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A [[snubber]] circuit is essential for [[Flyback converter]], to prevent the transistor from burning up. The overshoot on the transistor voltage, is due to the leakage inductance, <math>L_k</math>, of the transformer. |
A [[snubber]] circuit is essential for [[Flyback converter]], to prevent the transistor from burning up. The overshoot on the transistor voltage, is due to the leakage inductance, <math>L_k</math>, of the transformer. |
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+ | [[File:Flyback w snubber.svg]] |
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− | =Variables= |
+ | ==Variables== |
* <math>P_s</math> - max power dissipated by the snubber resistor |
* <math>P_s</math> - max power dissipated by the snubber resistor |
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* <math>R_s</math> - snubber resistor |
* <math>R_s</math> - snubber resistor |
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* <math>f_s</math> - switching frequency |
* <math>f_s</math> - switching frequency |
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* <math>V_g</math> - Input voltage to the converter |
* <math>V_g</math> - Input voltage to the converter |
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− | * <math>V_t</math> - max acceptable |
+ | * <math>V_t</math> - transistor max acceptable voltage |
* <math>I</math> - average input current |
* <math>I</math> - average input current |
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* <math>L_m</math> - magnetizing inductance of the transformer |
* <math>L_m</math> - magnetizing inductance of the transformer |
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− | * <math>L_k</math> - |
+ | * <math>L_k</math> - leakage inductance of the transformer |
− | * <math>V_{t-peak}</math> - |
+ | * <math>V_{t-peak}</math> - transistor peak voltage, spec from datasheet |
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− | =Design= |
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+ | ==Transistor snubber design== |
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;Leakage inductance |
;Leakage inductance |
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− | It is not easy to calculate the leakage inductance of a transformer, but it can be measured after the transformer is built, or if a prebuilt transformer is used, it can be obtained from a datasheet. It can be assumed that the leakage inductance is 3% of the magnetizing inductance, <math> |
+ | It is not easy to calculate the leakage inductance of a transformer, but it can be measured after the transformer is built, or if a prebuilt transformer is used, it can be obtained from a datasheet. It can be assumed that the leakage inductance is 3% of the magnetizing inductance, <math>L_m</math>. |
<math>L_k \approx 0.03 * L_m</math> |
<math>L_k \approx 0.03 * L_m</math> |
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− | <br /><br /> |
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+ | If a transformer is well designed, leakage inductance can be reduced to 1% of the magnetizing inductance. |
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+ | ===RCD snubber=== |
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;Snubber resistor |
;Snubber resistor |
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− | To calculate the snubber resistance, <math>R_s</math>, an acceptable max transistor voltage, <math>V_t</math>. You want to select a <math>V_t</math> that has a wide margin from the peak transistor voltage rating specified in |
+ | To calculate the snubber resistance, <math>R_s</math>, an acceptable max transistor voltage, <math>V_t</math>. You want to select a <math>V_t</math> that has a wide margin from the peak transistor voltage rating specified in its datasheet. It still must be greater than the transistors blocking voltage, <math>V_g + V/n</math><br /> |
<math>V_{t-peak} > V_t > Vg + V/n</math><br /> |
<math>V_{t-peak} > V_t > Vg + V/n</math><br /> |
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<math>V_s = V_t - V_g</math><br /> |
<math>V_s = V_t - V_g</math><br /> |
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<math>R_s = V_s^2/P_s</math> |
<math>R_s = V_s^2/P_s</math> |
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− | <br /><br /> |
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;Snubber capacitor |
;Snubber capacitor |
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− | <math>C_s >> |
+ | <math>C_s >> \frac{ 1 }{ f_s*R_s }</math> |
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− | <br /><br /> |
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+ | ;Snubber diode |
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+ | The diode voltage must be able to block voltage a high voltage, 1N4007 tends to work. |
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+ | *<math>P_s = \frac{1}{2} L_f I^2 f_s = (0.5)(30~{\rm \text{µ}H})(1.5~{\rm A})^2(100~{\rm kHz}) = 3.375~{\rm W}</math> |
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+ | ==Schottky snubber design== |
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+ | If you choose to use a schottky diode its a good idea to have a snubber. |
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+ | ;design to be added |
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− | *<math>P_s = 1/2L_fI^2f_s = (0.5)(30uH)(1.5A)^2(100kHz)=3.375W</math> |
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− | =References= |
+ | ==References== |
* [http://ecee.colorado.edu/~ecen4517/materials/flyback.pdf Flyback transformer design] |
* [http://ecee.colorado.edu/~ecen4517/materials/flyback.pdf Flyback transformer design] |
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− | * [http://www.st.com/stonline/books/pdf/docs/10733.pdf |
+ | * [http://www.st.com/stonline/books/pdf/docs/10733.pdf Converter Improvement Using Schottky Rectifier Avalanche Specification] |
+ | [[Category:Howto]] |
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− | Schottky Rectifier Avalanche Specification] |
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+ | [[Category:Electronics]] |
Latest revision as of 17:19, 15 December 2020
A snubber circuit is essential for Flyback converter, to prevent the transistor from burning up. The overshoot on the transistor voltage, is due to the leakage inductance, , of the transformer.
Variables
- - max power dissipated by the snubber resistor
- - snubber resistor
- - snubber capacitor
- - switching period
- - switching frequency
- - Input voltage to the converter
- - transistor max acceptable voltage
- - average input current
- - magnetizing inductance of the transformer
- - leakage inductance of the transformer
- - transistor peak voltage, spec from datasheet
Transistor snubber design
- Leakage inductance
It is not easy to calculate the leakage inductance of a transformer, but it can be measured after the transformer is built, or if a prebuilt transformer is used, it can be obtained from a datasheet. It can be assumed that the leakage inductance is 3% of the magnetizing inductance, .
If a transformer is well designed, leakage inductance can be reduced to 1% of the magnetizing inductance.
RCD snubber
- Snubber resistor
To calculate the snubber resistance, , an acceptable max transistor voltage, . You want to select a that has a wide margin from the peak transistor voltage rating specified in its datasheet. It still must be greater than the transistors blocking voltage,
Using this, you can calculate , and
- Snubber capacitor
- Snubber diode
The diode voltage must be able to block voltage a high voltage, 1N4007 tends to work.
Example
Flyback using the following specifications:
Calculations
- Select:
- Select:
- Select:
Schottky snubber design
If you choose to use a schottky diode its a good idea to have a snubber.
- design to be added