Current sharing in parallel diodes - STMicroelectronics

AN4381 Application note Current sharing in parallel diodes

Introduction The use of diodes in parallel is commonly found in power electronic design. An important consideration for this practice is the current sharing between diodes due to the difference of electrical characteristics. This application note highlights the cause of the behavior of several diodes are connected in parallel. Some recommendations will be given to help the designer to produce a safe design. An electro-thermal model is described which simulates the current and junction temperature of each diode for given application conditions. This tool is illustrated using an example.

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Contents

AN4381

Contents 1

2

Current sharing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1

The basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2

Thermal effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2.1

Thermal effects due to the negative temperature coefficient of VF

1.2.2

Thermal effects due to the dependency of ⍺ VF versus VF

. . . . . . .4

. . . . . . . . . . . . . . .5

1.3

Worst case configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.4

VF and QRR of diodes connected in parallel . . . . . . . . . . . . . . . . . . . . . . . . 6

1.5

Considerations of maximum dispersion: ∆VFmax (I0, 25 °C) . . . . . . . . . . . 9

1.6

Layout recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Electro-thermal model of diodes in parallel . . . . . . . . . . . . . . . . . . . . . 11 2.1

DC Forward characteristic model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

2.2

Full system model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3

Simulation results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.4

Comments about simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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Current sharing

1.1

The basics Consider a simple example of two 5 A, 600 V ultrafast diodes in a TO-220 package (STTH5R06) connected in parallel and mounted on a common heat sink. Figure 1 shows the forward voltage VF of two STTH5R06 diodes (D1 and D2) versus forward current IF at different junction temperatures. Diodes D1 and D2 have a difference of forward voltage equal to 550 mV at Tj1 = Tj2 = 25 °C. This reference value will be expressed as: ∆VF(5A, 25 °C) = 550 mV. This system satisfies the equations: Equation 1 IT(t) = IF1(t) + IF2(t) Equation 2 VF1(t,IF1,Tj1) = VF2(t,IF2,Tj2) In this first example IT(t) = 10 A = cst. For diode D1, having a lower forward voltage characteristic than D2, IF1 will be higher than IF2. When Tj1 = Tj2 = 25 °C (for example when the converter starts up) we get: Equation 3 IF1 = 6.7 A, IF2 = 3.3 A (IT = 10 A) and Equation 4 VF1(6.7 A, 25 °C) = VF2(3.3 A, 25 °C) = 2.18 V Figure 1. Two 5 A, 600 V ultrafast diodes (STTH5R06) connected in parallel D1(150°C) D1(125°C) D2(125°C) D1(25°C)

IF(A)

D2(25°C)

12 I T= I F1 + I F2 =10A 10

IF2(t)

IF1(t) IT(t)

D1

IF1=6.7A

I F1 =6.7A

8

IF1=6.1 A

D2

ΔVF(5A,125 °C)=200mV 6

Tj1

VF1

Tj2

VF2 4

ΔVF(5A,25 °C)=550mV

IF2=3.9A IF2 =3.3A

2

IF2 =3.3A VF(V)

0 0

0,5

1 1,5 1.25V VF1(6.1A,125°C) = VF2 (3.9A,125°C)=1.33V

(a)

2

2,5

3

3,5

VF1 (6.7A,25°C) = VF2(3.3A,25°C)=2.18V

(b)

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1.2

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Thermal effects Two thermal effects need to be considered. The first one due to the negative temperature coefficient of VF is very well known. The second one, due to dependency of ⍺ VF on VF is practically unknown.

1.2.1

Thermal effects due to the negative temperature coefficient of VF Assume that in thermal steady state equilibrium the junction temperature of these two diodes is equal to 125 °C. From Figure 1.b we can deduce: Equation 5 IF1 = 6.1 A, IF2 = 3.9 A (IT = 10 A) and Equation 6 VF1(6.1 A, 125 °C) = VF2(3.9 A, 125 °C) = 1.33 V For D1, having higher forward current than D2, Tj1 will be higher than Tj2. Assuming now Tj1 = 150 °C and Tj2 = 125 °C we get: Equation 7 IF1 = 6.7 A, IF2 = 3.3 A (IT = 10 A) and Equation 8 VF1(6.7 A, 150 °C) = VF2(3.3 A, 125 °C) = 1.25 V The negative temperature coefficient of the forward voltage drop ( ⍺ VF < 0) increases the current unbalance between each diode. It is practically true for every technology of diode excepted for silicon carbide diodes for which ⍺ VF > 0 in the operating area. The negative impact of this well-known thermal effect is in practice limited by the fact that all the diodes are generally mounted on a common heat sink (see Figure 12). The case temperature being approximately the same for each diode, the difference of junction temperature ∆Tj can be determined by: Equation 9 ∆Tj = RTH(j-c)∆P Where RTH(j-c) is the junction to case thermal resistance and ∆P the difference of power losses between each diode. Generally ∆Tj is lower than 25 °C, limiting the current unbalance due to the temperature.

