- After studying this section, you should be able to:
- • Describe typical rectifier applications.
- • Recognise rectifier polarity markings.
- • Describe typical rectifier Parameters.
- • Junction p.d.
- • Average Forward Current.
- • Repetitive Peak Forward Current.
- • Reverse Leakage Current.
- • Repetitive Peak Reverse Voltage.
- • Reverse Recovery Time.
- • Describe temperature effects on rectifiers.
- • Thermal runaway.
Figure 2.1.1. Silicon Rectifier Diodes
Silicon Rectifier Diodes
Rectifier diodes, like those shown in Figure. 2.1.1 are typically used in applications such as power supplies using both high voltage and high current, where they rectify the incoming mains (line) voltage and must pass all of the current required by whatever circuit they are supplying, which could be several Amperes or tens of Amperes.
As shown in Fig. 2.1.2, carrying such currents requires a large junction area so that the forward resistance of the diode is kept as low as possible. Even so, the diode is likely to get quite warm. A black resin case or even a bolt on heat sink helps dissipate the heat.
The resistance of the diode in the reverse direction (when the diode is ‘off’) must be high, and the insulation offered by the depletion layer between the P and N layers extremely good to avoid the possibility of reverse breakdown, where the insulation of the depletion layer fails and the diode is permanently damaged by the high reverse voltage across the junction.
Figure 2.1.2. Silicon Rectifier
Diode Polarity Markings
On the resin case of the diodes, the cathode is usually indicated by a line around one end of the diode casing. Alternative indications do exist however, on some resin encapsulated rectifier diodes a rounded end on the casing indicates the cathode as shown in Fig. 2.1.2. On metal stud rectifier diodes, the polarity of the diode may be shown by diode symbol printed on the case. The stud end of the diode is often the cathode, but his cannot be relied on, as Fig. 2.1.1 shows, it may be the anode! On bridge rectifier diodes the + and - (plus and minus) symbols shown on rectifier case indicate the polarity of the DC output and not the anode or cathode of the device, the AC input terminals are indicated by small sine wave symbols. One corner of the casing on some in line bridge rectifiers is also often chamfered off, but this should not be taken as a reliable guide to polarity, as rectifiers are available that use this indication as either the + or - output terminal.
Silicon rectifier diodes are made in many different forms with widely differing parameters. They vary in current carrying ability from milliamps to tens of amps, some will have reverse breakdown voltages of thousands of volts.
What the parameters mean.
Depletion layer (Junction) p.d.
The depletion layer or junction p.d. is the potential difference (voltage) that is naturally set up across the depletion layer, by the combination of holes and electrons during the manufacture of the diode. This p.d. must be overcome before a forward biased diode will conduct. For a silicon junction the p.d is about 0.6V.
Reverse leakage current (IR).
When a PN junction is reverse biased a very small leakage current (IR) will flow due mainly to thermal activity within the semiconductor material, shaking loose free electrons. It is these free electrons that form a small leakage current. In silicon devices this is only a few nano-Amperes (nA).
Maximum Repetitive Forward Current (IFRM).
This is the maximum current that a forward biased diode may pass without the device being damaged whilst rectifying a repetitive sine wave. IFRM is usually specified with the diode rectifying a sine wave having a maximum duty cycle of 0.5 at a low frequency (e.g. 25 to 60Hz) to represent the conditions occurring when a diode is rectifying a mains (line) voltage.
Average Forward Current (IFAV).
This is the average rectified forward current or output current (IFAV) of the diode, typically this would be the forward current when rectifying a 50Hz or 60Hz sine wave, averaged beteween the period when a (half wave) rectifier diode is conducting, and the period of the wave when the diode is reverse biased. Notice that this average value will be considerably less than the repetitive value quoted for IFRM. This (and other parameters) are also largely dependant on the junction temperature of the diode. The relationship between the various parameters and junction temperature is usually specified as a series of footnotes in manufacturers data sheets.
