ST AN1453 APPLICATION NOTE

	AN1453
®	APPLICATION NOTE

NEW FAMILY OF 150V POWER SCHOTTKY

By F. GAUTIER

INTRODUCTION

Nowadays, the Switch Mode Power Supply (SMPS) is becoming more widespread as a result of computer, telecom and consumer applications.

The constant increase in services (more peripherals) and performance, which offers us these applications, tends to move conversion systems towards higher output power.

In addition to these developments dictated by the market, SMPS manufacturers are in competition, their battlefield being the criteria of power density, efficiency, reliability and cost, this last being factor very critical.

Today, SMPS designers of 12V-24V output have practically the choice between a 100V Schottky or a 200V bipolar diode.

The availability of an intermediate voltage has become necessary to gain in design optimization.

This is why STMicroelectronics is introducing a new family of 150V POWER SCHOTTKY diodes, intended for 12V and more secondary rectification, in applications such as desktops, file servers or adaptors for notebook.

Consequently, this application note will underline the advantages of a 150V Schottky technology compared to a 200V ultra fast diode.

In order to do this, the example of a Flyback converter will be used, and the static and dynamic parameters of the 150V Schottky will be detailed, as well as their influence in this converter.

1. CONDUCTION LOSSES & EFFICIENCY GAIN

Schottky diodes are mainly used for output rectification. In a typical SMPS working with a switching frequency lower than 100kHz, conduction losses are generally the main losses in the diode. They are directly linked to the curve of forward voltage (VF) versus forward current (IF), and obviously the best gain in efficiency will be obtained with the lowest VF .

In the following examples, the conduction losses between a 150V Schottky and a 200V bipolar diode in a Flyback and a Forward converter will be compared.

The conduction losses in the diode are calculated from the classical formula:

Pcond = VT0 × IF(AV) + Rd × IIF(RMS)2

Vt0:threshold voltage with VF(@ IF) = VT0 + Rd.IF Rd: dynamic resistance with Rd = DVF / DIF

where VT0 and Rd are calculated from the current range of current view by the diode (Fig. 1), for better accuracy.

Figure 1 shows also, the typical current through the rectification diode and the corresponding IF(AV)

and I2IF(RMS) :

Fig. 1: Typical current through a rectification diode

			ID
		Ima x
		Imin
								t
	0					αID.T	T
IF(AV) =		aID	(Imax + Imin )
	2
IF(RMS)2	= aID (I2max + Imin2 + Imax × Imin )
	3
Rd =	VF(@ Imax) - VF(@ Imin)					VT0 = VF(@ imax)	- Rd × Imax
		Imax - Imin

NB:

-In the datasheet, the VT0 and Rd are maximum values given for IF and 2 IF at 125°C.

-In discontinuous mode Imin=0.

July 2001

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APPLICATION NOTE

1.1. Example 1: FLYBACK

The first example is a 24V/48W Flyback converter working in continuous mode (Vmains=90V) with the following conditions:

α ID = 0.4, Imax ID = 6.66A, IminID = 3.33A, Iout = 2A

Fig. 2: Rectification diode in a Flyback converter

ID	Io u t
Vin

Calculations per diode give:

IF(AV)per diode = 1A and IF(RMS) per diode = 1.6A

We can now calculate the efficiency gain (Δη(%)= ηref - η) for this Flyback converter which has a reference (ref) efficiency of 85% with STPR1020CT:

Fig. 3: Example of efficiency gain in Flyback converter

	VT0	Rd			η=85
Pout=48W	typ(V)	mΩ	Pcond	P	%
Vout=24V	1.5A, 3A,		(W)	(W)	Δη%
	125°C
STPR102CT
2x5A / 200V	0.58	46.5	1.4	0 (ref)	0 (ref)
PN diode
STPR162CT
2x8A / 200V	0.54	46.5	1.32	-0.08	+0.12
PN diode
STPS10150CT
2x5A / 150V	0.50	43	1.22	-0.18	+0.27
Schottky diode
STPS16150CT
2x8A / 150V	0.47	40	1.14	-0.26	+0.39
Schottky diode

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1.2. Example 2: FORWARD

In the following example, the conduction losses in a 12V/96W Forward converter are simulated:

