As a new technology, the high component count of SMPS made the technology more expensive than linear. But with the birth of the electronic age, component costs have dropped so low that the high raw material content of copper and iron in the linear transformer has made the SMPS technology more cost effective. Even with the disadvantages of being more complex and requiring more care to control EMI, the advantages of switch mode power supplies far outweigh linear power supplies in all but a few niche applications. Switching power supplies are made up of a number of different stages. If the input is an AC input, then the input stage needs to include both the input filter and a rectifier to convert to a DC input. DC to DC converters do not need the rectifier. The inverter stage turns around and immediately converts the now DC input back into an AC input by switching the DC input voltage on and off at a much higher frequency than the original AC input.

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The frequency of operation is often chosen to be in the 20kHz to 150kHz range, which is high enough to be outside the audible range and low enough to keep it outside of the FCC requirements for conducted EMI. After the inverter stage, the output stage rectifies and filters the output. If an isolated design is required, a transformer is placed between the rectifier and output stage.

This transformer can be much smaller, lighter and cheaper than the linear power supply transformer, due to the higher switching frequency. Between the output stage and the inverter stage is a controller which monitors the output and adjusts the switching action to keep the output at the desired level. Buck Buck converters are one of the simplest, cheapest and most common topologies. While this topology is not suited for applications where isolation is required, it is ideal as a DC to DC converter used to step- down voltages.

1. Two-Switch Forward Converter. Switch Mode Power Supply (SMPS) Topologies (Part II). FIGURE 2: BUCK CONVERTER TOPOLOGY: TON PERIOD.
2. Operation and Benefits of Active-Clamp. The forward converter is derived from the buck topology. Operation and Benefits of Active-Clamp Forward Power.

Forward Converter Design Note. 2 Forward Converter Topology. The single active switch is sufficient at lower power levels below 200W.

Not only can you achieve high efficiency levels, but also high power levels using a buck converter, especially with poly-phase topologies. The down side to buck converters is that the input current is always discontinuous, resulting in higher EMI.

However, EMI issues can be addressed with filter components such as chip beads, common mode chokes and filter chokes. The buck topology only requires a single inductor for single-phase applications, and catalog inductors for a wide range of applications are available. In addition, custom inductors can be developed for those special inductance versus current values that are required, as well as for applications requiring extra windings for sensing or supplying power to the controller. Boost The boost topology, like the buck topology, is non-isolating. Unlike the buck topology, the boost steps up the voltage rather than stepping it down. Because the boost topology draws current in a continuous, even manner when operating in continuous conduction mode, it is an ideal choice for Power Factor Correction circuits. Like the buck topology, there are many catalog choices for the inductor used in boost circuits, and where there is a special need, custom inductors are available as well.

Buck-Boost The buck-boost topology can either step the voltage up or down. This topology is particularly useful in battery powered applications, where the input voltage varies over time but has the disadvantage of inverting the output voltage. Another disadvantage to the buck-boost topology is that the switch does not have a ground, which complicates the drive circuit. Using only a single inductor like the buck and the boost topologies, the buck-boost inductor and EMI components are readily available. SEPIC/Ćuk The SEPIC and Ćuk topologies both use capacitors for energy storage in addition to two inductors.

The two inductors can be either separate inductors or a single component in the form of a coupled inductor. Both topologies are similar to the buck-boost topology in that they can step-up or step-down the input voltage, making them ideal for battery applications. The SEPIC has the additional advantage over both the Ćuk and the buck-boost in that its output is non-inverting. An advantage to the SEPIC/ Ćuk topologies is that the capacitor can offer some limited isolation. Catalog coupled inductors are available for the SEPIC and Ćuk topologies, and custom inductors are readily available for special needs.

Flyback The flyback topology is essentially the buck-boost topology that is isolated by using a transformer as the storage inductor. The transformer not only provides isolation, but by varying the turns ratio, the output voltage can be adjusted. Since a transformer is used, multiple outputs are possible.

