SC11 Ratings and Characteristics - Philips Semiconductors / NXP Semiconductors

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 dilem m a 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+ diffus ion on the opposite side. This

asymmetric diffusion allows better control of the

thick ness 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 g ive 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 us e of epitaxial tec hnology 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-m ode power supplies (e.g. a 5V output) where

they give an increase in efficiency due to the very low

forward drop, and low switching los ses. Power Schottk y

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 f or an electr onic device.

It is determ ined for specif ied values of environm ent 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

RSM

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.

RWM

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 T

max in order to prevent thermal runaway.

CURRENT RAT INGS

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.

F(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 Ratings and Characteristics

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.

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.

0VF / V

0 0.5 1.51.0

IF / A

slope Rs

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

PF = VO.IF + IF2.RS. where IF is the instantaneous forward

current.

It can be shown that the average forward dissipation f or

any current waveform is: PF(AV) = VO.IF(AV) + IF(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 for ward diss ipation 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 m easurable 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

minor ity carrier s present. Bef ore the device can bloc k 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 r educes 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 k nown 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 param eter which can be used to determ ine the

speed of the rectifier is the reverse recovery time (trr).

This is measured from the instant the current passes

through zero (from for ward to reverse) to the inst ant 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 (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.

100%

time

Irrm

trr

25% or 10%

Figure 2: Definition of trr, Qs and Irrm

SOFTNESS OF RECOVERY

In many switching circuits it is not just the magnitude but

the shape of the reverse recovery characteristic that is

impor tant. If the positive-going edge of the c har acter istic

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 pr oblems. T he meas urement 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 r ec overy time, Irrm increas es with increas ing

temperature, so the effects sometimes only become

apparent when the equipment gets hot. Irrm correlates

with stored char ge Q s. T hus c hoosing an Ultraf ast diode

with low Qs usually avoids this problem.

SW ITCHING LOSSES

The product of the transient reverse current and the

reverse voltage is power dissipation, most of which

occur s whilst the reverse r ecovery current is dec reasing

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.

time

Irrm

trr

-dIF/dt

area = Qs

Figure 3: Waveforms showing the origin of reverse

switching losses.

FORWARD RECOV E RY

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 f orward voltage drop is k nown as the

Philips Semiconductors

Power Diodes Ratings and Characteristics

forward r ecovery voltage (Vfr). The time f rom the inst ant

the current r eaches 10% of its steady-state value to the

time the forward voltage drops below a given value (

usually 5V or 2V) is k nown as the forward rec overy 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.

time

Vfr

10%

5V / 2V

tfr

Figure 4: Definition of Vfr and tfr