AN604 STMICROELECTRONICS | Alldatasheet

Document overview

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Technical content

Datasheet sections

  • 1 Diode forward characteristics
  • 1.1 Junction temperature dependence
  • 1.2 Diode forward charac teristics modeling: VT0(Tj), RD(Tj)
  • 2 Conduction losses: basic equations
  • 2.1 Application parameters: average and rms currents
  • 3 An application example
  • 3.1 Average and rms current calculation
  • 3.2 V T0 (Tj) and RD (Tj) calculation
  • 3.3 Conduction losses expression
  • 4 Revision history

into account the forward voltage dependence on temperature and the current waveform. during a switching cycle are shown in Figure 1. Figure 1. Ideal current and voltage wavefo rms of a diode in a switch mode power reason why it is important to accurately estimate them.

1 Diode forward characteristics

1.1 Junction temperature dependence

higher, the temperature coefficient becomes positive. This behavior is shown in Figure 2. Figure 2. Forward (I

1.2 Diode forward characteristics modeling: V T0(Tj), RD(Tj)

Figure 3. V T0(Tj) and RD(Tj) parameters application conditions. See the example in Chapter 3.

AN604 Diode forward characteristics Doc ID 3607 Rev 3 5/12 For any junction temperature VT0(Tj), RD(Tj) and the forward voltage drop VF(IF,Tj) can be calculated as follow: Equation 5 Equation 6 Equation 7 Where αVTO and αRD are thermal coefficients calculated from the 2 reference temperatures: Tjref1 and Tjref2. A common choice of Tjref1 and Tjref2 is 25 °C and 125 °C. These thermal coefficients are calculated with the following equations: Equation 8 Equation 9 Note: αVT0 < 0 and αRD > 0 whatever the diode technology. () jRef1jT0VjRef1T0jT0 TTα)(TV)(TV −⋅+= () jRef1jDRjRef1DjD TTα)(TR)(TR −⋅+= () () FDRT0VjRef1jjRef1FFjFF IααTT)T,(IV)T,(IV ⋅+⋅−+= () () jref1jref2 jref1T0jref2T0 T0V TT TVTVα − () () jref1jref2 jref1Djref2D DR TT TRTR −=α

Conduction losses: basic equations AN604 6/12 Doc ID 3607 Rev 3

2 Conduction losses: basic equations

Conduction losses are the average dissipated power in the diode during the forward conduction phase given in Equation 10: Equation 10 Equation 10 can also be written as follows: Equation 11 Where IF(AV) is the forward average current and IF(RMS) is the forward root mean square current flowing through the diode. Note: In case of a square waveform, a short form ula can be used to calculate conduction losses: Equation 12

2.1 Application parameters : average and rms currents

The average and rms currents are different for each application condition. They can be calculated using Equations 12 (average current) and 13 (rms current). Equation 13 Equation 14 Figure 4 presents simplified expression of average and rms currents of commonly observed waveforms in a power rectifier. In most cases, these waveforms can be used for a rough estimation. () dt(t)IT,IVT 1)(TP swT FjFF sw jCOND ⋅⋅= ∫ F(rms)jDF(av)jT0jCOND I)(TRI)(TV)(TP ⋅+⋅= δI)T,(IV)(TP F(AV)jFFjCOND ⋅⋅= ()∫ ⋅= swT F sw AVF dttIT 1I )( ()∫ ⋅= swT F sw RMSF dttIT 1I )(

Figure 4. Average and rms currents of commonly observed waveforms

  • ×++=
  • += IIIII III MinMax Min Max RMSF MinMax AVF t ID(t) IMax Tsw δ•Tsw 2II I2I MaxRMSF Max AVF δ δπ Half period sinusoidal waveform t ID(t) IMax Tsw δ·Tsw Triangular waveform II II MaxRMSF Max AVF δ δ

3 An application example

shows the ideal waveforms of the diode: IMin = 4 A, IMax = 11.8 A and δ = 0.6. Let us calculate the maximum conduction losses in the diode for this application. Figure 5. Flyback converter Figure 6. Ideal current and voltage waveforms of the diode in the flyback converter.

3.1 Average and rms current calculation

The first step is the calculation of the average and rms currents. The forward average current is the output current: IF(AV) = Iload = 4.7 A.

3.2 V T0 (Tj) and RD (Tj) calculation

The second step is the calculation of VT0 (Tj) and RD (Tj) in the application condition range. Figure 7. STPS30M100S forward voltage drop versus forward current datasheet. These values are summarized in Table 1. Table 1. V F(Max) values at IMin and IMax

αVT0 and αRD. Calculated values of these parameters are summarized in Table 2.

3.3 Conduction losses expression

Figure 8. Maximum conduction loss es versus junction temperature Table 2. V T0, RD, αVT0, and αRD parameters

4 Revision history

Table 3. Document revision history 24-Aug-2011 3 Completely revised for currently available products.