The Forward and Flyback converters are two popular topologies widelyused in isolated DC-DC power converters. Thesetopologies are favored by designers for their simplicity, ability tohandle multiple isolated outputs, and ease to optimize the duty cycleby selecting the transformer turns ratio.
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However, the drawback to this single switch approach is that thevoltage stress on the switch is the sum of the input voltage, thereflected transformer voltage and the turnoff voltage spike caused byleakage inductance.
Adding a second MOSFET switch onthe high side results in the Two-Switch Forward or Flyback topology, ofwhich the voltage stress on each MOSFET is clamped to the inputvoltage. The leakage inductance energy is also clamped and recycledback to the input to improve efficiency.
The dissipative snubber circuit that is often required in the singleswitch approach is no longer required. MOSFET switches with a ratedvoltage slightly higher than the input voltage can be employed in thetwo-switch topology, while a rating of greater than twice the inputvoltage is required for the single-switch topology.
For many applications the added complexity and part count ofTwo-Switch Forward and Flyback converters can be a small price to payfor the benefits received.
|Figure1. Two-Switch Forward Converter Topology|
Two-Switch Forward Converter
Figure 1 above shows theTwo-Switch Forward converter topology, which consists of the inputcapacitor CIN, two MOSFET switches Q1 and Q1, the power transformer T1,two clamp diodes D3 and D4, two rectifier diodes D1 and D2, and theoutput filter consisting of LO and Co.
Figures 2a and 2b below depictthe operation of the Two-Switch Forward converter. Both Q1 and Q2 areturned on and off simultaneously. When they are on, as shown in Figure2a, power is delivered to the load through the transformer and theoutput filter.
When the MOSFETs are turned off, as shown in Figure 2b, power flowin the primary circuit is cut off, and the voltage on the primarywinding will reverse until the dot end is clamped to return by D3 andthe non-dot end is clamped to VIN by D4. Therefore, each MOSFET willsee a turnoff voltage stress magnitude of VIN.
|Figure2. Operating Modes of the Two-Switch Forward Converter|
Not only is the energy from the transformer magnetizing inductanceclamped but more importantly the leakage inductance energy is alsoclamped and returned to the input power bus through diodes D3 and D4.Energy stored in the leakage inductance during the on-time does nothave to be dissipated in a resistive snubber or the MOSFETs themselves.
This advantage over a single switch approach reduces system powerlosses and reduces system noise, since the ringing normally associatedwith the release of the inductive energy is now clamped. Consequently,there is no need for snubber circuit and the EMI signature of theconverter is greatly reduced.
Transformer core reset in a single switch Forward converter isnormally accomplished with a tertiary reset winding. Generally thereset 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-timeof the transistor. The voltage stress on the MOSFET switch will betwice the input voltage plus the spike caused by the leakage energy.
By limiting the duty cycle of the MOSFET switch to less than 50% thetransformer core will always reset each cycle. The two-switch Forwardconverter resets the transformer in exactly the same way without theadditional reset winding, because the conduction of D3 and D4effectively applies the input voltage in reversed polarity to the powertransformer primary winding to reset the core.
Since the maximum drain to source voltage across the MOSFETs isclamped to VIN, there is no uncertainty as to what the peak voltagestress will be. This benefit can not be overstated. Peak voltage stressin a single switch approach is proportional to the value of leakageinductance, switching speed and circuit layout. Leakage inductance isdifficult to control and can often vary even after the design goes intoproduction.
At first glance, the series conduction loss of the high side MOSFETappears to be additional power dissipation. However, a study of MOSFETprocess characteristics reveals that the two-switch topology canactually results in a reduction of conduction losses. For asingle-switch Forward converter with a 36V to 75V input application, a200V MOSFET is often required provided the leakage inductance spike iscontrolled.
The die size, and hence the cost of a MOSFET, are proportional toboth the on-resistance (RdsON) and the voltage rating. While thetwo-switch approach requires two MOSFETs in series, the totalresistance of the two MOSFETs usually can be smaller than a singleswitch with twice the voltage capability, for a given die size.
