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The phase-shifted full bridge (PSFB) proven in **Determine 1** is in style in functions >500 W as a result of it will probably obtain comfortable switching on its enter switches for prime converter effectivity. Though switching losses are drastically diminished, you’ll be able to nonetheless count on to see high-voltage stress on the output rectifier, as its parasitic capacitance resonates with the transformer leakage inductance—modeled as L_{r} in Determine 1. The voltage stress of the output rectifier could possibly be as excessive as 2V_{IN}N_{S}/N_{P}, the place N_{P} and N_{S} are the transformer main and secondary windings, respectively.

Limiting the utmost voltage stress on the output rectifier historically requires a passive snubber [1] similar to a resistor-capacitor-diode (RCD) snubber, however using a passive snubber will dissipate energy, leading to an effectivity penalty.

**Determine 1 **A PSFB energy stage with a passive clamp and waveforms, using the passive clamp dissipates energy which ends up in an effectivity penalty. Supply: Texas Devices

Alternatively, you possibly can apply an energetic snubber to clamp the rectifier voltage stress with out dissipating any energy within the snubber circuit (assuming a great change) [2]. **Determine 2** reveals the insertion of an energetic clamp leg (ACL) shaped by a capacitor (C_{CL}) and a MOSFET (Q_{CL}) earlier than the output inductor. When the output winding voltage turns into non-zero, vitality will switch from the first winding to the secondary winding to energise the output inductor whereas additionally conducting present by the Q_{CL} physique diode to cost C_{CL}, even when Q_{CL} isn’t turned on. You’ll be able to activate Q_{CL} after its physique diode has already carried out present to make sure zero voltage switching (ZVS) on Q_{CL}.

**Determine 2 **A PSFB energy stage with an energetic clamp and waveforms, not like the passive snubber, the energetic snubber doesn’t dissipate the ringing vitality on the facility resistor however circulates the vitality within the LC resonant tank as a lossless snubber. Supply: Texas Devices

It’s necessary to activate Q_{CL} earlier than the present within the energetic clamp MOSFET (i_{CL})polarity modifications to permit the current-second stability on C_{CL} to be full by the start of the efficient responsibility cycle (D_{eff}T_{S}). In different phrases, Q_{CL} solely must be turned on lengthy sufficient for the current-second stability of the energetic snubber to work as supposed, clamping the output rectifier voltage to the C_{CL} voltage (V_{CL}). In different phrases, Q_{CL} doesn’t must conduct all through the complete D_{eff}T_{S}, however for a comparatively quick time frame as a substitute. As such, Q_{CL} can have a hard and fast on-time—that’s, the Q_{CL} on time (D_{ACL}T_{S}) is fixed—whereas preserving D_{eff}T_{S} all the time better than the length the place the current-second stability (D_{CSB}T_{S}) is full.

This strategy addresses one of many challenges when utilizing an energetic snubber in that the transformer winding present doesn’t rise monotonically—which is a matter in case you are utilizing peak current-mode management. This occurs as a result of the energetic snubber capacitor vitality additionally participates in energizing the output inductor, relatively than solely counting on vitality switch from the first facet. Since D_{eff}T_{S} is bigger than D_{CSB}T_{S}, peak present detection can happen when the transformer present is rising monotonically. And since you’ll be able to count on greater effectivity for a PSFB with a bigger D_{eff}, you’ll be able to design the PSFB to have a bigger D_{eff} at mid to heavy hundreds, the place D_{eff} >> D_{CSB}. At mild hundreds, the converter ought to function in discontinuous conduction mode, the place D_{eff} will likely be smaller than D_{eff} in steady conduction mode on the similar enter/output voltage situation. With the intention to maintain D_{eff}T_{S} better than D_{CSB}T_{S} even at mild hundreds, you should utilize frequency-reduction management or burst-mode management.

As a result of the C_{CL} ripple voltage impacts the overall voltage stress on the output rectifier, it’s essential to choose a large-enough C_{CL} for a low capacitor ripple voltage. You have to additionally choose C_{CL} such that the inductor-capacitor (LC) resonant interval shaped by L_{r} and C_{CL} is for much longer than the switching interval [3], expressed by **Equation 1**:

The rectifier voltage stress will clamp to round V_{IN}N_{S}/N_{P} with the energetic snubber, which is half of the voltage stress with none clamp circuit. In contrast to the passive snubber in [1], the energetic snubber doesn’t dissipate the ringing vitality on the facility resistor however circulates the vitality within the LC resonant tank as a lossless snubber. Due to this fact, you’ll be able to count on greater converter effectivity on a PSFB with an energetic snubber than a PSFB with a passive snubber in an an identical specification.