The flyback is the simplest and most common of the isolated topologies for low-power applications. While they are well suited for high-output voltages, the peak currents are very high, and the topology does not lend itself well to output current above 10A. One advantage of the flyback topology over the other isolated topologies is that many of them require a separate storage inductor. Since the flyback transformer is in reality the storage inductor, no separate inductor is needed. This, coupled with the fact that the rest of the circuitry is simple, makes the flyback topology a cost effective and popular topology. Forward The forward converter is really just a transformer isolated buck converter.

Like the flyback topology, the forward converter is best suited for lower power applications. While efficiency is comparable to the flyback, it does have the disadvantage of having an extra inductor on the output and is not well suited for high voltage outputs. The forward converter does have the advantage over the flyback converter when high output currents are required. Since the output current is non-pulsating, it is well suited for applications where the current is in excess of 15A. Push-Pull The push-pull topology is essentially a forward converter with two primary windings used to create a dual drive winding.

This utilizes the core of the transformer much more efficiently than the flyback or the forward converters. On the other hand, only half the copper is being used at a time, thereby increasing the copper losses significantly in a similar sized transformer. For similar power levels, the push-pull converter will have smaller filters compared to the forward converter. However the advantage that push-pull converters have over flyback and forward converters is that they can be scaled up to higher powers. Switching control can be difficult with push-pull converters, because care has to be taken not to turn on both switches at the same time. Doing so will cause the equal and opposite flux in the transformer, resulting in a low impedance and a very large shoot-through current through the switch, potentially destroy- ing it.

The other disadvantage to the push-pull topology is that the switch stresses are very high (2∙VIN), which makes the topology undesirable for 250VAC and PFC applications. Half-Bridge The half-bridge topology, like push-pull topologies, can be scaled up well to higher power levels and is based on the forward converter topology. This topology also has the same issue of the shoot-through current, if both switches are on at the same time. In order to control this, there needs to be a dead-time between the on-time of each switch. This limits the duty-cycle to about 45%.

Beneficially, the half-bridge topology switching stresses are equal to the input voltage and make it much more suited to 250VAC and PFC applications. On the flip side, the output cur- rents are much higher than the push-pull topology, thereby making it less suited for high current outputs. Resonant LLC The resonant LLC topology is a half-bridge topology that uses a resonant technique to reduce the switching losses due to zero voltage switching, even in no-load conditions. This topology scales up well to high power levels and has very low losses in devices that are on at all times. This topology is not as well suited for stand-by mode power supplies, as the resonant tank circuit needs to be energized continuously.

The resonant LLC also has the advantage over both push-pull and half-bridge topologies of being suitable for a wide range of input voltages. The down side to the resonant LLC topology is its complexity and cost.

Adding a high-side MOSFET switch to the conventional forward and flyback power converter design brings many benefits, especially in higher input-voltage applications. With the resulting two-switch approach, the leakage energy is recycled back to the input to improve efficiency and there is no need for a snubber circuit. And you'll reduce electrical stresses, too, as the switch clamps the voltage on the MOSFET switch to the input voltage. The new solution adds a few more parts, but it's a small price to pay for what you'll get out of it. The performance and cost advantages of such a two-switch approach in a DC/DC regulator design become more remarkable in an integrated solution in which the control circuit, gate drivers, and two MOSFET switches are on the same chip.

Overview Designers favor the forward and flyback converter topologies in isolated DC/DC power converters for their simplicity, flexibility to accommodate multiple isolated outputs, and the easy means by which users can optimize the duty cycle. The conventional forward and flyback converter employs a single MOSFET switch, which is primary-ground referenced for conveniently driving the gate. However, the drawback to this single-switch approach is that the voltage stress on the switch is the sum of the input voltage, the reflected transformer voltage, and the turnoff voltage spike caused by leakage inductance. We can clamp the voltage on each MOSFET to the value of the input voltage by adding a second MOSFET switch on the high side. With this two-switch topology, the leakage inductance energy is also clamped and recycled back to the input to improve efficiency.

