(Vg was different in the spec from the calculations. Changed to 150V) Tag: Visual edit |
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===Example=== |
===Example=== |
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− | + | Flyback using the following specifications: |
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+ | |||
− | <math>V_g= |
+ | <math>V_g = 150~{\rm V},\ V_{out} = 30~{\rm V},\ n = 0.2,\ f_s = 50~{\rm kHz},\ L_m = 320~{\rm \text{µ}H},\ I = 5~{\rm A},\ V_{t-peak} = 500~{\rm V}</math> |
====Calculations==== |
====Calculations==== |
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− | *<math>V_{t-peak} > V_t > Vg + |
+ | *<math>V_{t-peak} > V_t > Vg + \frac{V_{out}}{n}</math> |
− | *<math> |
+ | *<math>400~{\rm V} > V_t > 150 + \frac{30}{0.2}</math> |
− | *<math> |
+ | *<math>400~{\rm V} > V_t > 300</math> |
− | *Select: <math>V_t= |
+ | *Select: <math>V_t = 325~{\rm V}</math> |
− | *<math>L_k=0.03 |
+ | *<math>L_k = 0.03 \times L_m = (0.03)(0.001) = 30~{\rm \text{µ}H}</math> |
− | *<math>P_s = 1 |
+ | *<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> |
− | *<math>V_s = V_t - V_g= |
+ | *<math>V_s = V_t - V_g = 175~{\rm V} </math> |
− | *<math>R_s = V_s^2 |
+ | *<math>R_s = \frac{V_s^2}{P_s} = 9074~\Omega</math> |
− | *Select: <math>R_s = |
+ | *Select: <math>R_s = 10~{\rm k\Omega},\ 5~{\rm W}</math> |
− | *<math>C_s >> T_s |
+ | *<math>C_s >> \frac{T_s}{R_s} = \frac{10~{\rm \text{µ}s}}{10~{\rm k\Omega}} = 1~{\rm nF}</math> |
− | *Select: <math>C_s = |
+ | *Select: <math>C_s = 47~{\rm nF},\ 500~{\rm V}</math> |
==Schottky snubber design== |
==Schottky snubber design== |
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