FQA - Transformer and Inductor Design Calculations for Push-Pull and Full-Bridge Topologies
ITransformer Design Calculations
1.1Determining the Design Input Parameters
The following parameters must be defined before design:
- Input voltage range: ~
- Output voltage:
- Maximum output power:
- Switching frequency:
- Expected efficiency:
1.2Turns Ratio Calculation (Same for Push-Pull and Full-Bridge)
Considering the maximum duty cycle limit (usually, to reserve dead time), the turns ratio is:
Where is the rectifier diode forward voltage drop; the above formula assumes full-bridge diode rectification on the secondary. Push-pull requires two identical Np windings, while full-bridge needs only one Np
1.3Core Selection and AP-Method Calculation (Same for Push-Pull and Full-Bridge)
Use the Area Product (AP) method for initial core selection:
- : window utilization factor (0.2 for push-pull due to two primary windings, 0.3 for full-bridge)
- : maximum flux density swing (0.25T for PC95 material)
- : current density (400~500 A/cm²)
- : switching frequency (Hz)
Core material selection: - PC95 ferrite is commonly used for its low high-frequency loss,
1.4Turns Calculation (Same for Push-Pull and Full-Bridge)
Primary turns calculation
Based on the fundamental electromagnetic induction formula U=N*Ae*dBm/dt, the primary turns are determined by:
Where is the effective core cross-section (cm²); the factor 2 corresponds to half a cycle,
Note: the push-pull circuit uses a center-tapped primary; each half winding has Np turns
Secondary turns calculation
1.5Key Points of Winding Design
Push-pull transformer symmetry design
The push-pull topology demands extremely high symmetry between the two primary half-windings:
- Turns symmetry: turn deviation between the two halves must be <0.5%, otherwise flux imbalance saturation occurs
- Leakage inductance matching: leakage inductance difference between the two halves must be within 5%
- Flux imbalance suppression measures: 1. Core gap design: introduce an appropriate gap (mm) to increase DC reluctance and improve flux imbalance immunity 2. Series blocking capacitor: add a non-polarized capacitor in the primary loop to block the DC component 3. Current-mode control: cycle-by-cycle current sensing and balancing
Full-bridge transformer design points
Leakage inductance control: leakage in the full-bridge topology helps achieve ZVS
- Winding structure: sandwich winding is recommended to reduce proximity-effect loss
IIOutput Filter Inductor Design Calculations
2.1Inductance Calculation
The output filter inductor usually takes 20%~30% of the rated current as the ripple current peak-to-peak value.
Critical inductance calculation (based on U=L*dI/dt):
Where is the maximum allowed ripple current, typically:
2.2Core Selection
- Ku
- 0.2~0.4 for iron powder and alloy powder toroids
- J
- 300~400A/cm2 for natural cooling, 400~600 A/cm2 for thermally conductive glue or forced-air cooling
- Bmax
- 0.6~1.2 for iron powder and alloy powder cores; consult the datasheet for specifics
2.3Turns Calculation
Based on the AL value (inductance factor):
2.4Wire Diameter Selection and Loss Calculation
Wire diameter selection:
IIIDesign Example: 600W Full-Bridge DC-DC Converter
3.1Design Specifications
Output voltage: 120V DC
Input voltage: 20V~30V DC
- Switching frequency
- 100kHz
- Efficiency target
- >90%
- Ripple current
- <20%
3.2Transformer Design
Turns ratio calculation (taking ):
Core selection:
Select PQ35/30 (, )
Turns calculation (taking ):
3.3Output Inductor Design
Inductance calculation (20% ripple, equivalent frequency 200kHz):
Take .
Core selection:
0.035cm^4
Turns:
IVTransformer and Inductor Loss Calculations
See 'Electromagnetic Loss Calculation for Power Inductors and Transformers'
K, a, b are constants; refer to the manufacturer datasheet. Reference values for the PC95 core:
f in kHz, Bac in mT, Ve in cm3, Pcore in mW