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AC-DC Forward Power Supplies

AC-DC forward power supplies with a relatively simple circuit configuration are widely used for power supply applications with a wattage of a few hundred watts. AC-DC forward power supplies exhibit a higher transformer efficiency than flyback power supplies. Therefore, AC-DC forward power supplies are used for applications with a wattage of up to 500 W. Toshiba offers a wide range of MOSFETs with a VDSS of 600 V to 800 V ideal for AC-DC forward power supply applications.

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Blocking Diode PFC MOSFET PFC Controller ICs Main Switch MOSFET Photocoupler Gate Driver Rectification MOSFET



Name Outline Date of issue
Describes the features of the DTMOSV series and the improvements from the previous series 9/2017

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  • DTMOS Applications (Noise Reduction)
Describes the mechanism of noise generation and noise reduction techniques coming soon

Application Note

Application note
Name outline Date of issue
Provides hints and tips based on simulation results to help you reduce the chip temperature of discrete semiconductor devices. 01/2018
The high dv / dt between the drain and the source of the MOSFET can cause problems and explain the cause of this phenomenon and its countermeasures. 12/2017
Describes mechanism of avalanche phenomenon, I will explain durability and countermeasures against it 12/2017
describes how to reduce the chip temperature of discrete semiconductor devices. 12/2017
describes how to calculate the temperature of discrete semiconductor devices. 12/2017
discusses temperature derating of the MOSFET safe operating area. 12/2017
When a rapidly rising voltage is applied between the drain and source of the MOSFET,the MOSFET may malfunction and turn on, and its mechanism and countermeasures will be explained. 12/2017
Describes the guidelines for the design of a gate driver circuit for MOSFET switching applications and presents examples of gate driver circuits 11/2017

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Describes current imbalance in parallel MOSFETs and the mechanism of parasitic oscillation 11/2017

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Describes the oscillation mechanism of MOSFETs for switching applications 11/2017

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Describes thermal equivalent circuits, examples of channel temperature calculation and considerations for heatsink attachment 2/2017
Describes planar, trench and super-junction power MOSFETs 11/2016
Describes the absolute maximum ratings, thermal impedance and safe operating area of power MOSFETs 11/2016
Describes electrical characteristics shown in datasheets 11/2016
Describes how to select power MOSFETs, temperature characteristics, the impacts of wires and parasitic oscillation, avalanche ruggedness, snubber circuits and so on 11/2016


  • 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.

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  • Operation

    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

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