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1.2.2

Current sharing

Thermal effects due to the dependency of ⍺ VF versus VF The second thermal effect is linked to the dependency of temperature coefficient ⍺VF versus forward voltage drop VF. Figure 1.b shows that diode D1, having a lower voltage drop than D2, has at the same time, a lower absolute value of ⍺ VF. This law is confirmed by Figure 2 giving the relationship between ⍺ VF and VF for different STTH5R06. Each point corresponds to one diode. ⍺ VF is measured for a forward current of 5 A. This second thermal effect is favorable for current equilibrium and partially counteracts the first thermal effect. Figure 2. Dependency of ⍺ VF versus VF for STTH5R06 - VF (5A) (mV/ °C)

10,5

10,0

9,5

9,0

8,5

8,0 VF (V) 7,5 2

1.3

2,1

2,2

2,3

2,4

2,5

Worst case configuration To produce a safe design it is important to consider the worst case situation represented in Figure 3. D1 has the lower VF and diodes D2 to Dn have the higher VF. We will define by ∆VFmax(I0, 25 °C) the maximum forward voltage dispersion of all the diodes at 25 °C and at current rating I0, for example I0 = 5 A for an STTH5R06. This configuration is defined by: diode D1 with VF1(I0, 25 °C) and diodes D2 to Dn with: Equation 10

VF2(I0, 25 °C) = VFn(I0, 25 °C) = VF1(I0, 25 °C) + ∆VFmax(I0, 25 °C) At the thermal steady state equilibrium we now get: IT(t) = IF1(t) + IF2(t) + … + IFn(t) IF2(t) = … = IFn(t) Tj1 > Tj2 … Tjn

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Equation 11 VF1(IF1,Tj1) = VF2(IF2,Tj2) = … = VFn(IFn,Tjn) Tj2 = … = Tjn Figure 3. Worst case situation of n diodes connected in parallel n-1 diodes with same higher VF IT IF1(t) D1 Tj1

VF1

IF2(t)

IFn(t)

IFn-1(t)

D2

Dn-1

Dn

Tj2

VF2 Tjn-1

VFn-1 Tjn

VFn

D1 with lower VF

To ensure a safe design, the more stressed diode (D1) has to work within its specified limits in terms of junction temperature, rms current, and transient surge current capability. The goal of the simulation tool presented in Section 2 is to address this question.

1.4

VF and QRR of diodes connected in parallel We first compare the forward voltage characteristic of the two diodes D1 and D2 connected in parallel with the case of two medium diodes also connected in parallel. Figure 4.a shows again the forward characteristic of D1 and D2 with the corresponding medium characteristic. Figure 4.b shows VF of D1 connected with D2 when Tj1 = Tj2 = 125 °C and when Tj1 = 150 °C and Tj2 = 125 °C (∆Tj = 25 °C corresponding to the worst case with a common heat sink) which can be compared with the two medium diodes at Tj1 = Tj2 = 125 °C. The curves from Figure 4.b can be deduced from the curves of Figure 4.a. For example, VF at 8 A of D1 connected with D2 when and Tj1 = 150 °C and Tj2 = 125 °C is defined by: Equation 12 VF1(5.3 A, 150 °C) = VF2(2.7 A, 125 °C) = 1.15 V In this particular case, the resulting forward characteristic of the 2 medium diodes is approximately the same as for D1 connected with D2 at Tj1 = Tj2 = 125 °C but is higher when Tj1 = 150 °C and Tj2 = 125 °C.

