ST AN2025 APPLICATION NOTE

AN2025

®

APPLICATION NOTE

Converter Improvement Using

Schottky Rectifier Avalanche Specification

STMicroelectronics gives in product datasheets useful information for all their Schottky Rectifier families
to define their working limit in the avalanche area. A simple method to determine if a Schottky diode can
work in the avalanche area in a given Switch Mode Power Supply (SMPS) is described in this document.
Then an accurate method will be defined in order to estimate the maximum average avalanche power
losses. Finally, a concrete example will be illustrated to show how the choice of a Schottky diode can be
optimized in order to improve the efficiency of the converter.

I. Introduction

The design of SMPS is subjected to heavy constraints in order to improve the trade-off between the cost
and the power density. One way to respond to these aggressive specifications is to drive components
closer to their intrinsic limits. The use of Schottky diodes in the avalanche area is a good example of this
evolution.

II. Description of the specification tool

STMicroelectronics guarantees for each Schottky diode a reference avalanche power capability corre­sponding to a rectangular current pulse: P

(1µs, 25°C) (given at tp=1µs and Tj = 25°C) - See figure 1.

ARM

Derating curves shown in figure 2 and figure 3 give
the admissible avalanche power for each Schottky
diode versus the operating junction temperature
(T

) and the pulse duration (tp).

j

P

(1µs,25°C) for each part number as well as

ARM

derating curves are given in the respective data
sheet.
The designer must ensure that the guaranteed av­alanche power P
alanche power in the application P

P

AVALANCHE

ARM (tp,Tj

(application) < P

) is greater than the av-

AVALANCHE

(tp, Tj)

ARM

:

Figure 1: P
avalanche power)

(1µs, 25°C) (Maximum repetitive

ARM

SCOPE

V

Clamp

I

PP

P (1µs, 25°C) = V

ARM Clamp PP

t = 1µs

p

T = 25°C

j

x I

REV. 1AN2025/1004

1/11

AN2025 - APPLICATION NOTE

Figure

1.2

1

0.8

0.6

0.4

0.2

0

P

ARMtpTj

-------------------------------------------- -

2: Figure 3:

P

ARMtp

P(t,T) /

ARM p j

25 50

,()

versus T

25oC

,()

P (t , 25°C) versus T

ARM p j

T (°C)

j

75

100 125 150 175

j

P

---------------------------------------- -

P

P(t,T) /

ARM p j

0.01 0.1 1 10 100 1000

tpT

ARM

ARM

,()

j

1µsT

P (1µs, T , 25°C) versus t

ARM j p

10

1

0.1

0.01

0.001

versus T

,()

j

p

t (µs)

p

III. Simple method to estimate the maximum avalanche peak power

III.1. Setting the Problem

Most of the time, it is difficult to accurately determinate the avalanche power through the diode in the hard­ware circuit.
This is mainly due to measuring problems such as delay time between current and voltage probe, the very
low pulse duration and the snubber circuit impact. Generally, in SMPS applications, the maximum
avalanche peak power occurs for a diode having the lowest clamping voltage. Practically, this diode is very
difficult to find.

These are the reasons why STMicroelectronics proposes a simple method to estimate the maximum av­alanche peak power P
> P

AVALANCHE

) but sufficient to determine whether or not a given Schottky diode will sustain the applied

PEAK_AV

. In most of SMPS applications, this method will be pessimistic (P

PEAK_AV

avalanche energy. This method only covers Schottky diodes used in rectification function for SMPS (see
figure 4), where the pulse duration of the avalanche current t

is less than 1µs.

p

Figure 4: Typical secondary rectification topologies

R

C

S

S

L

F

or

L

F

D

R

R

S

C

S

D

F

Transformer Transformer

2/11

AN2025 - APPLICATION NOTE

III.2. Switching-off analysis (simple method)

III.2.1. Introduction

The figure 5 shows the equivalent circuit that can be used to simulate a secondary rectification function
when the diode turns off : L
by the capacitance Cj, R

The figure 6 shows the corresponding current and voltage waveforms taking into account the delay time
between current and voltage probes. When the total current (current in the diode + current in the snubber)
is at maximum (I

PEAK

voltage across the diode is equal to V

represents the leakage inductance of the transformer. The diode is modelized

F

and CS are the snubber components.

S

), the voltage across the leakage inductance is zero (dIT/dt = 0). Consequently the

.

S

i

T

= I

PEAK

⇔ vD = -V

S

Figure 5: Basic equivalent circuit

Figure 6: Total current (iT) and voltage (VD)
when the diode turns off

v

RRM

SPIKE

D

i with delay time

T

i without delay time

T

C

S

R

S

L

iT

F

V

S

L : leakage inductance of the transformer

F

C

j

v

D

Snubber

-VS

-V

-V

I

PEAK

III.2.2. Switch-off behavior when the diode works in the avalanche area

The figure 7 shows the switch-off behavior when the diode works in the avalanche area.
This characteristic is made up of 2 distinct phases.

Phase 1: t ∈ [t

At t = t

: iT = I

0

0

, t1]

0

Figure 7: Switch-off behavior when the diode
works in the avalanche area

vD,i

T

vD = 0

i

T

20V/div
1A/div
20ns/div

0V

0A

t

t

0

1

C

R

i

L

V

S

T

F

S

S

I

-V

I

PEAK

-V

RRM

Clamp

0

I

1

-V

S

C

j

v

D

v

D

t

Phase2Phase1

dI

2

t

/dt

T

3/11

AN2025 - APPLICATION NOTE

The first phase corresponds to the charging of the junction capacitance of the diode, Cj.
The voltage across the diode V
figure 7). As was explained above, when the total current is equal to I
Once the current has reached I
(see figure 7).

decreases until it reaches the clamping voltage of the diode -V

D

, it then increases to reach the value I1 corresponding to VD = -V

PEAK

, VD is equal to -VS.

PEAK

Clamp

(see

Clamp

Phase 2: t ∈ [t

At t = t

: iT = I

1

vD = V

1

1

, t2]

Clamp

C

S

S

i

AVALANCHE

R

i

L

T

F

V

S

V

Clamp

During this phase, the diode works in the ava­lanche region. Consequently, the voltage across
the diode is equivalent to a voltage generator
equal to V

Clamp

.
The total current increases linearly with a slope
equal to:

di

After t

V

T

--------

=

----------------------------

dt

, the voltage across the diode increases towards -VS (see figure 7).

2

Clamp

L

F

Vs

(see figure 7)

These considerations show that:

I

< I

1

PEAK

III.2.3. Estimation of the maximum avalanche peak power: P

PEAK_AV

The figure 8 shows in blue color the total current IT (diode + snubber) and in black line the real avalanche
current waveforms during the switching-off of the diode.

Figure 8: Total current and avalanche current waveforms when the diode works in the avalanche area

Clamp

Phase2

i

T

R

S

C

S

I

PEAK

I

AR

I

V-V

di

Phase2

I

1

i

AVALANCHE

dt

Clamp

T

=

i

T

S

L

F

i

AVALANCHE

V

t

t

2

4/11

t

1

t

< 1µs

p