Half-Bridge Push-Pull Converter

Main page |

How
to use the program |

Function
principals |

Mathematics
used in the program |

Help
for HF transformer

Top of page |

Application            |

Tips              |

Literature
Notes
|

Help
for choking coils

How to use the program
Reference: The shapes of current and voltage curves are
calculated using Faraday's Law. They do not represent an incremental simulation
like it is done normally by programs like P-Spice. In the calculations the
forward voltages of the diodes are considered with VF = 0.7V,
and the transistors are interpreted as ideal switches.

The values of all input fields can be changed.

If an input field is left empty, a default value is chosen. This value is
displayed after leaving the input field in question.

The switch mode power supply operates within a certain input range i.e.
between Vin_min and
Vin_max.
Note:

For the european mains of 230V +/-10% and behind the rectifier and the
smoothing (with a voltage ripple of 10%) the input voltage range is
between Vin_min = 250V and Vin_max
= 360V.

For wide range Switch Mode Power Supplies the input voltage range of the
mains is from 100Vac -10% (Japan) to 240Vac +6% (Great Britain). In this
case, the DC input range of the power supply is from
Vin_min =110V to Vin_max =360V.

For use of a power factor pre-regulator the input voltage range is
normally from Vin_min =360V to Vin_max
=400V.

The program needs the output values Vout and
Iout.

The switching frequency  f  is the operating frequency of
the transistor.

If the field "proposal" is activated for the inductor L, a value
for L and the corresponding current ripple ΔIL is
proposed. These values are laid out such that ΔIL =
0.4Iout with Vin_max as the input voltage.

If the field "proposal" for the input field
"N1/N2" is activated, the turns
ratio N1/N2 is proposed. This suggestion
is chosen such that the required output voltage can be achieved using
Vin_min as an input voltage.

If you do not agree with our proposals, you can change
N1/N2 or L as well as
ΔIL. The field "proposal" is then deactivated automatically.

The value Vin is the value for the calculation of the
current and voltage diagrams on the right side of the display.
Vin must lie between Vin_min and
Vin_max.

Top
of page

Application
The
Half-Bridge Push-Pull Converter belongs to the primary switched
converter family since there is isolation between input and output. It is
suitable for output powers up to 1kW.

Top
of page

Function principals

Illustration 1: Half-Bridge
Push-Pull Converter

For the
following analysis it will be assumed that the transistor is simplified as an
ideal switch and the diode has no forward voltage drop. In the program itself,
the diode will take into account a forward voltage drop VF =
0.7V.

The Push-Pull converter drives the high-frequency transformer with an AC
voltage, where the negative as well as the positive half swing transfers
energy. The capacitor-bridge generates, in its centre point, a voltage of ½
Vin.
The primary transformer voltage V1
can be +½ Vin, -½ Vin or zero
depending on whether the upper transistor, the lower transistor or neither is
on.
On the secondary side, the AC voltage is rectified, so that
V3 is a pulse-width-modulated voltage which switches between
½·Vin·(N2/N1) and zero.
Due to the rectification, the pulse-frequency of V3 is equal
to 2· f .
The Low-Pass filter, formed by the inductor
L and the output capacitor Cout, produces the average
value of V3. For continuous mode (IL never
becomes zero) this leads to:

The Duty cycle
of this converter may theoretically increase to 100%. In practice this is not
possible because the serial connected transistors, T1 and
T2, have to be switched with a time difference to avoid a
short circuit of the input supply.

Due to the fact that the duty cycle t1/T can
theoretically increase to 100%, it follows for the turns ratio that:

In the program,
this value is multiplied by a factor of 0.95, so that the proposed value for
N1/N2 includes a small margin which
guarantees the demagnetisation of the core, when the input voltage is minimal,
(remember: at minimum input voltage the duty cycle reaches its maximum).

For the allocation of the inductor L, the same rules
as for the Buck
Converter can be used. One also distinguishes between discontinuous
and continuous mode, depending on whether or not the inductor current
falls to zero during the on-time of the transistor.

During continuous operation:

In continuous mode the output voltage depends only on the duty cycle and
the input voltage, it is load independent.

The inductor current
IL has a triangular shape and its average value is determined
by the load. The change in inductor current ΔIL is dependent
on L and can be calculated with the help of Faraday's Law.
During
continuous mode, with Vout = Vin ·
(N2/N1) ·t1/T and a
chosen switching frequency  f  it can be shown that:

The change in inductor current is load independent. The output current
Iout is taken to be the average value of the inductor
current IL.

For a small load current, namely if
Iout < ΔIL/2, the current will fall to
zero during every period. This is what is known as discontinuous mode.
In this case the calculations stated above are no longer valid.
In that
moment, when the inductor current becomes zero, the voltage
V3 jumps to the value of Vout. The diode
junction capacitance of the secondary rectifier forms a resonant circuit with
the inductance, which is activated by the voltage jump at the rectifier. The
voltage V3 then oscillates and fades away.

Continuous Mode

Discontinuous
Mode

Illustration 2: Operating modes of the
Half-Bridge Push-Pull Converter

Top
of page

Tips

The larger the chosen value of the inductor L, the smaller the
current ripple ΔIL. However this results in a physically
larger and heavier inductor.

The higher the chosen value of the switching frequency
f , the smaller the size of the inductor. However the
switching losses of the transistor also become larger as  f
increases.

The smallest possible physical size for the inductor is achieved when
ΔIL = 2Iout at Vin_max.
However, the switching losses at the transistors are at their highest in this
state.

Choose ΔIL so that it is not too big. The suggestions
proposed by us have adequately small current ripple along with physically
small inductor size. With a larger current ripple, the voltage ripple of the
output voltage Vout becomes clearly bigger while the
physical size of the inductor decreases marginally.

It is best not to alter the turns ratio
N1/N2 proposed by us.

Top
of page

Mathematics used in the program
The following parameters must be entered
into the input fields:

Vin_min, Vin_max,
Vout, Iout and
f

Using these parameters, the program produces a proposal for
N1/N2 and L:

(the factor of
0.95 is taken into account to allow for the fact that the duty cycle
t1/T = 1 cannot be completely reached).

VF = 0.7 (Diode
Forward-voltage)

ΔIL =
0.4Iout

For the calculation of the curve-shapes, and
also for the calculation of "ΔIL for
Vin_max", two cases have to be distinguished, i.e.
continuous mode and discontinuous mode:

From this it follows that:

For ΔIL< 2Iout the converter
is in continuous mode and it follows that:

For ΔIL> 2Iout the converter
is in discontinuous mode and it follows that:

Main page |

How
to use the program |

Function
principals |

Mathematics
used in the program |

Help
for HF transformer

Top
of page |

Application            |

Tips              |

Literature
Notes
|

Help
for choking coils