Applications

Forward DC/DC Converters

A forward converter is a type of DC-DC converter that, like the flyback and half-bridge converters, can supply an output voltage either higher or lower than the input voltage and provide electrical isolation via a transformer. Although more complex than a flyback , the forward converter design can yield higher output power (generally up to 200W) along with higher energy efficiency.

The circuitry on the secondary (i.e. right) side is almost identical to a buck converter, and forward converters store and deliver energy in much the same way. The switching element, often a power MOSFET or IGBT, ideally is either opened or closed – off or on – so the forward converter will alternate between two different states.

When the switch is closed (MOSFET on-state), current flows through both the primary (leftmost) and secondary windings. In this state, energy is stored in the inductor and the capacitor, and no current flows through the tertiary (middle) winding due to the tertiary diode being reserve biased. It is important to note that, unlike a flyback, forward converters do not store any significant amount of energy in the transformer, and are therefore free from the corresponding limitations of doing so.

When the switch is opened, current is prevented from flowing in the primary and secondary coils. Power is supplied to the load by the inductor while the rightmost diode provides a return path for the current. However, the transformer coils still act as coupled inductors, so the tertiary winding is added in order to reset the flux stored in the transformer core, inducing a counter-clockwise flow of current that returns energy to the input voltage supply.

As the inductor continues to deliver its stored energy to the load, the inductor current will begin to drop. This current drop off continues until the next on-state, when the current rises to its previous value. This undesirable phenomenon is called output ripple, and is usually reduced to an acceptable level by placing a suitably large “smoothing” capacitor in parallel with the output. Output ripple can also be reduced by designing the converter to operate at higher switching frequencies, shortening the period of time spent in off-state.

PWM Control

V_IN

V_OUT

Forward Converter Topology

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This design is for reference only. The design, as well as the products suggested, has not been tested for compatibility or interoperability.

PWM Controllers for Forward DC/DC Converters

Pulse Width Modulation (PWM) is widely used in switch mode power supplies that use digital control to provide the switching action. PWM itself is a controlled digital output signal. The PWM controller controls the rapid switching in a power supply by sending a pulse to the gate driver that drives a power MOSFET (or other switching device like a bipolar transistor, IGBT, etc.) One advantage of PWM is that the signal is digital. Digital signals are more immune to noise, because a digital signal is either a binary “1” or “0.” Therefore noise can only change a digital signal if it is big enough to change a logical “0” to register at the receiving end as a logical “1”, or vice versa.

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MOSFETs for Forward DC/DC Converters

Metal-oxide-semiconductor field-effect transistors (MOSFETs) are by far the most common of transistors today, being used for flash memory, processors, random-access memory (RAM), and application-specific integrated circuits (ASICs), and more. MOSFETs can be conceptualized as a voltage-controlled device for limiting current flow.

MOSFETs are also for power switching circuits. Unlike bipolar junction transistors (BJTs), the competing type of power transistor, MOSFETs do not require a continuous flow of drive current to remain in the ON state. Additionally, MOSFETs can offer higher switching speeds, lower switching power losses, lower on-resistances, and reduced susceptibility to thermal runaway. In switched-mode power supplies (SMPSs), MOSFETS are often used as the switching elements as well as for power factor correction (PFC).

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Diodes for Forward DC/DC Converters

The first silicon-based electronic component, diodes are passive devices which are found in virtually every electronic product or device. The ideal diode allows current to flow freely in one direction and completely prevents current from flowing in the opposite direction. Although, at their core, semiconductor diodes consist of a single P-N junction, there is vast array of different diode types and designs. The zener diode, for example, is designed to also conduct current in the opposite direction when reverse-biased at or above a specific voltage threshold known as the "breakdown voltage". AC/DC power supplies often employ diodes in a bridge-type configuration to rectify the AC input.

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Capacitors for Forward DC/DC Converters

A capacitor is a passive electronic component that stores energy in the form of an electric field. As part of an electrical circuit, capacitors "oppose" changes in voltage by supplying (or drawing) current. An ideal capacitor is characterized simply by its capacitance value, the device's ability to store charge. However, a real-world capacitor has many additional characteristics, such as tolerance rating, working voltage, leakage current, temperature coeffecient, and equivalent series resistance (ESR) – each of which may carry a different level of importance for any given application.

Many types of capacitors exist to perform a variety of functions for a variety of different applications. Decoupling capacitors protect electrical circuits from destructive voltage spikes and transients. Similarly, coupling capacitors serve to block direct current, which can cause damage to certain electronics, while only allowing the AC signal to pass. AC-to-DC power supplies use a reservoir capacitor to smooth the output of a rectifier stage.

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Inductors for Forward DC/DC Converters

Like a capacitor, inductors are also passive energy-storing devices. Inductors, however, store energy in a magnetic field, and have the effect of opposing changes in current flow. An ideal inductor is characterized by a single value called inductance, which is measured in units called henries. Physical, real-world inductors generally consist of a coil of wire wrapped around a core of ferromagnetic material. However, not all inductors use a magnetic core, and the material used directly affects the non-ideal properties of the device such as eddy current losses, magnetic saturation, peak current, and high-frequency losses. Mutual inductance, formed by one or more inductors with coupled magnetic flux, is the principle that underlies another electronic device, the transformer. In a switched-mode power supply, one or more inductors can be used for both output filtering and energy storage (often implemented as a transformer).

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