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AC-DC Resonant Full-Bridge Power Supplies

Full-bridge circuits are more complex than half-bridge circuits. However, since full-bridge power supplies provide higher efficiency, they are mainly used for large-capacity applications (with over 1-kW capacity).

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Application Examples

  • Power supplies for base stations
  • EWS
  • High-capacity power supplies
  • Servers
  • Medical equipment

Circuit Overview

Although full-bridge AC-DC power supplies have a complex circuit configuration, low-voltage devices can be used to build them.

Since full-bridge AC-DC power supplies exhibit high efficiency, they are used for 300-W and higher-capacity power supply applications.

The circuit on the primary side of a transformer is comprised of Q1 to Q4, their parasitic diodes, and an inductor (L). The current flows change as shown in the figure according to the "on" and "off" states of the transistors.

Timing 

0 to t1:Q1,Q3:ON    Q2,Q4:OFF

t1 to t2:Q1,Q3:OFF

t2 to t3:Q1,Q3:OFF    Q2,Q4:ON

t3 to t4:Q2,Q4:OFF


Operation

1. Q1/Q3 ON

Q1 and Q3 are turned on. As a result, Vs is applied across the primary winding of the transformer Np.
At the same time, the energy stored in the primary winding is transferred to the secondary winding. Since the polarity of this voltage is positive, an electric current flows through D1 and L, charging C.

2. Q1/Q3 OFF

When Q1 and Q3 are turned off, the energy stored in the reactor LD is flowing a load current in the secondary side.

3. Q2/Q4 ON

Q2 and Q4 are turned on. As a result, Vs is applied across the primary winding of the transformer Np.
At the same time, the energy stored in the primary winding is transferred to the secondary winding. Since the polarity of this voltage is opposite to that at Step 1, a current flows through D2 and L, charging C.

4. Q2/Q4 OFF

When Q2 and Q4 are turned off, the energy stored in the reactor LD is flowing a load current in the secondary side.

The output voltage is approximated as follows:

t(on): Cycle period minus dead time,
T: Cycle period, Np: Primary winding, Ns: Secondary winding (Ns1=Ns2=Ns), Vs>>Vds, Vf

VOUT=[Vs x (Ns/Np) ]x ( 2xt(ON)/T)



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