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Current sharing Figure 4. Worst case situation of n diodes connected in parallel

12

D1(125°C) D1(25°C) D1(150°C) D2(125°C) D2(25°C)

IF(A)

16

IF(A)

D1 with D2 TJD1 =TJD2 =125°C

D1 with D2 TJD1 =150°C TJD2 =125°C

14 10

12 2 medium diodes TJD 1 =TJD2 =125°C

10

8

IF1 =5.3A

8

6

6 4 2 0

IF1 +I F2 =8A

4

IF2 =2.7A 0

0,5

1

1,5

2

2,5

2 VF(V)

VF(V)

3

0 3,5

VF1 (5.3A,150°C) = V F2 (2.7,125°C) = 1.15V

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

VF1 (5.3A,150°C) = V F2 (2.7,125°C) = 1.15V

(a)

(b)

We now use, the same approach to compare reverse recovery charges: QRR at IT = 10 A and VR = 400 V, versus dIF/dt for different configurations. Consider the method employed with an example. In the case of diode D1 (Tj1 = 150 °C) in parallel with D2 (Tj2 = 125 °C) for IT = 10 A we get IF1 = 6.7 A and IF2 = 3.3 A. Since IT = IF1 + IF2, we have: Equation 13 dIT/dt =dIF1/dt + dIF2/dt If dIT/dt = 300 A/µs, we get dIF1/dt = 200 A/µs and dIF2/dt = 100 A/µs. Figure 5.b shows the switching oscillograms of D1 at IF1 = 6.7 A, dIF1/dt = 200 A/µs, Tj1 = 150 °C and that of D2 at IF2 = 3.3 A, dIF2/dt = 100 A/µs, Tj2 = 125 °C. In these conditions accurate measurements give: Equation 14 QRR(D1)(6.7 A, 200 A/µs, 150 °C) = 125 nC, QRR(D2)(3.3 A, 100 A/µs, 125 °C) = 59 nC So QRR of D1 in parallel with D2 is: Equation 15 QRR(D1 + D2) = QRR(D1) + QRR(D2) = 184 nC Assume now two diodes D1 are connected together. Obviously the current will be well balanced so we have: IF1 = 5 A, dIF1/dt = 150 A/µs, Tj1 = 125 °C. Measurements give: Equation 16 QRR(D1)(5 A, 150 A/µs, 125 °C) = 80 nC Equation 17 So QRR(2 D1) = 2 • QRR(D1)(5 A, 150 A/µs, 125 °C) = 160 nC

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In the same way we obtain: Equation 18 So QRR(2 D2) = 2 • QRR(D2)(5 A, 150 A/µs, 125 °C) = 150 nC It is interesting to note that while D1 has a lower forward voltage drop than D2, the reverse recovery charges are higher for D1. Figure 5. Switching oscillograms of D1 and D2 IT= 10A

IF1 =I F2 = 5A

dI T/dt =300A/µs IT = 10A 200A/µs, TJD1 =150°C V R =400V

TJD2 =125°C dIT/dt =300A/µs IT = 10A V R=400V

300A/µs

IF1 = 6.7A

IF2= 3.3A dIT/dt =150A/µs

ID2

100A/µs, TJD2 =125°C

ID1

2A/div

5A/div

10 ns/div

50 ns/div

(a)

(b)

Using this method for different dIT/dt we get the diagrams shown in Figure 6. Figure 6. QRR at IT = 10 A and VR = 400 V versus dIT/dt for different configurations 240

QRR (nC)

220

D1 in parallel with D2 Tj(D1) = 150 °C, TjD2 = 125 °C

200

2 D1 in parallel Tj(D1) = 125 °C

180

160 D1 in parallel with D2 Tj(D1) = TjD2 = 125 °C

140

120

2 D2 in parallel Tj(D2) = 125 °C

dIT/dt (A/µs) 100 100

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Current sharing To summarize, the VF and QRR comparison between two STTH5R06 in parallel having a dispersion of ∆VF(5A, 25 °C) = 550 mV with the ideal case constituted by medium diodes indicates that these characteristics are close when junction temperatures are identical. When the junction temperature difference between D1 and D2 increases, VF tends to decrease slightly and QRR tends to increase slightly. We cannot make a general conclusion from a particular case, but this example shows the general trend and illustrates a method which can be employed for others configurations.

1.5

Considerations of maximum dispersion: ∆VFmax (I0, 25 °C) The above considerations show the key impact of the maximum dispersion ∆VFmax(I0, 25 °C) on the current imbalance. For a designer it is not always easy to know this value. However, the following information can be given: •

∆VFmax(I0, 25 °C) increases with the voltage rating (VRRM). That means the forward voltage dispersion of 600 V ultrafast diodes will be higher than the dispersion of 200 V ultrafast diodes. The same consideration can be applied between 200 V ultrafast diodes and 100 V Schottky diodes and between 100 V Schottky diodes versus 45 V Schottky diodes.

•

For ultrafast diodes having the same VRRM ∆VFmax(I0, 25 °C) is higher for faster diode families. For example, If we consider 600 V ST ultrafast diodes, R family (Rapid) has higher dispersion than the L family (Low VF).

