SC11 Ratings 1998 1 - Philips Semiconductors / NXP Semiconductors

1998 Dec 07

DISCRETE SEMICONDUCTORS

Ratings and Characteristics

Power Diodes

1998 Dec 07 2

Philips Semiconductors

Power Diodes Ratings and Characteristics

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

‘back-diffused’ 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 = 150 V) 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 efficiences

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 5 V 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 conduction

is uniquely defined by the device’s metal-semiconductor

‘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.

VRContinuous reverse voltage. The maximum

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

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Philips Semiconductors

Power Diodes Ratings and Characteristics

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 DC 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.

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 re-applied VRWM

following the surge.

IRRM Repetitive peak reverse current. The maximum

allowable peak reverse current including

transients which occur every cycle.

IRSM 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.

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=V

O+I

.RS, and

the instantaneous dissipation is PF=V

.IF+I

.RS. where

IF is the instantaneous forward current.

It can be shown that the average forward dissipation for

any current waveform is: PF(AV) =V

.IF(AV) +I

F(RMS)2.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

repetitively, either as a result of the average dissipation in

the device or as a result of high peak currents

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.

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

Fig.1 Piecewise linear approximation to diode

forward characteristic.

0VF / V

0 0.5 1.51.0

IF / A

slope Rs

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Philips Semiconductors

Power Diodes Ratings and Characteristics

rectification efficiency. At sine-wave frequencies up to

about 400 Hz 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 increase

recovery time.

•Reverse bias voltage (VR); low reverse voltage

increases recovery time

•Rate of fall of anode current (dIF/dt); high rates of fall

reduce recovery time, but increase stored charge.

•Junction temperature (Tj); high temperatures increase

both recovery time and stored charge.

Fig.2 Definition of trr, Qs and Irrm.

100%

time

Irrm

trr

25% or 10%

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. Irrm 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 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.

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Philips Semiconductors

Power Diodes Ratings and Characteristics

Fig.3 Waveforms showing the origin of reverse

switching losses.

time

Irrm

trr

-dIF/dt

area = Qs

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 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 5 V or

2 V) is known as the forward recovery time (tfr). The

forward recovery parameters are defined in Fig.4.

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 (Tj); The influence of temperature

is slight.

Fig.4 Definition of Vfr and tfr.

time

Vfr

10%

5V / 2V

tfr