Fig. 4: Rectification diode in a Forward converter

D1

IL Io u

Vin

D2

α D1 = 0.3, ILmax = 9A, ILmin = 7A, Iout = 8A

Calculations per diode give:

IF(AV)D1 = 2.4A, IF(RMS)D1 = 4.39A

IF(AV)D2 = 5.6A, IF(RMS)D2 = 6.71A

The difference of efficiency between a STPR1620CT (2x8A, 200V Ultrafast) and a STPS16150CT (2x8A, 150V Schottky) for a 12V output, are given in table Fig. 5:

Fig. 5: Example of efficiency gain in Flyback converter

	VT0	Rd			η=85
Pout=96W	typ(V)	mΩ	Pcond	P	%
Vout=12V	7A, 9A,		(W)	(W)	Δη%
	125°C
STPR1620CT	0.8	20	6.48	Ref	Ref
STPS16150CT	0.68	20	5.60	-0.95	+0.72

These two examples show that whatever the type of converter, a significant efficiency gain can be achieved only by replacing a 200V bipolar diode by a 150V Schottky.

APPLICATION NOTE

2. REVERSE LOSSES AND TJMAX

2.1. Reverse losses: Prev

The reverse losses can be determined by:

Prev = VR × IR × (1- a)

with:

(1-): duty cycle when the reverse voltage (VR) is applied

IR: leakage current versus VR and operating junction temperature (Tj)

VR: reapplied voltage accross the diode

Fig. 6 shows an example of reverse losses in a Flyback converter with the following conditions:

(1- a) = 0.4, VR = 80V, Tj = 125°C

Fig. 6: Example of reverse losses in a Flyback converter

	IRtyp per diode	Prev
	100V, 125°C	per diode
STPS10150CT	130µA	4.2mW

Thus, the reverse losses are very low due to the low value of the leakage current.

The following paragraph will show that due to these low values of reverse current, the thermal runaway limit is only reached for high junction temperature.

2.2. Tjmax before thermal instability is reached

Remembering that the stability criterion is given

by:

dPrev < 1

dTj Rth(j− a)

with:

Prev = VR.IR(VR,Tjmax) .(1- a)

The above formulae give the critical value of the leakage current before the thermal runaway limit is reached:

	IR(VR,Tjmax)	=		1
			VR	× c.Rth(j− a)	× (1- a)

The evolution of the leakage current versus Tj and VR is given by:

IR(VR ,Tj) = IR(VR ,125) expc(Tj−125)

From these physical laws, it can be deduced that:

Tjmax	= 125 +	1		× In	IR(VR ,Tjmax)
		c		IRmax (VR ,125°C)datasheet

Example:

Flyback converter with 2 diodes in parallel

(1- a) = 0.4, c = 0.069, VR = 80V

Rth(j− c)total = 2.4°C / W, Rth(c −a) = 7.6°C / W

Fig. 7: Example of Tjmax with STPS10150CT

For a dual	IRmax	IR(VR,Tjmax)	Tjmax
diode	(80V, 125°C)
STPS10150CT	1.3mA	45.28mA	176.5°C

This example shows that in a typical application, a 150V Schottky can be used up to 175°C. STMicroelectronics specifies in the datasheet Tjmax at 175°C.

3. SWITCHING BEHAVIOUR

3.1. Turn-on behaviour

The behaviour at turn-on is characterized by a low value of peak forward voltage (VFP) and forward reverse recovery time (tfr) (Fig. 8).

Fig. 8: VFP and tfr for STPS16150CT

IF=16A	tfr	VFP
dIF/dt=100A/µs	(ns)	(V)
Tj=25°C
Per diode
STPS16150CT	100	2.2

These values depends mainly on the dIF/dt. The switching losses at turn-on are always negligible.

3.2. Turn-off behaviour

The turn-off behaviour is a transitory phenomenon (ns), but repetitive depending on the switching frequency. It is a source of spike voltage, noise and for high switching frequency, of non-negligible switching losses.

In order to illustrate this phenomenon, the example of a Flyback converter will be used once again.

The difference in behaviour between a 150V Schottky and 200V bipolar diode will be compared for the three following points: spike voltage, EMC and switching losses.

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