Gate drive losses are obviously higher with two switches, but withthe lower Rds(ON) and the elimination of leakage inductance loss oftenresults in a gain of conversion efficiency. The elimination of snubbercomponents and control of the leakage inductance effects are bigbenefits of the two-switch topology especially at higher inputvoltages.
Higher input voltage applications often have more primary turnswhich tend to increase leakage inductance and loss. The benefits of thetwo-switch approach increase with increasing input voltage, but lowerinput voltage applications can often benefit as well.
Historically, driving the high side MOSFET has been a challenge forthe two-switch topology since the high side MOSFET requires a floatinggate driver. New monolithic IC regulators eliminate the headache of thehigh side MOSFET gate drive through the use of a boot-strap capacitortechnique controlled by a high speed level shift circuit. Figure 3 below shows the blockdiagram of the high side gate drive implementation.
|Figure3. Block Diagram of High Side Gate Drive Circuit|
The advantages of Two-Switch Forward become more remarkable in anintegrated solution where the complete control circuit, gate drive forboth high side and low side switches, and even the two high voltageMOSFETs, can all be integrated in the same IC.
By clamping voltage stress on the MOSFETs, the maximum input voltagerange of the power converter can approach the rated voltage of theMOSFETs, making full use of the MOSFET process capability. In contrast,the maximum input voltage range for a single-switch Forward converteris limited to less than half the rated voltage of the MOSFET.
A typical example of the fully integrated Two-Switch DC-DC regulatoris National Semiconductor's LM5015, which provides a high performancelow cost DC-DC regulator solution capable of a very wide input voltagerange from 4.25V to 75V.
Two Switch Flyback Converter
Figure 4 below shows aTwo-Switch Flyback converter topology, which consists of two MOSFETswitches Q1 and Q2, the power transformer T1, two clamp diodes D1 andD2, the secondary rectifier diode DO, the input filter capacitor CINand the output filter capacitor CO.
|Figure4. Two-Switch Flyback Converter Topology|
Both MOSFET switches are turned on and off simultaneously, as in theTwo-Switch Forward converter. The operation of the Flyback transformeris best described as two-winding coupled inductor. Energy is suppliedto the inductor in the primary circuit when the primary MOSFETs areactive, then the energy is released to the secondary when the primaryMOSFETs turn off.
The coupling between the primary and secondary windings is neverperfect; this leakage inductance can destroy the primary MOSFET in asingle switch approach if left unchecked. The clamp diodes in theTwo-Switch Flyback are used to recover the leakage energy back to theinput, and to clamp the turn-off peak voltage across each MOSFET atVIN.
All of the same benefits are realized in the Two-Switch Flyback asin the Two-Switch Forward. The voltage stresses on the MOSFET switchesare clamped to VIN, and the leakage inductance energy is returned tothe input instead of being dissipated in snubbers that are normallyrequired in the single switch approach.
The same techniques as shown in Fig. 3 can be used for the high sideMOSFET gate drive. The Two-Switch Flyback can be operated in eitherdiscontinuous or continuous conduction mode just like the Single-SwitchFlyback converter.
By adding a high side MOSFET switch, the Two-Switch Forward or Flybacktopology clamps the voltage stress on each MOSFET switch to the inputvoltage. With the two-switch approach, the leakage energy is recycledback to the input to improve efficiency and there is no need fordissipative snubber circuit that is often required in the single switchconverters.
The added complexity and part count of the Two-Switch Forward andFlyback converters can be a small price to pay for the benefitsreceived. The advantages of two-switch approach become more remarkablein an integrated solution in which the gate drive for both switches,and even the two MOSFET switches can be integrated in the same IC withthe control circuit.
The integrated solution allows the input voltage range to approachthe rating of the MOSFET process capability providing a small formfactor, high performance solution.
Robert Bell is an application engineer and Youhao Xi is a PrincipalApplications Engineer in the Power Management Group, NationalSemiconductor Phoenix Design Center.
Editor's Note: The authors of this article will bepresenting at the Embedded Power Conference to be held September 17-18in San Jose, Ca. which will include 20 courses by instructors from 20or more companies on all aspects of power management design as well asan experts panel. To attend, go to the Embedded Power Conferenceregistration page.