The dissipative snubber circuit that often required in the single-switch approach is no longer needed. MOSFET switches with a rated voltage slightly higher than the input voltage can be employed in the two-switch approach, while a rating of greater than twice the input voltage is required for the single-switch topology. Forward-converter circuitry The main components of the traditional single-switch forward converter topology (Fig.

1) are the input capacitor C IN, MOSFET Q 1, power transformer with a tertiary reset winding, T 1, reset clamp diode D 3, secondary rectifier diodes D 1 and D 2, and the output filter consisting of L o and C o. In practice, Q 1 normally requires a dissipative snubber circuit to limit the peak drain-to-source voltage stress when it is turned off. (Click on Image to Enlarge) Fig. 1: Conventional single-switch forward converter Figure 2 shows the two-switch forward converter topology. The secondary circuit is exactly the same as in the single-switch topology. However, the power transformer is simplified by eliminating the tertiary reset winding, thus reducing the transformer's cost.

The primary circuit now employs two MOSFET switches, of which the original Q 1 remains in series between the low side of the transformer and input return, as in the single-switch topology. MOSFET Q 2 is in series between V IN and the high side of the transformer primary. The original reset clamp diode, D 3, is now placed between the high side of the transformer primary and the input return, and the added second clamp diode, D 4, is placed between the input and the low side of the transformer primary. (Click on Image to Enlarge) Fig.

2: Two-switch forward converter topology In operation, both Q 1 and Q 2 are turned on and off simultaneously. When both MOSFETs are on, power is delivered to the load through the transformer and the output filter (Fig. When the MOSFETs are turned off, power flow in the primary circuit is cut off. The current flowing in the magnetizing inductance will cause the voltage on the primary winding to reverse until the dot end is caught at return by D 3 and the non-dot end is caught at V IN by D 4. Each MOSFET sees a maximum voltage at turn-off of V IN (Fig.

(Click on Image to Enlarge) Fig 3a: Current path when MOSFETs are turned on Fig 3b: Current path when MOSFETs are turned off Not only does this circuit clamp the energy from the transformer's magnetizing inductance but, more importantly, the leakage inductance energy is also clamped and returned to the input line through diodes D 3 and D 4. Energy stored in the leakage inductance during the on-time does not have to be dissipated in a resistive snubber or the transistors themselves. This circuit reduces system losses and reduces system noise over the single-switch approach, since the ringing normally associated with the release of the inductive energy is now clamped. Consequently, there is no need for a snubber circuit and electromagnetic interference (EMI) is greatly reduced.

Resetting the transformer core in a single-switch forward converter is normally accomplished with a tertiary reset winding. When Q 1 turns off, the voltage on the reset winding will reverse until it is clamped by diode D 3 to the input voltage. Generally the reset winding has the same number of turns as the primary winding.

Thus, the core will always reset with a reset time equal to the on-time of the transistor. The voltage stress on the MOSFET switch will be twice the input voltage plus the spike caused by the leakage energy. If the duty cycle of the MOSFET switch is limited to less than 50 percent, the transformer core will always reset each cycle.

The two-switch forward converter, on the other hand, resets the transformer in the same way without the additional reset winding. That is, D 3 and D 4 are conducting and effectively apply the input voltage in reversed polarity to the power transformer's primary winding to reset the transformer core. The maximum drain-to-source voltage across the MOSFETs is clamped to V IN. We cannot overstate this benefit. The peak voltage stress in a single-switch approach is proportional to the value of leakage inductance, switching speed, and circuit layout. The transformer's leakage inductance is difficult to control and can often vary even after the design goes into production. At first glance, the series conduction loss of the high-side MOSFET appears to manifest as additional power dissipation.

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However, a study of MOSFET process characteristics reveals that the two-switch topology can actually result in a reduction of conduction losses. For a single-switch forward converter with a 36- to 75-volt input application, a 200-volt MOSFET is often required, provided the leakage inductance spike is controlled.

