When designing high voltage, high power rectifiers or other types of high voltage converters where diodes are necessary, diodes can be used in series or parallel blocks to increase ratings. Because the exact characteristics of each individual diode cannot be guaranteed to be the same it is prudent to include additional simple circuitry to ensure reverse voltage and forward current sharing occurs. Without protection circuits individual diodes can be destroyed due to over voltage or over current conditions.
8.1 Series Connected Diodes
If diodes are connected in series as shown in figure 12 the combined effect is to increase the reverse blocking capability. When forward current flows in the forward direction both diodes conduct the same current and the forward voltage drops are very similar. However, reverse voltages across each individual diode could vary drastically dependant on the characteristic of each diode. In figure 11, it can be seen that the voltage drop across D2 will not cause breakdown however, avalanche breakdown will occur in diode D1.
Figure 11: The effect of using diodes with different characteristics on the reverse blocking capability
The simplest protection circuit is to connected high value resistors in parallel with each diode. Theoretically, if the exact characteristics of each diode are known it would be possible to design the resistors so that exact voltage division is achieved. Practically however, this is not possible and a simple design can be used.
Example:
If Vs = 4000V and the reverse leakage current at VRRM for each diode is specified as
IR = IRRM = 50mA . Then, letting IR1 = IR2 = 10 IR gives R1 = R2 = 2000/500 mA = 4 MW. A standard value of 4.7 MW is preferred. The power loss is: (10 IR)2 R1 = (500m)2 4.7M = 1.2W. A power rating of 2W is preferred.For further transient voltage equalization an additional series connected RC network can be placed across each diode in addition to R1 and R2.
8.2 Parallel Connected Diodes
Connecting diodes in parallel will increase the current carrying capability. If it is possible to match the diodes so that approximately equal current sharing is achieved this should be done, however, in the event that the exact characteristics are not know sharing resistors (with associated losses) can to be used. Figure 12 shows exaggerated characteristics to highlight the variation in current through each diode. Again a simple method of calculating resistance values can be used if all resistors are set equal.
Figure 12: Mismatched parallel connected diodes and a voltage sharing circuit
Example:
The maximum average current in one leg of a bridge rectifier is 3000A.
VR1 = VR2 = VR3 = 1V then R1 = R2 = R3 = 1/1000 = 1 mW.
For further reading:
[1] "Power Electronics: Converters, Applications and Design", Mohan, Undeland and Robbins, Wiley, 1989.
Copyright © G. Ledwich 1998.
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