•

Practically all ST common cathode diodes in the same package can be connected in parallel without any precaution. Since these diodes are on the same die, ∆VFmax(I0, 25 °C) is very low.

•

Low voltage Schottky diodes (VRRM < 60 V) can also be connected in parallel without precaution.

•

Sometimes, designers fix by establishing a maximum ∆VFmax(I0, 25 °C) for all diodes connected in parallel to ensure a safe design. A good value can be: ∆VFmax(I0, 25 °C) = 40 mV. With this value, the current will be equitably shared between each diode and the thermal effects will become negligible.

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Layout recommendation The basic recommendation is to use a symmetrical layout as illustrated in Figure 7. Figure 7. Good and bad layouts Good layout Same RS1=RS2=RS3

Bad layout Different R S1≠ RS2 ≠ RS3

IT IT

RS1

RS2 IF1

IF3

IF2 D2

D1 VF1

VF2

D3

RS3

RS1 D1

VF3

RS3

RS2 IF1

D2 VF1

IF2

D3 VF2

IF3 VF3

If the layout is not symmetrical, the connection resistors will increase the current imbalance. In contrast, a symmetrical layout will balance the current in each diode. To have a real impact, the values of these resistors have to be of the same order as the resistance of the diodes. Adding the small resistance in series with each diode can be a good way to balance the current. On the other hand this solution will generate more power losses and will decrease the efficiency of the converter. We can observe in Figure 4.b that the resistance of the diode RD increases with lower forward current IF. So paralleling will be easier for lower current. The dependency of RD versus IF can also be considered as a compensation effect

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Electro-thermal model of diodes in parallel

2

Electro-thermal model of diodes in parallel

2.1

DC Forward characteristic model The first step is to construct a forward characteristic model of the diode (see Figure 8). Figure 8. Model of the forward characteristic of the diode I F =(VF,Tj)

Tj

I F =(VF,T j )

VF

This is defined by the following current generator IF depending on Tj and VF Equation 19 2 3 4 IF(VF,Tj) = a0(Tj) + a1(Tj)VF + a2(Tj)VF + a3(Tj)VF + a4(Tj)VF + a5(Tj)√VF Each coefficient ai(Tj) is of the form: Equation 20 a0(Tj) = a00 + a01Tj + a02Tj2 + a03Tj3 Figure 9 and Figure 10 give respectively ai(25 °C) and a0j coefficients for a STTH5R06. Figure 9. ai(25 °C) coefficients of STTH5R06

40

IF(A)

Example: Tj = 25°C

35

D1(25°C)

30 25

a 0 (25°C)

0.443113633610841

a 1(25°C)

1.823450442549035

a2 (25°C)

-1.596962213365812

a 3 (25°C)

1.497001749151565

a 4 (25°C)

-0.139628758047056

a5 (25°C)

-1.59534418112371

20 15 10 5

VF(V)

0 0

1

2

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Figure 10. a0i coefficients of STTH5R06 0,6

a0(Tj)

0,4

Original curve 0,2

Fitted curve

0 -0,2 -0,4 -0,6 -0,8 -1 -1,2

Tj(°C)

a 00 a 01 a 02

1.330314150823049 -0.040753310751888 0.000157525320538

a 03

0.000000033311313

-1,4 0

25

50

75

100

125

150

175

200

Figure 11 illustrates the precision of this model. Figure 11. Forward characteristic comparison between model and measurement 40

IF(A)

175°C 150°C 125°C 100°C

75°C

50°C 25°C

30

O

Measurement Modeling

20

10

VF(V) 0

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2.2

Electro-thermal model of diodes in parallel

Full system model Having now obtained a good model of the forward characteristic of the diode, we can construct the model of the total system. Without loss of generality, we consider the simple example represented in Figure 12. It consists of two STT5R06 diodes mounted on the same heat sink. The total current IT(t) is rectangular with a switching period TS = 10 µs, a duty cycle δ = 0.5 and a peak current IP = 10 A. The full model of this system is also shown in Figure 12. We can distinguish the electrical model defined in the previous paragraph from the thermal model. In this example the case temperature of the heat sink is constant and is equal to Tcase = 100 °C. It is important to understand that these two models will work together. VF1 and VF2 depend on Tj1 and Tj2 which depend on the power losses in the diodes (P1 and P2). Thus Tj1 and Tj2 depend upon VF1 and VF2 Figure 12. Full system model Tj1