The die size (and hence the cost) of a MOSFET is proportional to both its on-resistance ( RDS ON) and the voltage rating. While the two-switch approach requires two MOSFETs in series the total resistance of the two MOSFETs is comparable to a single-switch with twice the voltage capability, for a given die size. Gate-drive power losses are obviously higher with two switches, but with the lower RDS ON and the elimination of leakage inductance loss the two-switch topology often results in a gain of conversion efficiency. Control of the leakage-inductance effects, and eliminating snubber components are big benefits of the two-switch topology especially at higher input voltages. Higher input-voltage applications often have more primary turns, increasing the leakage inductance and loss. The benefits of the two-switch approach increase with increasing input voltage, but lower input-voltage applications can often benefit as well.

Historically, driving the high-side MOSFET has been a challenge for the two-switch topology since the high side MOSFET requires a floating gate driver. New monolithic integrated circuit (IC) regulators overcome the challenges of the high-side MOSFET gate-drive through the use of a bootstrap capacitor technique controlled by a high-speed level-shift circuit. Figure 4 shows a block diagram of the high-side gate-drive implementation employed in National's LM5015 two-switch regulator.

(Click on Image to Enlarge) Fig 4: Block diagram, high-side gate drive circuit The advantages of the two-switch forward converter circuit clearly become more significant when the complete control circuit, gate-drive for both high-side and low-side switches, and the two high voltage MOSFETs are integrated in the same monolithic IC. Clamping circuitry limits voltage stress on the MOSFETs, and the maximum input-voltage range of the power converter can approach the rated voltage of the MOSFETs. In contrast, the maximum input-voltage range for a single-switch forward converter is limited to less than half the rated voltage of the MOSFET. The LM5015, a fully integrated two-switch DC/DC regulator, provides a high performance, low cost DC/DC regulator solution capable of a very wide input voltage range from 4.25 to 75 volts. Two-switch flyback operation Figure 5 shows a conventional single-switch flyback converter topology. The main components are the MOSFET switch Q 1, power transformer T 1, secondary rectifier diode D o, input filter capacitor C IN, and output filter capacitor C o.

In practice, Q 1 also requires a dissipative snubber circuit to limit the peak voltage stress when the device is turned off. (Click on Image to Enlarge) Fig 5: Conventional single-switch flyback converter Figure 6 shows a two-switch flyback converter topology. The power transformer and secondary circuit remain the same as in the single-switch approach. In the primary circuit, the original MOSFET switch Q 1 remains in series between the low side of the transformer and input return. An added transistor, Q 2 is in series between V IN and the high side of the transformer primary.

Clamp diodes D 1 and D 2 are placed between the low side of the transformer primary and input return, and between V IN and the high side of the transformer primary, respectively. (Click on Image to Enlarge) Fig 6: Two-switch flyback converter Both MOSFET switches are turned on and off simultaneously, as in the two-switch forward converter. The operation of the flyback transformer is best described as a two-winding coupled inductor. Energy is supplied to the inductor in the primary circuit when the primary MOSFETs are active, then the energy is released to the secondary when the primary MOSFETs are turned off.

The coupling between the primary and secondary windings is never perfect; this leakage inductance can destroy the primary MOSFET in a single-switch approach if left unchecked. The clamp diodes in the two-switch flyback design are used to recover the leakage energy back to the input line, and to clamp the turn-off peak voltage across each MOSFET at V IN. All of the same benefits are realized in the two-switch flyback as in the two-switch forward design.

The voltage stresses on the MOSFET switches are clamped to V IN and the leakage inductance energy is returned to the input line instead of being dissipated in snubbers normally required in the single-switch approach. The same technique can be used for the high-side MOSFET gate drive.

The two-switch flyback can be operated in either discontinuous or continuous conduction mode just like the single-switch flyback converter.