Repetitive Peak Reverse Voltage (VRRM)
The maximum peak voltage that may be repetitively applied to a diode when it is reverse biased (anode - cathode +) without damage to the device. This is an important parameter and refers normally to mains (Line) operation. E.g. a diode used as a half wave rectifier for rectifying the 230V AC mains voltage will conduct during the positive half cycle of the mains waveform and turn off during the negative half cycle. In a power supply circuit the cathode of the rectifier diode will usually be connected to a large electrolytic reservoir capacitor, which will maintain the cathode voltage of the rectifier at a voltage close to the peak voltages of the mains waveform. Remember that the 230V AC wave refers to the RMS value of the wave, so the peak value will be about 230V x 1.414 = approximately +325V. During the negative half cycle of the mains waveform the anode of the diode will fall to a maximum negative value of about -325V. Therefore there will be repetitive periods (50 or 60 times per second when the reverse voltage across the diode will be 325V x 2 = 650V. For this task therefore it would be necessary to use a rectifier diode with a VRRM parameter of at least 650V, and to ensure reliability there must be a safety margin for such an important component, so it would be wiser to select a diode with a VRRM of 800 or 1000V.
Maximum Working Peak Reverse Voltage (VRWM)
This is the maximum allowable reverse voltage. The reverse voltage across the diode at any time, whether the reverse voltage is an isolated transient spike or a repetitive reverse voltage.
Fig 2.1.3 Spike Suppression
Maximum DC Reverse Voltage (VR)
This parameter sets the allowable limit for reverse voltage and is usually the same value as VRRM and VRWM. Theoretically these maximum parameters could each be different, but as any voltage (instantaneous, repetitive or constant) that is greater by no more than about 5% than any of these parameters could potentially destroy the diode, it is always advisable to be cautious when fitting diodes and build in a reasonable margin to allow for unexpected spikes in voltage. One common safety measure to protect power supply rectifiers from externally generated spikes is to connect a small capacitance, high voltage capacitor, typically a disc ceramic type across each of the four diodes in a bridge rectifier as shown in Fig. 2.1.3.
Reverse Recovery Time (trr)
Fig 2.1.4 Reverse
Recovery Time (trr)
The time required for the current to fall to a specified low level of reverse current when switching from a specified forward current (diode turned on) to a specified reverse current (diode turned off, typically <10% of the value of the ‘on’ current). Typical trr times for rectifier diodes, though not as fast as small signal diodes, and depends somewhat on the voltages and currents involved, can be found to be in the tens of nanoseconds (ns) e.g. 30ns for a BYV28 3.5A IAF 50V rectifier and <60ns for a BYV44 dual 30A IAF 500V rectifier.
When a rectifier diode is used in a high speed switching operation such as in a switched mode power supply The reverse current should ideally fall to zero instantly. However when the diode is conducting (before switch off) there will be a large concentration of minority carriers on either side of the junction; these will be holes that have just crossed to the N type layer and electrons that have just crossed to the P type layer, and before they have been neutralised by joining with majority carriers. If a reverse voltage (VR) is now suddenly applied, as shown in Fig. 2.1.4, the diode should be turned off, but instead of the current through the diode falling instantly to zero, a reverse current (IR) is set up as these minority carriers are attracted back across the junction (holes back into the P layer and electrons back into the N layer). This reverse current will continue to flow, until all these charge carriers return to their natural side of the junction.
Each of these parameters can be affected by other factors, such as the ambient temperature in which the diode is operating, or the junction temperature of the device itself. Any semiconductor generates heat, especially those used in power supplies. Therefore it is essential that the design of such circuits takes into account the effects of temperature. One of the greatest problems is the prevention of Thermal Runaway where a diode (or any other semiconductor) increases its temperature, leading to an increase in current through the device, which leads to a further increase in temperature and so on until the device is destroyed. To prevent this problem each of the diode parameters references temperature, for example the reverse leakage current of a silicon PN diode is usually quoted at an ambient temperature of 25°C but is likely to approximately double for each 10°C above that figure. Also an increase in temperature will cause a decrease in the forward junction potential of about 2 to 3 mV for every 1°C of temperature increase. Temperature has an even greater effect on Schottky rectifiers.