Tj2

D1

D2

IT Ts=10 µs 10A Tcase

Time

δ =0.5 IT

P2(t)=VF2 (t).IF2 (t)

P1(t)=VF1 (t).IF1 (t) Tj1 Tj1

IF1 Tj2

IF2

VF1 D1

Rth(j-c)

Tj2

Cth(j-c)

Cth(j-c)

Rth(j-c)

Tcase

VF2 D2 Rth(j-c)=3°C/W Cth(j-c)=0.27sW/°C Electrical Model

Thermal Model

In this application note we consider only conduction losses. This hypothesis is justified not only for Schottky and silicon carbide diodes but also for the major applications using ultrafast diodes. Effectively, major switching losses, due to reverse recovery charges, are generally dissipated in the companion switch (MOSFET or IGBT) and do not affect the current imbalance of the diodes. For applications for which switching losses generated in the diodes are not negligible versus conduction losses, it is possible to complete the model. This more complex model is not covered in this application note.

2.3

Simulation results The simulations were performed with STTH5R06 having different values of ∆VFmax(5 A, 25 °C). Figure 13 shows the forward voltage characteristic of seven STTH5R06 diodes at Tj = 25 °C and gives ∆VFmax(5 A, 25 °C) versus the reference diode Dref having the highest VF.

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Figure 13. STTH5R06 with different ∆VFmax(5 A, 25 °C) I (A) 8 F

D3 D4 D5 D6 D7 D8 D Ref ΔVF(5A,25°C)=100mV

7 6 5 4 3 2 1 VF(V) 0 0,5

1

1,5

2

2,5

3

Figure 14 represents the variation current and junction temperature versus time of each diode for respectively ∆VFmax(5 A, 25 °C) equal to 100 mV, 300 mV, 400 mV and 600 mV. The current imbalance and the difference of junction temperature between Dref and the other diode increase with ∆VFmax(5 A, 25 °C). Even for ∆VFmax(5 A, 25 °C) = 600 mV, the difference of junction temperature is low (< 6 °C), in this example the thermal effect of negative ⍺ VF < 0 is weak. Up to ∆VFmax(5 A, 25 °C) = 300 mV the current imbalance is low

(IDref = 4.37 A, I6 = 5.63 A).

Figure 14. Simulation results IF(A) Tj(°C)

IF(A) Tj(°C)

20 120

20 120

ΔVF(5A,25°C)=100mV

15 110

TjD8

TjD1 = 112°C

TjD_Ref

TjD_Ref = 111 °C

10 100

ΔVF(5A,25°C)=300mV

90

0

80

TjD_Ref

TjD_Ref = 109,9°C

10 100

IT

ID8 =5,2A

5

90

0

80

ID6 =5,63A ID_Ref =4,37A

ID_Ref =4,8A

0

100

IF(A) Tj(°C)

20 120

200

300 Time (ms)

400

500

20 120

TjD_Ref = 109,56°C

10 100

80 0

400

500

TjD3

TjD1 = 113,68 °C

TjD_Ref

TjD_Ref = 107,84°C

10 100

IT

0

200 300 Time (ms)

15 110

TjD_Ref

90

100

(5A,25°C)=300mV ΔVF(5A,25°C)=600mV

TjD4 = 112,86 °C

15 110

5

0

IF(A) Tj(°C)

(5A,25°C)=300mV ΔVF(5A,25°C)=400mV TjD5

IT

ID5 =5,74A

5

90

0

80

ID3 =6,41A

ID_Ref =4,27A

14/16

TjD3 = 112,7 °C

15 110

IT 5

TjD6

I

=3,67A

D_Ref PROTECTION - Industrial Applications

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2.4

Conclusions

Comments about simulation This simulation can be easily extrapolated to more complex configurations integrating n diodes, more accurate thermal models (for example Cauer type circuit thermal model) including models of the heat sink, layout resistors, more complex IT(t) current wave forms. If designers are only interested in knowing steady state current and junction temperature it is possible to reduce the calorific capacity (CTH) to gain simulation time. This tool can be used to analyze both current imbalance and difference of temperature for transient surge currents.

3

Conclusions This application note shows the impact of forward voltage dispersion is generally more critical than thermal effects for the current imbalance problem. To perform a safe design it is important to be sure the most stressed diode works within its specified limits in the worst case situation. The simulation tool described allows estimation of the junction temperature and current of each diode for transient and steady state phases.

4

Revision history Table 1. Document revision history Date

Revision

30-Jul-2014

1

Changes Initial release.

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