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AC-DC Forward Power Supplies

AC-DC forward power supplies with a relatively simple circuit configuration are widely used for hundreds of watts power supply applications. Forward power supplies have less ripple since the capacitor is continuously charged. Compared to flyback power supplies, they exhibit a higher transformer efficiency and thus can provide up to 500 W.

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Forward Power Supplies

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Application Examples

  • Desktop PCs
  • Power supplies for game consoles
  • Multifunction printers
  • Industrial power supplies
  • Notebook PC adaptors
  • Information and communications equipment

Circuit Overview

When the switch is closed, the energy stored in the primary winding of a transformer is transferred to the secondary winding.

In contrast to flyback converters, the polarity of the secondary winding of a transformer in forward converters is opposite to that of the primary winding. Additionally, to build an AC-DC power supply, forward converters require an inductor (L) and a freewheel diode.

The output voltage of a forward converter is a function of the transformer turns ratio and the transistor on/off ratio.

where, Ns is the number of turns of the secondary winding of a transformer, Np is the number of turns of the primary winding, Ton and Toff are the "on" and "off" periods of the transistor.

Forward-coupled converters (forward converters) are used for hundreds of watts power supply applications.

Flyback Converter Block Diagram / Forward Converter Block Diagram


When Q1 is turned on, the energy stored in the primary winding of a transformer is transferred to the secondary winding.

1. Q1 ON

An electric current flows to Np.

The stored energy is transferred to the secondary winding at the same time, causing a current to flow through D2 and L1 and charging C1.

When power transistor Q1 is on, it supplies power to the output load. Therefore, the cathode voltage of the diode in a forward converter, Vor, is calculated as follows:


VDS1 is the Drain to Source voltage of Q1

Np : the number of Primary winding

VD2 is the Anode to Cathode voltage of D2

Nm : the number of Secondary winding

2. Q1 OFF

When Q1 is turned off, the current flowing through D2 decreases, causing both the primary and secondary windings of the transformer to develop a negative voltage relative to the winding's dot end. The energy stored in the transformer is delivered through D1 as an excitation current and resets the transformer. At this time, the voltage across Nr becomes Vdc+0.7 V.
The voltage across Np reverses, developing a voltage equal to the voltage across Nr. Consequently, the drain-source voltage of Q1 becomes 2Vdc (2Vdc+0.7V ≈2Vdc). When A1=A2, the drain voltage of Q1 (2Vdc) is maintained.

Output Voltage

The output voltage, Vo, is compared to the reference voltage, and its differential output signal is fed to a PWM controller via an error amplifier. The PWM pulse output, in turn, switches on and off the MOSFET (Q1). Vor is maintained at a constant voltage by this negative feedback loop. Vo is calculated as follows:

T : Cycle period  Ton : On-time of Q1 in one cycle

Forward Converter

Waveforms at Various Points in a Forward AC-DC Power Supply


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·Before creating and producing designs and using, customers must also refer to and comply with the latest versions of all relevant TOSHIBA information and the instructions for the application that Product will be used with or for.