EE 136 Project Presentation

• The Diagonal Half-Bridge Flyback Converter

• Non-isolated Self-Oscillating Flyback Converter with Primary Current-Mode Control

• Isolated Self-Oscillating Flyback Converter with Primary Current-Mode Control

• Presented By: Latt M. Tee Naw Sandar Shwe

The Diagonal Half-Bridge Flyback Converter

– This converter is suitable for power FETs operation

– Same design procedure applies as a single-ended flyback converter

Circuit Analysis for Half-Bridge Flyback Converter

• Switches is driven by control circuit, both “on” and “off” together

• Flyback action takes place during “off” period

• Cross-connected diodes D1 and D2 return excess flyback energy to supply lines

• D1 and D2 eliminates the need of an energy recovery winding or large snubbing components.

Schematic Diagram for Half-Bridge Flyback Converter

The Circuit Diagram When Switches ‘On’ & ‘Off’ and Voltage and Current Waveform

D1

D2D8

D4

D5

D6C1

C2

TFR1P2W1C3 R1

V1

S1

S2

D1

D2D8

D4

D5

D6C1

C2

TFR1P2W1

D3

C3 R1V1

TRAPEZ1

TRIANG1

Schematic DiagramVoltage and Current waveforms

(using Simplorer Software)

S1

S2

D1

D2D8

D4

D5

D6C1

C2

TFR1P2W1

D3

C3 R1V1

TRAPEZ1

TRIANG1

D6.I [A] 1e-036

-1.3e-018

D5.I [A] 1.3e-018

-1e-036

TFR1P2W1.V1 [V] 1p

-3p

TFR1P2W1.I1 [A] 2.9e-017

-1.2e-018

t [s] 0 4n0.5n 1n 1.5n 2n 2.5n 3n 3.5n

Advantages of this converter

• Using the diodes D1 and D2 is good for over-voltage stress.

• Flyback action returns the stored energy to the supply line.

• Do not need bifilar-wound energy recovery winding

• Reduce the cost and eliminate unreliable source.

Self-oscillating flyback converter

• Low component count without loss of performance

• Widely used in low power applications such as computers, video display terminals and so on

Frequency variation as a function of loadNon-isolated self-oscillating flyback converter

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120

load %

freq.

kHz

• Type C operation• Variable ‘on’ time,

‘off’ time, and repetition rate (freq.)

• Has a more desirable characteristic

Circuit Diagram of non-isolated flyback converter

Base drive current waveform of self-oscillating converter

Schematic and waveforms using Simplorer software

R1

R2

R3

R7

R5

R6

R4

R8 R9

R10

C1 C2E1

D1

BJT1

BJT2

TWT

TWT1

D2C3

+

-

OPV51

TFR1P11

D3

D4

C4E2

BJT1.I [A]

t [s]

2.8m

0

0.5m

1m

1.5m

2m

0 0.4m50u 0.1m 0.15m 0.2m 0.25m 0.3m

R2.I [A]

t [s]

0

-2.8m

-2m

-1.5m

-1m

-0.5m

0 0.4m50u 0.1m 0.15m 0.2m 0.25m 0.3m

Isolated self-oscillating flyback converter

• The input and output voltage does not depend on each other

• Optocoupler is used as a feedback loop• Transistor Q2 and R4 control the current-

mode• Input voltage +300 V DC and output

voltage +12V is applied to the circuit for self-oscillating

Non-isolated self oscillating circuit

The wave form of Q1 and Q2• Q1 Ic increase

Ie increase

• Vbe 0.6V

Q2 turn on

Simulation result

C1

C2

C3

C4

C5

C6

D1

D2R1

D3

Diode601

L1

R2

R3

R4

R5

R6

R7

R8

TWT

TWT1

Diode602

R9

D4

R11

R12

E2R10

C7

R5.V [V]

t [s]

0.1k

-0.25k

0

-0.15k

-0.1k

-50

0 9.5m1m 2m 3m 4m 5m 6m 7m 8m

NPN61

NPN63

NPN62

THY41

NPN61.VBE

t [s]

4k

0

1k

1.5k

2k

2.5k

3k

0 9.5m1m 2m 3m 4m 5m 6m 7m

NPN63.IC

t [s]

0.3

-50m

50m

0.1

0.15

0.2

0 9.5m1m 3m 5m 7m

NPN63.I_B

t [s]

0.14

-20m

20m

40m

60m

80m

0.1

0 9.5m1m 3m 5m 7m

Advantages

• The system as if the primary is a high-impedance current source.

• Results in a first order transfer function.

• The control circuit may have high frequency response.

• Line ripple rejection and stability improved.

• Primary current limiting is provided.

Conclusion

• Low cost

• Fewer components need

• Widely used in low power application