Philips Semiconductors Power Diodes Back diffused rectifier diodes A single-diffused P-N diode with a two layer structure cannot combine a high forward current density with a high reverse blocking voltage. A way out of this dilemma is provided by the three layer structure, the so-called P-I-N diode, where 'I' is a lightly doped (nearly intrinsic) layer. This layer, called the base, + is sandwiched between the highly doped diffused P + + + + + and N outer layers giving a P -P-N or P -N-N structure. Generally, the base gives the diode its high reverse voltage, and the two diffused regions give the high forward current rating. Such a three layer diode can be realised using a 'backdiffused' structure. A lightly doped silicon wafer is given + a very long N diffusion on one side, followed by a + relatively shallow P diffusion on the opposite side. This asymmetric diffusion allows better control of the thickness of the base layer than the conventional double diffusion method, resulting in a better trade-off between low forward voltage and high reverse blocking voltage. Generally, for a given silicon area, the thicker the base layer the higher the VR and the lower the IF. Reverse switching characteristics also determine the base design. Fast recovery diodes usually have N-type base regions to give 'soft' recovery with a narrow base layer to give fast switching. Ultra fast rectifier diodes Ultra fast rectifier diodes, made by epitaxial technology, are intended for use in applications where low conduction and switching losses are of paramount importance and relatively low reverse blocking voltage (VRWM = 150V) is required: e.g. Switched mode power supplies operating at frequencies of about 50 kHz. The use of epitaxial technology means that there is very close control over the almost ideal diffusion profile and base width giving very high carrier injection efficiencies leading to lower conduction losses than conventional technology permits. The well defined diffusion profile also allows a tight control of stored minority carriers in the base region, so that very fast turn-off times (35 ns) can be achieved. The range of devices also has a soft reverse recovery and a low forward recovery voltage. Schottky-barrier rectifier diodes Schottky-barrier rectifiers find application in low-voltage switched-mode power supplies (e.g. a 5V output) where they give an increase in efficiency due to the very low forward drop, and low switching losses. Power Schottky diodes are made by a metal-semiconductor barrier process to minimise forward voltage losses, and being majority carrier devices have no stored charge. They are therefore capable of operating at extremely high speeds. Electrical performance in forward and reverse Ratings and Characteristics conduction is uniquely defined by the device's metalsemiconductor 'barrier height'. Philips process minimises forward voltage drop, whilst maintaining reverse leakage current at full rated working voltage and Tj max at an acceptable level. Philips range of power schottky-barrier diodes can withstand reverse voltage transients and have guaranteed reverse surge capability. Power diode ratings A rating is a value that establishes either a limiting capability or a limiting condition for an electronic device. It is determined for specified values of environment and operation, and may be stated in any suitable terms. Limiting conditions may be either maxima or minima. All limiting values quoted in this data handbook are Absolute Maximum Ratings - limiting values of operating and environmental conditions applicable to any device of a specified type, as defined by its published data, which should not be exceeded under the worst probable conditions. VOLTAGE RATINGS VRSM Non-repetitive peak reverse voltage. The maximum allowable instantaneous reverse voltage including all non-repetitive transients; duration < 10 ms. VRRM Repetitive peak reverse voltage. The maximum allowable instantaneous reverse voltage including transients which occur every cycle, duration < 10 ms, duty cycle < 0.01. VRWM Crest working reverse voltage. The maximum allowable instantaneous reverse voltage including transients which may be applied every cycle excluding all repetitive and non-repetitive transients. Continuous reverse voltage. The maximum VR allowable constant reverse voltage. Operation at rated VR may be limited to junction temperatures below Tj max in order to prevent thermal runaway. CURRENT RATINGS IF(AV) Average forward current. Specified for either square or sinusoidal current waveforms at a maximum mounting base or heatsink temperature. The maximum average current which may be passed through the device without exceeding Tj max. IF(RMS) Root mean square current. The rms value of a current waveform is the value which causes the same dissipation as the equivalent d.c. value. IFRM Repetitive peak forward current. The maximum allowable peak forward current including transients which occur every cycle. The junction temperature should not exceed Tj max during repetitive current transients. Philips Semiconductors Power Diodes IFSM Non-repetitive forward current. The maximum allowable peak forward current which may be applied no more than 100 times in the life of the device. Usually specified with reapplied VRWM following the surge. Repetitive peak reverse current. The maximum allowable peak reverse current including transients which occur every cycle. Non-repetitive reverse current. The maximum allowable peak reverse current which may be applied no more than 100 times in the life of the device. IRRM IRSM 50 Ratings and Characteristics IF / A 30 slope Rs 20 10 Vo 0 0.5 1.0 The average junction temperature rise is the average dissipation multiplied by the thermal resistance; Rth j-mb or Rth j-hs. Subtracting the junction temperature rise from the maximum allowable junction temperature Tj max, gives the maximum allowable mounting base or heatsink temperature. The peak junction temperature rise for a rectangular current pulse may be found by multiplying the instantaneous power by the thermal impedance. Analysis methods for non-rectangular pulses are covered in the Power Semiconductor Applications handbook. Power diode characteristics A characteristic is an inherent and measurable property of a device. Such a property may be expressed as a value for stated or recognised conditions. A characteristic may also be a set of related values, usually shown in graphical form. 40 0 repetitively, either