ST AN3358 Application note

AN3358

Application note

Turbo2 600 V diodes: optimized solutions for PFC and other applications

Introduction

In a switched mode power supply, there are a great number of electronic functions where 600 V ultrafast diodes are used. Each diode has a specific function. In one application a parameter can be critical but secondary in another.

A rectifier manufacturer who wants to propose an optimized solution for each function needs to develop several families with different trade-offs (mainly between the forward voltage VF and reverse recovery charge Qrr).

STMicroelectronics’ Turbo2 600 V ultrafast diodes offer three different families in order to offer an optimal solution for each application.

After some general information about this new technology, a discussion of the PFC application, working in continuous mode, transition mode and fixed-off-time, is presented. In the case of continuous mode operation, hard switching and soft switching conditions are considered. Some other conventional functions are also touched upon.

September 2011

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Contents	AN3358

Contents

1	General information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		3
	1.1	Technology information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
	1.2	VF, Qrr trade-off for the three families . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
	1.3	Platinum doping and low leakage current . . . . . . . . . . . . . . . . . . . . . . . . .	3

2	Main applications of 600 V ultrafast diodes . . . . . . . . . . . . . . . . . . . . .			. 5
	2.1	Power factor corrector applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		5
		2.1.1	Boost diode in PFC working in continuous mode . . . . . . . . . . . . . . . . .	. 5
		2.1.2	Boost diode in PFC working in transition mode . . . . . . . . . . . . . . . . . . .	10
		2.1.3	Boost diode working in fixed-off-time (FOT) PFC . . . . . . . . . . . . . . . . .	12
	2.2	Other applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		13
3	Conclusion .		. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16
4	References . .		. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
5	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			17

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AN3358	General information

1 General information

1.1Technology information

The Turbo2 families are manufactured using simple rules to insure high quality and reliability. These diodes are planar structures on epitaxial layers. The wafers are thus subjected to reduced mechanical stress for planar diodes compared to mesa ones.

The use of epitaxial layers makes the VF/trr trade-off independent of the wafer thickness, the contrary of homogenous diodes. These properties make the manufacturing of large diameter wafers possible. So the wafers benefit from state-of-the-art technology on recent equipment.

Epitaxial diodes, which present good drift area thickness, are particularly suitable for diodes up to 600 V and exhibit a significantly superior VF/trr trade-off. The lifetime control of the carriers for the Turbo2 diodes is obtained through platinum (Pt) doping. Pt doping is required for high junction temperature applications because it results in low reverse current at elevated temperature and, in this way, presents a low thermal runaway risk.

1.2VF, Qrr trade-off for the three families

The three families are: STTHxxR06 (R stands for rapid with low Qrr), STTHxx06 (medium VF and Qrr), and STTHxxL06 (Low forward voltage).

Figure 1 shows where a trade-off occurs in three operational areas. A technology using gold doping is also shown.

Figure 1. VF - Qrr trade-off for an 8 A diode

VF	(V)
1.7		Typical values
						IF = 8 A
						VR = 400 V
						dIF/dt =	200 A/µs
						Tj = 125 °C
R Family				Gold doping
1.2
	Medium Family
	Platinum doping
				L Family			Qrr (nC)
0.7
0	200	400	600	800	1000	1200	1400	1600

1.3Platinum doping and low leakage current

Figure 2 shows the trade-off between leakage current IR and Qrr in several operational areas. The faster the diode, the higher the IR is. This rule is true for both gold and platinum doping. For the same Qrr, IR is approximately 100 times lower with platinum doping. The corresponding “R” family with gold doping would have a high maximum leakage current (18 mA at 125 °C and 400 V). As shown later in this Application note, with such a leakage current thermal instability can be reached for operating junction temperatures higher than 125 °C in a conventional application.

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General information	AN3358

It will also be shown that IR is also a critical parameter for diodes in axial and SMD packages.

Figure 2. IR - Qrr trade-off in several operational areas for an 8 A diode

100000	IRmax (µA)
			IF = 8 A
			VR = 400 V
10000			dIF/dt =	200 A/µs
			Tj = 125 °C
1000	R Family
	Medium Family		Gold doping
100		L Family
10
	Platinum doping		Qrr typ (nC)
1
0	500	1000	1500	2000

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AN3358	Main applications of 600 V ultrafast diodes

2 Main applications of 600 V ultrafast diodes

This section discusses the trade-offs in a common application. Boost power factor corrector (PFC) will be widely covered since it is a major application. A typical PFC circuit is shown in

Figure 3.

2.1Power factor corrector applications

Figure 3. Boost power factor corrector circuit

Vmains

L

Dboost

Vout

IRM

Vgate

2.1.1Boost diode in PFC working in continuous mode

Hard switching conditions

PFC applications are mainly designed in continuous mode when the power is greater than 200 W.

In such an application, it is well known that the greatest losses due to the diode are the switching losses in the transistor (Pontr) when it turns on. The reverse recovery current (IRM) of the boost diode flows into the MOSFET (Figure 3). Consequently, the best choice in most cases is the “R” family.

Switching losses due to IRM depend mainly on two parameters: the operating junction temperature Tj and the mains voltage Vmains.

Figure 4 and Figure 5 show that the switching losses for STTH8R06 quickly increase when

Tj increases and when Vmains decreases. These curves are drawn with a software tool realized by these authors.

If the PFC only works on 240 V mains, with a low operating junction temperature, switching losses will be less critical and the best trade-off could be the intermediate trade-off: STTHxx06.

However, most PFCs are designed to work in a wide mains voltage range (85 V-264 V) with an operating junction temperature (in the worst case) close to 100 °C. The “R” family will be the family usually recommended.

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Main applications of 600 V ultrafast diodes	AN3358
Figure 4. Switching losses versus Tj at turn off of the diode

14 Poff diode + Pontr due to the diode (W)

			STTH8R06D
12
10
8						Vmains = 90 V
					dI/dt = 400 A/µs
6						L = 0.5 mH
						Fsw = 100 kHz
4						VOUT = 400 V
						POUT = 400 W
2
0						Tj	(°C)
0	25	50	75	100	125	150	175

Figure 5. Switching losses versus Vmains at turn off of the diode

Poff diode + Pontr due to the diode (W)

12
			STTH8R06D		Tj = 100 °C
					dI/dt = 400 A/µs
10					L = 0.5 mH
					Fsw = 100 kHz
					VOUT = 400 V
8					POUT = 400 W
6
4
2
0					Vmains (V)
0	50	100	150	200	250	300

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