To grasp the components that decide the ACL present degree, you’ll must calculate the present move by the ACL itself. **Determine 3** illustrates waveforms across the ACL conduction interval.

**Determine 3 **Waveforms throughout an ACL present conduction interval. Supply: Texas Devices

Assuming that V_{CL} is a continuing and L_{m} = ∞, **Equation 2** derives the present in a single facet of the output rectifier (i_{SR2}) because the drain to supply voltage rises as:

By assuming i_{SR2} present decreases at a relentless price, **Equation 3** derives the time length of t_{2}-t_{1} as:

Since C_{CL} wants to keep up current-second stability, the sum of areas A1 and A3 will equal space A2. With all of this info, it’s potential to calculate the root-mean-square (RMS) worth of i_{CL}. As Equation 3 reveals, the synchronous rectifier (SR) output capacitance (C_{oss}) controls the height present on the ACL. If you choose a decrease C_{oss} SR FET, the ACL RMS present will likely be decrease and thus assist enhance converter effectivity.

**Determine 4** reveals waveforms of the Texas Devices (TI) 54-V, 3-kW phase-shifted full bridge with energetic clamp reference design, which is a 400-V enter, 54-V output, 3-kW PSFB converter utilizing an energetic clamp realized with TI’s C2000™ microcontroller. On this design, the transformer turns ratio is Np:Ns = 16:3. With the ACL FET turned on just for 300 ns inside the output inductor energizing interval, the output rectifier voltage stress (Ch1 in Determine 4) is proscribed to 80 V, even at a 3-kW load. The decrease voltage stress allows using SR FETs with a decrease voltage ranking and a greater determine of advantage to additional enhance the effectivity of the PSFB.

**Determine 4 **A 54-V, 3-kW phase-shifted full bridge with energetic clamp reference design steady-state waveforms. Supply: Texas Devices

This management methodology isn’t restricted to a full-bridge rectifier with one ACL; you may also apply it to an energetic snubber with different varieties of rectifiers similar to a present doubler [4] or a center-tapped rectifier. TI’s 3-kW phase-shifted full bridge with energetic clamp reference design with >270-W/in^{3} energy density has a 400-V enter, 12-V output, 3-kW PSFB converter with an energetic clamp the place the secondary facet makes use of a center-tapped rectifier. The output rectifier stress (Ch1 in **Determine 5**) is proscribed to 40 V at a 3-kW load.

**Determine 5 **A 3-kW phase-shifted full bridge with energetic clamp reference design with >270-W/in^{3} energy density steady-state waveforms. Supply: Texas Devices

**The advantage of an energetic clamp in a PSFB converter**

The implementation of an energetic snubber in a PSFB converter considerably reduces the utmost voltage stress on the output rectifiers. This discount in voltage stress allows using an SR FET with a decrease drain-to-source voltage ranking, which may have a greater determine of advantage. Whereas an energetic clamp can create challenges with the implementation of peak current-mode management, correct implementation allows using an energetic clamp and peak current-mode management in concord. This mixture achieves greater energy density and better effectivity in comparison with conventional PSFB implementations.

*Ben Lough acquired his M.S. in electrical engineering on the Ohio State College in 2016. He joined TI in 2016 engaged on AC/DC energy conversion, energy issue correction and remoted DC/DC design. He has authored over 15 technical articles at TI and exterior publications. He at the moment works as a programs engineer within the Energy Design Providers group at TI.*

**Associated Content material**

**References**

- Lin, Music-Yi, and Chern-Lin Chen. “Evaluation and Design for RCD Clamped Snubber Utilized in Output Rectifier of Part-Shift Full-Bridge ZVS Converters.” Revealed in IEEE Transactions on Industrial Electronics 45, no. 2 (April 1998): pp. 358-359.
- Sabate, J.A., V. Vlatkovic, R.B. Ridley, and F.C. Lee. “Excessive-Voltage, Excessive-Energy, ZVS, Full-Bridge PWM Converter Using an Lively Snubber.” Revealed in Sixth Annual Utilized Energy Electronics Convention and Exhibition (APEC), March 10-15, 1991, pp. 158-163.
- Nene. “Digital Management of a Bidirectional DC-DC Converter for Automotive Functions.” Revealed in
*twenty eighth Annual Utilized Energy Electronics Convention and Exposition (APEC)*, March 17-21, 2013, pp. 1360-1365. - Balogh, Laszlo. “Design Overview: 100 W, 400 kHz, DC/DC Converter with Present Doubler Synchronous Rectification Achieves 92% Effectivity.” Texas Devices Energy Provide Design Seminar SEM100, literature No. SLUP111, 1996.

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