AN1300 APPLICATION NOTE

2/9

In the parallel-resonant converter, the load is placed in parallel with the resonant tank capacitor. This is done

through a forward-mode transformer where the output rectifiers conduct simultaneously with the positive and

negative excursions of voltage on the primary winding. The load though, must represent as high an impedance

as possible so it does not load the tank capacitor too much which would excessively lower the tank circuit's "Q".

This is done by the use of a choke-input (L-C) filter following the output rectifiers. The choke input filter is high

impedance when viewed above its filter pole frequency. The output voltage is mathematically the area under

the curve of the output voltage waveform. The number of turns in the secondary winding is therefore higher

than in a series resonant converter.

In the series resonant converter, which is proposed here, the load is placed in series with the resonant induc-

tance and capacitance. Here the load must be low impedance. This is best done by the use of a capacitor-input

filter on the transformer's secondary, where the tank views the low impedance of the output capacitor(s) reflect-

ed to the primary winding of the transformer.

Both resonant topologies are driven with a symmetrical waveform above that tank circuit's resonance frequency.

The output power is controlled by where on the tank circuit's gain Vs frequency curve the converter is operating.

The closer, to the tank's resonance, the higher is the output power.

The series-resonant half-bridge causes the current to resonate in sinusoidal fashion when the tank circuit's volt-

grees, the primary and secondary leakage inductances and the inter-turn and inter winding capacitances. The

load impedance on the secondary of the transformer is also reflected to the primary circuit and becomes a por-

tion of the tank circuit. By incorporating these parasitic elements into the tank circuit, the noise typically gener-

ated by their unpredictable behavior within PWM converters is harnessed for real work by the converter.

The half-bridge capacitors (C1, C2) are in series with the tank circuit. Traditionally these are high enough in val-

ue such that the center node stays at a fixed voltage of approximately one-half the DC input voltage. These ca-

pacitors though could be reduced in value and used as the resonant capacitor itself, thus eliminating the

resonant capacitor. Going yet another step further, these capacitors are electrically in parallel within the reso-

nant circuit, and one of them can be eliminated with no detriment to the converter's operation. The upper ca-

pacitor would be the logical choice to eliminate from a reliability standpoint.

The external resonant inductor can also be eliminated, if one realizes that the leakage inductance is in series

with the primary winding, as is the external resonant capacitor. If one purposely makes a transformer with high

primary leakage inductance, one can then eliminate the external resonant inductor in some cases. For the pow-

er systems (220-240 VAC), where the transformer has more turns on the primary winding, developing a leakage

inductance of around 100

µH is possible. This is done by using a 2-sectioned bobbin where the primary winding

is placed in one-half of the bobbin and the secondary in the other half. This lowering of the coupling between

the primary and secondary and the core, raises the leakage inductance. For the low voltage AC power systems

(100 - 120 VAC), it can be more difficult to create the large leakage inductance, so an external inductor may still

need to be placed within the tank circuit.

The tank circuit's approximate resonance frequency, neglecting the dead time period, can then be then calcu-

lated by:

[Eq. 1]

Where: Lord is the series combination of Lelk + Lr(ext) if used

C'r is the AC value of the bridge capacitor(s)

This frequency will be the primary point of reference for the converter. The control circuit must always stay

above this point to maintain the advantage of zero current switching in the semiconductors.

The output load determines the "Q" of the power stage. Its equivalent resistance is reflected through the trans-

former. This is given by equation 2.

[Eq. 2]

f

o

1

2

π L'rC'r

-------------------------------

=

Q

L

r

C

r

------

R

pri

R

refl

+

-----------------------------

=