as a result of the average dissipation in the device or as a result of high peak currents 1.5 VF / V Figure 1: Piecewise linear approximation to diode forward characteristic FORWARD CURRENT RATINGS The forward voltage/ current characteristic of a diode may be approximated by a piecewise linear model as shown in fig:1. where RS is the slope of the line which passes through the rated current and VO is the voltage axis intercept. The forward voltage is then VF = VO + IF.RS, and the instantaneous dissipation is 2 PF = VO.IF + IF .RS. where IF is the instantaneous forward current. It can be shown that the average forward dissipation for 2 any current waveform is: PF(AV) = VO.IF(AV) + IF(RMS) .RS, where IF(AV) is the average forward current and IF(RMS) is the rms value of the forward current. Graphs in the published data show forward dissipation as a function of average current for square or sinusoidal waveforms over a range of duty cycles and form factors. To ensure reliable operation, the maximum allowable junction temperature Tj max should not be exceeded REVERSE RECOVERY When a semiconductor rectifier diode has been conducting in the forward direction sufficiently long to establish the steady state, there will be a charge due to minority carriers present. Before the device can block in the reverse direction this charge must be extracted. This extraction takes the form of a transient reverse current and this, together with the reverse bias voltage results in additional power dissipation which reduces the rectification efficiency. At sine-wave frequencies up to about 400Hz these effects can often be ignored, but at higher frequencies and for square waves the switching losses must be considered. The parameters of reverse recovery are defined in fig:2. STORED CHARGE The area under the IR versus time curve is known as the stored charge (Qs) and is normally quoted in microcoulombs or nanocoulombs. Low stored charge devices are preferred for fast switching applications. REVERSE RECOVERY TIME Another parameter which can be used to determine the speed of the rectifier is the reverse recovery time (trr). This is measured from the instant the current passes through zero (from forward to reverse) to the instant the current recovers to either 10% or 25% of its peak reverse value. Low reverse recovery times are associated with low stored charge devices. The conditions which need to be specified are: * Steady-state forward current (IF); high currents Philips Semiconductors Power Diodes Ratings and Characteristics increase recovery time. * Reverse bias voltage (VR); low reverse voltage increases recovery time. * Rate of fall of anode current (dI F/dt); high rates of fall reduce recovery time, but increase stored charge. * Junction temperature (T j); high temperatures increase both recovery time and stored charge. I dI F F dt trr time Qs 100% 25% or 10% I R I rrm occurs whilst the reverse recovery current is decreasing from the peak value (Irrm) to zero. In repetitive operation an average power can be calculated and added to the forward dissipation to give the total power. The peak value of transient reverse current is known as Irrm. The origin of reverse recovery losses is illustrated in fig:3. The conditions which need to be specified are: * Forward current (IF); high currents increase switching losses. * Rate of fall of anode current (dIF/dt); high rates of fall increase switching losses. This is particularly important in square-wave operation. Power losses in sine-wave operation for a given frequency are considerably less due to the much lower dIF/dt. * Frequency (f); high frequency means high losses. * Reverse bias voltage (VR); high reverse bias means high losses. * Junction temperature (Tj); high temperature means high losses. IF -dIF/dt Figure 2: Definition of trr, Qs and Irrm area = Qs trr SOFTNESS OF RECOVERY In many switching circuits it is not just the magnitude but the shape of the reverse recovery characteristic that is important. If the positive-going edge of the characteristic has a fast rise time (as in a so-called 'snap-off' device) this edge may cause conducted or radiated radio frequency interference (rfi), or it may generate high voltages across inductors which may be in series with the rectifier. The maximum slope of the reverse recovery current (dIR/dt) is quoted as a measure of the 'softness' of the characteristic. Low values are less liable to give rfi problems. The measurement conditions which need to be specified are as above. REVERSE RECOVERY CURRENT The peak value of the reverse recovery current (Irrm) is an important parameter in many switched mode power supply circuits. This is because the high transient current produced by a diode with a high Irrm can be interpreted by the circuit as a short circuit fault, which may cause the power supply to shut down or have apparently poor load regulation. Like the stored charge and reverse recovery time, Irrm increases with increasing temperature, so the effects sometimes only become apparent when the equipment gets hot. I rrm correlates with stored charge Qs. Thus choosing an Ultrafast diode with low Qs usually avoids this problem. SWITCHING LOSSES The product of the transient reverse current and the reverse voltage is power dissipation, most of which time IR Irrm VF VR VR Figure 3: Waveforms showing the origin of reverse switching losses. FORWARD RECOVERY At the instant a semiconductor rectifier diode is switched into forward conduction there are no carriers present at the junction, hence the forward voltage drop may be instantaneously of a high value. As the stored charge builds up, conductivity modulation takes place and the forward voltage rapidly falls to the steady state value. The peak value of forward voltage drop is known as the Philips Semiconductors Power Diodes forward recovery voltage (Vfr). The time from the instant the current reaches 10% of its steady-state value to the time the forward voltage drops below a given value ( usually 5V or 2V) is known as the forward recovery time (tfr). The forward recovery parameters are defined in fig:4. Ratings and Characteristics I F 10% The conditions which need to be specified are: * Forward current (IF); high currents give high recovery voltages. * Current pulse rise time (tr); short rise times give high recovery voltages. * Junction temperature (T j); The influence of temperature is slight. time tfr V F V V 5V / 2V F time Figure 4: Definition of Vfr and tfr fr